Attenuation correction in SPECT based on transmission studies and Monte Carlo simulations of build-up functions.

The quantitative information in SPECT images is distorted by photon attenuation and contribution of photons scattered in the object. It is, therefore, important to know the distribution of different attenuating tissues in order to be able to perform a proper attenuation correction. A correction method, based on correcting one pixel at a time by using density maps and build-up functions, has been developed. The density map has been produced by transmission measurements of the object using an external, solid 57Co flood source mounted on the scintillation camera head. The outline of the object is accurately defined by the map since the density values outside the object are very close to zero. The build-up of photons scattered in the object has been simulated by a Monte-Carlo code. SPECT-studies with 99mTc, 201T1 and 111In line sources in different parts of a non homogeneous Alderson phantom have been performed. The emission images have been corrected for photon attenuation using the measured density maps and the simulated build-up functions. The results show that quantitative measurements of the radioactivity in nonhomogeneous area can be accomplished to within +/- 10% for different radionuclides by using the attenuation correction described.