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CE Directed Reading Techniques, Benefits, and Challenges of PET-MR Positron emission tomography (PET) scans can now be acquired in unison with magnetic resonance (MR) scans as a single resource. This hybrid PET-MR solution combines the anatomic detail and functional data of MR scans with the biologic or physiological information offered by PET scans. This article describes aspects of implementing a PET-MR imaging program, including various technical and operational challenges, scheduling and workflow solutions, room construction and equipment, and finally clinical applications of this novel modality. The Directed Reading also discusses the possible future role of PET-MR in the clinical setting. This article is a Directed Reading. Your access to Directed Reading quizzes for continuing education credit is determined by your membership status and CE preference. Tessa Ocampo, MBA, CNMT Katie Knight, BS, R.T.(N)(MR), CNMT, LMT Rachel Dunleavy, BS, R.T.(R)(N), CNMT Shetal N Shah, MD After completing this article, the reader should be able to: Describe the function and features of positron emission tomography2magnetic resonance (PET-MR) imaging. Compare PET-MR technology and features to PET–computed tomography. Explain PET-MR design and procedures. Discuss the technical and operational challenges related to PET-MR imaging. List the benefits of PET-MR to patients and radiology departments. T he idea of merging data sets to enhance information is not new. For example, weather forecasters routinely combine radar, cloud coverage, and topography data to produce composite maps. These maps provide more information in a single image than any individual data set can supply. So, too, the fusion of medical images offers clinicians a better picture of anatomy and physiology.1 Images from several modalities potentially can be fused, including computed tomography (CT), magnetic resonance (MR) imaging, positron emission tomography (PET), single photon emission computed tomography (SPECT), ultrasonography, and radiography images. Most frequently, data sets are merged to take advantage of the specific strengths of each modality. For example, CT and MR images demonstrate excellent anatomical detail but do not indicate physiologic function. In contrast, PET provides information about pathophysiological RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 processes such as metabolic activity but offers limited structural information.1 One approach to image fusion is the combined, or hybrid, scanner. In this system, the scanning components from 2 different modalities are joined and used either in tandem or as a single unit. The equipment acquires data sequentially or simultaneously, with the patient positioned on the same table for both scans. The computer software of the scanner then fuses the resulting data sets to create images that contain both structural and functional information.2 The concept of a combined PET-CT scanner was initially proposed in the early 1990s. Although hybrid imaging equipment has been available for only a short period of time, PET-CT scanners have largely supplanted PET-only equipment. In fact, stand-alone PET units were no longer sold commercially after 2006. PET-CT technology has proved particularly useful for oncology imaging, both for diagnosis and 393 CE Directed Reading Techniques, Benefits, and Challenges of PET-MR treatment planning, and there is a growing body of evidence that PET-CT provides more accurate information than either CT or PET images acquired separately.2 Following the widespread acceptance of PET-CT, hybrid PET-MR scanners have recently been introduced for clinical use.2 This Directed Reading discusses various aspects of PET-MR imaging, including technical and operational challenges of the modality. Because PET-MR is a relatively novel approach, the article references protocols and guidelines from the Cleveland Clinic Imaging Institute as examples; readers should note that protocols and guidelines are usually specific to an institution and manufacturer. To fully understand this new application of hybrid imaging, however, it is important to begin with a discussion of PET-CT. Positron Emission Tomography Positron emission tomography is a 3-D nuclear medicine examination that detects photons emitted by the stabilization of various intravenously injected radioisotopes such as fluorine 18 (18F), carbon 11 (11C), and yttrium 86 (86Yt). Unlike anatomic imaging modalities such as CT and MR, PET imaging shows the pathophysiologic processes that precede anatomic changes. Therefore, PET has become an important tool for the detection, localization, diagnosis, and characterization of several pathologies that occur at the microscopic and molecular levels. Although doctors rely on PET scans most often to help manage cancer, this imaging modality is increasingly being used in the diagnosis and treatment of neurological abnormalities and cardiovascular pathology. Today, fluorodeoxyglucose F 18, or 18F-FDG, is the most ubiquitously used radiopharmaceutical in PET imaging, allowing clinicians to assess and manage various solid and hematologic malignancies before initiating therapy, after treatment, and during follow-up. 18F-FDG is a glucose analog, with an 18F atom substituted at the second hydroxyl group (-OH) of glucose. 3,4 The rationale for using 18F-FDG in cancer imaging is based on the Warburg effect, which maintains that to meet relatively higher metabolic demands, cancerous and inflamed cells overexpress a number of cell membrane and intracellular proteins known as glucose transporters. The overexpression of these proteins 394 allows greater cellular uptake of glucose relative to noncancerous cells. After intravenous injection, 18F-FDG is preferentially taken up by cancerous cells; however, once the 18F-FDG is intracellular, neither 18F-FDG nor its byproduct undergo normal catabolic or metabolic transformation, and it cannot be converted into a form that can exit the cell. This so-called “metabolic trapping” at the cellular level permits 18F-FDG to accumulate in abnormal cells.3,4 On the atomic level, the 18F begins to stabilize by releasing a positron, which in turn collides with an orbital electron. The resulting annihilation reaction releases energy in the form of two 511-keV photons, which are emitted in nearly opposite directions. An array of photomultiplier tubes in the PET scanner gantry identifies pairs of interactions occurring at nearly the same time, a process known as annihilation coincidence detection. The photomultiplier tubes then convert and amplify the signal into an electrical signal. After advanced signal processing and computer analysis, the signal’s point of origin is estimated and this information is converted into an image. 4 In PET imaging, the photon pair must exit the patient simultaneously to be detected as a true coincidence event. The detectors cannot identify true events if the photons are absorbed in the body or if they are scattered out of the field of view. This attenuation causes an inaccurate count of true events, which in turn increases image noise, artifacts, and image distortion. Therefore, the acquired PET data must be corrected to accurately measure 18F-FDG activity.5 To correct for attenuation, the PET-CT scanner uses x-rays from the CT scan to create an attenuation map, which displays density differences in the body. Generally speaking, tissues deeper in the body or close to relatively dense structures such as a metal prosthesis are subject to more attenuation than the skin surface or certain other tissues (eg, lungs). The correction process adjusts the event counts, depending on the degree of attenuation of the tissue.5 The correction process also is used to determine the standard uptake value, a relatively simple method for determining the amount of 18F-FDG activity within an area of interest. The standard uptake value is the ratio of the mean radioactivity within a region of interest and RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 CE Directed Reading Ocampo, Knight, Dunleavy, Shah the injected dose of radioactivity per kilogram of body weight. Thus, it provides an indication of tumor activity, although there is some variation in the measurement. For example, a patient’s weight affects the value in that thinner individuals have lower standard uptake values (SUVs) than do heavier patients.5 Computed Tomography The cross-sectional and multidimensional aspects of CT provide excellent structural detail. 6,7 The modality’s effectiveness led to increases in its use that have since stabilized. In 2011, CT examination volume peaked at more than 85 million studies in the United States. By 2013, the number had decreased more than 10% to 76 million studies. 8 The success of CT has been met with concerns about radiation exposure from the examinations. In addition to efforts aimed at optimizing dose, there has been increased emphasis on justification of patient exposure and appropriate use.12 CT radiation is emitted as an x-ray spectrum. The spectrum for a given unit varies by manufacturer, which means the CT values also vary by manufacturer. CT units can be standardized, however, when values are converted to Hounsfield units (HUs) for the reconstructed image.2 The x-ray tube rotates around the patient, creating a fan-beam cross-sectional image. With today’s helical multidetector scanners, a full rotation takes less than one second, and total examination times are relatively short. This technology has paved the way for CT’s use in dynamic studies for cardiac applications, CT angiography, and examinations such as virtual colonoscopy.10 Use of intravenous or oral contrast agents can further improve subject contrast between anatomy, increase sensitivity of pathology, and improve accuracy in interpreters’ detection of lesions.11 Multidetector units and faster scanning times increased the modality’s effectiveness as a diagnostic tool, particularly for trauma imaging. When CT technology was fused with functional imaging, including PET and SPECT, the merging of metabolic function information from nuclear medicine examinations with the anatomic detail of CT scans improved diagnostic information substantially.9 RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 The usefulness of the CT beam in penetrating and accurately imaging structures deep within the body aids its ability to provide attenuation information for the fused modality. Attenuation is much more likely to occur in deep organs and tissues than it is in those closer to the surface. The attenuation image can be obtained in seconds and with lower radiation exposure than is needed for a full diagnostic image.5 PET-CT Scanning Radiologists have noted the benefits of PET-CT,11 and numerous studies over the past decade have demonstrated the added clinical benefit of fused (or hybrid) PET-CT imaging over PET or CT imaging alone in managing cancers, neurological conditions, and cardiovascular pathologies. 