Dual-Energy CT: Lower Limits of Iodine Detection and Quantification

Abstract

Background Assessments of the quantitative limitations among the six commercially available dual-energy (DE) CT acquisition schemes used by major CT manufacturers could aid researchers looking to use iodine quantification as an imaging biomarker. Purpose To determine the limits of detection and quantification of DE CT in phantoms by comparing rapid peak kilovoltage switching, dual-source, split-filter, and dual-layer detector systems in six different scanners. Materials and Methods Seven 50-mL iohexol solutions were used, with concentrations of 0.03-2.0 mg iodine per milliliter. The solutions and water sample were scanned five times each in two phantoms (small, 20-cm diameter; large, 30 × 40-cm diameter) with six DE CT systems and a total of 10 peak kilovoltage settings or combinations. Iodine maps were created, and the mean iodine signal in each sample was recorded. The limit of blank (LOB) was defined as the upper limit of the 95% confidence interval of the water sample. The limit of detection (LOD) was defined as the concentration with a 95% chance of having a signal above the LOB. The limit of quantification (LOQ) was defined as the lowest concentration where the coefficient of variation was less than 20%. Results The LOD range was 0.021-0.26 mg/mL in the small phantom and 0.026-0.55 mg/mL in the large phantom. The LOQ range was 0.07-0.50 mg/mL in the small phantom and 0.20-1.0 mg/mL in the large phantom. The dual-source and rapid peak kilovoltage switching systems had the lowest LODs, and the dual-layer detector systems had the highest LODs. Conclusion The iodine limit of detection using dual-energy CT systems varied with scanner and phantom size, but all systems depicted iodine in the small and large phantoms at or below 0.3 and 0.5 mg/mL, respectively, and enabled quantification at concentrations of 0.5 and 1.0 mg/mL, respectively. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Hindman in this issue.

Figure 1a:

Phantom configuration for the (a) large and (b) small phantoms. Labels in a refer to the nominal iodine concentration present in each sample (in milligrams of iodine per milliliter).

Figure 1b:

Phantom configuration for the (a) large and (b) small phantoms. Labels in a refer to the nominal iodine concentration present in each sample (in milligrams of iodine per milliliter).

Figure 2a:

Comparison of the measurements performed in the (a) small and (b) large phantoms on scanner 4 at 80 and 140 kVp with tin filtration. Note the lower standard deviation of the blank sample and the tighter distributions of the repetitions of the higher concentrations in the small phantom that lead to a calibration curve with a lower fitting error and, ultimately, a lower limit of detection. The limit of blank (LOB) was defined as the upper limit of the 95% confidence interval of the water sample. The limit of detection (LOD) was defined as the concentration with a 95% chance of having a signal above the LOB. The limit of quantification was defined as the lowest concentration where the coefficient of variation was below 20%.

Figure 2b:

Comparison of the measurements performed in the (a) small and (b) large phantoms on scanner 4 at 80 and 140 kVp with tin filtration. Note the lower standard deviation of the blank sample and the tighter distributions of the repetitions of the higher concentrations in the small phantom that lead to a calibration curve with a lower fitting error and, ultimately, a lower limit of detection. The limit of blank (LOB) was defined as the upper limit of the 95% confidence interval of the water sample. The limit of detection (LOD) was defined as the concentration with a 95% chance of having a signal above the LOB. The limit of quantification was defined as the lowest concentration where the coefficient of variation was below 20%.

Figure 3a:

Comparison of the scanner 3 calibration curves at 140 kVp for the (a) small and (b) large phantoms. Note the plateau in the measured iodine concentration below 0.25 mg/mL in the small and 0.50 mg/mL in the large phantom. The limit of blank (LOB) was defined as the upper limit of the 95% confidence interval of the water sample. The limit of detection (LOD) was defined as the concentration with a 95% chance of having a signal above the LOB. The limit of quantification was defined as the lowest concentration where the coefficient of variation was below 20%.

Figure 3b:

Comparison of the scanner 3 calibration curves at 140 kVp for the (a) small and (b) large phantoms. Note the plateau in the measured iodine concentration below 0.25 mg/mL in the small and 0.50 mg/mL in the large phantom. The limit of blank (LOB) was defined as the upper limit of the 95% confidence interval of the water sample. The limit of detection (LOD) was defined as the concentration with a 95% chance of having a signal above the LOB. The limit of quantification was defined as the lowest concentration where the coefficient of variation was below 20%.