Cardiovascular Positron Emission Tomography (PET)

Richard C. Brunken

QUESTIONS

1. Which of the following statements best characterizes rubidium-82, a radiotracer frequently used for cardiac PET imaging?

A. The tracer is eluted from a portable generator, in which decay of the “parent” isotope molybdenum-99 yields the “daughter” isotope rubidium-82.

B. Following intravenous (IV) administration, rubidium-82 is retained in the myocardium by mitochondrial sequestration.

C. As part of the quality control (QC) process, the first sample of the day eluted from the rubidium-82 generator is examined for “break-through” of radionuclide contaminants.

D. The 2-minute half-life of rubidium-82 facilitates patient throughput in busy imaging centers, permitting rest and stress PET myocardial perfusion imaging in as little as 30 minutes.

View Answer

1. ANSWER: C. Rubidium-82 chloride is a short-lived (half-life = 76 seconds) radioactive analog of potassium that relies upon the Na⁺-K⁺ ATPase transporter for cellular uptake. Rubidium-82 is eluted from a small portable generator in which strontium-82, the “parent” radionuclide, is absorbed onto a SnO₂ column. When strontium-82 decays (half-life = 25 days), the “daughter” radionuclide rubidium-82 can be washed (eluted) off the column using normal saline. The column also contains strontium-85, a radionuclide (half-life = 65 days) that is generated as a by-product during the production of strontium-82. Problems with the generator column can release both strontium-82 and strontium-85 into the eluate. These radionuclide contaminants are tested for on a daily basis by examining the first eluted sample of the day.

2. Which of the following PET tracers can be used to quantitatively assess both myocardial perfusion and metabolism?

A. ¹⁸F-2-fluoro-2-deoxy-2-D-glucose (FDG)

B. ¹¹C-acetate

C. ¹¹C-heptagluconate

D. ¹¹C-hydroxyephedrine

View Answer

2. ANSWER: B. ¹¹C-acetate is a marker of myocardial oxidative substrate metabolism. Following the uptake of ¹¹C-acetate by the myocardium, initial rates of clearance of carbon-11 activity from the tissue quantitatively reflect oxidative substrate flux through the tricarboxylic acid cycle. In clinical studies of patients with CAD, rates of ¹¹C-acetate clearance have proven useful for identifying dysfunctional myocardial segments which will exhibit an improvement in function following revascularization. Because ¹¹C-acetate has a high first-pass myocardial extraction fraction, images obtained early following tracer injection can also be used to measure regional tissue blood flow, in ml/min/g tissue. FDG is a tracer of myocardial glucose metabolism, while ¹¹C-hydroxyephedrine is used to depict sympathetic innervation of the myocardium. ¹¹C-heptagluconate is a tracer of myocardial fatty acid utilization.

3. At a myocardial blood flow of 3.7 mL/min/g tissue, which of the following myocardial perfusion tracers would exhibit the highest net tissue uptake (flow × extraction)?

A. Thallium-201 chloride

B. Nitrogen-13 ammonia

C. Technetium-99m sestamibi

D. Oxygen-15 water

View Answer

3. ANSWER: D. Oxygen-15 water is a freely diffusible tracer of myocardial blood flow whose net tissue uptake is near unity even at hyperemic blood flows. For each of the other tracers, net tissue uptake gets progressively smaller for each unit increase in tissue blood flow. As a result, linearity is lost and tracer uptake no longer reflects flow. This roll off or plateau effect is a greater problem with technetium-99m tracers especially when performed in conjunction with pharmacologic stress, which has a greater increase in coronary blood flow relative to exercise.

4. A 64-year-old patient with congestive heart failure and malignant ventricular arrhythmias undergoes cardiac PET imaging with rubidium-82 at rest (RstAC) and ¹⁸F-2-fluoro-2-deoxy-2-D-glucose (FDGAC). The FDG images were acquired following oral glucose loading. Representative cardiac PET images are shown in Figure 6.1. Which of the following findings would be most likely to be identified in the abnormal anteroapical and basal inferior myocardial regions with the matching defects in perfusion and metabolism on the PET images?

Figure 6.1

A. An increase in signal intensity on delayed gadolinium-enhanced magnetic resonance images

B. An improvement in regional wall motion on a postpremature ventricular contraction on contrast left ventriculography

C. Thallium-201 redistribution on rest and 4-hour redistribution single photon emission computed tomography myocardial perfusion imaging (SPECT MPI)

D. An end-diastolic wall thickness of 8.0 to 8.5 mm on electrocardiogram-gated magnetic resonance images

View Answer

4. ANSWER: A. The cardiac PET images depict severe matching perfusion and metabolic defects in the anteroapical and basal inferior regions, consistent with myocardial infarction. On delayed contrast-enhanced magnetic resonance images, an increase in signal intensity is typically found in areas of nonviable tissue (fibrosis or scar) and would be consistent with the infarction noted on the PET images. By contrast, postextrasystolic potentiation of regional function, thallium-201 redistribution, and an end-diastolic wall thicknesses of 8.0 to 8.5 mm would all be suggestive of significant residual tissue viability and not transmural scar.