18F-FDG PET-CT imaging in particular is a faster and more accurate technique than either modality alone. 18F-FDG PET-CT better differentiates malignant from nonmalignant 18F-FDG activity and is effective in detecting primary and secondary cancerous lesions and demonstrating tumor extent. The efficacy of PET-CT has resulted in management changes for 30% to 35% of patients with various solid tumors.13,14 The addition of contrast agents provides differentiation between the lesions and surrounding structures on CT, which is particularly important in head and neck imaging and in imaging of the abdomen and pelvis. The most important benefit of using CT contrast agents in PET-CT imaging is the precise anatomic localization of pathology. In the head and neck, intravenous contrast agents are used to differentiate between malignant lesions and adjacent blood vessels, the thyroid gland, salivary glands, and muscles. In the abdomen and pelvis, intravenous and oral contrast agents can help accurately delineate lesions adjacent to bowel loops, the stomach, mesenteric and iliac blood vessels, and parenchymal organs. The availability of contrast-enhanced CT data improves confidence to accurately localize a PET-positive lesion in approximately 25% of patients.11 CT contrast agents provide value in imaging tumors with minimal or no increase in 18F-FDG uptake. False negative FDG PET-CT scans can be seen with several tumors, including bronchoalveolor carcinoma, mucinous colorectal cancer, and renal cell carcinoma. The 395 CE Directed Reading Techniques, Benefits, and Challenges of PET-MR additional CT information allows identification of the specific radiopharmaceutical uptake location within the anatomic background.11 If a tumor or its metastases are 18F-FDG negative, the availability of diagnostic CT data from combined PET-CT imaging can improve lesion detection and characterization. Lesion detection is enhanced with increased attenuation differences between anatomic structures, and the pattern of contrast enhancement can aid in lesion characterization. PET-CT, with and without contrast enhancement, improves patient management better than conventional imaging. PET-CT allows physicians more guidance when choosing a treatment plan for their patients and plays a vital role in surgery, biopsy procedures, and radiation therapy planning. Contrast-enhanced CT scans in PET-CT are important in planning a patient’s treatment options because accurate image fusion is mandatory for guiding the surgeon or interventional radiologist to the precise tumor region. PET-CT also improves planning for patients undergoing radiation therapy. Performing a contrastenhanced CT allows an accurate differentiation of tumor tissues to the adjacent organs and is vital to planning a target volume for radiation therapy and preventing exposure to radiation-sensitive organs.11 Treatments can then be more focused on the area of interest to improve sparing of normal tissues. PET-CT Procedure Typically, patients receive preparation instructions for PET-CT scans for the day before the scan through the day of the scan. Patients scheduled for PET scans are cautioned to avoid strenuous activities, drink at least 5 glasses of water, and have a high-protein dinner the day before the appointment.15 Patients are asked to fast for a minimum of 4 hours before the 18F-FDG injection (the standard dose range of 18F-FDG is 6 to 18 mCi, based on patient weight).15 The patient’s blood sugar is measured by the PET technologist before the injection. The patient’s blood glucose should be under 200 mg/dL; if the patient’s glucose level is above 200 mg/dL, a PET/nuclear medicine physician or radiologist should be consulted.15 An appropriate glucose level is essential because glucose inhibits the uptake of 18F-FDG in cells.16 396 After the 18F-FDG injection, patients are instructed to lie flat in a dimly lit room for 60 minutes. Immediately before scanning begins, patients are instructed to void and to remove any metal objects. A low-dose, non−contrast-enhanced CT scan is first acquired for attenuation correction. An attenuation artifact can occur if a highly attenuated object such as a metallic orthopedic device is in the path of the CT beam.3,5 Hip prostheses, dental implants, cardiac pacemakers, contrast-enhanced vessels, and truncation can cause attenuation artifacts. 3 Without attenuation correction, the perceived distribution of 18F-FDG inside the body might not be a true representation.17 For Siemens Biograph scanners, the CT scan parameters are as follows: 120 kVp, CARE Dose 4D applied mAs (maximum of 120 mAs), collimation of 32 1.2 mm, and reconstructed images with 5-mm slice thickness and 4-mm reconstruction increments.15 The PET scan is then acquired using time-of-flight (TOF) imaging. TOF reconstruction improves signalto-noise ratio and lesion detectability and achieves better timing resolution.18 TOF is a technique in thirdgeneration PET scanners that considers the amount of time required for each gamma ray to reach the detector. Scintillation crystals in PET scanner detectors determine the precision with which photons are simultaneously detected. TOF imaging pinpoints the arrival time difference and can increase resolution of final PET images.19,20 The PET scan acquisition occurs using a series of bed positions, each for a set time period. The times can range from 1 to 5 minutes each, depending on department protocol. Timing is based on the patient’s height and weight, the dose amount administered, the time from injection, and whether the patient’s arms are above the head or down at the sides. PET postprocessing is performed with iterative reconstruction (Gaussian filter, TrueX reconstruction method, 2 iterations, and 21 subsets).15 For example, if a patient has a bed time of 3 minutes and it takes 7 bed positions to cover the area of interest, the total scan time is 21 minutes. PET and CT images are fused together, and coronal, sagittal, and axial fields are constructed with 5-mm slice thicknesses and with a distance between images of 5 mm.15 RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 CE Directed Reading Ocampo, Knight, Dunleavy, Shah Drawbacks of PET-CT Although 18F-FDG PET-CT imaging offers exquisite sensitivity in whole-body imaging with a single piece of equipment, this technique has limitations. First, numerous studies have shown that 18F-FDG PET-CT has limited specificity and relatively low spatial resolution (particularly in evaluating lesions less than 8 mm). Second, although PET-CT images have excellent sensitivity, 18F-FDG PET-CT has shown relatively low specificity for accurate lesion characterization, including nonspecific uptake in inflammatory or infectious processes within the chest, abdomen, and pelvis. Third, the PET-CT scan is obtained sequentially, typically over the course of 25 to 30 minutes, and is subject to artifacts from patient motion, such as breathing and physiologic changes during the scan (eg, filling of the urinary bladder). Finally, in North America, PET-CT scans are typically obtained with a large field of view only, so these scans tend to be used by physicians as survey examinations, with limited ability to assist in evaluating small, focal pathology, such as in various abdominopelvic viscera. The recent fusion of 18F-FDG PET scans with low-dose transmission CT scans introduces 2 additional challenges: cumulative exposure to ionizing radiation and relatively poor soft-tissue contrast and spatial resolution of the correlative anatomic images.21 PET-MR Understanding the limitations of PET-CT imaging, scientists, engineers, physicians, and imaging device manufacturers began work on combining PET and MR as the next logical step in hybrid imaging. Conceptually, combining 18F-FDG PET with MR holds great clinical promise in the evolving realm of individualized medical care. Advances in PET detector technology and the discovery of novel PET agents, such as fluoromisonidazole F 18 (FMISO) for hypoxia and 18F-labeled alphamethyl tyrosine (FMT) for angiogenesis, combined with advances in MR technology and sequences could further improve PET-MR imaging.22 MR is based on the inherent ability of hydrogen molecules in various soft tissues to orient along an applied external magnetic field and to subsequently relax to a state of equilibrium when the magnetic field is removed. In clinical practice, the magnetic field has an intensity RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 of 1.5 T or 3 T.22 Personnel in more advanced clinical research settings acquire images with magnets at a field intensity of up to 9.4 T.22 Hydrogen nuclei absorb energy from radiofrequency energy signals. This causes magnetic moments in the nuclei to move and realign. The clinical MR image is formed by analyzing or parsing the resulting radiofrequency energy signals based on a number of factors. These factors include the inherent chemical shift properties and varying densities of hydrogen protons in several of the body’s tissues. For example, longitudinal or spin-lattice relaxation (T1) is the shifting of magnetic moments from high to low energy states, and transverse or spin-spin relaxation (T2) occurs when intrinsic magnetic fields of nuclei interact with one another. Motion and flow phenomena also affect the image.23,24 The use of MR-based soft-tissue segmentation has been clinically successful.24 By assigning, or segmenting, voxels to tissue types, various tissues within the body (eg, soft tissue, bone, and lungs) from whole-body T1-weighted imaging can be manipulated to create a “pseudo-CT” image, which can then be overlaid with the PET image to create the fused or hybrid PET-MR image.25,26 PET-MR Design Designing a system that can combine the excellent soft-tissue contrast of MR with the molecular data available in PET images is technologically challenging. Inherent interference and cross-talk can occur from the proximity of the MR magnets and the ferromagnetic photomultiplier tubes in PET detectors.27,28 Other considerations include time of examination, ability to acquire images from both modalities simultaneously, minimizing patient motion, and potential idleness of one modality while the other is in use.2,24 Addressing any one of these challenges can introduce another. For example, physically separating the PET and MR units and using a rotating bed that requires moving the patient from one modality to another eliminates many of the technical alterations required for fusion of PET and MR technology. However, it does not facilitate simultaneous acquisition of data.2 As PET-MR has evolved, researchers and developers have chosen either this approach of 397 CE Directed Reading Techniques, Benefits, and Challenges of PET-MR sequential scanning, also called a tandem or shuttle design, or simultaneous