Validation of CT-based attenuation correction for multi-pinhole PSF reconstruction for small-animal SPECT

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 ^99mTc 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 ^mTc 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 ^99mTc 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 ^mTc 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 ¹²⁵I phantoms but did not apply SC to the ¹²⁵I data. With no correction applied, an axial slice of a 3 cm diameter cylinder uniformly filled with ¹²⁵I dipped 25 % near the center of the phantom. After applying AC, the ¹²⁵I image profile no longer dipped but rather was overcorrected by 4.5%. Similarly, the reconstructed image of the ¹²⁵I implants with no correction underestimated the activity of each source by 23% on average, while the ¹²⁵I 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 ^99mTc and ¹²⁵I studies.