Calibration of megavoltage cone-beam CT for radiotherapy dose calculations: Correction of cupping artifacts and conversion of CT numbers to electron density

摘要: Megavoltage cone-beam CT (MV CBCT) is used for three-dimensional imaging of the patient anatomy on treatment table prior to or just after radiotherapy treatment. To use MV CBCTimages dose calculation purposes, reliable electron density (ED) distributions are needed. Patient scatter, beam hardening and softening effects result in cupping artifacts distort number ED conversion. A method based transmission images presented correct these without using knowledge object’s geometry. The scatter distribution originating from calculated with pencil kernels that fitted measurements. radiological thickness extracted subtracted then converted primary reconstruction. These corrections performed an iterative manner, regarding geometry composition object. was tested various homogeneous inhomogeneous phantoms varying shapes compositions, including a phantom different inserts, large variations, anthropomorphic head phantom. For all phantoms, artifact substantially removed linear relation between found. After correction deviations reconstructed true values were reduced up 0.30 units 0.03 majority phantoms; residual difference equal amount noise images. evaluated terms absolute accuracy cylinders size; errors decreased 7 % below 1 center objects uncorrected corrected images, respectively, maximum differences 17 2 , respectively. corrects extracts information geometries composition, making suitable accurate purposes.