Determination of proton stopping power ratio with dual-energy CT in 3D-printed tissue/air cavity surrogates.

Abstract

To study the accuracy with which proton stopping power ratio (SPR) can be determined with dual-energy computed tomography (DECT) for small structures and bone-tissue-air interfaces like those found in the head or in the neck. Hollow cylindrical polylactic acid (PLA) plugs (3cm diameter, 5cm height) were 3D printed containing either one or three septa with thicknesses tsepta =0.8, 1.6, 3.2, and 6.4mm running along the length of the plug. The cylinders were inserted individually into a tissue-equivalent head phantom (16cm diameter, 5cm height). First, DECT scans were obtained using a Siemens SOMATOM Definition Edge CT scanner. Effective atomic number (Zeff ) and electron density (ρe ) images were reconstructed from the DECT to produce SPR-CT images of each plug. Second, independent elemental composition analysis of the PLA plastic was used to determine the Zeff and ρe for calculating the theoretical SPR (SPR-TH) using the Bethe-Bloch equation. Finally, for each plug, a direct measurement of SPR (SPR-DM) was obtained in a clinical proton beam. The values of SPR-CT, SPR-TH, and SPR-DM were compared. The SPR-CT for PLA agreed with SPR-DM for tsepta ≥3mm (for CT slice thicknesses of 0.5, 1.0, and 3.0mm). The density of PLA was found to decrease with thickness when tsepta <3mm. As tsepta (and density) decreased, the SPR-CT values also decreased, in good agreement with SPR-DM and SPR-TH. Overall, the DECT-based SPR-CT was within 3% of SPR-TH and SPR-DM in the high-density gradient regions of the 3D-printed plugs for septa greater than~3mm in thickness.