Room: Exhibit Hall | Forum 6
Purpose: 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.
Methods: Hollow cylindrical polylactic acid (PLA) plugs (3 cm diameter, 5 cm height) were 3D-printed containing either one or three septa with thicknesses (t-septa) = 0.8 mm, 1.6 mm, 3.2 mm and 6.4 mm running along the length of plug. The cylinders were inserted individually into a tissue equivalent head phantom (16 cm diameter, 5 cm height). First, DECT scans were obtained using a Siemens SOMATOM Definition Edge CT scanner. Effective atomic number (Zeff) and electron density (Ï?e) 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 Ï?e 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.
Results: SPR-CT for PLA agreed with SPR-TH for t-septa â‰¥ 3 mm (for all three CT slice thicknesses utilized). At t-septa < 3 mm, the SPR-CT became lower, diverging from SPR-TH. This is attributed to the fact that the density of PLA is no longer a constant when t-septa < 3 mm; it decreases with t-septa. Such an actual reduction in density also led to a reduction in SPR-DM, in agreement with the behavior of SPR-CT.
Conclusion: Overall, the DECT-based SPR determination did accurately determine SPR in regions of high-density gradients such as bone-air interfaces, provided the objects along the interfaces were greater than ~3mm in thickness.