Cardiac Blood Pool Imaging: An Overview
Assessing left ventricular function is integral in the evaluation of CAD because the severity of abnormal function is a strong predictor of prognosis.
Nuclear techniques noninvasively and reliably yield these measures for both the right and left ventricles. Quantitative indicators of cardiac function include heart rate, cardiac output, arterial pressure, left ventricular end-diastolic pressure, stroke volume and ejection fraction. Ventricular volumes, ejection fraction and exercise hemodynamics are the most important measures of cardiac performance, providing global and regional information about cardiac pump function.
The first attempt to assess cardiac function using radiotracers was made in 1948 by the renowned American Cardiologist Dr. Myron Prinzmetal. With an IV bolus of sodium-24, and with an ink-writing GM tube placed over a patient’s pericardium, Dr. Prinzmetal demonstrated deflections representing right and left ventricular motion.
Radionuclide ventricular function studies can be broadly divided into two types:
- Multigated Blood Pool Imaging (MUGA Scan)- RESTING and EXERCISE
- First Pass Ventricular Function Studies -RESTING and EXERCISE
Blood Pool Imaging Reveals qualitative indicators of function such as WALL MOTION ABNORMALITIES (hypokinesis, akinesis, and aneurysm) of the left ventricular wall and INTRACARDIAC SHUNTS.
The RADIONUCLIDE ANGIOGRAM is a physiologic, safe, noninvasive procedure that is easy to perform and to repeat without undesirable side effects or discomfort to the patient. It is much less expensive than either cardiac catheterization or conventional x-ray contrast angiography. The MUGA scan has been used for decades to evaluate global
and regional wall motion, to calculate the excursion of blood from the left or right ventricles, determine ventricular volumes and to obtain a stroke volume ratio. With the first pass technique we may quickly assess wall motion abnormalities, left and right ejection fractions and ventricular volumes, calculate transit times and cardiac output and evaluate intracardiac shunts.
Different technical characteristics of first-pass and equilibrium methods account for their respective advantages and disadvantages. However, both provide ventricular volumes and temporal variations of volume necessary for functional assessments. Since chamber blood volumes are proportional to the density of radioactive emissions emanating from the chamber, their determination is independent of geometric formulas. This enables calculation of ventricular volumes in patients with irregular or misshapen ventricles, and for studies acquired during peak exercise, when diagnostically and prognostically important cardiac functional abnormalities typically are evoked.
Following the intravascular injection of the radiotracer, the passage of radioactivity through the cardiopulmonary chambers is monitored by a scintillation camera, and images are sequentially recorded on computer disk. Images of the initial-transit or first pass of radioactivity and equilibrium blood pool pictures are assessed qualitatively and/or quantitatively.
FIRST PASS RADIONUCLIDE ANGIOGRAPHY shows a tight bolus of injected radiotracer in its first transit through the right heart and lungs, then back through the left heart and out the aorta. Both techniques may be performed at rest or exercise, and same day rest and exercise studies are possible. Because more sophisticated equipment is needed to perform the first pass exam correctly, the planar MUGA scan is more commonly found in the average nuclear medicine department or testing office.