The estimation of SPECT statistical reliability

To provide a quantitative understanding for the interdependence of radiopharmaceutical uptake, detector efficiency, and reconstruction parameters on SPECT statistical accuracy, a unified approach to estimating %rms noise has been developed. The procedure consists of the following steps: 1) Determine an acceptable geometric model for the organ system including radionuclide concentration Qs. 2) Select desired transverse and axial resolutions. 3) Select an acceptable imaging time T/sub scan/. 4) Compute the total number N/sub t/ of expected gamma photons in slice using: N/sub t/ = π.R/sub s/.R/sub eff/.L/sub slice/.Q/sub s/.ε.T/sub scan/.A/sub body/ where ε is the detection efficiency, R/sub s/ is the physical source radius, R/sub eff/ is the reduced source radius resulting from self-absorption, L/sub slice/ is the slice thickness, and A/sub body/ is the attenuation factor of surrounding body tissue. 5) Compute the %rms noise using an equation that includes the effects of spatial filtering and attenuation compensation. Two cylindrical phantoms (10 cm dia.) containing concentrations of Tc-99m (Q/sub s/=0.5 and 1.0 μCi/cc) were sequentially placed in a water-filled cylinder (20 cm dia.), and scanned (T/sub scan/=1175 sec.) with a SPECT system. Values of N/sub t/ equal to 120 (117) and 236 (238) kcnts were obtained using the mathematical model (phantom data). Using a generalized Hann filter (f/sub c/=3.1 cycle/cm) values of %rms noise equal to 21 and 15% were mathematically computed for the two concentrations, compared with measured values of 25 and 13%. These results indicate that the mathematical model is useful in 1) evaluating SPECT performance, 2) selecting SPECT imaging parameters, and 3) predicting the utility of proposed SPECT pharmaceuticals