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SMU-DDE-Assignments-Scheme of Evaluation PROGRAM SEMESTER SUBJECT CODE & NAME BK ID DRIVE MARKS Q. No 1. A Bachelor in Medical Imaging Technology V BMI 502– Computed Tomography Imaging and Patient Care B2082 WINTER 2015 60 Criteria Marks Total Marks Explain the technical factors of CT brain perfusion scans. The most common technique associated with CT perfusion scanning is based on the first pass of a contrast bolus through the brain tissue. With this technique a 50-mL IV bolus of a non-ionic low osmolality contrast is injected at 4 to 5 mL/s. A helical scanner is used to produce a dynamic set of images at a single location. A 5-second scan delay is used and slices are typically 5 mm thick. Typical scan durations are in the range of 40 to 45 seconds. The slices are produced by repeatedly scanning the same region at the same table position, a technique some manufacturers refer to as the cine mode. Multislice scanners allow several z position slices to be scanned simultaneously. Scans are typically acquired at 5-mm sections to lessen beam hardening artifacts, and then reformatted into 10-mm-thick sections for viewing to improve the signal-to-noise ratio. The brain perfusion protocol begins with an unenhanced scan of the whole brain. Although the level for scanning the enhanced portion of the study may be selected at the time of examination based on the unenhanced CT findings, a transverse slice through the level of the basal ganglia contains territories supplied by the anterior, middle, and posterior cerebral arteries, thus offering the opportunity to interrogate each of the major vascular regions. The outlined areas on the upper two rows of images are the basal ganglia. A major limiting factor of older helical scanners was the 10- to 20-mm maximum anatomic coverage. Often a second bolus of contrast was necessary so that the patient could be scanned at a different location, typically in a more cephalad direction above the lateral ventricles. The first step in post processing the acquired images using the perfusion software is to select a reference artery and a reference vein. The reference artery should be 1) seen in crosssection, 2) one of the first to enhance, 3) produce a curve with a high enhancement peak, or 4) produce a curve with a narrow width. The most common choice of reference artery is the anterior (Unit 2, Section 2.3) 10 10 SMU-DDE-Assignments-Scheme of Evaluation 2. A cerebral artery. The reference vein should be the largest venous structure available or one that produces an enhancement curve with the highest peak. The superior sagittal sinus is the vein most often used as the reference. An ROI is placed on the reference artery and the reference vein so that contrast-enhancement curves can be generated. Analysis of the contrast enhancement curves guides the selection of pre enhancement and post enhancement images. The pre enhancement image is that last image before contrast arrives. The post enhancement image is the point immediately after the first pass of the contrast bolus when the timeattenuation graph begins to flatten. The perfusion software then generates color-coded maps demonstrating: a) Regional cerebral blood volume, or rCBV b) Blood mean transit time, or MTT, through cerebral capillaries c) Regional cerebral blood flow or rCBR. Quantitative data can be extracted from the maps by placing multiple ROIs in the brain parenchyma. d) Brain perfusion studies are frequently ordered with a CTA of the circle of Willis or the carotid arteries. In these situations the CTA should be performed first and the brain perfusion study can immediately follow. Discuss the general methods of thoracic scanning. CT of the airways Technical parameters used for CT imaging of the airways include the use of thin sections (1.25 mm or less), a fast acquisition that allows the entire lungs to be scanned during a single breath-hold, optimal spatial resolution, and the use of post processing techniques. Overlapping z axis image reconstruction of 50% is typical. Neither IV nor oral contrast media are routinely required. IV contrast may be used in cases of airway tumors. Airway imaging is routinely performed at both inspiration and expiration. CT is generally accepted as the best imaging technique for assessment of disease of the central airways and is most commonly used to look for narrowing that may occur in patients who have been intubated in the past. Applying post processing techniques, such as volume rendering, may be referred to as CT bronchography. Virtual bronchoscopy is accomplished with similar post processing techniques, but is different in that it offers an internal rendering of the tracheobronchial walls and lumen High-resolution CT High-resolution CT (HRCT) is used to evaluate the lung parenchyma in patients with known or suspected diffuse lung diseases such as fibrous is and emphysema. Like airway (Unit 3, Section 3.2) 5+5 10 SMU-DDE-Assignments-Scheme of Evaluation 3. imaging, HRCT protocols use thin sections (1.5 mm or less), a fast acquisition to reduce motion artifact, and optimal spatial resolution. In addition to the thin sections, spatial resolution is optimized by the selection of an edge-enhancing algorithm (such as a bone algorithm) and a display field of view (DFOV) that is just large enough to include the lungs. In some institutions, HRCT protocols are incremental, meaning images are obtained with an interval of 10 mm or more between slices and only approximately 10% of the lung parenchyma is scanned. This technique is intended to provide representative areas of lung disease. However, because evidence of some types of diffuse lung disease