Demonstration of iodine K-edge imaging by use of an energy-discrimination X-ray computed tomography system with a cadmium telluride detector
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- Abudurexiti, A., Kameda, M., Sato, E. et al. Radiol Phys Technol (2010) 3: 127. doi:10.1007/s12194-010-0088-8
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An energy-discrimination K-edge X-ray computed tomography (CT) system is useful for increasing the contrast resolution of a target region by utilizing contrast media. The CT system has a cadmium telluride (CdTe) detector, and a projection curve is obtained by linear scanning with use of the CdTe detector in conjunction with an X-stage. An object is rotated by a rotation step angle with use of a turntable between the linear scans. Thus, CT is carried out by repetition of the linear scanning and the rotation of an object. Penetrating X-ray photons from the object are detected by the CdTe detector, and event signals of X-ray photons are produced with use of charge-sensitive and shaping amplifiers. Both the photon energy and the energy width are selected by use of a multi-channel analyzer, and the number of photons is counted by a counter card. For performing energy discrimination, a low-dose-rate X-ray generator for photon counting was developed; the maximum tube voltage and the minimum tube current were 110 kV and 1.0 μA, respectively. In energy-discrimination CT, the tube voltage and the current were 60 kV and 20.0 μA, respectively, and the X-ray intensity was 0.735 μGy/s at 1.0 m from the source and with a tube voltage of 60 kV. Demonstration of enhanced iodine K-edge X-ray CT was carried out by selection of photons with energies just beyond the iodine K-edge energy of 33.2 keV.
KeywordsX-ray CTCdTe detectorPhoton countingEnergy discriminationIodine K-edge CT
Monochromatic X-rays are very useful for carrying out energy-selective imaging, and various monochromatic X-ray generators have been developed corresponding to specific radiographic objectives. Currently, quasi-monochromatic K-series characteristic X-rays are selected by use of a K-edge monochromatic filter, and the average photon energy of the Kα rays is determined by the target element. Without using the K-edge filter, we have developed a K-ray generator [1, 2] utilizing the angular dependence of bremsstrahlung X-rays. The bremsstrahlung intensity decreases with increasing electron-accelerating (tube) voltage.
For performance of high-speed radiography with X-ray durations below 1 μs, several different flash X-ray generators have been developed [3–5]. In particular, linear plasma X-ray generators [6–8] produce extremely clean nickel and copper K-rays without using filters because bremsstrahlung rays are absorbed effectively by weakly ionized metal plasma. From a spherical plasma X-ray generator [9, 10], intense tungsten and tantalum K-rays are produced. Because these rays are absorbed effectively by gadolinium-based contrast media, high-speed gadolinium K-edge angiography has been carried out with X-ray durations of approximately 100 ns.
In conjunction with single silicon crystals, synchrotrons produce quite clean monochromatic parallel X-ray beams, and the photon energy is selected by use of Bragg’s angle. By use of X-ray photons with energies just beyond the iodine K-edge energy of 33.2 keV, enhanced iodine K-edge angiography [11–13] has been performed for observation of small blood vessels below 100 μm in diameter with high contrast. On the other hand, we have developed a steady-state cerium X-ray generator [14, 15] and have succeeded in performing cone-beam K-edge angiography by using cerium Kα rays with an average energy of 34.6 keV.
Monochromatic radiography is realizable by means of energy-discriminating imaging, and a reflection-type X-ray camera  utilizing a cadmium telluride (CdTe) detector has been developed for carrying out two-dimensional X-ray fluorescence analysis (XRF). Next, we developed a penetration-type, energy-discriminating X-ray camera , and iodine K-edge radiography has been performed. Recently, a 64-channel CdTe linear detector has been developed and applied to a material-discriminating X-ray computed tomography (CT) system . However, it is not easy to perform iodine K-edge CT because of low photon energy resolution. Therefore, the energy resolution should be improved so that molecular-level X-ray CT can be performed.
