Comparative study in physical performance and dosimetric characterization between diagnostic and therapeutic CT scanners

Abstract

CT simulator is used to acquire a volumetric image for dose distribution calculation. With virtual simulation software, beam placement and treatment design could be performed. The therapeutic CT simulation process required large gantry bore to simulate all patients in a comfortable treatment position. The purpose of this study is to evaluate the physical performance and dosimetric characterization of large-bore CT simulator and compare with the diagnostic CT scanner in the same setting. The image quality of an 80 cm bore CT scanner (GE LightSpeed RT) and a 70 cm bore diagnostic type scanner (GE LightSpeed Plus) were performed using manufacturer QA phantom and independent image quality phantom (CATPHAN 500). The evaluated parameters were; slice thickness accuracy, CT number accuracy and linearity, uniformity and noise, low contrast resolution, and high contrast resolution. The computed tomography dose index was measured in phantom for standard head andbody imaging protocols. The measurement with CATPHAN phantom is the standard tool to compare the quality of both scanners. The result showed no significantly difference for the quality of both units. Measured slice thickness was within acceptable criteria of +- 0.5 mm. Both units exhibited comparable CT number accuracy, linearity and uniformity. The contrast scales were 1.92x10[superscript -4]cm[superscript -1]/CT no. for LightSpeed Plus and 1.91x10[superscript -4]cm[superscript -1]/CT no. for LightSpeed RT. The 80 cm bore scanner showed slightly increased in noise compared to 70 cm bore scanner. For low-contrast resolution, LightSpeed Plus obtained 8 mm at 0.3% contrast compared to 9 mm at the same contrast for LightSpeed RT. For high contrast scale, LightSpeed Plus showed 7.14 lp/cm compare to 7.29 lp/cm at 5% MTF for LightSpeed RT. The results showed that LightSpeed RT gave slightly increased high contrast resolution but slightly decreased in low-contrast resolution, the percent noise was higher and theradiation dose was lower when compared with LightSpeed Plus. The results from manufacturer QA phantom agree with CATPHAN phantom and proved to be reliable for routine constancy check. The LightSpeed RT was a successful CT-simulation process, the CT-scanner consistently produced patient images with the highest possible quality and accurate geometrical information.