A comparison of carbon ions versus protons for integrated mode ion imaging
Authors: Simard Mikaël , Fullarton Ryan , Volz Lennart , Schuy Christoph , Robertson Daniel G. , Toltz Allison , Baker Colin , Beddar Sam , Graeff Christian , Fekete Charles‐Antoine Collins 
Published: February 2025

Background

Incorporating image guidance into ion beam therapy is critical for minimizing beam range uncertainties and realizing the modality's potential. One promising avenue for image guidance is to capture transmission ion radiographs (iRads) before and/or during treatment. iRad image quality is typically maximized using a single‐event imaging system, which involves tracking individual ions, albeit the approach is generally not suited to clinical beam settings. An alternative faster and clinically compatible method is integrated mode imaging, where individual pencil beam data is acquired, rather than single ion data. To evaluate the usefulness of transmission ion imaging for image guidance, it is crucial to evaluate the image quality of integrated mode iRad systems.

Purpose

We report extensive image quality metrics of integrated mode carbon ion radiographs (cRads) and compare them with proton radiographs (pRads).

Methods

iRads were obtained at the Marburg Ion Beam Therapy Center using a plastic volumetric scintillator equipped with CCD cameras. The detector captures orthogonal views of the 3D energy deposition in the scintillator from individual pencil beams. Four phantoms were scanned using a field of view and a beam spacing of 1 mm. First, 9 tissue‐substitute inserts were used to evaluate water equivalent thickness (WET) accuracy. Radiographs of those inserts were reconstructed for beam spacings ranging from 1 to 7 mm to evaluate the impact of spacing on quantitative accuracy. For spatial resolution, custom 3D printed line pair (lp) modules ranging from 0.5 to 10 lp/cm were scanned. To evaluate low contrast detectability, a custom 3D printed low contrast module consisting of 20 holes with depths ranging from 1 to 8 mm and diameters from 1 to 10 mm was scanned. iRads of an anthropomorphic head phantom were also obtained.

Results

Spatial resolution and low contrast detection are systematically improved for cRads compared to pRads. Image resolution was 3.7 lp/cm for cRads and 1.7 lp/cm for pRads in the center of the field of view. Spatial resolution was found to vary with the object's location in the field of view. While pRads could mostly resolve low contrast holes of 10 mm in diameter, cRads could resolve holes of up in 4 mm diameter. WET accuracy is similar for both ion species, with a root mean squared error of approximately 1 mm. WET accuracy was stable (maximum of 0.1 mm increase) across beam spacings, although important under‐sampling artifacts were observed for iRads reconstructed using large beam spacings, especially for cRads. iRads of the anthropomorphic head phantom showed improved apparent contrast using cRads, especially to identify bony structures.

Conclusions

This work is the first investigation of image quality metrics such as spatial resolution and low contrast detectability for integrated mode cRads, with a full comparison with pRads. Enhanced image quality is obtained with cRads compared to pRads, although pRads still maintain high WET accuracy and deliver image quality within acceptable bounds.