Nonisotropic attenuation in SPECT: phantom tests of quantitative effects and compensation techniques.

A quantitative study of nonisotropic attenuation in SPECT imaging is presented. The study includes a case where the spatial distribution of the attenuation coefficient is nonuniform, as well as a case where the photon path length in the attenuating medium is variable as a function of direction. The effects are studied using phantoms with known source activity and density distributions. Reconstructed images of the phantoms with and without attenuation compensation are compared with the source distribution. Three methods are used to provide partial attenuation compensation, using effective attenuation coefficients. These coefficients include some of the effects of photon scatter, but scatter is not explicitly treated. One attenuation compensation method involves a multiplicative postprocessing correction using an assumed constant attenuation coefficient. A modification of this technique is implemented using the correct nonuniform attenuation map to determine the multiplication factors. A single-iteration technique is used to provide a more complete compensation. The results indicate that nonuniform attenuation can produce significant distortion in line spread functions and in larger distributed sources. This distortion can alter volume determinations, quantitation measurements, and the shape of small objects, and can cause misplacement of counts into regions of low density. The distortion cannot be eliminated by the multiplicative postprocessing correction, but the single-iteration technique can significantly decrease the distortion.