Validation of CT-based Attenuation Correction for Multi-Pinhole PSF Reconstruction for

We recently reported a numerical ray-tracing algorithm for calculating the point-spread function (PSF) used in 3-D ordered subsets expectation maximization (OSEM) reconstruction of single and multi-pinhole collimated single photon emission computed tomography (SPECT) images. In this work, we evaluated the performance of our PSF reconstruction method with and without X-ray CT-based attenuation correction (AC) and dual energy window scatter correction (SC). X-ray CT data was acquired to create the attenuation maps. SPECT data was acquired using 99m Tc phantoms and 5-pinhole tungsten collimators with 1.0 mm diameter pinholes. With no corrections applied, an axial image slice of a 3 cm diameter cylinder uniformly filled with 99m Tc showed a 13% dip near the center of the phantom. When AC and SC were applied, the cross-section through an axial slice showed a desirable flattened profile that dipped only 3%. We also scanned a mouse with 99m Tc implantable sources that had negligible self-attenuation. The sources were first scanned in air for calibration. Our results show that the reconstructed SPECT images with no corrections underestimated the activity for each 99m Tc implanted source by 12% on average, while the image with AC and SC underestimated the activity by only 3.3% on average. We repeated all of the experiments with 125 I phantoms but did not apply SC to the 125 I data. With no correction applied, an axial slice of a 3 cm diameter cylinder uniformly filled with 125 I dipped 25% near the center of the phantom. After applying AC, the 125 I image profile no longer dipped but rather was overcorrected by 4.5%. Similarly, the reconstructed image of the 125 I implants with no correction underestimated the activity of each source by 23% on average, while the 125 I image with AC overestimated the activity in the sources by 4.6% on average. These results have shown that our PSF reconstruction method with CT-based attenuation correction improved the quantitative accuracy of SPECT images for representative 99m Tc and 125 I studies.