DIAGNOSTIC NUCLEAR MEDICINE

continuous exchange of thallium across the myocardial cell membrane (10—12).Within 2 or 3 hr after injection, a potentially different concentration of thallium results from an exchange equilibrium determined by a balance betweencontinuouscellularuptakeandefflux of thai hum. Thus the delayed thallium distribution reflects, rather than blood flow, an equilibrium condition deter mined by intracellular thallium exchange. The deter mination of the entire uptake and washout curve may, therefore, provide more complete characterization of myocardial blood flow and cellular integrity. This paper describes a method utilizing both relative temporal as well as relative spatial quantitation in order to capitalize on the kinetic as well as the static information intrinsic to thallium studies.. METHODS Backgroundsubtraction.In Tl-201 myocardialim ages, a significant fraction of the total counts within a myocardial target region arise from “background―—or, The need for a more quantitative approach to thai lium-20i myocardial imaging is currently recognized. Quantitation can be performed by comparing the relative activity of different myocardial segments in the same image (spatial) or by determining the relative activity of thesamesegmentasa functionof time (temporal). Temporal quantitation is required to characterize thai hum redistribution, which is commonly observed (I— 7). Theredistributionof thalliumin delayedimagesoc curs because a normal myocardial segment has a char acteristic pattern of thallium uptake and washout dif fering from that of an abnormal myocardial segment. Theinitial uptakeof thalliumisnearlyproportionalto regional myocardial blood flow (I, 8, 9), and following the initial myocardialextractionof thallium therei