Differential uptake and apparent 201Tl washout after thallium reinjection. Options regarding early redistribution imaging before reinjection or late redistribution imaging after reinjection.

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Abstract
BACKGROUND Because thallium reinjection enhances the identification of viable myocardium, many laboratories have adopted the routine practice of performing reinjection imaging instead of 3-4-hour redistribution imaging. This approach assumes that the stress-reinjection protocol provides the necessary information regarding both exercise-induced ischemia and myocardial viability. Because apparent "washout" of thallium may occur between redistribution and reinjection studies, we examined the limitations created by eliminating 3-4-hour redistribution images. METHODS AND RESULTS We studied 50 patients with chronic stable coronary artery disease by exercise thallium tomography, radionuclide angiography, and coronary arteriography. Immediately after the 3-4-hour redistribution images, 1 mCi thallium was injected at rest, and images were reacquired both 10 minutes and 24 hours after reinjection. The stress, redistribution, reinjection, and 24-hour images were then analyzed quantitatively, and the magnitude of change in regional thallium activity after reinjection was termed "differential uptake." Of the 127 abnormal myocardial regions on the stress images, 55 (43%) demonstrated either complete or partial reversibility on 3-4-hour redistribution images. After reinjection, 14 of these regions (25%) demonstrated apparent thallium washout due to low differential uptake of thallium, which was only 46 +/- 20% of that observed in normal regions. As a result, the relative thallium activity, which was 55 +/- 13% during stress (relative to normal regions) and increased significantly to 75 +/- 13% on 3-4-hour redistribution studies (p less than 0.001), decreased to only 58 +/- 13% after thallium reinjection. At 24 hours, redistribution again developed in all 14 regions, resulting in a relative thallium activity of 71 +/- 16% (p less than 0.03), which was similar to that achieved on 3-4-hour redistribution images. Twelve of the 14 regions (86%) exhibiting apparent washout after reinjection were supplied by a totally occluded coronary artery, of which eight (67%) had normal wall motion at rest. In contrast, only 41% of the regions with either improved or unchanged thallium uptake after reinjection were supplied by a totally occluded coronary artery (p less than 0.05). CONCLUSIONS These data indicate that regions with thallium defects that are reversible on 3-4-hour redistribution images may demonstrate apparent washout of thallium after reinjection due to low differential uptake. Although this occurs in only a small fraction of regions (8%) identified as abnormal on exercise images, these regions represent approximately 25% of regions showing redistribution. Such defects would appear irreversible if redistribution imaging is not performed before reinjection. However, these same myocardial regions also redistribute further after reinjection and are identified as reversible on 24-hour images. Thus, one of two imaging options, either stress-redistribution-reinjection imaging or stress-reinjection-24-hour imaging, may be used for a comprehensive assessment of myocardial ischemia and viability.

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