Effect of Phantom Size and Tube Voltage on the Size-Conversion Factor for Patient Dose Estimation in Computed Tomography Examinations

This study aimed to establish the conversion factors to normalize the output dose of volumetric computed tomography dose index (CTDIvol) to the patient dose (i.e. size-specific dose estimate (SSDE)) for various phantom diameters and tube voltages.

In-house cylindrical acrylic phantoms with physical diameters ranging from 8 to 40 cm were developed in this study. Each phantom had a hole in the center and four holes in the peripheral areas. The phantoms were scanned by a Siemens Somatom Definition AS CT Scanner using different tube voltages (i.e. 80, 100, 120, and 140 kVps) and with 200 mAs and 10 mm slice thickness. In addition, the doses in every hole and phantom were measured using a Raysafe X2 CT Sensor. The weighted SSDE (SSDEw) values were calculated using the five holes in every measurement. The size-conversion factors for the body and the head CTDI phantoms were established by dividing the SSDEw for various sizes with the SSDEw at the water-equivalent diameter of 33.90 cm and 16.95 cm, respectively.

The results revealed that the size-conversion factor exponentially decreased with an increase in the phantom size. It was also found that the size-conversion factor was affected by the tube voltages. Furthermore, the different size-conversion factor between 80 and 140 kVp was more than 15% in very thin and obese patients.

Higher accuracy of the size-specific dose estimation can be achieved considering the impact of the tube voltages beside the size of the patient.