Geldenhuys E M, Lötter M G, Minnaar P C
Department of Biophysics, University of the Orange Free State, Bloemfontein, South Africa.
J Nucl Med. 1988 Apr;29(4):538-41.
The method of determining count rate curves suggested by the National Electrical Manufacturers Association (NEMA) is based upon a single reference count rate. Inaccuracies in the reference value are propagated to the rest of the curve values. NEMA also requires the measurement of individual attenuation factors for every absorber. Errors can easily occur during the measurement and during the subsequent calculations which utilize these factors. An alternative approach is suggested: count rate curves are based upon at least five low count rate values and the determination of individual attenuation factors is eliminated. Count rate curves are generated from graphs of the 1n of the observed count rate against the number of equally thick copper absorber plates, by back-extrapolation of a line which is fitted to the linear low count rate data. The method was successfully implemented in practice. Agreement was found between the results of the NEMA method and the alternative method suggested.
