Room: AAPM ePoster Library
Purpose: radiography could allow for direct measurement of the water-equivalent path length (WEPL) in tissue, which can then be used to determine relative stopping power (RSP). This allows for treatment modality-specific image and potentially reduced range uncertainty associated with the CT HU to RSP conversion. Previous work has shown the validity of this approach in a scattering system, and here we extend this to a pencil-beam scanning (PBS) system.
Methods: first step is developing an imaging plan to send to the machine. Uniform 1 cm spot-spacing and a 5 cm range shifter were used in each plan. The energy layers were chosen to cover a range of water-equivalent path lengths (WEPLs) expected to see in our phantom measurements. An out-of-field spot is used to gate the detected signal, allowing us to reconstruct the image per each energy layer. Reconstruction techniques are tested on various phantoms to characterize quantitative accuracy, spatial resolution, and general image quality.
Results: acquisition delivers ~1 cGy per image which takes ~90 seconds to deliver (Figure. 2). A mean error of 0.38 mm and percentage error of 0.83% (Table. 1) was achieved for WEPL and RSP determination.
Conclusion: accurate WEPL imaging is possible with the proposed method. Layer-by-layer reconstruction yields accurate results and intensity-based reconstruction gives better spatial resolution. Future work will focus on reducing the dose, time and expanding from radiography to tomography.