myocardial perfusion & viability scintigraphy

Indications

• ECG stress testing for patients with suspected coronary artery disease
  • failed treadmill testing
  • cannot undergo treadmill testing (exercise intolerant)
    • chest pain walking up 2 flights of stairs is not exercise intolerant
  • baseline ECG that renders treadmill testing inappropriate (LBBB)
  • yearly testing not unreasonable in high-risk patients
• triage in patients presenting to the emergency room with chest pain & ECG without evidence of acute myocardial ischemia (non-ST segment elevation myocardial infarction, NSTEMI)

prognostic value similar to dobutamine stress echocardiography

prognostic value similar to coronary CT angiography

Contraindications

• baseline second degree heart block is a contraindication to use of adenosine
• contraindications to pharmaceutical vasodilators
  • active wheezing
  • bronchospasm
  • theophylline use
  • sick sinus syndrome
  • high degree AV block
  • systolic blood pressure < 90 mm Hg
  • aortic stenosis
    • limited ability to further increase contractility
    • decrease in diastolic filling & wall-tension development with reflex tachycardia
• contraindications to use of dobutamine include
  • severe baseline hypertension
  • unstable angina
  • arrhythmias
• relevant issue is whether coronary anatomy is amenable to revascularization
  • coronary angiography indicated

* exercise cannot be used in patients with LBBB because perfusion defects not related to obstructive coronary artery disease may be seen in the interventricular septum with exercise

* chest pain induced by emotional stress, unrelated to exercise is not an indication for pharmacologic perfusion scintigraphy

Clinical significance

• false positive results may be obtained in patients with left bundle branch block & severe left ventricular hypertrophy

Procedure

caffeine should be withheld 24 hours prior to procedure

single photon emission computed tomography may be method of choice

Perfusion imaging agents include:

• Tl-201
• Tc-99 sestamibi
• Tc-99 teboroxime
• Tc-99 tetrofosmin

A resting image of myocardial uptake of Tl-201 administered in the form of thallous chloride is compared witha post vasodilation uptake of Tc-99 in the form of sestamibi. Vasodilation* may be induced by:

• exercise
• persantine
• adenosine
• dobutamine#

* A 3-5 fold increase in blood flow normally occurs.

# beta-blocker must be held prior to the test

3 mCi of Tl-201 in the form of thallous chloride is administered IV. Cardiac muscle has an extraction efficiency of 80-90% for Tl+. Approximately 4% of administered Tl+ is taken up by the myocardium. Myocardial uptake is dependent upon blood flow. A redistribution occurs reflecting the local K+ pool (i.e. viable myocardium). Redistribution appears to be complete in 3-5 hours. Therefore imaging should be begun within 5 minutes of Tl-201 injection.

Emitted radiation is sensed with an Anger type gamma counter with either medium or high resolution collimators. Images are obtained from the anterior & at least other projections 40 & 70 degrees so that all cardiac segments are well visualized.

Persantine 0.57 mg/kg is injected IV over 4 minutes. At 7 minutes, 25 mCi of Tc-99-labeled sestamibi is injected. Sestamibi is extracted less efficiently than Tl+ & almost no redistribution occurs. This permits a larger time window between isotope injection & imaging for uptake with vasodilation.

With vasodilation, normal arteries vasodilate, but stenotic vessels do not. Nitroglycerin should be avoided as it will dilate stenotic vessels as well.

Resting images may be then compared with images after vasodilation. Three different perpendicular views including short axis, & two long axes views are obtained. A comparison is made of the resting & vasodilation images in order to identify regions of inviable myocardium & regions of myocardium at risk for ischemia during stress.

Interpretation

Table

resting image	stress image	interpretation
normal	normal	no ischemia
normal	perfusion defect	stress-induced ischemia
perfusion defect	perfusion defect	myocardial infarct
normal	LV dilation	*

* small or no focal ischemia, possible diffuse ischemia, possible multivessel CAD

Complications

• adenosine or dipyridamole may cause chest pain, dyspnea or flushing
• radiation exposure
  • patients exposed to more radiation with than with coronary CT angiography
• patient dissatisfaction
  • patients rate the procedure less favorable than with coronary CT angiography

More general terms

• pharmacologic stress testing
• scintigraphy (radionuclide test)

More specific terms

• mycocardial single photon emission computed tomography (SPECT)

Additional terms

• radionuclide ventriculography; cardiac blood pool imaging (RVG, MUGA)