scanning. Sequential scanning might involve placing each modality in adjoining rooms, shielding of photomultiplier tubes, and using a patient bed that has immobilization to minimize patient motion during transfer.2,24 Simultaneous scanning has required more technical adjustments and expense at the research and development level, but fully hybrid units are becoming available for clinical use. Generally, these units include either an insert or fully integrated technology as manufacturers have introduced various solutions to overcome the proximity of photomultiplier tubes and MR magnets. Designers also have investigated algorithms to address attenuation correction of acquired data.2 Sequential Scanning Some early versions of PET-MR scanners used trimodality sequential imaging and required 2 rooms to house the equipment and acquire patient images.29 In this construct, the patient is placed on a movable gantry to have a PET-CT scan. Once the scan is completed, the patient remains on the entry gantry apparatus while it is moved to a different room, where an MR scan is acquired. The 2 scans are postprocessed and combined for interpretation. This is thought to be a low-cost solution, although it involves a larger geographic footprint (approximately 572 square feet or 4.3 m 13 m), longer scan times, and slower workflow, and introduces the technical limitations of scans obtained sequentially in different rooms. GE also has a simultaneous PET-MR system called the SIGNA PET/MR, which has a smaller footprint than the company’s Trimodality Discovery.30 Philips also uses sequential scanning for PET-MR.29 The Ingenuity TF PET/MR, which is similar to the Gemini TF PET/CT, uses existing hardware for each scanner, but requires only one room.29 In this construct, the PET and MR scanners are located in the same room, although the scanners are spaced apart and shielded from one another to prevent interference of the magnetic field with the PET photomultiplier tubes. The scanners are connected by a movable gantry that pivots 180° on a common axis. Once the PET scan is completed, the gantry (with the patient) rotates 180° and the technologist acquires the MR images. 398 The Philips system uses a transmission scan for attenuation correction. Overall, it is a relatively low-cost solution that uses existing PET and MR technology but requires a large geographic footprint. The footprint of the scanning room will be larger than a simultaneous PET-MR system because the Philips Ingenuity PET-MR system is comprised of 2 separate bores with a 6-foot imaging table that connects the bores together.31 The sequential PET-MR scanner’s configuration requires minimal adjustments to create a hybrid PET-MR image. Minimal modification of the existing software package can define the scan sequence, manage the bed displacement, and display the fused results from the PET and MR scans.22 This technique is prone to artifacts from sequential scanning and can result in slow workflow and longer scan times, so the patient might have to undergo repeated scans to investigate a particular finding. Sequential systems do not disrupt the functionality of traditional photomultiplier tubes, which facilitates appropriate tube calibration and operation. Traditional photomultiplier tubes are based on scintillators, which tend to detect nuclear annihilation more quickly, generally enabling TOF imaging more easily than with fully integrated scanners.16,29 Simultaneous Scanning Simultaneous PET-MR scanning has been constructed with the use of a split superconducting magnet, field-cycled MR, or by inserting or attaching the PET detector ring to the radiofrequency coil of the MR unit. The MR superconducting coil is built in 2 separate elements; between each element is an axial space of several centimeters in which a PET scintillation ring can be placed. This system was tested for preclinical imaging at the neuroscience department at the University of Cambridge and has a design requirement of less than 1 T. The design of a lower field magnet has specialized gradients that are specific to small animal imaging (ie, a mouse).22 The field-cycled acquisition, which is also used for preclinical research, has 2 separate and dynamically controllable magnets that are used for polarization. This separation enables interleaving in the acquisition of MR data.22 Simultaneous PET-MR also can be achieved by taking both scintillator crystals and the associated RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 CE Directed Reading Ocampo, Knight, Dunleavy, Shah photodetectors (PET detectors) and inserting them behind the radiofrequency coil of the MR scanner. This can be achieved by reducing the radius of the radiofrequency coil to provide space for the detector.22 For example, the Siemens hybrid PET-MR scanner (Biograph mMR) combines PET and MR modalities in a single gantry, which leaves a geographic footprint of approximately 355 square feet. At this size, the unit can fit into most existing MR or PET scanner suites.26,29 The Biograph mMR adds PET detector rings and water-cooled photodetectors made of avalanche photodiodes, which are not affected by magnetic fields, to the MR gradient and body coils.2 This technique requires that the avalanche photodiodes be MR compatible and small enough to fit inside the gantry of an existing MR scanner. The Biograph mMR was constructed with a 70-cm bore, and the detectors only comprise 10 cm of the bore.29 Although avalanche photodiodes have a relatively poor timing resolution, which inhibits their functionality for TOF imaging and degrades the PET performance interference between the PET and MR images, Siemens states that the effect is almost inconsequential with the system.29 The coincidence window is approximately 5.86 ns.32 Although this hybrid scanner requires greater engineering resources, the manufacturer says that the system’s fusion architecture is less prone to the artifacts associated with sequential imaging. Future designs will make more use of advancements in photomultiplier tube technology known as silicon Geiger-mode avalanche photodiodes.29 This design will achieve minimum interference between MR and PET.29 Silicon tubes are much more advanced in terms of signal-to-noise ratio, timing resolution, and TOF acquisition than are traditional photomultiplier tubes.29 Scans taken with silicon tubes within a magnetic field with the use of gradients and radiofrequency pulses have demonstrated acceptable image quality.29 GE Healthcare recently introduced a new scanner model called the SIGNA PET/MR, which combines silicon photomultiplier tubes in the PET detector with a 3-T strength MR magnet in a single unit. The scanner offers simultaneous TOF imaging.33 As new technology such as alternative photomultiplier tubes facilitate PET-MR design, developers can begin RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 to address the technical challenges associated with the fused modalities. Two of the primary considerations to date have been attenuation correction and minimization of artifacts. PET-MR Scanning Procedures Patient care depends partly on PET-MR unit design and safety and operational considerations. Patient preparation for a PET-MR examination is nearly the same as preparation for PET-CT, including appropriate fasting, glucose control, and rest time between 18F-FDG administration and imaging. MR contraindications, such as implanted ferromagnetic devices and MR safety, are areas of special concern.24,32 The MR safety checklist includes implanted and external metallic objects. Patients must remove all ferromagnetic objects before entering the PET-MR room. MR safety precautions must be maintained to avoid risk of accidents or MR quenching, which is rapid loss in the magnet’s superconductivity and the generation of heat. Quenching can occur spontaneously if there is a fault in the equipment, or in an emergency the MR scanner can be quenched by activating the magnet’s stop button. When the emergency stop button is activated, liquid cryogens that cool the magnet coils boil off rapidly, releasing helium quickly into the surrounding air.34 Negligence in conducting patient or personnel safety checks could result in fatal accidents. Further, the repair of a quenched MR unit is very expensive and should be avoided at all costs.34 Additional considerations might include light sedation for patients who have claustrophobia. Patient preparation for the MR portion of the scan is more time consuming for technologists than is that of CT, largely because of coil placement and assistance with headphones.24 The Table compares PET-CT and PET-MR guidelines at the Cleveland Clinic.35 Attenuation Correction In PET-CT, the CT data provides information about the gamma ray properties of various tissues in the body, which helps to generate attenuation maps and information needed to correct for attenuation. Because MR does not rely on absorption of ionizing radiation in tissues to generate images, it is more difficult for MR to provide attenuation maps in PET-MR imaging.2 399 CE Directed Reading Techniques, Benefits, and Challenges of PET-MR Table PET-CT and PET-MR Protocols at Cleveland Clinic 35 PET-CT PET-MR Fasting time 4 hours 4 hours Dose 6-18 mCi 6-18 mCi Time 60 min 60 min Reconstruction 5-mm slice thickness, 4-mm increments 2-mm slice thickness, 2-mm increments PET bed time 1-5 min 3-5 min Emission correction Gaussian filter; TrueX reconstruction method; iterations: 2, subsets: 21 Gaussian filter; TrueX reconstruction method; iterations: 2, subsets: 21 Reconstruction slices Coronal, axial, sagittal Coronal, axial, sagittal Reconstruction image thickness 5 mm 3.1 mm Distance between images 5 mm 5 mm Reprinted from Cleveland Clinic Imaging Institute: Nuclear Medicine Regional Body FDG PET-CT Oncology Guidelines. Cleveland Clinic, Cleveland, Ohio. Effective July 3, 2014. Researchers continue to study how to correct for attenuation for PET-MR. Several approaches to address the challenge of obtaining an attenuation map in PET-MR have been proposed.17 One method, sometimes called atlas-based algorithms, uses a standard transmission based on the patient’s anatomy, which then is associated with predefined attenuation maps. The atlas-based algorithms were developed to integrate a global anatomical knowledge derived from a reference data set. 36 The approach uses an atlas registration and pattern recognition deriving a pseudo-CT image, which is then converted to an attenuation map at the appropriate energy levels associated with the PET radiopharmaceutical.2,17 Another method uses segmentation techniques to classify the voxels of the MR image into various tissue types.17 Sequential systems such as the Philips Ingenuity use a 3-D multistation and spoiled gradient echo MR sequence as a transmission scan for 400 attenuation correction. The 3-D multistation MR sequence is automatically segmented into 3 classes: air, lungs, and soft tissue, which results in an MR attenuation map. 37 One of the major pitfalls thus far in PET-MR vs PETCT is the comparison of SUVs in MR attenuation correction. 