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The Influence of True Counting Rate and the Photopeak Fraction of Detected Events on Anger Camera Deadtime

Michael Reese Medical Center, Chicago, Illinois

Correspondence: For reprints contact: John E. Arnold, Div. of Nuclear Medicine, Michael Reese Medical Center, 2929 S. Ellis Ave., Chicago, Ill., 60616.

ABSTRACT

As true counting rate (N) is increased, an increasing proportion of photopeak events go unrecorded by the Anger camera due to its pulse pair resolving time (T). With increasing radio-active source strength, the observed counting rate (R) is shown to reach a maximum (R_max) and then decrease. The resolving time (T) can be estimated by T = (eR_max)^-1 and used in the equation R = Ne^-NT to predict observed from true counting rate. T does not vary with counting rate until values of N well above those that would produce R_max are encountered. However, it is shown that variation in the window width setting or in the degree of scatter in and around the radioactive source will alter the value of T by altering the photopeak fraction (the fraction of detectable events which will fall within the window of the pulse-height analyzer.) Hence methods to correct quantitative data for loss of counts due to instrument deadtime should use correction factors appropriate for the photopeak fraction encountered during the study. Methods of achieving this are discussed.