may not be uniform in distribution throughout the lung, this method of sampling may result in characteristic foci of the disease not being imaged. More recently, as MDCT scanners have become common place, the technique known as volumetric HRCT is replacing the HRCT axial protocols. Volumetric HRCT protocols use a helical mode to acquire images of the entire lung, rather than representative slices. Because these helical protocols cover the entire lung, they result in a more complete assessment of the lung. Lung nodules that could be missed between slices in incremental protocols are not missed with volumetric HRCT, and the central airways can be evaluated at the same time. In addition, they allow post processing techniques such as maximum (MIP) and minimum (MinIP) intensity projection reformation. Although there are clear advantages to the use of volumetric HRCT over an interspaced technique, the increased radiation exposure is a consideration. Many volumetric HRCT protocols decrease the tube current (mA) to reduce the radiation dose. Many HRCT protocols (both volumetric and axial) include more than one series of scans. In all patients there is a gradual increase in attenuation and vessel size from anterior to posterior lung regions owing to the effect of gravity on blood flow and gas volume. Therefore, HRCT protocols are routinely obtained at full inspiration. However, expiratory images are useful in many instances. For example, expiratory images better depict bronchiolitis and air trapping. Discuss cardiac CT technique. (Unit 5, Section 5.2) SMU-DDE-Assignments-Scheme of Evaluation A 4. A General-purpose CT protocols can often be used in imaging of abdominal, thoracic, and cerebral vessels in which image quality is not substantially influenced by cardiac motion or vessel pulsation. However, for studies of the heart and coronary arteries, dedicated cardiac CT acquisition techniques are needed to produce images free of motion artifact. Pharmacologic heart rate control: Essentially the CT scanner is a fast camera, but as fast as state-of-the-art scanners are, they cannot take a motion free picture when the heart is beating too fast. Therefore, many institutions use β -blockers as part of their cardiac CT protocols. β-blockers are used to lower the heart rate to less than 65 to 70 beats per minute (bpm) and to make the rhythm more regular. ECG gating: To minimize cardiac motion artifact, most cardiac CT protocols use images acquired during the point of the cardiac cycle with the least cardiac motion. Most often this point is during end-diastole, but may also be at end-systole. In other words, just before or just after the left ventricle is fully contracted. However, different structures may be most still at slightly different phases of the cardiac cycle. Contrast administration: Most cardiac CT protocols require the intravenous administration of iodinated contrast agents. Standard screening for contraindications to the contrast agent (e.g., renal impairment, iodine allergy) is necessary. An intravenous line is placed using a large-lumen (20-gauge or larger) flexible cannula in a vein of sufficient diameter to accommodate a relatively high injection rate. Breath-hold: It is very important that the patient suspend respiration during scan acquisition. Breathing during the scan will result in motion artifact. Careful breath-hold instructions should be given to the patient before the scan begins. It is often helpful to have the patient practice holding his or her breath in moderate inspiration. Discuss the imaging studies for acute appendicitis. Many diseases other than acute appendicitis may produce signs and symptoms indistinguishable from those of acute appendicitis. In situations for which a clear diagnosis of appendicitis cannot be made by the patient’s history, physical examination, and laboratory evaluation, radiologic testing particularly ultrasonography and CT can be useful. Plain radiographs of the abdomen are frequently obtained as part of the general evaluation of a patient with an acute abdomen, but they are rarely helpful in diagnosing acute appendicitis. Barium enema is unreliable in the diagnosis of acute appendicitis and has been replaced by ultrasonography and CT. Ultrasonography is inexpensive and widely available, does not require contrast agents, and poses no special risk to the 10 10 (Unit 8, Section 8.2) 10 10 SMU-DDE-Assignments-Scheme of Evaluation 5. A fetus in pregnant patients. It is particularly well suited for evaluating right lower quadrant or pelvic pain in pediatric and female patients. However, the technique used to visualize the appendix, called graded compression sonography, is highly operator dependent. Some of the difficulties with ultrasonography are related to the fact that a normal appendix must be identified to rule out acute appendicitis. This is difficult in obese or very muscular patients or when there is an associated ileus that produces shadowing secondary to overlying gas-filled loops of bowel. Accuracy of ultrasound also decreases with retrocecal or pelvic locations of the appendix. In addition, the following scenarios can occur: a false-positive scan can occur in the presence of periappendicitis from surrounding inflammation; a dilated fallopian tube can be mistaken for an inflamed appendix; or inspissated stool can mimic an appendicolith. Finally, patients often complain of discomfort resulting from the transducer pressure during ultrasound evaluation. Despite these limitations, when institutions are comfortable with ultrasound for the evaluation of suspected appendicitis, ultrasonography continues to serve as the primary imaging study in children and in young or