Fig 1.
(a) Image of the prototype photon-counting CT system. (b) Illustration of ROIs used for uniformity measurement using the cylindrical water phantom with 150 mm diameter; uniformity value (the absolute value of [center (1) mean CT#-periphery (2–5) mean CT#]). 140 kVp, 1 mA, slice thickness: 0.625 mm, display window: W/L = 400/0 HU.
Fig 2.
(a) Three different cylindrical phantoms (diameter: 12, 17, and 22 cm) for iodine quantification accuracy. (b) Low-energy threshold (bin 1: 30–50 keV), middle-energy threshold (bin 2: 50–65 keV), and high-energy threshold (bin 3: 65–140 keV) images of the 12 cm phantom. (c) Iodine images and PMMA images. (d) Comparison of the measured iodine concentration with the true concentration.
Fig 3.
(a) Cylindrical PMMA phantom with 150 mm diameter and (b) transaxial image at a single-channel image for the 30–140 keV energy level. The phantom was specifically designed for basis material calibration and contained drilled holes filled with iodine (I) and gadolinium (Gd) at several concentrations: iodine (I-1: 3.7 mg/mL, I-2: 10.1 mg/mL, I-3: 18.9 mg/mL) and gadolinium (Gd-1: 3.0 mg/mL, Gd-2: 7.5 mg/mL, Gd-3: 13.8 mg/mL).
Fig 4.
(a) Gammex phantom with iodine and calcium at various concentrations: Iodine (I-1: 2.0 mg/mL, I-2: 5.0. mg/mL, I-3: 10 mg/mL, I-4: 15 mg/mL) and calcium (Ca-1: 300 mg/mL, Ca-2: 100 mg/mL, Ca-3: 50 mg/mL). (b) The single channel image for the 30 to 140 keV energy level (display window: W/L = 1000/0 HU): Iodine (I-1: 51 HU, I-2: 128 HU, I-3: 268 HU, I-4: 402 HU) and calcium (Ca-1: 899 HU, Ca-2: 333 HU, Ca-3: 195 HU). Calcium and iodine are difficult to distinguish. (c) An overlaid image that clearly separates iodine and calcium using images of the three energy levels (bin 1, bin 2, and bin 3) (pink: iodine, blue: calcium).
Fig 5.
(a) Adult male ATOM head phantom (ATOM phantom) that artificially generates metal artifacts and targets iodine-enhanced oral tumor. (b) Two metal implants and iodine were inserted into the ATOM phantom. (c) Image for the 30 to 140 keV energy level. We placed two metallic implants, one on the left, and the other on the right region of the lower jawbone (red arrow); iodine-enhanced tissue (yellow arrow) is located between them. Display window: W/L = 600/150 HU.
Fig 6.
Scheme of metal artifact reduction and tissue characterization image for multi-energy CT.
The NMAR method and MD are applied to the bin 1, bin 2, and bin 3 reconstructed images of different energy levels and created VMIs.
Fig 7.
Multi-energy bin 1, bin 2, and bin 3 images acquired from the PCD CT system.
VMI was created using three acquired images. (a) The higher the energy from bin 1 to bin 3, the smaller the metal artifact (red arrow). Compared with bin 2 and bin 3, bin 1 (yellow arrow) shows enhanced iodine in the oral tumor portion. (b) VMI from 40 keV to 140 keV created from three energy bin images reconstructed from an iodine-enhanced ATOM phantom with a left oral tumor (red arrow). VMIs of the same scan/slice are shown at 40, 60, 80, 100, 120, and 140 keV. Display window: W/L = 600/150 HU.
Fig 8.
(a) Imaging of adult male ATOM phantom acquired at an energy level of 30 to 140 keV. White dotted lines outline the ROIs placed for objective image analysis: ROI1, area of iodine-enhanced tissue; ROI2s, normal tissue (area where metal artifact is less affected), and ROI3s surrounding air for measuring background noise. (b) Image with NMAR method applied to image A. (c) Virtual monochromatic image of 40 keV with MD applied to bin 1, bin 2, and bin 3 without NMAR. (d) Iodine-enhanced virtual monochromatic image (yellow arrow) at 40 keV, which applied NMAR and MD to bin 1, bin 2, and bin 3. The imaging shows how metal artifacts were removed and iodine was enhanced. (a´), (b´), (c´), and (d´) are magnified images of (a), (b), (c), and (d) focusing on the iodine-enhanced tissue. Display window: W/L = 600/150 HU.
Fig 9.
The energy spectrum of the CdTe detector with FWHM of 4.51% at 122.06 keV.
Fig 10.
Count rate performance of the CdTe detector.
The output count rates were obtained by averaging the 25 detector pixels. The count rate was recorded as a function of input count rate (5, 15, 25, 55, 73, and 91 mA) for 120 kVp tube voltage, and 1 s data acquisition time.
Fig 11.
Stability of mean pixel (25 pixels) counts per frame in 20 s for varying tube currents at 120 kVp tube voltage.
Mean pixel counts and SD were 90 ± 1.0 (2 mA), 217 ± 1.4 (5 mA), 411 ± 1.9 (10 mA), 583 ± 2.1 (15 mA), and 874 ± 2.5(25 mA), respectively.
Fig 12.
Contrast-to-noise ratio (mean ± standard deviation) for quantitative image quality of the multi-energy CT imaging in Fig 8.
Table 1.
Detailed values (HU) of mean ± SD and CNR at each ROI.
These are used for quantitative evaluation of image quality of the multi-energy CT imaging shown in Fig 8.
Fig 13.
(a) Hybrid chip image using Co-57 (3.7 MBq) source. (b) Energy peak according to temperature. As the temperature rises, it shifts to the left.
Fig 14.
Detector–response curves of two different detector pixels.
This non-linear behavior requires a sophisticated non-linearity correction, unlike the conventional energy-integrating detector.