Impact of a silver filter with deep learning reconstruction on non-contrast MDCT of adrenal glands in terms of image quality, density values and radiation dose

Abstract Background To investigate whether the use of a silver beam filtration with deep learning reconstruction (DLR) impact the normal adrenal gland on unenhanced MDCT in terms of density, image quality and radiation dose. Methods This unicentric retrospective study evaluated two unenhanced MDCT acquisitions of 123 patients performed with a conventional filtration (Group A) and with a silver filtration (Group B) on a 320-detector row CT scanner with DLR. A total of 246 series, corresponding to 492 adrenal glands, were analyzed. We recorded the HU values of the normal adrenal gland, subcutaneous fat and extra corporeal air as well as the radiation dose and assessed image quality. Results The subjective image quality was excellent, with no significant difference between the two acquisitions with a mean score on the Likert scale of 4.6 ± 0.3 and 4.1 ± 0.6 for groups A and B, respectively ( p = 0.09). Interobserver agreement for image quality was good and quite similar for both groups (group B: k= 0.86; group A: k= 0.90). The mean density of adrenal gland was 15.65 ± 5.5 HU with conventional filtration (Group A) and 13.05 ± 4.5 HU for silver filtration (Group B). For the right adrenal gland, the median values were 15.8 HU and 12.9 HU for group A and B, respectively (p=0.01). For the left adrenal gland, the median values were 15.5 HU and 13.2 HU for group A and B, respectively (p=0.03). A subgroup of 135 adrenal glands (including both right and left side) with a density value ≤ 15HU using conventional filtration (Subgroup A) showed a statistically significant lower median value with silver filtration (Subgroup B). Inside this ≤ 15 HU subgroup, 59 adrenal glands had a density value equal or lower than 10 HU of which 48 belonged to subgroup B. A reduction in HU values of 18% was observed with the silver filter in the overall cohort (492 adrenal glands), and 26% in the subgroup ≤ 15 HU. Similarly, the mean score for image noise was 7.6 ± 1.5 for conventional filter and 8.3 ± 0.9 for silver filtration, with no statistically significant difference ( p = 0.08). Radiation dose was significantly reduced by 35% in examinations performed with silver filtration when compared to conventional filter. Conclusion The spontaneous density of normal adrenal gland is lower than previously reported. Implementing silver filtration with DLR in CT acquisition achieves a significant radiation dose reduction while preserving image quality, along with a decrease in adrenal gland HU values, often below 10 HU. This must be taken into account as this is the threshold of the key reference for adenoma. Trial registration This study is part of a routine clinical practice. This unicentric retrospective study was approved by our local institutional review board, and written informed patient consent was not required. The patients were orally informed that their second CT examination will be performed using the newest technology available on the CT machine in our department.