38 SUVs are a way to closely quantify how much 18F-FDG uptake activity there is in a region of interest based on the patient’s weight and the amount of 18FFDG administered to the patient. When PET detectors are calibrated correctly, the amount can be calculated at the workstation. Image noise and artifacts can affect SUVs.5,38 Methods used in PET-MR for attenuation correction might not account well for cortical bone, and errors can occur when imaging in the area of large bony structures such as the spine, pelvis, or femur.24 Until algorithms improve to account for attenuation and SUVs, interpreting physicians must carefully evaluate fused PET images by considering clinical concerns and findings along with images.5 Research has demonstrated that SUV reproducibility is difficult when evaluating 18F-FDG PET-CT studies at different sites with various scanners, and differences of up to 30% have been detected in phantom models.38 However, other groups have shown high reproducibility of SUVmean and SUVmax values in malignant tumors when repeated measurements are performed with the same scanner. Research also has shown that a long time span between injection of 18F-FDG and performing the PET-MR scan might lead to a decrease in SUVs within normal tissue and within lesions.38 Artifacts Fusing 2 completely different imaging modalities introduces challenges related to image artifacts, including misregistration, patient motion errors, and reconstruction complications. Some artifacts are introduced during attenuation correction. For example, atlas-based algorithms can cause artifacts if a patient has implants or variations from a normal organ structure because of a history of resection. Segmentation-based algorithms might be associated with artifacts from metal implants, bony anatomy, or the patient’s lungs.2 Motion artifacts are of major concern, especially with sequential image capture.2 On simultaneous RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 CE Directed Reading Ocampo, Knight, Dunleavy, Shah acquisition, such as with the Siemens Biograph mMR, the PET acquisition is designed to occur along with the MR sequences. The PET sequence is set to run for 3 to 5 minutes per table position. According to Shetal N Shah, MD, Cleveland Clinic’s PET-MR protocol specifies that the patient should breathe normally for the MR attenuation correction-PET acquisitions and then hold the breath on expiration during MR. These sequences can range from 15 to 20 seconds per breath hold. MR sequences are obtained simultaneously. The time adjustment can vary among facilities and depends on the hospital’s standards of operations as established by a nuclear physicist and radiologist (oral communication, January 2015). The PET scan is fused to a short MR sequence such as the 2-point Dixon sequence (see Figure 1).32 This sequence provides water- and fat-weighted images for both attenuation correction and anatomic fusing. Dixon sequences can be obtained and segmented into 4 compartments: air, fat, lung, and soft tissue. MR-based attenuation correction with the 2-point Dixon sequence for each bed position is comparable to the type of anatomic correction achieved with low-dose CT scans in PET-CT.2,38 One study evaluated the anatomic A localization and comparable SUVs to compare simple anatomic correction. 38 The purpose of the Dixon sequences is to correct for attenuation and facilitate fusion of the PET and MR images. The MR images obtained during the 2-point Dixon sequence are limited in diagnostic value.24 Gadolinium contrast can interfere with Dixon sequences, and should be withheld until after completion of the sequence acquisition.2 Patient motion from breathing or claustrophobia often causes MR artifacts. The MR sequences are performed on expiration to compensate for the motion artifact caused by breathing; it has been shown that most PET data also are collected at expiration. 39 Several techniques are used for motion correction for both PET and MR imaging. A study of PET-CT by Liu et al showed that 1295 patients had respiratory traces on the images, which means that most patients spend a substantial amount of breathing time dwelling at the end-expiration location. 39 The study indicated that imaging at end-expiration tends to produce less motion on images. Quiescent period gating extracts PET data only from periods when the patient’s breathing is inactive to form image volume. This optimizes the image resolution, decreases misregistration B C D Figure 1. How a positron emission tomography–magnetic resonance (PET-MR) image is created. A. Segmented 2-point Dixon MR images are acquired (illustrated here for the head and neck) and processed to create a -map image (B). This -map serves as an attenuation correction map that is fused with the corresponding corrected FDG-PET image (C), resulting in the creation of a fused FDG PET-MR image (D). Images courtesy of Shetal N Shah, MD. RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 401 CE Directed Reading Techniques, Benefits, and Challenges of PET-MR between different image sets, improves SUVs, and causes minimal increases in noise. 39 Whether the data are collected sequentially or simultaneously, misregistration issues must be addressed and corrected where possible. Image artifacts related to patient anatomy or pathology also can be a problem in PET-MR. The patient’s body habitus and implants such as hip prostheses or spinal hardware can cause a substantial amount of artifact. Any metal introduced into the magnetic field causes a truncation artifact. Metal artifacts also can distort the attenuation correction map, which typically involves the 2-point Dixon sequence or the 3-D multistation spoiled gradient. The MR system might misinterpret the inhomogeneity and misclassify tissues (eg, classify fat as water). This inaccurate attenuation map can lead to quantitative errors and possible artifacts. Radiologists must evaluate the attenuation map to determine whether these errors are acute or the result of misregistration.39 Operational Challenges The addition of a PET-MR scanner to an imaging department involves many operational challenges, including physical installation, technologist training, reimbursement issues, and shared responsibilities among technologists and physicians. Additionally, managing the PET-MR process involves collaboration among personnel from 2 distinct imaging specialties. Professionals from imaging departments must work together to make decisions about issues such as budgeting and personnel. Implementation Some have argued that the addition of a PET-MR scanner causes a disruption in workflow and scheduling for nuclear medicine and MR departments.40 Workflow innovations have to be carefully considered to overcome these concerns. Further, relationship building between departments is key to successful implementation. The transition of patients from one department or technologist to another should be seamless, with no interruption in the quality of care provided. An internal team consisting of staff from various levels, and including radiologic technologists, physicians, 402 administrators, and others, should be created to determine which model of PET-MR scanner to purchase and where to install it. At Cleveland Clinic, committees involving multiple departments and personnel were established to make these decisions (Shashi Khandekar, nuclear medicine administrator at the Cleveland Clinic, oral communication, November 2013). Only a few fully integrated PET-MR scanners are available on the market, and the cost of these scanners is comparable to the cost of a PET-CT scanner.24,41 Therefore, medical imaging departments must gather data to ensure they have sufficient patient population to justify the costs. The PET-MR scanner models are large, and one of the factors to consider is the room size and whether the scanner can fit in the existing space or whether extensive construction is needed to accommodate the scanner. The scanner’s use should generate enough revenue to fund the initial capital costs and ongoing operational costs such as upgrades to the equipment or software. Personnel from several departments must collaborate when deciding where to place the PET-MR scanner. Although nuclear medicine and MR departments are generally managed separately, the addition of a PETMR scanner requires combining resources from these 2 imaging departments. The PET-MR scanner must be installed in an area that is convenient for both departments and feasible for daily workflow. At Cleveland Clinic’s main campus, the PET-MR scanner is located in the MR department, which is near the nuclear medicine department. It was decided that the scanner should be placed in the MR department because the MR personnel are qualified in MR safety (S Khandekar, oral communication, November 2013). The close proximity of the 2 departments facilitates efficient workflow. There was no need to build another hot lab for dose storage or monitoring laboratory for uptake, as these are considered shared resources by both departments (S Khandekar, oral communication, November 2013). Personnel Once the scanner is installed, the next task is establishing shared responsibilities for personnel in the nuclear medicine and MR departments. Nuclear medicine technologists should be responsible for sending RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 CE Directed Reading Ocampo, Knight, Dunleavy, Shah and receiving radiopharmaceutical shipments and calibrating daily dose constancy to monitor the limits of dose calibrators for PET patients. Nuclear medicine technologists are trained and certified to administer radiopharmaceuticals and monitor the patient’s uptake phase, and they should continue to be responsible for this portion of the PET-MR examination. The flood phantom used to perform quality control tests on the PET-MR scanner contains a radioactive source, and certified nuclear medicine technologists have the proper training and authorization to handle these sources. It is the responsibility of the MR technologist to help patients complete their MR safety screening form to ensure that patients are properly cleared for a PET-MR scan. Patients might have implants, devices, or objects that are not visible and can be hazardous within the MR zone. Because the MR magnet is always on, it is important to stress proper clearance of all patients and personnel before allowing entrance to the room. For example, aneurysm clips, cardiac pacemakers, implantable cardiac defibrillators, or other devices can be contraindications for PET-MR. Physicians should consider all patient factors and might advise patients who have these devices to have a PET-CT.34,42 Because of these safety concerns, the PET-MR scanner should be operated by a technologist who is trained in MR; therefore, in most cases, MR technologists should acquire PETMR scans. 