Optimization of Cone Beam Computed Tomography Scan Protocols for the Intraoperative Detection of Residual Stones in Percutaneous Nephrolithotomy

Purpose: The primary surgical treatment modality for large or complex renal stones is percutaneous nephrolithotomy (PCNL). Cone beam computed tomography (CBCT) allows for intraoperative imaging and can be used to enhance intraoperative assessment of a stone-free status during a PCNL procedure. However, scanning protocols have not yet been optimized for this purpose. The high degree of stone and patient characteristics require a tailored approach. This study aimed to select the most suitable CBCT protocols for imaging stone fragments intraoperatively during PCNL.Methods: A phantom insert with 100 calcifications varying in size and density was placed in an anthropomorphic abdominal phantom. Nine different CBCT protocols were used with varying dose and copper filter settings, and each scan was repeated five times with a small translation between each scan. Detectability of the calcifications was scored by visual assessment and visibility curves were generated for each protocol, depicting the minimum size and density at which calcifications were still detectable. Image noise and contrast-to-noise ratios (CNRs) were calculated for each protocol, as well as estimated effective patient doses per CBCT scan.Results: Calcification detectability and CNRs decreased with a decrease in radiation dose, whereas noise ratios increased. Three suitable scanning protocols were selected, a high-dose or soft-stone CBCT protocol resulting in an effective dose of 10.7 millisievert (mSv) per CBCT scan, a medium-dose CBCT protocol resulting in an effective dose of 4.6 mSv and a low-dose or hard-stone CBCT protocol resulting in an effective dose of 2.2 mSv.Conclusion: Radiation dose for intraoperative CBCT for imaging calcifications can effectively be lowered while maintaining calcification visibility, by implementing low-dose CT protocols with lowered dose settings and the application of a copper filter. The three proposed scanning protocols provide options to select a scan protocol based on stone type and patient characteristics.