Purpose: Photon-counting-detector (PCD) CT poses unique features compared to conventional energy-integrating-detector (EID) CT. Thus, the quality control needs to be adapted accordingly. This study presents our experience on quality control of a clinical PCD-CT.
Methods: The ACR CT Accreditation Phantom (CTAP) was scanned with a commercial dual-source PCD-CT using routine abdomen and head protocols. Energy thresholded images (named T3D) along with virtual monoenergetic images (VMIs) between 40-120 keV were reconstructed and complete quality control was conducted. Modulation transfer function (MTF) was computed using a high-resolution mode with a sharp kernel (Br96) to evaluate the limiting in-plane spatial resolution. The multi-energy capability provided by PCD-CD was evaluated by scanning a body phantom (40X30 cm²) with four iodine inserts (concentrations: 2-15 mg/cc).
Results: The measured CT number for Teflon (bone) insert (~1000 HU) on T3D images fell outside of the expected range of 850-970 HU, while all measurements on 70 keV VMI were within the ranges. A larger number of slice thickness wires were visible on PCD-CT than on comparable EID-CT. The MTF curve indicated a limiting resolution of nearly 40 lp/cm in high-resolution mode which exceeds the 12 lp/cm maximum on the CTAP phantom. For multi-energy scans of the 40-cm phantom, iodine CT numbers were accurately measured on all VMIs with an averaged percentage error of 1.4%, and iodine concentrations had an averaged root mean squared error of 0.5 mg/cc.
Conclusion: While the ACR CTAP phantom has been routinely used in CT quality control, precautions need to be taken in its uses on clinical PCD-CT and protocols/parameters should be properly selected to meet current accreditation requirements. For quality control purposes, VMIs (e.g. 70 keV) may be more suitable than energy-thresholded 120 kV images. Additional evaluations such MTF measurements and multi-energy scans are also recommended to fully evaluate scanner performance.
Funding Support, Disclosures, and Conflict of Interest: This study was supported by the National Institutes of Health under award numbers R01 EB028590 and supported in kind by Siemens Healthineers GmbH, who own the evaluated system under the terms of a sponsored research agreement with the Mayo Clinic.
Quality Control, CT, Photon Detectors