40 Radiation safety training is required for all PET-MR technologists (nuclear medicine and MR) who are performing any part of the PET-MR scan. 43 Radiation surveys must be conducted in areas where workers are exposed to radiation levels that might result in radiation doses in excess of 10% of the occupational dose limits or where an individual works in an environment with a dose rate of 0.025 mSv/hour or more. 42 This training is necessary because patients having PET-MR examinations receive injections of radiopharmaceuticals, and there is a chance of radioactive contamination. Radioactive contamination can occur with an improperly placed intravenous line or improper handling of the radiopharmaceutical or a patient’s urine or other bodily fluid. The patient becomes radioactive once he or she is injected, and a chance of contamination can occur before and after the injection. RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 Because there are specific responsibilities required of a technologist who operates a PET-MR scanner, it is ideal for the technologist to be certified in both nuclear medicine and MR. Acquiring quality images from the distinct modalities requires intimate knowledge of each, and technologists must be familiar with the proper use of each type of scanner to ensure patient safety. A technologist with certification in nuclear medicine and MR imaging can assume all responsibilities related to patient care and equipment operation. This technologist can send and receive radioactive shipments, inject the radiopharmaceutical, monitor patients during uptake, perform quality control tests on the unit, ensure MR safety, acquire scans, and monitor for radioactive contamination. All PET-MR technologist skill sets are regulated by state and national licenses and certifications. Licenses ensure that these individuals possess a basic level of education, knowledge, and skills.16 Each state has different license requirements for imaging technologists; as of this article’s publication, 38 states require fully or partially licensed radiographers, and 21 states require that fully or partially licensed nuclear medicine technologists perform PET-MR scans.16 The Society of Nuclear Medicine and Molecular Imaging Technologist and the Section for Magnetic Resonance Technologists are developing pathways for co-certifications in PET-MR.40 Requiring imaging technologists to be dual certified or requiring 2 technologists to be present for every PETMR examination could limit patient access to the PETMR scanner.16 Expanding technologists’ skill sets to include more modalities should improve patient access to hybrid scanners. Another issue that must be addressed when a PETMR scanner is added to an imaging department is interpretation of the PET-MR images. At some sites, a physician specializing in nuclear medicine reads the PET scan and a radiologist with significant MR experience reads the MR scan; the physicians review the case and combine their findings for the final report. 40 Shyam Srinivas, MD, PhD, a nuclear medicine physician at Cleveland Clinic, suggests 3 options for reading PET-MR scans (oral communication, December 2013). First, a 2-person team that includes a nuclear medicine physician and a radiologist can read PET-MR scans as 403 CE Directed Reading Techniques, Benefits, and Challenges of PET-MR a team, as described above. Alternatively, a radiologist familiar with PET-CT imaging or a nuclear medicine physician familiar with MR scans can likely interpret PET-MR scans. No matter the interpretation procedure selected, active collaboration between nuclear medicine physicians and radiologists is necessary to evaluate the diagnostic content of fused images. 44 Developing this knowledge will lead to a new division of competencies regarding organ pathology for interpreting physicians. 44 MR cross-training courses for nuclear medicine physicians are in development (S Srinivas, oral communication, December 2013). Cleveland Clinic nuclear medicine physician Sankaran Shrikanthan, MD, stated that although PET-MR scans should initially be evaluated jointly by nuclear medicine physicians and radiologists, eventually the scans can be interpreted by the physician who has the appropriate skill set (oral communication, December 2013). Radiation Safety The PET-MR technologists should monitor the PET-MR scanner suite after each scan is performed. If a spill is observed, technologists should take radiation safety measures to ensure proper radiation decontamination. In the PET setting, a Geiger counter is the most frequently used instrument for detecting radioactive contamination; however, at the time this article was published, there were no commercial-grade Geiger counters available on the U.S. market that could be used near an MR scanner. Some of the metal components of a Geiger counter are not classified as MR safe and cannot be used in the PET-MR area. The inside of a Geiger counter comprises metal resistors, solder joints, connector wires, and screws that are unsafe in an MR suite. 45 A wipe smear test and a gamma camera well counter can detect the presence of radioactive material and can be used in place of a Geiger counter. Wipe smears are usually made of absorbent materials. A cotton swab, gauze, or commercially available wipe smears can be used as long as the wipe material can fit easily inside the counting instrument. Wipe tests, used to check for removable contamination, are performed by wiping the surface of an object and assessing the amount of radioactive material on the wipe with an appropriate 404 instrument. The wipe test should be performed using medium pressure and should cover an area of at least 100 cm2 . The wipe smear is then counted in the gamma well counter. In most cases, wiped surfaces should include the PET-MR table, floor, walls, laboratory furniture, and equipment. 42 If there is concern about contamination, the suspected object or article of clothing can be moved outside the PET-MR suite and the magnet’s field (safety zone) and a Geiger counter can then be used safely to detect any contamination. It is important to measure the suspected object in an area where the background level is low; if an injected patient is present in the room when measuring the contaminated object, the patient’s presence interferes with the measurement of activity. 42 If contamination is found, the contaminated article can be placed in a bag and stored in a room designated for radiation decay. Items contaminated by radiation should be stored for at least 10 half-lives of the specific isotope involved. Most radiopharmaceutical use in PET-MR imaging involves isotopes with short half-lives that should decay within 24 hours or less. For example, 18F-FDG has a half-life of approximately 110 minutes, and typically within 18 to 20 hours the exposure’s measurement is at background levels. Nuclear medicine departments have a decay room that can serve as a shared resource for nuclear medicine and PET-MR. Reimbursement In 1970, the United States spent $75 billion on health care. 46 It is predicted that in 2015 U.S. health care expenditures will reach $4.2 trillion. 46 Health insurance premiums also have increased through the years and many Americans are uninsured. 46 These economic challenges have led to a decrease in reimbursements, upon which the practice of radiology depends.47 These macro-economic pressures affect radiology’s sustainability and the increasing availability of new and emerging imaging technologies. 46,47 Revenue always must be considered when adding a new scanner to an imaging center. Revenue achieved by a medical practice depends on the amount of reimbursement that is provided for a medical procedure. 47 In the case of PET-MR, the technical and professional distribution of revenue is based on a percentage of factors RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 CE Directed Reading Ocampo, Knight, Dunleavy, Shah involved in the procedure (technologist) and interpretation (physician) (S Khandekar, oral communication, November 2013). In general, reimbursement issues for imaging are driven by clinical studies that demonstrate improved health outcomes with the use of a particular imaging modality.16 The practice of radiology is largely influenced by Accountable Care Organizations (ACOs). 48 The goal of ACOs is to provide high-quality and cost-effective health care services. 48 Modifications in Medicare and insurance reimbursements also can limit the use of imaging services.29 One of the goals is to reduce use of services that might be unnecessary or inappropriate for management of patients’ medical care. Multiple procedure payment reduction is a reimbursement model designed to capture savings when multiple services are provided in the same session. 48 Current procedural terminology codes for imaging services are assigned to provide reimbursement for imaging centers. 49 At the time this article was published, there were no codes for PET-MR; separate codes were being used. 40 Reimbursement codes for clinical PET, diagnostic CT, clinical MR, and diagnostic MR are used in place of PET-MR codes (S Khandekar, oral communication, November 2013). Although there is evidence that a full-body PET-MR can extend an individual’s life expectancy and affect the amount of health care services used over a lifetime, evidence is needed to show that the diagnoses and information gained from PET-MR also help avoid multiple unnecessary imaging examinations. A year of life is worth approximately $50,000 to $200,000 a year, and the health care cost of increasing life by an additional year is an average of $19,000. 46 Many say the additional cost is worth the extension of a person’s life. 46 Proving the clinical value of emerging imaging techniques requires rigorous comparative research that can demonstrate that the new modality is superior to current technology. 48 A number of centers use PET-MR scanners for research purposes. 40 Such research could lead to advanced disease detection and monitoring that other modalities are limited in achieving. For example, PET-MR might contribute to research in the therapeutic effects of targeted gene transfer, stem cell transplantation, and cell replacement.50 PET-MR could be used in RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 future clinical practice to demonstrate the viability and differentiation of transplanted cells. The cells’ activity and growth can be monitored using PET imaging. 