In our research, the major objectives were as follows: development of an energy-discriminating X-ray CT system with an energy resolution of 1.2 keV, decreases in the absorbed dose for patients by a decrease in the tube current, and performing enhanced K-edge imaging by using iodine contrast media. Therefore, we developed an energy-discrimination X-ray CT system utilizing a CdTe detector, which can be used for enhanced iodine K-edge CT.
Energy-discrimination X-ray CT system
X-ray CT system
Novel X-ray generator for photon counting
X-ray intensity and spectra
Figure 7 shows relation between the mass attenuation coefficients of iodine and X-ray spectra at selected energies; the coefficient curve is discontinuous at the iodine K-edge of 33.2 keV. Figure 8 shows X-ray spectra used for tomography; low-density iodine molecules can be detected for X-ray spectra with energies just beyond the K-edge. In energy-discrimination CT, X-ray photons with an energy range from 33.3 to 43.3 keV were selected by use of the MCA. These photons are absorbed effectively by iodine molecules, and iodine-based contrast media are observed with high contrast. X-ray photons with a range from 43.4 to 60.0 keV reduce the image contrast of iodine molecules. In the X-ray spectra, because two K-edges of cadmium and tellurium are observed, the sensitivity of the CdTe detector varies around the two edges. However, the energies of the two edges are below the iodine K-edge energy, and the decreases in the sensitivity beyond tellurium K-edge energy are negligible for carrying out iodine K-edge imaging.
For performing energy-discrimination CT, the absorbed dose for patients should be minimized by decreasing the tube voltage. Next, the effective photon count for K-edge imaging should be maximized, and X-ray spectra with a peak energy of approximately 30 keV will be useful for performing dual-energy subtraction with use of photons having energies around the K-edge energy of 33.2 keV. Therefore, the tube voltage was determined as 60 kV for carrying out enhanced K-edge CT by use of the CdTe detector.
We developed an energy-discriminating X-ray CT system that utilize a CdTe detector with an energy resolution of 1.2 keV to perform enhanced iodine K-edge CT by using photons with energies just beyond the K-edge energy of 33.2 keV. In this CT, the X-ray exposure time for obtaining one tomogram was 15 min for scan and for rotation steps of 1.0 mm and π/60 (rad), respectively, and the time increased with decreases in the two steps. However, the exposure time decreased with increases in both the scanning and angular velocities.
Energy-discrimination CT is realizable with use of the photon-counting CT in conjunction with an MCA for determining both the photon energy and the energy width. In the photon-counting X-ray CT, the image quality improves with increasing photon count per scan step. In our research, the photon counting time and the maximum count were constant, and their values were 0.2 s and 1.0 kilo-count (kc) per second, respectively. Thus, the photon count per measuring point is 0.2 kc; this value seems to be a lower limit for photon-counting imaging. On the other hand, the maximum count rate for energy discrimination is limited by the maximum rate of the MCA. A high-count-rate MCA for reducing exposure time is desirable for the future. In this CT system, the X-ray projection curve is obtained by use of one detector, and multi-slice CT is realizable with multiple detectors.
The spatial resolution improves with decreases in the scan step, the rotation step, and the diameter of the lead pinhole which is used for preventing pileups of the event signal by decreasing the photon count rate. In this CT, because a 0.7-mm-diameter pinhole was used, the diameter should be decreased corresponding to the scan step. In addition, the photon count per scan step should be increased to beyond 0.2 kc by increases in the tube current when a small-diameter pinhole is used.
To prevent signal pileups, the photon-counting X-ray generator is driven at a low tube-current range from 1.0 to 100 μA, and the X-ray flux for performing CT can be decreased easily at the photon-counting. Although iodine K-edge CT was carried out, gadolinium-based contrast media for magnetic resonance angiography  can easily be used for gadolinium K-edge CT with X-ray photons just beyond the gadolinium K-edge energy of 50.3 keV.
This work was supported by Grants-in-Aid for Scientific Research and Advanced Medical Scientific Research from MECSST, Health and Labor Sciences research grants, grants from the Keiryo Research Foundation, The Promotion and Mutual Aid Corporation for Private Schools of Japan, the Japan Science and Technology Agency, and the New Energy and Industrial Technology Development Organization.