Purpose: Improve tumor localization in In-111 ProstaScint SPECT scans through improved reconstruction and identification/removal of non-specific blood pool volumes using simultaneously acquired Tc-99m tagged red blood cell (RBC) SPECT scans.
Methods: We chose 30 patients with a history of prostate cancer who had undergone CT/MR and simultaneous Tc-99m RBC/In-111 ProstaScint SPECT scans due to rising PSA. To estimate the impact of reconstruction methods on anatomic definition and artifacts, SPECT volume data sets were reconstructed using ordered set-expectation maximization (OS-EM) with varying numbers of iterations and subsets, and these were compared against each other and against standard filtered back projection (FBP) reconstruction. Non-blood pool bladder activity in the Tc-99m scans was suppressed prior to subtraction from the In-111 scans by using an averaging algorithm within an ellipsoid volume encompassing the bladder. Outside the ellipsoid volume, Tc-99m voxel values were subtracted from the corresponding In-111 voxels after normalization of the data sets based on peak activity within the descending aorta.
Results: OS-EM reconstruction using 3 iterations and 45 subsets showed improved representation of anatomy compared with FBP. Bladder suppression reduced artifacts in the prostate bed. The subtraction method reduced the blood pool signal, confirmed visually by superimposition with matched CT/MR scans. Preliminary results using the Coefficient of Variation (CV) and a Student's t-test, show that the superimposition landmark distance differences are significantly different after subtraction.
Conclusions: OS-EM reconstruction together with bladder suppression and subtraction of the blood pool may help improve the specificity of ProstaScint SPECT interpretation and increase its utility in radiation therapy treatment planning.