50 Radiologists might consider increasing their guidance and management of PET-MR referrals to increase the modality’s use.48 In addition, radiologists can help ordering physicians understand the appropriate use and benefits of PET-MR and demonstrate how this modality can contribute to patient care in the clinical setting as well as in the research arena. Ordering physicians should stay current on new imaging examinations that can provide a more accurate diagnosis. Benefits of PET-MR The main goals for hybrid PET-MR are to decrease the amount of time patients spend in multiple scanners and the amount of radiation the patient receives, all while maintaining high image quality and obtaining accurate and useful diagnostic information.50 Compared with PET-CT, PET-MR decreases the dose of ionizing radiation to patients. Achieving a highquality attenuation map without the use of ionizing radiation is especially valuable for pediatric patients and for patients who require multiple routine PET scans, such as those with cancer who might be examined several times a year to monitor the disease’s response to treatment.27,50 The results from the scan can conclude either a remission or a discovery of more malignancies. PET-MR offers potential advantages over PET-CT in the imaging of cancer. Research has demonstrated that PET-MR is useful in staging cancers and tracking progress after treatments.51 These advantages are most apparent in cancers for which MR and functional MR are considered superior to CT, specifically when dealing with soft-tissue contrast. Head and neck cancers, breast cancer, colorectal cancer, liver lesions, and lymphoma are some of the areas for which PET-MR might be superior to PET-CT for staging and restaging (see Figure 2).51 PET-MR also can be beneficial for patients who have claustrophobia and require anesthesia for imaging; combining imaging modalities decreases the number of examinations, radiation exposure, and anesthesia frequency and length.29,50 This is especially beneficial for pediatric patients who need to undergo anesthesia for both PETCT and MR.50 When hybrid imaging is used, the safety of 405 CE Directed Reading Techniques, Benefits, and Challenges of PET-MR A B Figure 2. Whole body FDG PET-MR of a 54-year-old woman with Figure 3. A. Coronal T2-weighted spin-echo MR image of a 66-year- breast cancer. This T1-weighted coronal image shows physiologic FDG uptake in viscera, soft tissue, and bone. Patient had stage II breast cancer at initial treatment with an FDG avid primary tumor (not shown) and an FDG avid right axillary lymph node (arrow). Image courtesy of Shetal N Shah, MD. old woman with non−small cell lung cancer (arrow). B. Coronal fused FDG PET-MR image of the patient’s chest illustrates hypermetabolic nodular soft-tissue thickening of the right apical, lateral, and mediastinal pleura at the apex, with associated T2-weighted hyperintense, loculated pleural effusion that is not FDG avid (arrow). Image courtesy of Shetal N Shah, MD. the patient is substantially increased because the anesthesia staff does not have to transport the patient from one department or suite to another while the patient is under anesthesia. This reduction in scans from PET-CT and MR to only PET-MR can lead to cost savings. Cost savings include reducing the professional fee for anesthesia for pediatric patients (from 2 procedures to 1),50 and open appointment times for additional patients that result from merging 2 studies into a single examination. PET-MR imaging offers several technical benefits over PET-CT. Techniques such as diffusion-weighted imaging, dynamic contrast enhancement, and spectroscopy, along with use of novel pharmaceutical agents (ie, fluoromisonidazole F 18 [FMISO] and 18F-labeled alpha-methyl tyrosine [FMT]), could lead to shorter acquisition times, higher resolution scans, lower cumulative ionizing radiation, superior soft-tissue contrast, greater patient convenience, and lower costs. In 406 RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 CE Directed Reading Ocampo, Knight, Dunleavy, Shah addition, clinicians can vary fields of view or sequence for each MR examination dependent on the diagnoses. PET-MR has proved beneficial in providing diagnoses and staging for neurologic diseases such as epilepsy, dementia, and Alzheimer disease. PET-MR also has demonstrated value in diagnosis of oncologic diseases such as pelvic, prostate, colorectal, and gynecologic cancers.29,52 Research shows that MR images provide high spatial resolution for evaluation of tumor volume and extent of staging (see Figure 3).29 A study by Torigian et al compared 18F-FDG PETMR imaging with 18F-FDG PET-CT imaging in assessment of cancerous invasion of surrounding tissue.29 The PET-MR images were superior at demonstrating tumor involvement. The authors found that the sensitivity and specificity of PET-MR images were the highest compared with the sensitivity and specificity of 4 other modalities, at up to 90%.29 The study also provided a level of high diagnostic confidence when using 18F-FDG PET-MR or MR imaging compared with 18F-FDG PET-CT or CT.29 PETMR provides superior cancer screening and staging for patients and is a useful tool for radiologists and referring physicians. The combination of the 2 advanced modalities could facilitate evaluation of disease at the micro and picomolar levels, with distinct advantages in accelerating drug development and potentially providing a robust surrogate biomarker tool.29 Future of PET-MR MR has been established for years as the preferred modality for identifying neurological pathology and monitoring its progression. 50 PET-MR already is useful in brain tumor imaging, and the hybrid modality can be used for radiation therapy planning and surgical mapping in more precise areas of the brain.53 Combining data from PET and MR scans can help quantify tumor proliferation and vascularity, and antitumor effects, thus helping clinicians to understand tumor biology, evolution, and therapeutic response on an individual basis.50 Simultaneous scanning with PET and MR is superior to PET-CT at detecting motion. This feature has been particularly promising for Alzheimer disease research. By detecting the onset and the extent of pathology, PETMR allows for more accurate disease staging by evaluating both functional and molecular abnormalities.50 RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 PET-MR also might be useful in evaluating patients with epilepsy. Many patients having examinations for epilepsy are children or young adults who require sedation before imaging. Combining PET and MR scans could reduce the number of times sedation would be required for imaging. In addition, when a patient with epilepsy is being considered for surgery, MR scans often are performed to determine sites of structural damage in the brain from the disease, and PET-CT scans are performed to identify the exact location of the epileptogenic foci (the precise locations in the cerebral cortex responsible for epileptic seizures). 50 Like PET-CT, PET-MR imaging can be useful for the detection of the seizure foci. 50 With fused imaging, the patient can undergo a single scan instead of 2 separate examinations. In stroke patients, the ischemic penumbra (an area of cerebral tissue that is injured but still viable) can be identified by PET and is valuable in helping physicians distinguish salvageable brain tissue. In an emergency setting, PET scans are not as readily available as CT and MR scans, which typically are run 24 hours a day in larger facilities with emergency departments. The limited hours of PET operation also limit the amount of available PET radiopharmaceuticals with short half-lives. If these radiopharmaceuticals were available in trauma centers with PET-MR scanners, simultaneous PET-MR sequences for ischemic stroke patients could optimize treatments. If advances are demonstrated in stroke diagnosis with the use of PET-MR, they could lead to expanded hours and volume of PET-MR operation.53 Cardiovascular preclinical research is one of the most active and challenging fields because of the potential for medical discoveries. 54 Cardiac researchers are assessing the effectiveness of PET-MR for cardiac diagnostic imaging. PET-CT is a useful modality to assess perfusion, metabolism, and myocardial blood flow. Stand-alone cardiac MR currently is the preferred modality for the assessment of cardiac chamber and myocardial masses. The combination of PET and MR might provide imaging researchers with a greater understanding of cardiac structure and function. PET-MR for cardiac imaging involves less radiation exposure for the patient and offers a higher soft-tissue contrast in cardiac rest/stress scans than does PET-CT.54 407 CE Directed Reading Techniques, Benefits, and Challenges of PET-MR PET-CT can identify hibernating myocardium, myocardial tissue that is impaired but viable, with the use of 18F-FDG. MR’s higher spatial resolution and improved soft-tissue contrast can facilitate the identification of acute myocardial infarction or scarring from a previous infarct. 54 Cardiac researchers also are finding uses of PET-MR in cardiac stem cell transplantation and other gene therapies. Advances in noninvasive imaging support further cardiac assessments without surgery. 53 The combination of PET imaging with the high resolution of MR and functional MR sequences can further improve image quality and diagnostic accuracy.53 Use of PET alone has limited gynecological tumor detection because of lower spatial resolution and bladder artifacts A near the uterus and ovary.40 The introduction of PET-CT has improved bladder artifacts along with current innovations in image processing technology, but introduction of PET-MR could improve detection of other gynecological tumors better than the use of PET-CT (see Figure 4). In addition, in endometrial cancer, PET-MR images can accurately display enlarged lymph nodes. PET-MR has a high sensitivity in displaying cervical cancer and measuring the extent of uterine body invasion, which is difficult to identify using PET-CT images.40 PET-MR holds promise for the detection of recurrent pelvic lesions. In the past, localized pelvic lesion recurrence generally has been difficult to diagnose with CT or MR images. Visibility of fibrosing or necrotic lesions is limited in CT and MR scans, but PET images C Figure 4. Fused coronal B 408 (A) and sagittal (C) T2-weighted PET-MR images of an elderly woman with biopsy proven vulvar cancer illustrate a focal hypermetabolic lesion along the right vulva from known primary vulvar cancer (blue arrows). Fused axial (B) and sagittal (C) T2-weighted PET-MR images demonstrate bilateral hypermetabolic hilar metastatic lymphadenopathy (white arrows). Images courtesy of Shetal M Shah, MD. RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 CE Directed Reading Ocampo, Knight, Dunleavy, Shah with FDG uptake better display recurrent lesions. Improved health outcomes from accurate detection and extent of malignancy can improve life expectancy because treatments can be more precisely modified to suit the individual’s disease.52 The future of PET-MR likely will be affected by changes in PET isotopes. As PET-MR scanner technology improves, pharmaceutical