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A Line-Source Method for Aligning Onboard-Robotic-Pinhole and Other SPECT-Pinhole Systems

S Yan

S Yan*1, J Bowsher1,2, W Giles1, F Yin1,2, (1) Medical Physics Graduate Program, Duke University, Durham, NC, (2) Department of Radiation Oncology, Duke University Medical Center, Durham, NC

TH-E-BRA-1 Thursday 1:00:00 PM - 2:50:00 PM Room: Ballroom A

Purpose: To achieve on-board molecular imaging in the radiation therapy room, we are developing a robotic multi-pinhole SPECT system. Alignment of the SPECT system to LINAC/CBCT coordinates is essential for target localization and image reconstruction. Pinhole alignment methods typically use point sources, which can be challenging to make and to localize. We propose a method that utilizes line sources and as few as one pinhole projection. This method is generally applicable to SPECT pinhole imaging.

Methods: Line source positions are taken as known by CBCT or some other transmission imaging procedure. A model is developed relating line source positions and their pinhole-SPECT projections to pinhole-SPECT alignment parameters. Based on this model and a radon-transform detection of projected lines, alignment parameters (three detector rotations, three detector translations, and in some cases three pinhole translations) are estimated. To evaluate the method, line-source projections are computer-simulated, with blur and noise included, for three and five lines, and for several pinhole-SPECT rotations/translations. Alignment parameters are obtained and compared to true values.

Results: For six parameters (3 translations, 3 rotations), errors were 0.3±0.6mm, 0.3±0.4mm, 1.1±2.2mm, 0.2±0.1deg, 0.3±0.6deg, 0.5±0.5deg using three line sources and 0.2±0.3mm, 0.5±1.2mm, 0.9±1.6mm, 0.1±0.2deg, 0.4±0.7deg, 0.2±0.4deg using five line sources. For nine parameters and five line sources, errors were 6.2±10.5mm, 8.4±16.7mm, 29.2±44.0mm, 1.9±2.8deg, 0.7±0.9deg, 0.7±1.6deg, 3.7±4.5mm, 4.8±3.3mm, 16.7±25.1mm. Replacing the line-source projection positions determined by the radon transform with true values, these nine parameters were estimated with no error using five line sources.

Conclusions: Estimation of alignment parameters can be achieved using one pinhole projection of line-sources. Alignment errors are largely associated with limited accuracy of the radon transform in determining positions of line-source projections. For robotic multi-pinhole SPECT, this method shows promise to estimate relative and absolute alignment of each pinhole within the multi-pinhole system.

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