Room: AAPM ePoster Library
Purpose: Flat panel detectors are commonly used for cone-beam CT, but they have limited dynamic range, leading to overexposure in air or near the skin line. For many scans, this leads to inferior image quality near the surface of the patient, excess dose, and extra scatter. For example, in weight-bearing CT of the knees, the patella may be difficult to visualize. We have constructed a prototype dynamic range reducer (DyRaR) that can attenuate the intensity of the beam outside or near the edge of the patient while conforming to the patient’s shape.
Methods: The prototype DyRaR comprises 8 brass wedges (4 on each lateral side) that can be independently controlled by linear actuators. Brass was selected for its machinability, and the wedges have a maximum thickness of 1.5 mm that taper down medially to zero thickness. The DyRaR was mounted to the collimator of a C-arm system, and cone-beam CT scans of a Catphan phantom were acquired with and without the wedges attenuating the edge regions. Separate air scans with the DyRaR were acquired to normalize the CBCT scans.
Results: The DyRaR reduced the projection signal in air by 11.0× at 102 kV, which is approximately equivalent to the attenuation of 12 cm water. Without correction for the DyRaR, large artifacts were observed in the reconstructed images. With correction, the artifacts are largely addressed, although some remain due to beam hardening and differences in the wedge positions from the air scan. This suggests the wedges can be used to reduce dynamic range, although image processing may be needed to find the exact position of the wedges.
Conclusion: This work demonstrates an initial prototype and evaluation of a dynamic range reducer for CBCT. In future work, the DyRaR will be used to scan more complex object shapes.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by NIH R01 AR065248 and Siemens Healthineers