companies will market different isotopes for PET imaging. Future radiopharmaceuticals might be labeled to be disease specific at the molecular level. For example, improved new agents other than F-18 are labeled to detect estrogen receptors used for diagnosis of aggressive endometrial cancer. Some of these new PET agents are methionine and choline.52 Modifying the PET radiopharmaceuticals allows a better understanding of the biological characteristics of tumors and other diseases and allows for more effective personalized therapy.52 New radiopharmaceuticals and MR-based atrophy quantification might enable evaluation of psychological disorders in the future.55 One PET radiopharmaceutical that could become a valuable clinical research tool to evaluate neurologic processes noninvasively is flutemetamol F 18. In multiple studies, flutemetamol F 18 demonstrated the ability to cross the blood-brain barrier. This allows clinical research into the development of Alzheimer-type dementia.55 In addition, the radiopharmaceutical FMISO is a noninvasive quantification tool for hypoxia in cardiac tissue.55 FMISO also might be valuable for cardiac studies to evaluate patients for problems such as ischemia or cardiomyopathy. Adding FMISO to MR imaging, particularly to multimodal MR, can facilitate use of carbogen gas breathing during blood-oxygen level dependent effect MR, which can accurately detect hypoxia. does PET-CT, decreases scan time, and reduces ionizing radiation exposure. The fusion of these 2 important and effective modalities is the beginning of a new chapter in medical diagnostic imaging. The future outlook of PET-MR as an independent modality holds a great deal of potential. With this potential comes many operational and technical challenges. PET scanning has evolved into a key diagnostic tool for the medical community, and the fundamental success of PET scanning, especially when fused with CT, was a pivotal point in the field of nuclear medicine. The fusion of these separate modalities paved the way for PET-MR, which provides greater sensitivity and specificity than Tessa Ocampo, MBA, CNMT, has been a nuclear medicine technologist for more than 10 years at the Cleveland Clinic in Cleveland, Ohio. She received her bachelor of science degree in advanced medical imaging technology in nuclear medicine and magnetic resonance imaging at the University of Cincinnati. She also has a master’s degree in business administration from Cleveland State University. Ocampo has worked on various interdepartmental projects at the Cleveland Clinic. She also is an adjunct faculty member at Cuyahoga Community College in the nuclear medicine technology program. Katie Knight, BS, R.T.(N)(MR), CNMT, LMT, has been an MR technologist/PET-MR technologist at the Cleveland Clinic for 4 years. She received her associate degree in applied science in nuclear medicine while attending Cuyahoga Community College in Cleveland and a bachelor’s degree in nuclear medicine with a minor in health care management at Siena Heights University in Adrian, Michigan. Knight also attended the Cleveland Clinic School of Diagnostic Imaging, obtaining her certificate in magnetic resonance imaging. Rachel Dunleavy, BS, R.T.(R)(N), CNMT, is a nuclear medicine technologist at the Cleveland Clinic who specializes in PET and diagnostic CT. She earned her associate of applied science degree in 2006 and bachelor’s of radiologic and imaging sciences degree in 2007, both from Kent State University-Salem Campus. Shetal N Shah, MD, is an academic fellowship-trained abdominal radiologist with extensive clinical and research experience in multiple imaging modalities including CT, MR, PET, and ultrasonography, with specific research interest in oncologic imaging and response assessment. He codirects the Cleveland Clinic PET Center and is the medical director of the Cleveland Clinic PET-MR program. Reprint requests may be mailed to the American Society of Radiologic Technologists, Communications Department, at 15000 Central Ave SE, Albuquerque, NM 87123-3909, or e-mailed to [email protected]. © 2015 American Society of Radiologic Technologists RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 409 Conclusion CE Directed Reading Techniques, Benefits, and Challenges of PET-MR References 1. Maisey M, Leong J. Introduction to image fusion. Medical Image Fusion. Albuquerque, NM: American Society of Radiologic Technologists; 2003. 2. Gaertner FC, Fürst S, Schwaiger M. PET/MR: a paradigm shift. Cancer Imaging. 2013;13(1):36-52. doi:10.1102/14707330.2013.0005. 3. Kapoor V, McCook BM, Torok FS. An introduction to PET-CT imaging. Radiographics. 2004; 24(2):523-543. doi:10.1148/rg.242025724. 4. Shah SN, Nair RT. Use of positron emission tomography imaging in gynecologic cancers. In: Fielding JR, Brown DL, Thurmond AS, eds. Gynecologic Imaging. Philadelphia, PA: Elsevier; 2011:473-482. 5. Yao LL, Gay SB, Vu QDM, Anderson MW, Powell SM, Patel PN. PET/CT basics. Attenuation correction. University of Virginia Health Sciences Center, Department of Radiology Web site. https://www.med-ed.virginia.edu/courses/rad /PETCT/Attenuation.html. Accessed October 2, 2014. 6. Amis ES, Butler PF, Applegate KE and the ACR Blue Ribbon Panel on Radiation Dose in Medicine. American College of Radiology white paper on radiation dose in medicine. J Am Coll Radiol. 2007;4(5):272-284. 7. Platten D. Basic principles of CT scanning. ImPACT course. http://www.impactscan.org/slides/impactcourse /basic_principles_of_ct/img2.html. Published October 2005. Accessed December 2, 2014. 8. New report indicating decline in CT utilization should discourage further Medicare reimbursement cuts. 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American Society of Radiologic Technologists. http://www.asrt.org/docs/default -source/whitepapers/asrt_ct_consensus.pdf?sfvrsn=2. Published 2008. Accessed December 3, 2014. 410 13. Hillner BE, Tosteson AN, Tosteson TD, et al. Intended versus inferred care after PET performed for initial staging in the National Oncologic PET Registry. J Nucl Med. 2013;54(12):2024-2031. doi:10.2967/jnumed.113.123430. 14. Hillner BE, Siegel BA, Liu D, et al. Impact of positron emission tomography/computed tomography and positron emission tomography (PET) alone on expected management of patients with cancer: initial results from the National Oncologic PET Registry. J Clin Oncol. 2008;26(13):21552161. doi:10.1200/JCO.2007.14.5631. 15. Cleveland Clinic Nuclear Medicine Regional Body PET-CT Oncology Guidelines. 16. Bolus NE, George R, Washington J, Newcomer BR. PetMRI: the blended-modality choice of the future? J Nucl Med Technol. 2009;37(2): 63-71. doi:10.2967/jnmt.108.060848. 17. Mollet P, Keereman V, Bini J, Izquierdo-Garcia D, Fayad Z, Vandenberghe S. Improvement of attenuation correction in time-of-flight PET/MR imaging with positron-emitting source. J Nucl Med. 2014; 55(2):329-336. doi:10.2967/jnuc med.113.125989. 18. Daube-Witherspoon M, Surti S, Perkins A, Karp JS. Determination of accuracy and precision of lesion uptake measurements in human subjects with time-of-flight PET. J Nucl Med. 2014; 55(4):602-207. doi:10.2967/jnuc med.113.127035 19. Positron emission tomography. In: Cherry S, Sorenson J, Phelps M. Physics in Nuclear Medicine. 4th edition. Philadelphia, PA: Elsevier; 2012: 307-310. 20. Stanford School of Medicine Molecular Imaging Instrumentation Laboratory. Advanced time-of-flight (ToF) PET photon detectors. Stanford University. http://miil.stan ford.edu /research/tofdetector.html. Accessed December 5, 2014. 21. Basu S, Kwee TC, Surti S, Akin EA, Yoo D, Alavi A. Fundamentals of PET and PET/CT imaging. Ann N Y Acad Sci. 2011;1228:1-18. doi:10.1111/j.1749-6632.2011.06077.x. 22. Delso G, Ziegler S. PET/MR System Design. In: Carrio I, Ros P, ed. PET/MRI Methodology and Clinical Application. New York: Springer; 2014:1-16. doi:10.1007/978-3-64240692-8. 23. Westbrook C. MRI at a Glance. 2nd ed. West Sussex, United Kingdom: Wiley-Blackwell;2010. 24. Martinez-Möller A, Eiber M, Nekolla SG, et al. Workflow and scan protocol considerations for integrated whole-body PET/MRI in oncology. J Nucl Med. 2012;53(9):1415-1426. doi:10.2967/jnucmed.112.109348. 25. Schwenzer NF, Schmidt H, Claussen CD. Whole-body MR/ PET: applications in abdominal imaging. Abdom Imaging. 2012;37(1):20-28. doi:10.1007/s00261-011-9809-7. RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 CE Directed Reading Ocampo, Knight, Dunleavy, Shah 26. Siemens. Biograph mMR technical details. http://www .healthcare.siemens.com/magnetic-resonance-imaging /mr-pet-scanner/biograph-mmr/technical-details. Accessed January 13, 2014 27. Daftary A. Pet-MRI: Challenges and new directions. Indian J Nucl Med. 2010;25(1):3-5. doi:10.4103/0972-3919.63590. 28. Pichler BJ, Kolb A, Nagele T, Schlemmer HP. PET/MRI: paving the way for the next generation of clinical multimodality imaging applications. J Nucl Med. 2010;51(3):333-336. doi:10.2967/jnucmed.109.061853. 29. Torigian DA, Zaidi H, Kwee TC, et al. PET/MR imaging: technical aspects and potential clinical applications. Radiology. 2013;267(1):26-44. doi:10.1148/radiol.13121038. 30. GE. SIGNA PET/MR. http://www3.gehealthcare.com /en/products/categories/magnetic_resonance_imaging/. Accessed December 29, 2014. 31. Philips Ingenuity TF PET/MR. http://incenter.medical.phil ips.com/doclib/enc/fetch/2000/4504/577242/577252/5772 53/588155/588156/452296297291_IngenuityTF_PETMR _Brochure_LR.pdf%3fnodeid%3d10101343%26vernum% 3d1. Accessed January 8, 2015. 32. Delso G, Furst S, Jakoby B, et al. Performance measurements of the Siemens mMR integrated whole-body PET/MR scanner. J Nucl Med. 2011;52:1914-1922. doi:10.2967/jnucmed .111.092726. 33. GE Healthcare. SIGNA PET/MR. http://www3.gehealth -care.com/en/products/categories/magnetic_resonance _imaging/signa_pet-mr#tabs/tab3EE81FE2DEB44F6581D 9E646EA4BE20B. Accessed December 5, 2014. 34. ReviseMRI. What is quenching? http://www.revisemri.com /questions/safety/quenching. Accessed February 20, 2014. 35. Cleveland Clinic Imaging Institute: Nuclear Medicine Regional Body FDG PET-CT Oncology Guidelines. Effective date July 3, 2014. 36. Wagenknecht G, Kaiser H, Mottaghy F et al. MRI for attenuation correction in PET: methods and challenges. MAGMA. 2013; 26(1)99-113. doi:10.1007/510334-012-0353-4. 37. Kalemis A, Delattre BM, Heinzer S. Sequential whole-body PET/MR scanner: concept, clinical use, and optimization after two years in the clinic. The manufacturer’s perspective. MAGMA. 2013;26(1):5-23. doi:10.1007/s10334-012-0330-y. 38. Kershah S, Partovi S, Traughber BJ, et al. Comparison of standardized uptake values in normal structures between PET/ CT and PET/MRI in an oncology patient population. Mol Imaging Biol. 2013;15(6):776-785. doi:10.1007/s11307-0130629-8. 