Nuclear Medicine: Cardiac Imaging Part 1
Q&A review of nuclear cardiac imaging for radiology board review.
Show Notes/Study Guide:
Prior to a nuclear cardiac myocardial perfusion exam, what are key patient preparation steps that improve the subsequent imaging?
Patients should be instructed to fast for something like 4 hours to reduce uptake in the GI system/liver. If possible, have patient refrain from taking certain medications to include long-acting nitrates, caffeine, calcium channel and beta blockers as these will reduce sensitivity for detection of a perfusion defect/coronary artery stenosis.
Is Tc-sestamibi or Tc-tetrofosmin more likely to show liver and bowel uptake on a cardiac imaging study?
Tc-tetrofosmin has more rapid liver and bowel clearance compared to sestamibi and therefore will have less competing /adjacent activity in the liver and bowel on a cardiac imaging study.
What is the purpose of adding physiologic or pharmacologic stress to a nuclear cardiac exam?
Stressing the patient with exercise (physiologic) or pharmacologic stress increases the sensitivity for detection of coronary artery stenosis. Without stress, nuclear cardiac imaging can detect something like stenosis >90%. With stress sensitivity improves to be able to detect a stenosis >50%.
Name some common pharmacologic vasodilators used for myocardial perfusion imaging?
Adenosine, dipyridamole and regadenoson are common pharmacologic vasodilators used for myocardial perfusion imaging.
What pharmacologic stress agent is a specific adenosine receptor agonist with a lower risk of inducing bronchospasm?
Regadenoson is a specific A2A adenosine receptor agonist with a lower risk of inducing bronchospasm compared to adenosine or dipyridamole. Note that adenosine typically is associated with the highest risk of side-effects, including a risk of AV block. Dipyridamole inhibits the breakdown of adenosine, thus raising adenosine levels to cause vasodilation.
Which agent has a longer half-life: regadenoson or adenosine?
Regadenoson has a half-life of 2-3 minutes and therefore may be given as a single IV bolus. Adenosine has a half-life of only 10 seconds and therefore is given as an IV drip.
Which agents can be given to reverse the effects of regadenoson and other pharmacologic vasodilators?
Aminophylline is a reversal agent for regadenoson. Note that oral and IV caffeine also can provide reversal of the effects of regadenoson. Therefore, caffeine must be avoided prior to myocardial perfusion imaging when using any of the pharmacologic vasodilators in order to obtain adequate pharmacologic vasodilation/stress.
Which agent has a longer half-life: aminophylline or dipyridamole?
Aminophylline has a shorter half-life than dipyridamole. Therefore, when giving aminophylline to reverse the effects of dipyridamole, breakthrough symptoms can occur so you must continue to monitor for breakthrough when using aminophylline as a reversal agent.
Which pharmacologic agent increases stress by increasing heart rate and myocardial contraction as opposed to vasodilation?
Dobutamine is a beta 1 agonist that can be used to induce stress for myocardial perfusion imaging. This essentially causes a similar increase in heart rate and contractility to exercise instead of causing direct coronary vasodilation as with adenosine, dipyridamole and regadenoson.
Which patients may be best suited for pharmacologic stress with dobutamine?
Dobutamine may be considered for myocardial perfusion stress imaging in patients who cannot tolerate exercise for stress imaging who also have COPD and/or asthma to avoid the risk of bronchospasm that exists with the vasodilatory agents. Dobutamine can also be used in patients who have had caffeine intake within the past 12 hours. Note that dobutamine should not be used in patients on beta blockers as this would prevent the beta-1 induced increase in heart rate and contractility that is necessary to induce stress.
During a 1-day myocardial perfusion scan, is a higher dose of radiotracer administered during the stress or during the rest portion of the study?
Typically, you would start with a rest portion of the study at lower dose. Subsequently, a much higher dose would be given to overwhelm the rest counts and now provide perfusion information under stress. Note that a normal stress study effectively rules-out significant coronary artery stenosis, so if stress is performed first and is normal the rest portion of the study would not be necessary. 2-day protocols often would do stress first, and then rest the following day, only if stress is abnormal.
Basically, for a study with both stress and rest imaging, one either needs to wait about 4 half-lives for the radiotracer to decay from the first imaging study prior to performing the subsequent study to make sure there is not contamination on the second imaging from activity left over from the initial imaging. Alternatively, one can administer a dose something like 3-4 times as high for the second portion of the study and these will essentially overwhelm and diminish any residual counts that may be received from the initial imaging.
For more information on myocardial perfusion imaging, pharmacologic agents, patient contraindications for various agents, and protocols: please read here: https://www.asnc.org/files/Stress%20Protocols%20and%20Tracers%202009.pdf
