Purpose

Single-event ion imaging offers a great potential to directly reconstruct the relative stopping power (RSP) for ion-beam therapy. For such technique, among the lightest ions, helium imaging (HeCT) was recently hypothesized to be optimal. The purpose of this work was to filter the fragmentation noise present for helium imaging. To demonstrate the method, experimental data obtained with a prototype scanner as well as data obtained by Monte Carlo simulation were used.

Material & Methods

The preclinical proton CT (pCT) scanner built by the U.S. pCT Collaboration was operated with helium beams at the Heidelberg Ion-Beam Therapy Center (HIT). Simulations with this prototype were performed using the TOPAS simulation toolkit. A custom water phantom, and the Catphan® CTP404 (sensitometry) and CTP528 (line-pair) modules were reconstructed with HeCT. In order to identify and remove individual events caused by ion fragments, the multistage energy detector of the pCT scanner prototype was adapted to operate as a ∆E – E telescope.

Results

For the HeCT of the water phantom, events from ion fragments were shown to cause ring artifacts up to a deviation of ~4.5% relative error in simulation and up to ~8% in experiment. With the developed ∆E – E filtering method, fluctuations were reduced to less than 0.2% with respect to the mean in simulation, and to less than 0.5% in simulation. For the CTP404, the ∆E – E filter again removed ring artifacts otherwise arising and greatly improved the mean RSP accuracy measured through the phantom plastic inserts from 2.39% to 0.45% for experimentally acquired and to 0.40% for simulated HeCT. The spatial resolution measured through the CTP528 module was unaffected by the filtering of ion fragments. For all phantoms, fragment filtering reduced noise.

Conclusions

For HeCT, ion fragmentation causes ring artifacts and reduces RSP accuracy as demonstrated with simulated and experimental data. An effective fragmentation filter was developed that removed the fragmentation noise, improved the RSP accuracy to better than 0.5%, and did not negatively affect spatial resolution. With the filter, the prototype scanner developed by the U.S. pCT collaboration is capable also of high quality helium imaging.