Patient alignment based on Helium-beam radiography (αRAD)
Institution: German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology; National Center for Radiation Research in Oncology NCRO; Heidelberg Institute for Radiation Oncology HIRO; University of Heidelberg, Germany
Abstract
Purpose: In previous works, low-dose helium-beam radiography (αRAD) showed the potential to measure water-equivalent thickness (WET) maps along the beam axis. Moreover, visualizing anatomical structures with spatial resolutions of approximately 0.5 lp/mm (MTF10%) is viable, partly due to the beneficial scattering characteristics of helium ions. Building on these bases, the feasibility of patient alignment with high-resolution αRAD was investigated.Material and methods: Three pairs of thin silicon pixel detectors (WET<0.8mm) constitute our detection system, which provide path information and energy deposition (dE) of single ions when traversing the object. WET maps were measured based on recently established calibration functions between measured dE and WET. To overcome limitations in the dynamic range of WET for each initial energy, four initial energies of up to 197.01 MeV/u were used. To assess the performance in terms of patient alignment, two αRADs (24mm x 24mm) of an anthropomorphic head phantom with well-defined relative rotations/translations were performed at the Heidelberg Ion-Beam Therapy Center (HIT). An in-house implementation of a 2D-to-3D image registration was established, which enabled the estimation of rotations/translations between the αRADs and the planning X-ray CT. A pixel-value-based similarity measure indicated the most-likely phantom positioning in five degrees of freedom with respect to the planning CT. The comparison between the measured positioning correction and well-known relative rotations/translations was used to evaluate the quality of results.
Results: The similarity between the αRADs and DRRs were quantified by the sum of squared differences (SSD). The accuracy and precision of the position corrections were determined to be 0.27° (0.06mm) and 0.32° (0.06mm) for rotations(translations), respectively. This high performance can be achieved if αRADs present contrasts of bone and soft tissue.
Conclusions: Our initial study on patient positioning using αRAD showed promising accuracies and precisions, and revealed a high potential for a future clinical application.
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