Survey of normalized CTDI<sub>vol</sub> values across four major computed tomography vendors for use in the MIRDct software.

Abstract

<h4>Background</h4>Computed tomography (CT) is an essential imaging modality for disease diagnosis, treatment efficacy, and image-based guidance of various medical procedures. The locally deposited radiation dose in tissues, as estimated by the computed tomography dose index (CTDI), can vary considerably across exposures delivered by CT scanners from different vendors, even if the scans are performed using similar technique factors, such as tube potential and tube current. The volumetric CTDI (CTDI_vol) is a common dose metric that reports an average radiation dose (in mGy) delivered to a specific volume within a test phantom. The CTDI_vol is important in dosimetry applications as the organ absorbed dose within the patient has been shown to scale in near-linear proportion, creating a basis for comparing organ doses across different scan protocols and scanner models.<h4>Purpose</h4>To develop a database of tube current-time product (mAs) normalized CTDI_vol values for currently utilized CT scanner models for each of the four primary CT vendors for use in the MIRDct organ dosimetry software available at MIRDsoft.org. This data forms the basis of the MIRDct code, which reports organ doses across a range of computational phantoms based upon axial organ dose coefficient libraries generated through Monte Carlo radiation transport for a reference CT scanner. Organ doses delivered by alternate CT scanner vendors and models may then be reported using ratios of normalized CTDI_vol values under similar technique factors.<h4>Methods</h4>Scanners were selected from four major CT manufacturers: Philips Healthcare, GE Healthcare, Canon Medical Systems, and Siemens Healthineers. Technique parameters were also selected for each scanner that closely matched values used in the generation of an equivalent CT source term (small to large bowtie filters; 80-140-kVp tube voltage; and 10-mm to 40-mm beam collimation). For each scanner chosen, the appropriate technique factors and protocols were selected, and the console-reported CTDI_vol values were recorded and normalized to a set value of 100 mAs. The normalized CTDI_vol data collected for use within the MIRDct code were analyzed for noticeable patterns, features, and trends, and were compared to similar normalized CTDI_vol datasets used within the National Cancer Institute NCICT software and the Virtual Phantoms, Inc. VirtualDose software.<h4>Results</h4>For all given CT scanner and technique factor combinations, there was strong agreement in normalized CTDI_vol values across all three codes: between 0% and 12% difference for the compared scanners. Ratios of CTDI_vol values for various CT scanner vendors and models to the corresponding CTDI_vol values for the MIRDct reference scanner (Cannon Aquilion One Genesis) were also compared on the basis of either the 16-cm head PMMA phantom or the 32-cm body PMMA phantom. The mean quotient of these normalized CTDI_vol ratios (head ratios to body ratios) was found to be approximately 1.06, and thus either ratio may be applied in reporting patient organ dose by MIRDct.<h4>Conclusions</h4>A database of normalized CTDI_vol (mGy/100 mAs) was created for 17 models of CT scanners from four manufacturers at varying tube potentials, collimations, x-ray bowtie filters, and phantom sizes for use in the MIRDct software.