Abstract
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. This work is supported by NIH grant R21‐CA156390‐01A1.
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