Half-fan-based intensity-weighted region-of-interest imaging for low-dose cone-beam CT

Abstract

We developed an intensity-weighted region-of-interest (IWROI) imaging method in exact half-fan geometry to reduce imaging radiation dose to the patient in cone-beam CT (CBCT) for image-guided radiation therapy. By use of an intensity-weighting filter and an exact half-fan scanning, one can reduce the overall imaging radiation dose to the patient substantially. An intensity-weighting (IW) filter made of copper was mounted on the X-ray tube unit in the onboard imager (OBI) system. Since a half-fan full-scan mode is the choice of scanning for most treatment sites including the chest and the abdomen due to its enlarged field-of-view (FOV), the half-fan scan mode has been used in this study. The IW filter was placed so as to cover outside the ROI within the FOV. In addition to mounting the IW filter, the lead-blade collimation of the OBI was adjusted to produce an exact half-fan scanning geometry for further reduction of radiation dose. A humanoid pelvis phantom was used for the IWROI imaging experiment. The rebinned backprojection-filtration (BPF) algorithm in circular CBCT was implemented for image reconstruction. Correction of image artifacts caused by the heterogeneity in beam-quality was also made. Image registration between the IWROI image and the reference image was conducted to validate the clinical utility of the IWROI image for image-guidance. Additionally, we measured the radiation dose by use of the EBT2 film to estimate the dose reduction. The IWROI image of the phantom was successfully reconstructed after beam-quality correction, and it was registered to the reference image within an acceptable tolerance. The dosimetric measurements revealed that the dose is reduced approximately by 42% in the inner ROI and by 33% in the outer ROI compared to the unfiltered conventional half-fan scan. The proposed half-fan-based IWROI imaging technique can be a valuable option for low-dose CBCT in IGRT applications.