Nuclear Medicine: Cardiac Imaging Part 3
Part 3 review of nuclear cardiology for radiology and nuclear medicine education. Download the free study guide on these episodes on nuclear cardiology here.
Show Notes/Study Guide:
What is the primary purpose of a MUGA scan?
The primary purpose of a MUGA scan is to provide an ejection fraction calculation. MUGA stands for multi-gated acquisition scan and uses gating to provide evaluation of cardiac function and contractility after equilibration of radiotracer in the intravascular space.
What radiotracer is classically used for a MUGA exam?
MUGA classically uses a Tc-99m tagged red blood cell technique.
If you see a photopenic halo around the cardiac blood pool what does this suggest on a MUGA scan?
A photopenic halo around cardiac blood pool is a potential manifestation of a pericardial effusion.
If structures such as the pulmonary artery, thoracic aorta, left atrium, or right ventricle overlap the left ventricle on a MUGA acquisition will this falsely lower or falsely increase estimation of ejection fraction on a MUGA scan?
Vascular structures that overlap the left ventricle on a MUGA scan, whether great vessels or other cardiac chambers, will give the false appearance that tracer is not exiting the left ventricle and this will therefore lower the estimated ejection fraction. If you are asked this question, simply think it through and realize that in this scenario of vascular overlap, tracer will not be clearing the left ventricle as it should and this would therefore allow for underestimation of left ventricle ejection fraction.
If the background region of interest is erroneously placed over the spleen will this falsely lower or falsely increase estimation of ejection fraction on a MUGA scan?
Erroneously placing the background region of interest over the spleen will falsely increase ejection fraction calculation. The reason for this false increase in calculated ejection fraction is that there will be over-subtraction of background that is in the denominator of the ejection fraction equation, therefore this will lower the denominator and increase the calculated ejection fraction. The equation is (end diastolic counts – background) – (end systolic counts – background)/ (end diastolic counts – background).
Is tagging of Tc to red blood cells for a MUGA scan typically performed using in vivo or in vitro labeling?
For MUGA one can get by with in vivo labeling of red blood cells to Tc. This is different from a tagged red blood cell scan performed for a GI bleed which requires a higher labeling efficiency and requires in vitro labeling.
What are some key features to identify the short axis, vertical long axis, and horizontal long axis on nuclear cardiac imaging?
Short axis: look for the round donuts or ring-like configuration of the left ventricle. Vertical long axis: to me looks like an arrow pointing to the right. The apex of the heart is at the right of the screen. Horizontal long axis: look for the two parallel vertical lines, similar to the two parallel long lines of an “H” (H=Horizontal).
What is the leading cause of attenuation artifact on nuclear cardiac imaging?
Breast tissue overlying the heart is likely the top cause of attenuation artifact.
What is the top cause of reconstruction artifact on a myocardial perfusion scan?
Bowel or liver uptake “stealing” counts from the heart causes reconstruction artifact.
What is the calculation for maximal heart rate used to determine whether somebody has reached sufficient cardiac stress for a Bruce treadmill protocol?
220 minus the patient’s age. In general, it is desired to reach 85% of this maximal heart rate value for the stress portion of myocardial perfusion imaging.
What is the most commonly used nuclear medicine agent for evaluation of cardiac sarcoidosis?
FDG is the agent most commonly used in nuclear medicine to evaluate for cardiac sarcoidosis. On an FDG-PET/CT study active cardiac sarcoidosis shows heterogeneous myocardial uptake. Note that unlike amyloidosis and other infiltrative diseases of the heart, myocardial sarcoidosis is typically patchy and this patchy involvement accounts for the heterogeneous, slightly patchy uptake seen on a positive FDG-PET/CT study. A negative scan shows no increased myocardial uptake of FDG.
What patient preparation steps need to take place in order to prepare for FDG-PET/CT imaging for cardiac sarcoidosis?
Dietary modifications in which patients follow a high fat and low carbohydrate diet the day prior to imaging, with an overnight fast, is necessary. This switches myocardial metabolism away from glucose metabolism to free fatty acid metabolism. The purpose is to decrease the physiologic uptake of glucose in the heart with the rationale that areas of cardiac sarcoidosis will not switch their metabolism away from glucose uptake, thus allowing maximal signal to noise with areas of cardiac sarcoidosis taking up the FDG while areas of normal myocardium will take up less FDG as normal myocardium has preferentially switched to fatty acid metabolism. This is termed myocardial suppression and you need good myocardial suppression in order to get a high quality FDG-PET/CT scan for cardiac sarcoidosis. Some protocols also use IV heparin prior to imaging that can further suppress myocardial FDG uptake.
True or false: patients being evaluated for cardiac sarcoidosis typically complete a myocardial perfusion scan in addition to an FDG-PET/CT study?
True. FDG images for inflammation related to sarcoidosis but this does not give information about perfusion of the heart. Comparing information regarding inflammation from FDG and perfusion from a nuclear perfusion agent such as Rubidium on PET or a Technetium agent allows differentiation of normal tissue from scar tissue or areas of active sarcoidosis. This is also important for staging as sarcoid progresses from normal (perfusion and FDG both normal) to early stage (mild perfusion defect, mild increased FDG uptake) and progressive stages wherein perfusion defects worsen, and FDG uptake progressively gets more pronounced. Finally, end stage fibrosis from sarcoidosis would show a severe perfusion defect with minimal to no FDG uptake.
What is the preferred agent for nuclear cardiac imaging of amyloidosis?
Tc 99m pyrophosphate (PYP) is the main agent used in the US for nuclear imaging of ATTR amyloidosis. Tc 99m pyrophosphate binds to deposited amyloid transthyretin protein (ATTR) in the myocardium.
What are key features of a positive Tc 99m pyrophosphate scan for ATTR cardiac amyloidosis?
An uptake ratio of 1.5 or higher of the heart to contralateral chest on a planar view is positive for ATTR deposition in the heart. Semi-quantitative measures are also used (0-absent cardiac uptake, 1 uptake less than bone, 2 uptake equal to bone, 3 uptake greater than bone) but for board purposes I think remembering the 1.5 cutoff of heart to contralateral lung uptake is the most important measure to remember.
See this great article for more information on cardiac sarcoidosis and nuclear imaging: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4009625/