39. Liu C, Alessio A, Pierce L, et al. Quiescent period respiratory gating for PET/CT. Med Phys. 2010;37(9):5037-5043. doi:10.1118/1.3489508. RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 40.Kaplan D. PET/MRI: implementing, reading, and billing new technology. http://www.diagnosticimaging.com/petmr /petmri-implementing-reading-and-billing-new-technology. Published June 13, 2013. Accessed December 6, 2013 41. Kannivelu A, Kok Ty, Padhy AK. The conundrum of PET/ MR. World J Nucl Med. 2012;11(1):1-2. doi:10.4103/14501147.98717. 42. Model Procedure for Area Surveys Appendix R. Nuclear Regulatory Commission Consolidated Guidance about Material Licenses. NUREG. 2002;1556(9):R1-R6. 43. Howe DB, Beardsley M, Bakhsh SR. U.S. Nuclear Regulatory Commission, Office of Federal and State Materials and Environmental Management Programs. Consolidated Guidance About Materials Licenses: Program-Specific Guidance About Medical Use Licenses. Final Report, NUREG-1556;2008(9)Revision 2:8-61. 44.Mansi L, Ciarmiello A, Cuccurullo V. PET/MRI and the revolution of the third eye. Eur J Nucl Med Mol Imaging. 2012;39(10):1519-1524. doi:10.1007/200259-012-2158-x. 45. Ludlums Measurements, Inc. Model 44-9 Alpha, Beta and Gamma Detector. http://www.medphys.ludlums.com/imag es/stories/product_manuals/M44-40Series.pdf. Published March 2014. Accessed August 8, 2014. 46.Goyen M. Health-Care Costs and Impacts. In: Carrio I, Ros P, ed. PET/MRI Methodology and Clinical Application. New York: Springer;2014:153-158. doi:10.1007/978-3-642-40692-8. 47. Camponovo E. The business of radiology: cost accounting. J Am Coll Radiol. 2004;1(8):567-575. doi:10.1016/j .acr.2004.03.007. 48. Qayyum A, Yu JP, Kansagra AP, et al. Academic radiology in the new health care delivery environment. Acad Radiol. 2013;20(12):1511-1520. doi:10.1016j.acra.2013.10.003. 49. American Medical Association. CPT: current procedural terminology. www.ama-assn.org/ama/pub/physician-resources /solutions-managing-your-practice/coding-billing-insurance /cpt.page. Accessed December 23, 2013 50. Catana C, Drzezga A, Heiss WD, Rosen BR. PET/MRI for neurologic applications. J Nucl Med. 2012;53(12)1916-1925. doi:10.2967/jnumed.112.105346. 51. Pace L, Nicolai E, Aiello M, Catalano OA, Salvatore M. Whole-body PET/MRI in oncology: current status and clinical applications. Clin Transl Imaging. 2013;1:31-44. 52.Nogami Yuya, Iida M, Banno K, et al. Application of FDGPET in cervical cancer and endometrial cancer: utility and future prospects. Anticancer Res. 2014; 34(2):585-92. 53. Afaq A, Syed R, Bomanji J. PET/MRI: a new technology in the field of molecular imaging. Br Med Bull. 2013;108:105171. doi:10.1093/bmb/ldt032. 411 CE Directed Reading Techniques, Benefits, and Challenges of PET-MR 54. Nappi C, El Fakhri G. State of the art in cardiac hybrid technology: PET/MRI. Curr Cardiovasc Imaging Rep. 2013;6(4): 338-345. doi:10.1007s12410-013-9213-5. 55. Hernandez-Pampaloni M, Nardo L. PET/MRI radiotracer beyond 18F-FDG. PET Clin. 2014; 9(3):345-349. doi:10.1016/j.cpet.2014.03.010. 412 RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 Directed Reading Quiz 15802-01 2.0 Category A credits 2.0 MDCB credits Expires April 30, 2017* Techniques, Benefits, and Challenges of PET-MR To earn continuing education credit: Take this Directed Reading quiz online at www.asrt.org/drquiz. Or, transfer your responses to the answer sheet on Page 418 410M and and mail mail toto ASRT, ASRT, POPO Box Box 51870, 51870, Albuquerque, NM 87181-1870. New and rejoining members are ineligible to take DRs from journal issues published prior to their most recent join date unless they have purchased access to the quiz from the ASRT. To purchase access to other quizzes, go to www.asrt.org/store. *Your answer sheet for this Directed Reading must be received in the ASRT office on or before this date. Read the preceding Directed Reading and choose the answer that is most correct based on the article. 1. Positron emission tomography (PET) is a 3-D nuclear medicine examination that detects photons emitted by the stabilization of various injected radiopharmaceuticals such as: 1. fluorine 18. 2. carbon 11. 3. yttrium 86. a. b. c. d. 1 and 2 1 and 3 2 and 3 1, 2, and 3 2. The rationale for using fluorodeoxyglucose F 18 (FDG) in cancer imaging is known as the ______ effect. a.Harnack b.Warburg c.Lugburg d.Dixon 3. In PET, the ______ releases energy in the form of two 511-keV photons, which are emitted in nearly opposite directions. a. pair production b. scatter effect c.positronium d. annihilation reaction 4. A relatively simple method for determining the amount of FDG activity within an area of interest is: a. attenuation analysis. b. catabolic transformation. c. standard uptake value. d. Hounsfield units. 5. To standardize computed tomography (CT) values across manufacturers, values can be converted to: a. Hounsfield units. b. attenuation maps. c. standard uptake values. d. dose calibrations. continued on next page RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 413 Directed Reading Quiz 6.The most important benefit of using CT contrast agents in PET-CT is the ability to display the precise ______ of pathology. a. size progression b.angiogenesis c. anatomic localization d. metabolic function 7. During a PET-CT procedure, which of the following occurs after FDG injection? a. PET imaging begins immediately. b. CT or MR imaging begins immediately. c. The patient is instructed to sit in the imaging waiting area for 30 minutes. d. The patient is instructed to lie flat in a dimly lit room for 60 minutes. 8. Time of flight (TOF) is a technique in third-generation PET scanners that considers: a. the length of time required for full FDG uptake. b. the amount of time required for each gamma ray to reach the detector. c. how long it takes to reconstruct the image. d. time required for attenuation correction images taken in CT. 9. Limitations of 18F-FDG PET-CT include: 1. limited specificity. 2. motion artifacts. 3. low sensitivity. a. b. c. d. 1 and 2 1 and 3 2 and 3 1, 2, and 3 10. Magnetic resonance (MR) is based on the inherent ability of ______ molecules in soft tissue to orient along an applied external magnetic field. a.carbon b.hydrogen c.nitrogen d.magnesium 11. The longitudinal or spin-lattice relaxation (T1) is the: a. interaction of intrinsic fields of nuclei interacting with one another. b. interaction of extrinsic fields of nuclei interacting with one another. c. shifting of magnetic moments from high to low energy states. d. shifting of magnetic moments from low to high energy states. 12. Some challenges of designing a PET-MR system include: 1. simultaneous acquisition of data. 2. minimizing patient motion. 3. interference caused by the proximity of the 2 systems. a. b. c. d. 1 and 2 1 and 3 2 and 3 1, 2, and 3 13. Sequential scanning has required more technical adjustments and expense at the research and development level. a.true b.false 14. Which of the following statements are true about traditional PET photomultiplier tubes? 1. The tubes are based on scintillators. 2. The tube’s scintillators tend to detect nuclear annihilation slowly. 3. Scintillators enable TOF imaging easily. a. b. c. d. 1 and 2 1 and 3 2 and 3 1, 2, and 3 continued on next page 414 RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 Directed Reading Quiz 15. New photomultiplier tubes for simultaneous PET-MR are more advanced than traditional tubes and are made from: a.silicon. b. water-cooled scintillators. c.coils. d.titanium. 16. Which of the following happens when the MR magnet is quenched? a. Liquid nitrogen that cools the magnet coils boils off rapidly, releasing helium. b. Liquid cyrogens that cool the magnet coils boil off rapidly, releasing nitrogen. c. Liquid cyrogens that cool the magnet coils boil off rapidly, releasing helium. d. Gaseous argon that cools the magnet coils boils off rapidly, releasing nitrogen. 17. PET-CT and PET-MR protocols at the Cleveland Clinic call for an average fasting time of ______ hours before examinations. a.2 b.4 c.6 d.12 18. A type of attenuation correction method in PET-MR that uses a standard transmission based on the patient’s anatomy is called: a. segmentation technique. b. voxel-based mapping. c. atlas-based algorithms. d. standard uptake calibration. 19. Methods used in PET-MR for attenuation correction might not account well for: a. trabecular bone. b. cortical bone. c.water. d.fat. 20. Recent research also has shown that a long time span between injection of FDG and PET-MR may lead to a decrease in ______ within normal tissue and within lesions. a. subject contrast b. standard uptake values c. motion artifacts d. attenuation mapping accuracy 21. Fusing PET and MR images introduces challenges such as: 1.misregistration. 2. patient motion errors. 3. reconstruction complications. a. b. c. d. 1 and 2 1 and 3 2 and 3 1, 2, and 3 22. MR-based attenuation correction with the ______ sequence for each bed position is comparable to the type of anatomic correction achieved with low-dose CT scans in PET-CT. a. contrast-enhanced b.spin-echo c. 2-point Dixon d.Warburg 23. Which of the following can cause image artifacts in PET-MR imaging? 1.prostheses 2. spinal hardware 3. a patient’s body habitus a. b. c. d. 1 and 2 1 and 3 2 and 3 1, 2, and 3 continued on next page RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4 415 Directed Reading Quiz 24. ______ should be responsible for sending and receiving radiopharmaceutical shipments. a. MR technologists b. CT technologists c. Nuclear medicine technologists d. Medical physicists 25. Which of the following can be contraindications for PET-MR? 1. aneurysm clips 2. cardiac pacemakers 3. implanted cardiac defibrillators a. b. c. d. 1 and 2 1 and 3 2 and 3 1, 2, and 3 26. Which of the following is not true regarding personnel who work in PET-MR? a. All PET-MR personnel should have radiation safety training. b. The nuclear medicine technologist should operate the PET-MR scanner. c. PET-MR technologists ideally should be certified in nuclear medicine and MR. d. Professional societies have developed pathways for dual PET and MR certification. 28. If the PET isotope used has a half-life of 110 minutes, how long should department staff store and monitor contaminated items to ensure the exposure measures at background level? a. 2 to 4 hours b. 6 to 8 hours c. 10 to 12 hours d. 18 to 20 hours 29. At the time this article was published, PET-MR reimbursement was: a. nonexistent; the examination was considered only valid for research. b. based solely on PET current procedural technology codes. c. based on special new technology codes. d. based on separate codes for PET, CT, and MR. 30. PET-MR modalities could allow disease detection at a micro and picomolar level. a.true b.false 27. Inside the PET-MR suite, technologists can detect the presence of radioactive material using a: 1. traditional Geiger counter. 2. wipe smear test. 3. gamma camera well counter. a. b. c. d. 416 1 and 2 1 and 3 2 and 3 1, 2, and 3 RADIOLOGIC TECHNOLOGY, March/April 2015, Volume 86, Number 4