5. Which of the following myocardial perfusion imaging protocols will deliver the highest effective dose of ionizing radiation to an adult patient?

A. Stress and 4-hour redistribution thallium-201 SPECT imaging (3.5 mCi of thallium-201 chloride)

B. 1-day rest/stress technetium-99m SPECT tetrofosmin imaging (10.0 mCi rest dose, 27.5 mCi stress dose)

C. 2-day rest/stress technetium-99m tetrofosmin SPECT imaging (25.0 mCi rest dose, 25.0 mCi stress dose)

D. Rest and stress rubidium-82 PET imaging (50 mCi rest dose, 50 mCi stress dose)

View Answer

5. ANSWER: A. Because of the long half-life of thallium-201, the effective dose from protocol A is greater than that of the other options. The estimated effective dose for the stress and redistribution thallium-201 study is 22 mSv; for the 1-day rest-stress technetium-99m tetrofosmin SPECT imaging protocol 9.9 mSv; for the 2-day technetium-99m tetrofosmin SPECT imaging protocol 13.5 mSv; and for the rubidium-82 cardiac PET study 4.6 mSv.

REFERENCES:

Einstein AJ, Moser KW, Thompson RC, et al. Radiation dose to patients from cardiac diagnostic imaging. Circulation. 2007;116:1290-1305.

Senthamizhchelvan S, Bravo PE, Esaias C, et al. Human biodistribution and radiation dosimetry of 82Rb. J Nucl Med. 2010;51:1592-1599.

6. A 59-year-old man is concerned about the possibility of coronary artery disease (CAD). His 64-year-old brother recently sustained a myocardial infarction, and an evaluation performed by his family physician indicates an intermediate probability for CAD. His family physician orders a computed tomography (CT) scan for coronary calcium scoring, and the coronary calcium score is 612. If the patient is referred for an adenosine rubidium-82 PET myocardial perfusion study, which of the following statements would best describe the relationship between the CT findings and the anticipated results of the PET study?

A. The likelihood that the PET study will show ischemia is <2%.

B. The probability that the PET study will demonstrate ischemia is between 25% and 50%.

C. The likelihood that the PET study will demonstrate ischemia exceeds 50%.

D. Once the patient’s conventional cardiovascular risk factors are considered, the calcium score provides no incremental benefit for predicting the presence of inducible ischemia on the PET study.

View Answer

6. ANSWER: B. The likelihood that a patient with a calcium score >399 and <1,000 will demonstrate a reversible perfusion defect on pharmacologic stress PET perfusion imaging is almost 50%, based upon a study by Schenker and colleagues of 695 consecutive patients with an intermediate likelihood of CAD. In this study, the coronary calcium score provided a small, but incremental benefit for predicting ischemia on the PET scan over patient age, gender, and other conventional cardiac risk factors.

REFERENCE:

Schenker MP, Dorbala S, Hong EC, et al. Interrelation of coronary calcification, myocardial ischemia and outcomes in patients with intermediate likelihood of coronary artery disease: a combined positron emission tomography/computed tomography study. Circulation. 2008;117:1693-1700.

7. The 59-year-old man from the previous question undergoes an adenosine rubidium-82 cardiac PET imaging study. The study is normal, without a reversible perfusion defect to suggest ischemia. Which of the following best describes the patient’s annual risk for death/myocardial infarction?

A. 0.2%

B. 1%

C. 5%

D. 8%

View Answer

7. ANSWER: C. Schenker showed that in patients with an intermediate likelihood of CAD, the annualized event rate for patients with nonischemic PET perfusion studies and calcium scores between 400 and 999 was 5% (95% confidence intervals: 1.6% to 15.5%). Coronary calcium scores provided incremental prognostic information over that provided by the assessment of myocardial perfusion alone.

REFERENCE:

8. A 59-year-old man with atypical chest pain is referred for PET imaging 3 weeks after a stent to the left anterior descending (LAD) artery. He has a history of mixed hyperlipidemia, type II diabetes, hypothyroidism, and adrenal cortical insufficiency. Dipyridamole stress (StrAC) and rest (RstAC) rubidium-82 PET images are shown in Figure 6.2. Which of the following best describes the findings on the PET imaging study?

Figure 6.2

A. Normal study

B. Reversible defect, mid and distal LAD distribution

C. Fixed defect, distal LAD distribution

D. Fixed perfusion defect in a diagonal distribution

View Answer

8. ANSWER: D. The PET imaging shows a small fixed perfusion defect localized to the basal anterior wall. This corresponds to the loss (“jailing”) of a small first diagonal branch of the LAD at the time of the intervention; following the procedure, there was a small rise in the patient’s cardiac enzyme levels.

