Left ventricular cavity-to-myocardial count ratio: a new parameter for detecting resting left ventricular dysfunction directly from tomographic thallium perfusion scintigraphy.

Patients with reduced left ventricular function or aneurysms have cavities that appear dark on SPECT thallium scintigrams. We hypothesized that a quantitative index, which relates thallium activity in the left ventricular cavity to that in the myocardium (C/M ratio), could provide information on left ventricular function. A group of 80 patients who had both exercise SPECT thallium imaging and cardiac catheterization were studied. The C/M ratio was obtained from the short-axis tomogram on both exercise and rest images. Counts in a 2 x 2 pixel region of interest in the left ventricular cavity were divided by the number of counts in the "hottest" area of the myocardium. Plotting the angiographically determined ejection fraction against the C/M exercise and rest ratios, we observed a linear correlation between ejection fraction and both C/M ratios, r = 0.65 for C/M exercise and r = 0.67 for C/M rest ratio (p < 0.00001). Using data from 12 normal cardiac catheterization patients, we established the lower limit of normal; 50% for ejection fraction and 0.40 for the C/M ratios. A C/M exercise ratio < or = 0.40 identified 26 of 31 patients with an ejection fraction < or = 50%. A C/M exercise ratio > 0.40 identified 39 of 49 patients with an ejection fraction > 50%. These calculations yielded a sensitivity of 83% and specificity of 78% for the C/M exercise ratio. A similar analysis for C/M rest ratio revealed sensitivity of 61% and specificity of 92%. The present study shows that an abnormal C/M ratio correctly distinguishes patients with abnormal from normal ejection fractions with an accuracy of 81%. The C/M ratio is easily obtained, requires minimal processing time and is highly reproducible. These attributes may enable this index to add supplementary information regarding left ventricular function in addition to perfusion from thallium imaging.