Evaluation of rigid and non-rigid changes in a small animal irradiation system based on proton radiographies: influence of detector performance

Speakers:
Prasannakumar Palaniappan
Marco Riboldi

Abstract

A multi-stage 2D-3D deformable image registration (DIR) is implemented using orthogonal in-room 2D proton radiographies (pRads) based on a prototype pre-clinical imaging system capable of single particle tracking in order update the 3D treatment planning computed tomography (CT). Monte Carlo simulations of pRads relying on an X-ray micro CT acquisition of a mouse calibrated to relative stopping power (RSP) are calculated for the brain region, considering the ideal and realistic detector models of the in-house developed small animal imaging system. Artificial deformations are applied along with a rigid transformation to mimic inter-fractional changes. Two similarity metrics in radiographic domain, i.e. the root mean square error (RMSE) and the normalized mutual information (NMI), are adopted to update the moving planning CT with respect to the fixed pRads in an iterative 2D-3D DIR algorithm, where the first stage is devoted to rigid image registration (RIR). Results are compared to conventional 3D-3D DIR where the proton computed tomography (pCT) is reconstructed based on 180 projections (2º of spacing covering 360º) and used as fixed image. The RMSE in image domain shows that 2D-3D DIR using orthogonal pRads is able to achieve similar performance compared to 3D-3D DIR (accuracy > 90%) when compensating the applied rigid and non-rigid changes. No appreciable differences are observed when comparing the ideal vs. realistic detector models, with the RMSE in projection domain leading to smaller residual RSP differences compared to the NMI. This works demonstrates the feasibility of the 2D-3D DIR in the compensation of rigid and non-rigid changes for small animal proton irradiation with on-site image guidance based on proton radiographic imaging.

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