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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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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.
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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
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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.
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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
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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
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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
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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
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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
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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
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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.
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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
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References
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28. Pichler BJ, Kolb A, Nagele T, Schlemmer HP. PET/MRI:
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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/.
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_Brochure_LR.pdf%3fnodeid%3d10101343%26vernum%
3d1. Accessed January 8, 2015.
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of the Siemens mMR integrated whole-body PET/MR scanner. J Nucl Med. 2011;52:1914-1922. doi:10.2967/jnucmed
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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.
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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.
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Directed Reading Quiz
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Expires April 30, 2017*
Techniques, Benefits, and
Challenges of PET-MR
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 Take this Directed Reading quiz online at www.asrt.org/drquiz.
 Or, transfer your responses to the answer sheet on Page 418
410M
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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