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

PURPOSE: 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 water-equivalent 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 % -MTF10% ) 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 ± $\,{\pm}\,$ 0.00%. Applying the calibration to an ion radiograph (iRad) of a more complex WET distribution, the spatial resolution was determined to be MTF10% = 0.49 ± $\,{\pm}\,$ 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.

A technique for spatial resolution improvement in helium-beam radiography.

PURPOSE: Ion-beam radiography exhibits a significantly lower spatial resolution (SR) compared to x-ray radiography. This is mostly due the multiple Coulomb scattering (MCS) that the ions undergo in the imaged object. In this work, a novel technique to improve the spatial resolution in helium-beam radiography was developed. Increasing helium-beam energies were exploited in order to decrease the MCS, and therefore increase the SR. METHODS: The experimental investigation was carried out with a dedicated ion-tracking imaging system fully composed of thin, pixelated silicon detectors (Timepix). Four helium beams with increasing energies (from 168.8 to 220.5 MeV/u) were used to image a homogeneous 160 mm PMMA phantom with a 2 mm air gap at middle depth. An energy degrader (ED) was placed between the rear tracking system and the energy-deposition detector to compensate for the longer range associated with more energetic ions. The SR was measured for each beam energy. To take into account the overall impact on the image quality, the contrast-to-noise ratio (CNR), the single-ion water equivalent thickness (WET) precision and the absorbed dose in the phantom were also evaluated as a function of the initial beam energy. FLUKA Monte Carlo simulations were used to support the conceptual design of the experimental setup and for dose estimation. RESULTS: In the investigated energy interval, a total SR increase by around 30% was measured with increasing beam energy, reaching a maximum value of 0.69 lp/mm. For radiographs generated with 350 µGy of absorbed dose and 220 µm pixel size, a CNR decrease of 32% was found as the beam energy increases. For 1 mm pixel size, the CNR decreases only by 22%. The CNR of the images was always above 6. The single-ion WET precision was found to be in a range between 1.2% and 1.5%. CONCLUSIONS: We have experimentally shown and quantified the possibility of improving SR in helium-beam radiography by using increasing beam energies in combination with an ED. A significant SR increase was measured with an acceptable decrease of CNR. Furthermore, we have shown that an ED can be a valuable tool to exploit increasing beam energies to generate energy-deposition radiographs.

Potential survival of the lichen Caloplaca flavovirescens under high helium-beam doses.

Testing the limits of survivability in space is the primary focus in astrobiological research. Although a number of previous studies have examined terrestrial life survival in an extraterrestrial environment, only a few have investigated how life systems respond to high doses of alpha cosmic ray, the main component of cosmic rays. We used respiration and photosynthetic rates as indicators of the vital signs of the lichen Caloplaca flavovirescens, which is a symbiotic life form including fungi and algae. Our experiment demonstrated that the photosynthetic rate decreased with increased helium-beam doses, whereas the respiration rate was relatively unaffected. Specifically, under a helium-beam dose greater than 10 Gy, the respiration rate remained nearly constant regardless of further increases in the radiation rate. Our results indicate that the different metabolic systems of terrestrial life forms might exhibit different survival characteristics when they are in space.

Proof of principle of helium-beam radiography using silicon pixel detectors for energy deposition measurement, identification, and tracking of single ions.

PURPOSE: Hadron therapy has the capability to provide a high dose conformation to tumor regions. However, it requires an accurate target positioning. Thus, the precise monitoring of the patient's anatomical positioning during treatment is desirable. For this purpose, hadron-beam radiography with protons (pRad) and ions (iRad) could be an attractive tool complementing the conventional imaging technologies. On the pathway to an envisaged clinical application, several challenges have to be addressed. Among them are achieving the desired spatial resolution in the presence of multiple Coulomb scattering (MCS), performing radiographs with a sufficient thickness resolution at clinically applicable dose levels, and the search for combinations of particularly suitable hadrons and detectors. These topics are investigated in this work for a detection system based on silicon pixel detectors. METHODS: A method of iRad based on energy deposition measurements in thin layers is introduced. It exploits a detection system consisting of three parallel silicon pixel detectors, which also enables particle tracking and identification. Helium ions, which exhibit less pronounced MCS than protons, were chosen as imaging radiation. A PMMA phantom with a mean water-equivalent thickness (WET) of 192 mm, containing maximal WET-variations of ±6 mm, was imaged with a 173 MeV/u helium ion beam at the Heidelberg Ion-Beam Therapy Center. WET-differences in form of 2.3 mm × 2.3 mm steps were aimed to be visualized and resolved in images of the energy deposition measured behind the phantom. The detection system was placed downstream of the imaged object in order to detect single ions leaving it. The combination of the measured information on energy deposition, ion type, and the track behind the phantom was used for the image formation, employing a self-developed data-processing procedure. RESULTS: It was shown that helium-beam radiography is feasible with the reported detection system. The introduced data preprocessing purified the detector signal from detector artifacts and improved the image quality. Additionally, the rejection of hydrogen ions originating from nuclear interactions was shown to increase the contrast-to-noise ratio (CNR) by at least a factor of 2.5. This enabled the resolution of relative thickness differences of 1.2% at a dose level typical for diagnostic x-ray images. The spatial resolution was improved by taking into account the direction of single helium ions leaving the phantom. A spatial resolution (MTF10% ) of at least 1.15p mm-1 for the presented experimental set-up was achieved. CONCLUSION: A successful feasibility study of helium-beam radiography with the introduced detection system was conducted. The methodology of iRad was based on energy deposition measurements in thin silicon layers. The tracking of single ions and the method of the ion identification was shown to be important for helium-beam radiography in terms of spatial resolution and CNR.