Nuclear: Cardiac Flashcards
What are the most common radiotracers used in myocardial perfusion studies?
- Tc-99m sestamibi
- Tc-99m Tetrofosmin
Note: Both of these work by passively diffusing into cells and then localizing to mitochondria (they don’t redistribute like thallium, which isn’t used much anymore).
Which is better for myocardial perfusion: sestamibi or tetrofosmin?
Tetrofosmin (it is cleared more rapidly from the liver, decreasing the chance of hepatic uptake artifact)
How long after injection should myocardial perfusion scans be done?
30-90 min after injecting Tetrofosmin or Sestamibi (allowing for clearance of background)
How bad must coronary artery stenosis be for it to be visible on myocardial perfusion scanning?
Non-stress: 90%
Stress: 50%
How long before a myocardial perfusion scan should a pt be NPO
NPO for 4 hours prior to the exam (to reduce GI blood flow)
Which medications should be stopped prior to myocardial perfusion scan (if possible)?
- Beta blockers
- Calcium channel blockers
- Long acting nitrates
Note: These medications mess with the sensitivity of the stress portion of the scan.
What medication is used for the stress portion of a myocardial perfusion scan (if unable to do the treadmill)?
Regadenoson (a specific adenosine receptor agonist that acts as a coronary vasodilator) or dobutamine
Note: Regadenoson is more specific (has less bronchospasm) than adenosine or dipyridamole, which used to be used.
Why is it important to know a pt has a left bundle branch block prior to a myocardial perfusion scan?
LBBB can cause a false positive reversible perfusion defect at the septum (anteroseptal region), which is more common with some stress agents
Note: This is probably due to irregular relaxation during diastolic coronary filling.
Should dobutamine be used for the stress portion of a myocardial perfusion scan in a pt with a left bundle branch block?
No, you should use a coronary vasodilator (regadenoson, adenosine, or dipyridamole)
Note: Dobutamine causes more false positives in these pts (because it increases the heart rate more, which causes more septal irregularities in LBBB).
Fixed defect (on stress and rest) on a myocardial perfusion scan…
Scar from old infarction
Reversible defect (seen on stress, better on rest) on a myocardial perfusion scan…
Ischemia
Fixed defect with reversible defect around it on a myocardial perfusion scan…
Infarction with peri-infarct ischemia
Left ventricular cavity appears larger on stress than on rest on a myocardial perfusion scan…
Think transient ischemic dilatation (from diffuse subendocardial hypo perfusion)
Note: This is associated with high risk disease (left main or 3 vessel disease).
Fixed dilatation of the left ventricular cavity on stress and rest on a myocardial perfusion scan…
Dilated cardiomyopathy
Right ventricular activity similar to the left ventricle during the rest portion of a myocardial perfusion scan…
Think right ventricular hypertrophy
Lots of bowel/liver activity on a myocardial perfusion scan…
Pt isn’t stressed (exercise or medication) enough to shift blood flow from the bowel/liver to the heart
Normal perfusion but poor contractility on a myocardial perfusion scan that improves after a few weeks…
Think stunned myocardium
Fixed defect (rest and stress) on myocardial perfusion imaging, but avid FDG uptake…
Hibernating myocardium
Note: This is a chronic process resulting from severe CAD and chronic hypoperfusion (myocardial tissue is alive but conserving energy by not contracting because it doesn’t have enough blood supply).
Fixed defect on myocardial perfusion imaging and hot on thallium imaging…
Hibernating myocardium
Note: Myocardial tissue is alive (able to take up thallium via Na/K ATPase) but is not contracting to conserving energy because it doesn’t have enough blood supply.
What nuclear imaging study can be done to estimate left ventricular ejection fraction?
MUGA (multigated acquisition scan)
Note: This is basically a nuclear angiogram using Tc-99m RBCs. It is more accurate than myocardial perfusion imaging for estimating LVEF.
