Respective roles of scatter, attenuation, depth-dependent collimator response and finite spatial resolution in cardiac single-photon emission tomography quantitation: a Monte Carlo study

The purpose of this study was to investigate the relative influence of scat ter, attenuation, depth-dependent collimator response and finite spatial re solution upon the image characteristics in cardiac single-photon emission t omography (SPET). An acquisition of an anthropomorphic cardiac phantom was performed together with corresponding SPET Monte Carlo simulations. The car diac phantom and the Monte Carlo simulations were designed so that the effe ct of scatter, attenuation, depth-dependent collimator response and finite spatial resolution could be studied individually and in combination. The im pact of each physical effect and of combinations of effects was studied in terms of absolute and relative quantitative accuracy, spatial resolution an d signal-to-noise ratio (SNR) in the resulting images. No corrections for t hese effects were assessed. Results obtained from Monte Carlo simulations a nd real acquisitions were in excellent agreement. Attenuation introduced ab out 90% activity underestimation in a 10-mm-thick left ventricle wall while finite spatial resolution alone introduced about 30% activity underestimat ion. Scatter had a negligible impact on quantitative accuracy in the recont ructed slices when attenuation was present. Neither bull's eye map homogene ity nor contrast between a hot and a cold region were affected by depth-dep endent collimator response or finite spatial resolution. Bull's eye map hom ogeneity was severely affected by attenuation but not by scatter. Attenuati on and scatter reduced contrast by about 20% each. Both attenuation and sca tter increased the full-width at half-maximum (FWHM) characterizing the spa tial resolution of the imaging system by approximate to 1 mm each but the m ain effect responsible for the observed 11-mm FWHM spatial resolution was t he depth-dependent collimator response. SNR was reduced by a factor of appr oximate to 2.5 because of attenuation, while scattered counts increased SNR by approximate to 10%. In conclusion, the quantification of the relative i nfluence of the different physical effects showed that attenuation is defin itely the major phenomenon affecting cardiac SPET imaging accuracy, but tha t finite spatial resolution, scatter and depth-dependent collimator respons e also contribute significantly to the errors in absolute and relative quan titation and to the poor spatial resolution.