Deadtime of scintillation camera systems–definitions, measurement and applications

Fast quantitative dynamic studies with scintillation camera systems require deadtime correction. Although such systems are semiparalyzable, they are most conveniently treated either as paralyzable or nonparalyzable. Paralyzing deadtime (Ï„) is that deadtime during which a system is unable to provide a second output pulse unless there is a time interval of at least Ï„ between two successive events. Paralyzable systems are characterized by Poisson statistics, so that the “true” counting rate N = R eN Ï„, where R is the observed counting rate. Nonparalyzing deadtime (T) is that deadtime during which a system is insensitive after each observed event. The period of insensitivity is not affected by any additional “true” events before full recovery occurs. For nonparalyzable systems the corrected counting rate N = R / (1 − RT). A two source method protocol is presented for deadtime measurement. The paralyzing deadtime is calculated by a 5 dimension Newton–Raphson iteration. The nonparalyzing deadtime is calculated by a quadratic equation. Approximation equations are also presented not requiring a computer. Deadtimes are fitted to polynomial equations as dependent variables of measured counting rate. Algorithms incorporating the polynomials are presented for the deadtime correction of histogram curves. Using either the paralyzing or the nonparalyzing approach, precise deadtime corrections are demonstrated.