Scan Range and Radiation Dose of the Ultra-High-Resolution Series in Pancreatic Photon-Counting Detector CT: A Prospective Two-Group Comparison of an Ultra-Low-Dose Planning Series.

Background/Objectives: The ultra-high-resolution (UHR) mode of photon-counting detector computed tomography (PCD-CT) is the most dose-intensive series of a multiphase pancreatic protocol in relation to its scan length, and the pancreas cannot be localised on a topogram. We assessed whether an ultra-low-dose (ULD) unenhanced planning series reduces its scan range and dose without compromising complete depiction of the pancreas. Methods: In this prospective single-centre study, 61 pancreatic protocol examinations in 56 patients were performed on a first-generation dual-source PCD-CT. Following a protocol change, the UHR scan range was planned from the topogram in group A (n = 30) and from a ULD unenhanced series in group B (n = 31). Dose indices, scan length, margins to the pancreas and completeness of depiction were recorded. Results: The UHR scan range was 27% shorter in group B (114 vs. 160 mm; p < 0.001), driven by the cranial margin (-58%; p < 0.001) rather than the caudal margin (p = 0.076). Neither the volume CT dose index nor the size-specific dose estimate of the UHR series differed (p = 0.74 and p = 0.35), yet its dose-length product fell by 25% (p = 0.003) and its share of the dose excluding the mandatory venous phase fell from 62.0% to 50.7% (p < 0.001). The total dose was 7.5% lower without reaching significance (p = 0.37). Incomplete depiction occurred in 5 of 30 vs. 5 of 31 examinations (p = 1.00). The body habitus was closely matched. Conclusions: A ULD planning series was associated with a more tightly targeted UHR pancreatic phase, with a 27% shorter scan range and a 25% lower dose-length product for that series, and with no detected increase in incomplete depiction of the pancreas. With topogram-based planning alone, over-scanning was most pronounced at the cranial border. The reduction was confined to the ultra-high-resolution series and was statistically significant there; the total examination dose was lower with ultra-low-dose planning, but this difference was not statistically significant and no conclusion about total dose should be drawn from it.