Matched comparison of low kVp imaging and CT spectral imaging in image quality and radiation dose at abdominal CT

Objective To investigate the image quality and radiation dose of spectral CT with automatic spectral imaging mode selection (ASIS) and adaptive statistical iterative reconstruction (ASIR) at abdominal CT as compared with low-kVp CT with ASIR. Methods Two hundred patients underwent the plain and arterial-phase (AP) and portal venous-phase (PVP) contrast-enhanced abdominal CT scan were analyzed prospectively. The patients were randomly assigned to the control group (low-kVp CT scan) and study group (spectral CT scan), if BMI ≤ 23.9 kg/m2, patients were assigned into group A (80 kVp) or group B (spectral CT with ASIS); if 24.0 kg/m2 ≤ BMI<28.9 kg/m2, patients were assigned into group C (100 kVp) or group D (spectral CT with ASIS)(n=50 each). Groups A and B, groups C and D were matched by gender, age, body mass index, cross sectional area and contrast agent dose respectively. Low-kVp images and monochromatic images (40 to 60 keV) were all reconstructed by using ASIR. Radiation dose and quantitative parameters (image noise in HU and contrast-to-noise ratio of the liver, aorta and portal vein) were compared by using t test while overall image quality was assessed by Mann-Whitney U test between the matched groups. Results Radiation dose in the group B was significantly higher than group A (increased by 10%, P 0.05). Compared to the control group, image noises in the study group were higher at 40 keV and 50 keV (P 0.05). At the energy level of 40 keV, the study group showed higher CNRs, but lower overall image quality scores than the control group except for the similar image quality scores between groups A and B during AP (P<0.05). At the energy level of 50 keV, the CNRs and overall image quality scores in the study group were higher than or similar to the control group. At the energy level of 60 keV, the study group showed lower or similar CNRs, but higher or similar overall image quality scores compared with the control group. Conclusions The radiation dose of spectral CT with ASIS technique was slightly higher than 80 kVp CT but similar to 100 kVp CT. By combining ASIR technique, monochromatic image at 50 keV can maintain or improve CNR and overall image quality as compared with low-kVp images. Key words: Tomography, X-ray computed; Comparative study; Radiation dosage; Image quality