Room: AAPM ePoster Library
Purpose: Varian Halcyon mandates patient imaging prior to treatment using kilovoltage or megavoltage-CBCT guidance. Halcyon equips traditional filter-back projection (kV-CBCT) and new iterative-cone beam CT (iCBCT) image reconstruction algorithms. This prompts exposure setting investigation in iCBCT imaging with intention of reducing patient dose without compromising image quality compared to standard kV-CBCT protocol.
Methods: Vendor provided Quart/CatPhan phantom were imaged with kV-CBCT and iCBCT with a standard patient workflow. Quart/CatPhan simulated a patient’s head and pelvis sites, respectively. Phantoms were imaged with kV-CBCT under standard exposures of 126mAs/1080mAs for head/pelvis protocols, respectively. Using iCBCT for exposure settings from lowest to highest available, phantoms were re-imaged. BBs were placed to introduce artifacts in contrast regions for image quality. SNR and spatial resolution were evaluated for each exposure setting and phantom.
Results: iCBCT created higher SNR values for all exposure settings with respect to kV-CBCT clinical settings on both phantoms. On average SNR increase was observed to be 24.9±11.8 and 127.9±47.5 for Quart and CatPhan, respectively. Quart exhibited approximate SNR linear increase with increased iCBCT exposure head protocol. CatPhan using iCBCT pelvis protocol demonstrated an SNR plateau lower than the highest tested exposure setting, indicating reduced patient's dose. Spatial resolution was consistent for lower exposures as all scans demonstrated 6 line-pairs/cm. Additionally, lower streaking artifact presence was observed with iCBCT scan protocols at lower exposure settings than kV-CBCT.
Conclusion: Phantoms were introduced to a standard patient workflow to investigate potential exposure settings to reduce patient dose. Phantom contrast regions and line pair groups quantified and confirmed iCBCT provided similar image quality to kV-CBCT for lower exposure setting. We recommend lowering the pre-set exposure setting at Halcyon Linac when imaging with iCBCT protocols. Further study is ongoing into replacing MV-CBCT with iCBCT protocols to further lower patient dose when less streaking artifacts are present.
Backprojection, Cone-beam CT, Spatial Resolution