Regional wall motion abnormality on a resting MUGA study…
- Infarction (usually)
- Stunned myocardium
- Hibernating myocardium
Common causes of an artifactually lowered LVEF on a MUGA study
Poor left anterior oblique view (you should be able to see a region of photopenia representing the ventricular septum)
Note: If the left ventricle is overlapping the right ventricle or left atrium, the LVEF will be artifactual lowered.
Common causes of an artifactually elevated LVEF on a MUGA study
Poor background region of interest
Note: If the background region of interest is placed over the spleen, this will cause oversubtraction of background and artifactually increase the LVEF.
What view is used to estimate LVEF on a MUGA study?
Left anterior oblique (the best view to visualize the inter ventricular septum)
Note: You can tell positioning is good if you can identify the photopenic ventricular septum.
What is abnormal about this MUGA study?
Photopenic halo (large pericardial effusion)
Rubidium-82
A potassium analog that acts similar to thallium-201 (both are taken up by the Na/K pump) and can act as a viability radiotracer
Note: Rarely used because it is expensive and has a very short half life (75 seconds).
What are the two radiotracers made by a generator?
- Tc-99m
- Rubidium
Blue vascular territory
Left anterior descending artery
Note: The LAD supplies the ventricular apex.
Yellow vascular territory
Right coronary artery
Purple vascular territory
Left circumflex artery
Arrow
Breast-attenuation artifact (decreased activity in the anterior wall, can also affect septal and lateral walls)
Note: Confirm this by ensuring there are no ECG changes or wall motion abnormalities. If you aren’t sure, repeat in the prone position.
How can you confirm that decreased activity in the anterior wall on myocardial perfusion imaging is due to breast-attenuation artifact and not true ischemia?
If there are no ECG changes or wall motion abnormalities, you can call it artifact.
If you aren’t sure, repeat the study in the prone position.
What went wrong?
The pt moved during the study (the left ventricle is offset between frames)
What went wrong on this myocardial perfusion scan?
ECG gating issue
Note: This is the “flickering artifact” (numerous scattered bright pixels).
What went wrong?
Breast-attenuation artifact
What is causing the apparent perfusion defect in the inferior wall in this male pt?
Left hemidiaphragm motion
Note: This is not significant using the appropriate male database, but appears significant if the female database is used.
What is suboptimal about this myocardial perfusion scan?
There is a lot of sub diaphragmatic activity (in the liver/bowel)
Note: You can have the pt exercise a little to shift blood flow from the liver/bowel to the heart.
What should you do if there is respiratory motion artifact on a myocardial perfusion scan?
Repeat the study immediately
Note: You don’t have to give more radiotracer because it should remain fixed for 2 hours.
What is the cause of this septal perfusion defect?
Artifact from a left bundle branch block
Where is there often an artifactual perfusion defect in pts with a left bundle branch block?
Fixed septal defect that spares the apex
What often causes artifactually decreased myocardial perfusion due to increased perfusion of the ventricular septum?
Hypertrophic cardiomyopathy
Note: Wall motion should be normal.
MOA for dipyridamole
Inhibition of the breakdown of adenosine (allowing adenosine, a potent vasodilator, to accumulate)
Note: Less significant side effects than adenosine.
What is the most significant possible side effect of adenosine?
AV block
Note: Fortunately adenosine has a short half-life, so the AV block will only be temporary.
MOA for dobutamine
Beta 1 agonist (increases heart rate and myocardial contraction)
What medication is best to use for myocardial stress testing in pts who have had caffeine within the past 12 hours?
Dobutamine (unless pt is on a beta blocker)
Note: You cannot use regadenoson, adenosine, or dipyridamole if the pt has had caffeine recently.
Which pharmacological stress agent cannot be used if the pt is on a beta blocker?
Dobutamine (a beta 1 agonist)
Which pharmacological stress agent is best for pts with COPD or asthma?
Dobutamine (beta 1 agonist)
Which pharmacological stress agent should be avoided in pts with a left bundle branch block?
Dobutamine (increased heart rate makes LBBB artifacts worse)
What is the antidote for adenosine (if needed due to AV block)
Aminophylline
Note: The half life is even shorter than that for adenosine, so make sure to continue close monitoring of the pt.
