Experimental helium‐beam radiography with a high‐energy beam: Water‐equivalent thickness calibration and first image‐quality results

Abstract

A clinical implementation of ion-beam radiography (iRAD) is envisaged to provide a method for on-couch verification of ion-beam treatment plans. The aim of this work is to introduce and evaluate a method for quantitative waterequivalent thickness (WET) measurements for a specific helium-ion imaging system for WETs that are relevant for imaging thicker body parts in the future. Methods: Helium-beam radiographs (𝛼Rads) are measured at the Heidelberg Ion-beam Therapy Center with an initial beam energy of 239.5 MeV/u. An imaging system based on three pairs of thin silicon pixel detectors is used for ion path reconstruction and measuring the energy deposition (dE) of each particle behind the object to be imaged. The dE behind homogeneous plastic blocks is related to their well-known WETs between 280.6 and 312.6 mm with a calibration curve that is created by a fit to measured data points. The quality of the quantitative WET measurements is determined by the uncertainty of the measured WET of a single ion (single-ion WET precision) and the deviation of a measured WET value to the well-known WET (WET accuracy). Subsequently, the fitted calibration curve is applied to an energy deposition radiograph of a phantom with a complex geometry. The spatial resolution (modulation transfer function at 10 % -MTF 10% ) and WET accuracy (mean absolute percentage difference-MAPD) of the WET map are determined. Results: In the optimal imaging WET-range from ∼280 to 300 mm, the fitted calibration curve reached a mean single-ion WET precision of 1.55 ± 0.00%. Applying the calibration to an ion radiograph (iRad) of a more complex WET distribution, the spatial resolution was determined to be MTF 10% = 0.49 ± 0.03 lp/mm and the WET accuracy was assessed as MAPD to 0.21 %. Conclusions: Using a beam energy of 239.5 MeV/u and the proposed calibration procedure, quantitative 𝛼Rads of WETs between ∼280 and 300 mm can be measured and show high potential for clinical use. The proposed approach with the resulting image qualities encourages further investigation toward the clinical application of helium-beam radiography.

Energy painting: helium-beam radiography with thin detectors and multiple beam energies

“…To be able to image complex objects with a wide WET range, multiple beam energies can be used and combined into one high-precision image. This technique, which we call energy painting, was suggested in previous publications of our group (Amato et al 2020, Knobloch et al 2022 and is presented in this article for the first time. Note that the concept of multiple initial beam energies was also suggested for calorimeter and range-telescope based imaging systems (Dickmann et al 2021, DeJongh et al 2021 and flat-panel detectors (Telsemeyer et al 2012).…”

“…Therefore, calibrations translating energy deposition to WET values are essential for this work. They were acquired using the method described by Knobloch et al (2022), in contrast to which not only one, but five calibration curves were established. This process, which will be explained in the following, is also schematically depicted in figure 4.…”

“…The positive quantity x = −dE has the units of an energy and corresponds to the energy deposition measured by the detection system. In contrast to the calibration procedure described by Knobloch et al (2022), the established calibration curves are only applied to mean energy deposition values. The calibration function's argument was amended, because a slight systematic underestimation of WET values was observed by Knobloch et al (2022) if the calibration function, which was established with mean energy deposition values, was applied to single-ion values.…”

“…In contrast to the calibration procedure described by Knobloch et al (2022), the established calibration curves are only applied to mean energy deposition values. The calibration function's argument was amended, because a slight systematic underestimation of WET values was observed by Knobloch et al (2022) if the calibration function, which was established with mean energy deposition values, was applied to single-ion values. The non-linear nature of the calibration curve does not allow for switching the order of operations (application of calibration curve and averaging).…”

“…Subsequently, the calibration accuracy A and single-ion WET precision (SIWP) were determined. One approach for this was already described in Knobloch et al (2022). However, the application of the calibration curve to pixel values instead of single ions does no longer allow for this straight forward approach, as no single-ion WET distribution can be obtained anymore.…”