Quantitative performance of photon‐counting CT at low dose: Virtual monochromatic imaging and iodine quantification

Abstract

BackgroundQuantitative imaging techniques, such as virtual monochromatic imaging (VMI) and iodine quantification (IQ), have proven valuable diagnostic methods in several specific clinical tasks such as tumor and tissue differentiation. Recently, a new generation of computed tomography (CT) scanners equipped with photon‐counting detectors (PCD) has reached clinical status.PurposeThis work aimed to investigate the performance of a new photon‐counting CT (PC‐CT) in low‐dose quantitative imaging tasks, comparing it to an earlier generation CT scanner with an energy‐integrating detector dual‐energy CT (DE‐CT). The accuracy and precision of the quantification across size, dose, material types (including low and high iodine concentrations), displacement from iso‐center, and solvent (tissue background) composition were explored.MethodsQuantitative analysis was performed on two clinical scanners, Siemens SOMATOM Force and NAEOTOM Alpha using a multi‐energy phantom with plastic inserts mimicking different iodine concentrations and tissue types. The tube configurations in the dual‐energy scanner were 80/150Sn kVp and 100/150Sn kVp, while for PC‐CT both tube voltages were set to either 120 or 140 kVp with photon‐counting energy thresholds set at 20/65 or 20/70 keV. The statistical significance of patient‐related parameters in quantitative measurements was examined using ANOVA and pairwise comparison with the posthoc Tukey honest significance test. Scanner bias was assessed in both quantitative tasks for relevant patient‐specific parameters.ResultsThe accuracy of IQ and VMI in the PC‐CT was comparable between standard and low radiation doses (p < 0.01). The patient size and tissue type significantly affect the accuracy of both quantitative imaging tasks in both scanners. The PC‐CT scanner outperforms the DE‐CT scanner in the IQ task in all cases. Iodine quantification bias in the PC‐CT (−0.9 ± 0.15 mg/mL) at low doses in our study was comparable to that of DE‐CT (range ‐2.6 to 1.5 mg/mL, published elsewhere) at a 1.7× higher dose, but the dose reduction severely biased DE‐CT (4.72 ± 0.22 mg/mL). The accuracy in Hounsfield units (HU) estimation was comparable for 70 and 100 keV virtual imaging between scanners, but PC‐CT was significantly underestimating virtual 40 keV HU values of dense materials in the phantom representing the extremely obese population.ConclusionsThe statistical analysis of our measurements reveals better IQ at lower radiation doses using new PC‐CT. Although VMI performance was mostly comparable between the scanners, the DE‐CT scanner quantitatively outperformed PC‐CT when estimating HU values in the specific case of very large phantoms and dense materials, benefiting from increased X‐ray tube potentials.

Photon-Counting Detector CT for Liver Lesion Detection—Optimal Virtual Monoenergetic Energy for Different Simulated Patient Sizes and Radiation Doses

“…Future studies will be needed to find the optimal VMI energy for other abdominal pathologies and quantification tasks. 35 In conclusion, detectability of liver lesions depended on the radiation dose and phantom size and was considerably improved by selecting the optimal VMI energy. Based on our findings, we suggest VMI at 65 or 70 keV as the optimal VMI energy to detect hypoattenuating or hyperattenuating liver lesions.…”

Iodine quantification of renal lesions: Preliminary results using spectral-based material extraction on photon-counting CT

“…found similar results investigating the potential of PCCT for low-dose quantitative spectral tasks in a recent phantom study. They reported comparable iodine quantification accuracy between standard and low radiation doses on PCCT when outperforming accuracy on dual-energy CT. [ 16 ]…”

Dual-Energy and Photon-Counting Computed Tomography in Vascular Applications—Technical Background and Post-Processing Techniques

“…There is hope for further improvement in this field as PCCT scanners develop. In DECT, a scanner obtains two sources of information (e.g., by dual X-ray tubes with different voltages); a PCCT scanner can gain even more information by establishing more than two energy thresholds during the separation of detected photons [ 51 ]. The VMIs produced by PCCT scanners have been extensively studied as they are expected to have improved spatial resolution and lower image noise with no need for a higher radiation dose.…”

“…Many studies found that PCCT produces higher-quality VMIs than DECT scanners at low radiation doses [ 52 , 53 ]. Vrbaski et al [ 51 ] reported that PCCTs have comparable accuracy in VMI imaging between low and standard radiation dose levels, whether or not the DECT performance was dependent on the radiation dose. Liu et al [ 54 ] found that VMIs obtained with PECT in a cardiac phantom for coronary CT with different heart rates had especially high spatial and temporal resolutions with a low applied radiation dose.…”