Evaluation of a limited-area backprojection reconstruction method in quantitative SPECT

Because of the physical characteristics of collimator aperture, spatial resolution of single photon emission CT (SPECT) data degrades with increasing source-to-collimator distance. One way to reduce such blurring is to apply a modified filtered backprojection within a limited area defined by the line through the center of rotation parallel to the collimator surface and the surface itself. The feature of this limited-area backprojection method is that processing time is almost as short as that of the conventional filtered backprojection method. In this work, feasibility of the limited-area backprojection method was evaluated for quantitative SPECF with scatter and attenuation compensations. SPECT quantitative measurement was evaluated using a brain phantom in which the specific activity ratio of gray matter to white matter was 4.0. Scatter compensation was performed using the triple energy window (TEW) method. For correcting attenuation in the limited-area backprojection method, a modified iterative Chang method was applied. Chang attenuation map is generated using a line integral of attenuation at each projection within the 180-degree rotation area near collimator surface, and the iterative process is applied to only within the limited area. Results showed that SPECT value ratio of gray matter to white matter with the conventional method was 3.1, while that of the proposed method was 3.8, which was closer to the true ratio (4.0). In the phantom study, the proposed method showed improvement in quantitative measurement, indicating that the proposed method can compensate adequately for image blurring caused by collimator aperture. Since the proposed method requires only a short processing time, the method would be suitable for routine clinical use.