Session Item

Particle imaging can increase precision in proton and ion therapy. Interactions with nuclei in the imaged object result in image noise and reduced image quality, especially for multi-nucleon ions that may fragment. This work proposes a filter based on the physics of electromagnetic interactions to identify and remove ions, that have undergone nuclear interaction, and hence contribute to image noise.

The filter combines a prior reconstruction with scattering and straggling theory to determine the likelihood that a particle only interacts electromagnetically (primary). A threshold (P_t) is then set to reject those particles with a low likelihood (secondary). The filter performance was compared with the state-of-the-art 3σ filter which removes particles based on the water equivalent thickness (WET) and angular distributions per pixel. We reconstructed proton and helium radiographs from simulated data of the XCAT thorax phantom. Experimental proton and helium CT scans of a Catphan 404 Sensitometry module (The Phantom Laboratory, Salem, NY, USA) were also evaluated. The radiographs and the tomographic reconstructions were evaluated based on intra-pixel noise.

The proton radiographs showed minor improvement in noise (<1 mm standard deviation in WET) but a larger benefit was seen in helium radiographs (~4 mm standard deviation in WET) due to better filtering of secondary fragments (Fig. 1). The tomographic data shows significantly reduced noise for both ions with relative stopping power (RSP) noise of approximately 0.02 in the uniform part of the phantom (Fig. 2). The prior filter enabled greatly reduced imaging dose without relevant loss in image quality: even a reduction of the number of proton histories by a factor of 9, reducing the imaging dose to ~0.5mGy, only doubled RSP noise. For comparison, with the current state-of-the-art filtering technique, the same reduction resulted in a factor 8 increase in noise.

Fig 1. Noise profiles generated by averaging the standard deviation in each bin along the Y-axis for simulated radiographs of the XCAT phantom using the 3σ and prior filter. The No Filter plot is split across axes to better visualise the profiles of the filtered data.

Fig 2. Central slice of the proton and helium CT of the CTP404 Sensitometry module (right) and noise profiles (left) with the prior filter and the 3σ filter applied. The dashed line indicates the position on the phantom.

The prior filter significantly reduced image noise compared to the current state-of-the-art filter in helium radiography whereas, its performance in proton radiography is comparable. However, noise is significantly reduced for both proton and helium tomographic imaging. The filter’s design overcomes one of the drawbacks of the 3σ filter: its need to have a representative distribution of ions in every bin. The filter is adjustable and could be optimised for bespoke applications including providing opportunity for low dose online imaging for ion therapy.