Experimental comparison of photon versus particle computed tomography to predict tissue relative stopping powers

Purpose: Measurements comparing relative stopping power (RSP) accuracy of state-of-the-art systems representing single-energy and dual-energy computed tomography (SECT/DECT) with proton CT (pCT) and helium CT (HeCT) in biological tissue samples. Methods: We used 16 porcine and bovine samples of various tissue types and water, covering an RSP range from 0.90 +/- 0.06 to 1.78 +/- 0.05. Samples were packed and sealed into 3D-printed cylinders (d=2 cm, h=5 cm) and inserted into an in-house designed cylindrical polymethyl methacrylate (PMMA) phantom (d=10 cm, h=10 cm). We scanned the phantom in a commercial SECT and DECT (120 kV; 100 and 140 kV/Sn (tin-filtered)); and acquired pCT and HeCT (E similar to 200 MeV/u, 2 circle steps, similar to 6.2x106 (p)/similar to 2.3x106 (He) particles/projection) with a particle imaging prototype. RSP maps were calculated from SECT/DECT using stoichiometric methods and from pCT/HeCT using the DROP-TVS algorithm. We estimated the average RSP of each tissue per modality in cylindrical volumes of interest and compared it to ground truth RSP taken from peak-detection measurements. Results: Throughout all samples, we observe the following root-mean-squared RSP prediction errors +/- combined uncertainty from reference measurement and imaging: SECT 3.10 +/- 2.88%, DECT 0.75 +/- 2.80%, pCT 1.19 +/- 2.81%, and HeCT 0.78 +/- 2.81%. The largest mean errors +/- combined uncertainty per modality are SECT 8.22 +/- 2.79% in cortical bone, DECT 1.74 +/- 2.00% in back fat, pCT 1.80 +/- 4.27% in bone marrow, and HeCT 1.37 +/- 4.25% in bone marrow. Ring artifacts were observed in both pCT and HeCT reconstructions, imposing a systematic shift to predicted RSPs. Conclusion: Comparing state-of-the-art SECT/DECT technology and a pCT/HeCT prototype, DECT provided the most accurate RSP prediction, closely followed by particle imaging. The novel modalities pCT and HeCT have the potential to further improve on RSP accuracies with work focusing on the origin and correction of ring artifacts. Future work will study accuracy of proton treatment plans using RSP maps from investigated imaging modalities.