A Practical Rule-of-Thumb to Adapt Contrast Media Dose in Photon-counting Detector CT: The 10-to-5 Rule

To develop a simple rule-of-thumb on how to reduce the contrast medium (CM) dose in photon-counting detector CT (PCD-CT) when lowering the energy of the reconstructed virtual mono-energetic images (VMI) while maintaining the contrast-to-noise ratio (CNR) for parenchymal CT and CTA.

Spectral abdominal and chest CT phantoms were scanned using a portal venous phase (PVP) abdominal and a high-pitch CTA protocol, respectively, on a first-generation dual-source PCD-CT. The phantoms contained cylindrical rods with iodine in water equivalent material (0.5/1.0/2.0/5.0/10.0/15.0 mg I/mL) and ICRU muscle tissue. The phantoms were complemented with 2 fat equivalent rings to mimic different patient sizes. Iodine contrast, image noise, noise power spectra (NPS), and iodine CNR were investigated in VMIs with different energies (40 to 60 keV in steps of 5 keV). This was done for different iodine concentrations, phantom sizes, x-ray tube voltages (120 kV and 140 kV) and radiation doses. In addition, 15 abdominal and 15 CT angiographic patient scans [body mass index (BMI) range: 17 to 37 kg/m 2 ] were retrospectively analyzed to determine the CNR at different VMI energies.

Contrast at a given iodine concentration and VMI energy was independent of phantom size, radiation dose, and acquisition voltage (kV). With decreasing VMI energy, the maximum of the NPS curves increased, while their shape remained similar, indicating higher noise but similar noise texture. The CNR increased with lower VMI energy for a given iodine concentration and phantom size, while CNR decreased with increasing phantom size for a given VMI energy and iodine concentration. When the VMI energy was lowered by 5 keV steps in the range of 60 to 40 keV, similar CNR could be maintained when reducing the iodine concentration at each step by 11.7% to 13.7% for abdominal PVP scans and 11.8% to 14.5% for CTAs. CNR analysis of the patient scans confirmed these findings: a 5 keV reduction in VMI energy led to a mean±SD 11.4%±0.4% and 13.7%±1.0% increase in CNR for abdomen PVP and CTA scans, respectively. This can be translated to a corresponding reduction in CM dose when a constant CNR is aimed for. From these results, a simple, robust rule-of-thumb was derived, the 10-to-5 rule: For the evaluated PCD-CT protocols, CNR can be maintained with about 10% less CM dose for each reduction of the VMI energy by 5 keV.

This phantom study, which was complemented with a retrospective proof-of-principle patient study, showed that a simple, easy to implement 10-to-5 rule-of-thumb might be used in daily practice for contrast-enhanced PCD-CT. It allows for individual adaptation of the CM dose to the VMI energy applied.