Effect of 16-bit computed tomography imaging of metallic implants on dose distribution in radiotherapy

Abstract

Objective To reconstruct 16-bit images of metal implants using the extended function of computed tomography (CT) imaging, and to analyze the effect of the metal CT value on calculation of dose distribution by evaluation of metal CT values in different scanning conditions. Methods A stainless steel rod and a titanium rod were inserted in a phantom. The 12-and 16-bit images and CT value distribution of metal implants were obtained by scanning the phantom using 120 kV tube voltage and 230 mA tube current. The 16-bit images and CT value distribution of metal implants were obtained by scanning the phantom using fixed tube current (230 mA) with varied tube voltage (100, 120, and 140 kV) or fixed tube voltage (120 kV) with varied tube current (180, 230, and 280 mA). In the Varian treatment planning system, a single-field plan and a parallel-opposed field plan were designed based on the CT images. The dose distribution was calculated and compared by the paired t test. Results The CT values of the stainless steel rod and the titanium rod were both 3071 HU in 12-bit CT images. In 16-bit CT images; however, the CT value of the stainless steel rod was significantly larger than that of the titanium rod. There were no significant differences in CT value of 16-bit image and dose distribution in radiotherapy plan between three scanning conditions with different tube currents. Under three scanning conditions with different tube voltages, the maximum CT values were 13568, 13127, and 12295 HU for the stainless steel rod and 8420, 7140, and 6310 HU for the titanium rod, respectively. Conclusions High-density metal implants cannot be distinguished by 12-bit images, while the distribution of metal CT value can be obtained by 16-bit images. The dose distribution of metal implants based on 12-bit images is different from that based on 16-bit images. Changes in tube voltage cause substantial changes in the CT value for metal implants, leading to changes in dose distribution in radiotherapy. Variation of tube current within a certain range causes slight changes in metal CT value and dose distribution. Key words: Metallic implants; Tomography; X-ray computed; Scanning condition; Radiotherapy dose distribution