LESSON 6f page 4

CARDIAC BLOOD POOL IMAGING

GATED TOMOGRAPHIC RADIONUCLIDE ANGIOGRAPHY (SPECT MUGA)

It has recently become possible to combine gated blood pool imaging with photon emission computed tomography (SPECT). It allows the automated identification of the left ventricular surface in gated tomographic radionuclide ventriculograms. Data can be displayed in three dimensions. It has been shown that the regurgitation fraction in patients with mitral and aortic insufficiency is accurately measured by gated radionuclide angiography SPECT imaging. Although the resolution of gated SPECT is not as high as magnetic resonance imaging, it is relatively inexpensive and provides an accurate picture of ventricular function.

FORMULATION PROBLEMS WITH TAGGED RBC'S

FORMULATION PROBLEM	CLINICAL MANIFESTATION
Carrier 99Tc (especially with AC)	labeling efficiency; free pertechnetate
Al3+	RBC agglutination
Very acidic pH	RBC hemolysis
Inadequate stannous	free pertechnetate
Excessive stannous	plasma activity, spleen uptake
Excessive 99mTc activity added	rate and extent of labeling
Improper mixing order	liver uptake; labeling efficiency
Low cell concentration	rate and extent of labeling
Radiolytic decomposition and/or oxidation	free pertechnetate
Inadequate incubation time	free pertechnetate
Incubation at < 37 C	rate of labeling; free pertechnetate
Heparin vs ACD	labeling efficiency; extravascular activity; urinary excretion
ACD vs heparin	stability; blood/background ratio
EDTA as the anticoagulant	labeling efficiency
Excess ACD	sequestration in spleen; labeling efficiency
Large volume of pertechnetate solution	rate and extent of labeling
Inadequate volume of blood	labeling efficiency
EDTA as a sequesterant	blood pool retention; splenic accumulation

DRUG INTERFERENCE   Agents that alter, by direct pharmacological effect, cardiac function and have the potential to interfere with interpretation of the gated blood pool study:

- beta blockers (propranolol, 48 hr)
- calcium channel blockers (verapamil, 48-72 hrs)
- nitrates (nitroglycerin, 12 hr)

REASONS FOR ERROR IN EJECTION FRACTION DETERMINATION

OVERESTIMATION OF EF VALUES

• Background ROI placed too close to or over activity in spleen or descending aorta
  

• ROI too large (but does not include LA)
  

• Patients with postero-basal aneurysm

UNDERESTIMATION OF EF VALUES

• Inclusion of LA in left ventricular ROI, particularly at end diastole
  

• Poor separation of left and right ventricles
  

• Using a fixed ROI
  

• End-systolic ROI too large
  

• Inclusion of ascending aorta in end-diastolic ROI
  

• Placement of background ROI over photopenic area
  

• ECG gating problems
  

• Poor definition of true end-systolic volume (inadequate temporal resolution)
  

• Patients with antero-apical aneurysm or markedly enlarged LV

 

MUGA TROUBLE SHOOTING
Problem	Possible Causes	Remedies
Blurry Study low counts poor target to background	dose infiltrated very obese patient off peak bad lot of Pyrophosphate heparin or other drug	reinject 5 mCi Tc-99m or re-peak send vials to drug company redo with labeled HSA
Not gating well missing beats	R wave too low	Switch leads to find highest R wave
Not gating well atrial fibrillation	wide variability R-R’s	widen beat acceptance window
Not gating well atrial, ventricular arrhythmias	wide variability R-R’s	Patience If persistent biventricular bigeminy, reschedule patient
Not gating well paced rhythm	dual chamber; gating on atrial and ventricular artifacts	Call EP group to put magnet on pacemaker to change VVI
Big hot spleen	malignancy	In the lateral view make sure the heart is in FOV
Heart not beating in lateral view	ECG lead off	put lead back on and repeat view
Complex cardiac anatomy and difficulty finding best septal angle	complex CHF marked dilation of one chamber	Ask MD for assistance Take ANT or LAT No zoom for anatomy
Poor gating during first stages exercise	poor lead contact	Stop exercise, replace all leads and electrodes, have patient exercise briefly to make sure signal stable before restarting. Drop of normal saline on skin under electrode.
In 3 view MUGA one image looks “asystolic”	Lost gate or ECG lead	Repeat that view