9. PET imaging can be performed in a two-dimensional (2D) mode or in a three-dimensional (3D) mode. Which of the following statements best describes the use of 2D versus 3D PET imaging?

A. In the 2D mode, there are lead or tungsten septa between the detector rings.

B. As compared with doses used for imaging in the 2D mode, the doses employed for PET imaging in the 3D mode generally need to be larger.

C. The 2D mode is the type used most frequently for PET oncology imaging.

D. The 2D mode is preferred when PET imaging is performed in children.

View Answer

9. ANSWER: A. PET scanners that operate in the 2D mode, also known as “septa in” PET scanners, have lead or tungsten septa interposed between the detector rings. The septa serve to reduce coincidence events between detectors in a given ring and the adjacent rings, decreasing the number of scattered events. Scanners that operate in the 3D mode, “septa out,” do not have interposed septa. This permits a greater number of coincidence events between detectors in differing rings, serving to increase the sensitivity of the device as well as the number of scatter events. In general, lower doses of the radioactive tracers can be used for imaging on 3D machines than on 2D machines. Use of 3D scanners is especially attractive for imaging for pediatric patients, as it can reduce the radiation exposure. Most oncologic PET imaging studies are performed on 3D scanners, in order to maximize sensitivity for detection of abnormal tracer uptake.

REFERENCE:

ASNC Imaging Guidelines for Nuclear Cardiology Procedures. PET myocardial perfusion and metabolic imaging. 2009. Available online at: http://www.asnc.org

10. Which of the following crystal types used in PET scanners has the highest stopping power for annihilation photons?

A. Bismuth germanate (BGO)

B. Lutetium oxyorthosilicate (LSO)

C. Gadolinium oxyorthosilicate (GSO)

D. Lutetium yttrium orthosilicate (LYSO)

View Answer

10. ANSWER: A. While BGO crystals have the highest stopping power, their energy and timing resolution are not as favorable as the other crystals listed. Use of the newer LSO, GSO, or LYSO crystals reduces dead times and allows higher count rates, prompting their use in 3D PET scanners.

REFERENCE:

ASNC Imaging Guidelines for Nuclear Cardiology Procedures. PET myocardial perfusion and metabolic imaging. 2009. Available online at: http://www.asnc.org

11. Which of the following types of stress is an indicator of coronary artery endothelial function when used in conjunction with PET myocardial perfusion imaging?

A. IV dipyridamole

B. IV adenosine

C. cold pressor test

D. IV regadenoson

View Answer

11. ANSWER: C. The cold pressor test involves immersion of a hand or foot into ice water (2°C) for 2 minutes. The stress perfusion tracer is typically injected IV at 1 minute, with imaging for an additional minute while the hand or foot remains immersed in the cold water. Cold water immersion stimulates the release of norepinephrine from cardiac sympathetic nerve terminals, which in turn results in vasodilation of the coronary circulation via the endothelium-mediated release of nitrous oxide (NO). The vasodilation induced by dipyridamole, adenosine, and regadenoson is largely independent of endothelial function.

REFERENCE:

Al-Mallah MH, Sitek A, Moore SC, et al. Assessment of myocardial perfusion and function with PET and PET/CT. J Nucl Cardiol. 2010;17:498-513.

12. Which of the following QC measures would not be appropriate for a PET/CT scanner?

A. Center of rotation

B. System sensitivity

C. Intrinsic scatter fraction

D. Accuracy of attenuation correction

View Answer

12. ANSWER: A. Center of rotation is a QC procedure used to assess the performance of rotating SPECT MPI cameras. It is not used for PET scanners that have a ring detector system. The other QC procedures are used to assess the performance of PET/CT scanners.

13. In which of the following patients might an artifact be observed on myocardial PET images if x-ray CT is used for attenuation correction?

A. A 56-year-old woman with bilateral silicone breast implants

B. A 72-year-old man with a permanent pacemaker with RA and RV leads

C. A 39-year-old woman with cardiac sarcoidosis, ventricular arrhythmias, and an implanted defibrillator with an RV lead

D. An 82-year-old man with hypertensive heart disease and pronounced mitral annular calcification

View Answer

13. ANSWER: C. Use of transmission images obtained with low-dose CT for attenuation correction of cardiac PET images allows more rapid study acquisition than transmission images obtained with a rotating line source of activity. However, objects with a very high density, such as a defibrillator coil, may not be adequately characterized by the CT transmission images. The x-ray CT transmission images overcompensate for the density of the object, resulting in hot spots on the attenuation-corrected images.

REFERENCE:

DiFilippo F, Brunken RC. Do implanted pacemaker leads and ICD leads cause metal-related artifact in cardiac PET/CT? J Nucl Med. 2005;46:429-435.

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