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Low dose protocol for high resolution CT thorax: influence of matrix size and tube voltage on image quality and radiation dose

Background: High-resolution CT (HRCT) thorax has increase demand due to its advantage in diagnosing chronic respiratory diseases. The feasibility of matrix size with different tube voltage in the HRCT protocol of thorax is unknown. Therefore, this study aimed to compare the effect of matrix sizes and tube voltage on image quality and radiation dose on adult HRCT thorax.   Methods: A Phantom experiment was performed, followed by a patient scan. For phantom and patient scan, a total of six protocols with two tube voltage settings, 120 kVp and 100 kVp, with a combination of three matrix sizes, 512, 768, and 1024 were used. In this study, 180 adult patients who had HRCT thorax scan were considered. Dose data was collected, and quantitative image analysis was performed by drawing region of interests on the acquired phantom and patient images. Qualitative image analysis was performed independently by two blinded radiologists.   Results: The dose report of the phantom experiment revealed that the 100kVp with selected matrix size delivered 15.64% and 15.62% less radiation dose in terms of volumetric computed tomography dose index (CTDIvol) and dose length product (DLP), respectively, compared to 120kVp settings with selected matrix sizes. Similarly, for the patient population, the CTDIvol and DLP difference noted for 120kVp and 100kVp with different matrix sizes was statistically significant (p<0.001). For quantitative image quality, the difference noted was also statistically significant among two kVp settings. The mean score for subjective image assessment was greater than 4.5 for diagnostic acceptability and streak artefacts.   Conclusion: The result suggests that the 100 kVp with 512 X 512 matrix size is preferable in the HRCT Lung to achieve the optimal diagnostic image quality with a reduction of almost 40% of the dose to the patients compared to 120 kVp techniques.