Design Of A Collimated Line Source For Transmission Ct Imaging Of The Head With A Spect System

Single photon emission computed tomography (SPECT) has the potential to provide absolute quantitative information about the function of an organ. However, two factors which limit accurate quantification are the attenuation of the gamma rays emitted from the radiopharmaceutical concentration and the inability to define the anatomic region being quantified. Presented in this thesis is an attenuation correction that compensates SPECT brain images for attenuation and a transmission imaging system that (a) provides the distribution of attenuation coefficients required for the attenuation compensation and (b) simplifies regional identification by registering the functional SPECT images onto the anatomic transmission images. The attenuation correction algorithm is an exact compensation for a radiopharmaceutical concentration that is evenly distributed throughout the brain tissue and completely surrounded by an attenuating, nonradioactive bone medium. It explicitly corrects projections of the brain for bone and tissue attenuation and it improves the uniformity and increases the count density of the resulting SPECT images. The transmission CT imaging system consists of a collimated line source and a conventional SPECT camera equipped with a fan beam collimator (FBC). In the transverse image plane the resolution of the transmission system is equal to the intrinsic camera resolution ({dollar}\approx{dollar}4mm), but resolution in the longitudinal plane (coincident to the axis of rotation of the SPECT camera), without the line source collimator, is characterized by the camera and FBC resolutions ({dollar}\approx{dollar}8mm at 150mm above the collimator). The line source collimator (LSC) is a one-dimensional collimator that improves the longitudinal resolution of the system. Computer simulations were used to design a LSC that provided the system with isotropic resolution ({dollar}\approx{dollar}4mm in image plane) while maximizing the geometric efficiency. The attenuation compensation algorithm requires that the skull attenuation coefficient and thickness be known, and images of the human head demonstrated that this information can be measured using the transmission imaging system. However, due to limitations of the present SPECT system, a brain SPECT image that has been attenuation corrected with the techniques described herein is not presented. Finally, a mathematical formalism to compute the geometric modulation transfer function of a transmission imaging system with parallel hole collimation was derived. The formalism is a valuable tool in the design of such transmission systems and it replaces the need to use computer intensive simulations to predict the system resolution.;In this thesis I have demonstrated the feasibility of acquiring transmission CT images with a SPECT system. The transmission images will be used to determine the attenuation coefficients for the attenuation correction and registration of the SPECT brain images to anatomical images, thereby improving the quantification of regional cerebral blood flow