Myocardial viability assessed by exercise-stress SPECT with thallium reinjection

To determine the clinical significance of exercise-stress single photon emission computed tomography (SPECT) with thallium (Tl) reinjection method in assessing myocardial viability, 26 patients with myocardial infarction underwent both SPECT at stress, using the Tl reinjection method, and SPECT at rest. Tl uptake was visually analysed by a four-grade system: normal, mild hypoperfusion, severe hypoperfusion and defects. The number of abnormal perfusion areas on stress images was 34 for mild hypoperfusion, 42 for severe hypoperfusion and 119 for defects. Redistribution images 4 hr after stress (4 hr-delayed images) showed 2/34 (6%), 13/42 (31%) and 20/119 (17%) for mild hypoperfusion, severe hypoperfusion and defects, respectively. The corresponding figures showed 18 (53%), 15 (36%) and 24 (20%) areas, respectively, on redistribution images after reinjection of a second dose of Tl (reinjection images), and none (0%), 3 (7%) and 12 (10%) areas, respectively, on resting images. Reinjection images showed lower rate of perfusion improvement in complete vessel stenosis than incomplete vessel stenosis. The rate of perfusion improvement was 2% in incomplete vessel stenosis and 10% in complete vessel stenosis on resting images, revealing that resting images showed a higher rate of redistribution than reinjection images. Reinjection imaging with Tl-201 was found to have a sensitivity comparable to resting imaging in assessing myocardial viability. However, it appears to underestimate myocardial viability in approximately 10% of patients with complete vessel stenosis, from the viewpoint of the degree of Tl uptake. (N.K.)