Figure 1 shows an experimental setup for photon-counting radiography using a DE CdTe array detector (XCounter, Actaeon), and the radiographic conditions are shown in Table 1. The X-ray generator (Rtech, RXG-0120) has a 0.1-mm-diam-focus tube with a 0.5-mm-thick beryllium window and produces extremely low-energy photons. Therefore, a 1.0-mm-thick aluminum (Al) filter is used to absorb low-energy photons and to reduce the incident dose for objects. The distance between the X-ray source and the array detector is 1.00 m, and the object is attached to the detector to decrease the magnification ratio.

The tube current was regulated to the maximum value of 0.50 mA to increase the count rate for DE radiography. Using I-based contrast media, the tube voltage was 60 kV, and the threshold energies were 10 and 33 keV. On the other hand, the threshold energies were 30 and 50 keV using Gd media, and the tube voltage was 80 kV. The K-edge energies of I and Gd are 33.2 and 50.2 keV, respectively, and X-ray photons with energies just beyond the K-edge energies are absorbed effectively by the contrast media.

The maximum frame rate increases with increasing dose rate, and the frame rate for the DE radiography was determined as 100 frames per second (fps).

Because the averaging-frame number per image (radiogram) is set to 200 to improve the image quality, the exposure time for DE radiography is 2.0 s.

Various specifications are shown in Table 2. The dimensions of the CdTe array are 25 × 25 mm², and the detector is powered through an Ethernet port. The pixel dimensions are 0.1 × 0.1 mm², and each pixel has an individual charge-sensitive amplifier and a DE counter. In the photon-counting DE mode, the maximum frame rate is below 1 kfps. However, the maximum rate and the minimum capture time are 2.5 kfps and 50 μs, respectively, in the piled-up single mode.

The measurement of X-ray dose rate is important to calculate incident dose for patients. The X-ray dose rate from the X-ray generator was measured using an ionization chamber (Toyo Medic, RAMTEC 1000 plus) with the Al filtration at a tube current of 0.50 mA. The chamber was placed 1.0 m from the X-ray source, and we measured the dose rate with changes in the tube voltage from 50 to 100 kV.

To measure X-ray spectra with changes in the tube voltage, we used a readily available CdTe detector (Amptek, XR100-T) with a 0.5-mm-diam lead pinhole placed 1.0 m from the X-ray source. The event pulses from the shaping amplifier are input to a multichannel analyzer (MCA; γPGT, MCA4000) to perform pulse-height analysis. The photon energy was determined by two-point calibration using Kα₁ photon energies of tungsten (W; 59.3 keV) and I (28.6 keV).

In the DE-CT, we used real dog-heart and rabbit-head phantoms. These phantoms were made approximately twenty years ago, and the operation on animals was carried out in accordance with the animal experiment guidelines of our university.

Figure 2 shows the X-ray dose rate after penetrating the Al filter at a constant tube current of 0.50 mA. X-ray dose rate increased with increasing tube voltage and

was not proportional to the second power of the tube voltage. At tube voltages of 60 and 80 kV, the X-ray dose rates were 52.7 and 78.8 μGy/s, respectively.

The X-ray spectra measured using the CdTe detector are show in Figure 3. The maximum photon energy increased with increasing tube voltage from 60 to 80 kV. At a tube voltage of 80 kV, we observed W-K photons.

Figure 4 shows selected X-ray spectra for DE radiography using I media, and the X-ray photons with energies ranging from 10 to 60 keV are useless for carrying out I-K-edge radiography (Figure 4(a)). On the contrary, the X-ray photons at a range of 33 - 60 keV are useful for performing I-K-edge radiography (Figure 4(b)).

The selected X-ray spectra using Gd media are shown in Figure 5. Although

the X-ray photons beyond 30 keV are not so useful for imaging Gd media (Figure 5(a)), the photons beyond 50 keV are useful for performing Gd-K-edge radiography (Figure 5(b)).

Radiography was performed at a constant tube current of 0.50 mA. Radiograms are obtained as JPEG files, and the maximum and minimum gray-value densities are defined as 255 (white) and 0 (black), respectively.

Figure 6 shows radiography of two glass vials filled with two-different-density I media (iopamidol) of 15 and 30 mg/ml. The gray-value-density analysis using a program of Image J is shown in the same figure. At an energy range of 10 - 60 keV, it was easy to image glass walls, and the gray-value densities were almost the same. Utilizing I-K-edge radiography at a range of 33 - 60 keV, the density difference between the media increased, and the glass walls were not so visible.

Radiography (angiography) of a dog-heart phantom is shown in Figure 7, and the coronary arteries are filled with I-based microspheres of 15 μm in diameter. At a range of 10 - 60 keV, the muscle density was low, and the arteries were not visible. When the energy range was increased to 33 - 60 keV, the muscle density increased, and the arteries were observed at high contrasts.

Figure 8 shows radiography of two glass vials filled with Gd media (meglumine

gadopentetate) of two different densities of 15 and 30 mg/ml. At a range of 30 - 80 keV, the image density decreased slightly with increasing Gd density, and it was easy to image glass walls. Next, utilizing Gd-K-edge CT at a range of 50 - 80 keV, the density substantially decreased with increasing Gd density, and it was difficult to image the glass walls.

Radiography of a rabbit-head phantom is shown in Figure 9, and blood vessels are filled with gadolinium oxide (Gd₂O₃) microparticles of 1 - 10 μm in diameter. At a range of 30 - 80 keV, the muscle density was low, and it was difficult to image the vessels in the nose clearly. When the energy range was increased to 50 - 80 keV, the muscle density increased, and the vessels were observed at high contrasts.