pregnant women. CT is more accurate than ultrasonography. Also, because it is not as dependent on the skill and experience of the operator, CT is more reproducible from hospital to hospital. The diagnostic accuracy rate of CT for acute appendicitis is reported to range between 93% and 98%. Findings on CT increased the certainty of diagnosis more than findings on ultrasonography. Although ultrasonography is recommended as the initial imaging study in children, young women, and pregnant women, CT is most often recommended as the initial study in all other patients. In addition, CT is recommended for patients in whom sonographic evaluation is suboptimal or indeterminate, or for those patients in whom perforation is suspected. Explain the challenges in examination of shoulder and knee. Shoulder Noncontrast CT is often requested for the evaluation of bony trauma. Thin slices are acquired in the axial plane beginning at the acromioclavicular joint and extending a few centimeters below the most inferior fracture line (determined by careful examination of the last few slices or asking a radiologist for assistance). CT arthrography of the shoulder is useful for evaluation of the joint capsule and intracapsular structures and for finding loose bodies within the joint. CT arthrography can be performed either with a single or (Unit 10, Section 10.3) 5 10 SMU-DDE-Assignments-Scheme of Evaluation 6. A double contrast technique (0.5 to 3.0 mL of iodinated contrast material and approximately 10 mL of room air). Thin axial slices begin at just above the acromioclavicular joint and end just below the glenoid fossa. For either indication the patient is positioned supine on the CT table. The arm to be examined is downward alongside the body, the opposite arm is extended over the patient’s head to reduce the x-ray beam absorption as much as possible. Knee Although MRI is the primary modality for the evaluation of internal derangement of the knee, CT remains the modality of choice in certain situations, such as tibial plateau fractures. The primary indication of knee CT is to assess the degree and alignment of fracture fragments, particularly at the articular surfaces. Knee CT is also performed to assess the integrity of the bone around prosthesis. The display field of view (DFOV) is focused on one knee only and must include the patella, both femoral condyles, and the proximal tibia through the fibular head. The patient typically lies supine on the scanner table with legs extended, knees side-by-side, and enters the scanner feet first. CT of the knee is sometimes performed immediately after an arthrogram of the knee in which iodinated contrast or air has been injected directly into the joint space. Explain FDG PET imaging. Working of PET PET creates an image from the radiation given off when positrons (i.e., antimatter electrons) encounter electrons in the body. To do this, patients are given a radiopharmaceutical with a short half-life, made up of a radionuclide (in this case 18F) linked to a pharmaceutical agent (deoxyglucose). The radionuclide emits positrons that encounter electrons in the body. They annihilate each other, producing highenergy photons (i.e., annihilation photons) that can be detected by the imaging device. The pharmaceutical portion of the radiopharmaceutical (deoxyglucose) allows localization that favor glycolysis. PET is unique because it creates images of the body’s physiologic functions, such as blood flow and metabolism. PET uses unique radiopharmaceuticals different from those used in traditional nuclear medicine. PET radiopharmaceuticals can be labeled with isotopes that are basic biologic substrates. These isotopes mimic natural molecules such as sugars, water, proteins, and oxygen. As a result, PET is often capable of revealing more about the cellular level metabolic status of a disease compared 5 (Unit 12, Section 12.3) 10 10 SMU-DDE-Assignments-Scheme of Evaluation with the capabilities of other imaging modalities. Currently, the most commonly used PET radiopharmaceutical is F-FDG. FDG imaging pitfalls Tumor cells, however, are not the only cells that exhibit an increased uptake of FDG. Because FDG maps glucose metabolism, its distribution can be altered by any physical activity. Normal physiologic accumulation of FDG occurs in the brain, muscles, salivary glands, myocardium, gastrointestinal tract, urinary tract, brown adipose tissue, thyroid gland, and gonadal tissues. It is important to recognize and understand normal variants of FDG uptake and benign disease to avoid mistaking them with pathologic processes. The timing of a patient’s last meal before a PET study will have a considerable effect on the quantities of glucose and insulin in the circulation, thereby affecting FDG uptake. Similarly, the patient’s state of hydration can alter the distribution of FDG in the body by altering the patient’s excretion of the tracer. To avoid confounding results that may complicate image interpretations, patients are required to fast from 4 to 6 hours before their PET examinations. If the patient’s blood sugar level is more than 200 mg/dL before the injection of FDG, this could limit the study’s sensitivity. Although all diabetic patients should control their blood sugar level with oral hypoglycemic medication or insulin, the timing of an insulin injection may affect the PET scan. An insulin injection close to the time of FDG administration induces diffusely increased uptake of FDG in the skeletal muscles. Therefore, insulin should be given as far from the time of FDG injection as feasible. *A-Answer Note –Please provide keywords, short answer, specific terms, specific examples (wherever necessary) ***********