To image blood vessels, we performed fundamental experiments of a red-ray computed tomography (RRCT) scanner using 650-nm-laser and high-sensitivity-photodiode (PD) modules. The line laser beam is irradiated to an object, and the photons penetrating through the object are detected using the PD module through a 1.0-mm-diameter graphite pinhole and a 0.7-mm-diameter 5-mm-length graphite collimator for the PD. The spatial resolutions were primarily determined by the collimator diameter for the PD and were approximately 0.7×0.7 mm2. RRCT was performed by repeating the reciprocating translations and rotations of the object, and the ray-sampling-translation and rotation steps were 0.1 mm and 0.5°, respectively. The image contrast was regulated using the digital amplifier, and the visible diameter of object was 0.5mm.
Photon-counting x-ray computed tomography (PCCT) is useful for selecting optimal energy photons to image various portions of the target object, and we performed fundamental experiments of PCCT to carry out gadolinium (Gd) K-edge CT using Gd-based contrast media. The scanner mainly consists of an x-ray generator with a 0.1-mm-focus tube, a turntable, a cadmium-telluride (CdTe) flat panel detector (FPD) with pixel dimensions of 100 Pm, and a personal computer. An object on the turntable is irradiated by the x-ray generator, 720 radiograms are taken using the FPD, and tomograms are reconstructed. We used 1.3-time magnification tomography, the effective pixel dimensions were approximately 80 Pm, and Gd-K-edge CT was carried out using Gd-based contrast media at a tube voltage of 100 kV, a tube current of 0.40 mA, and a threshold energy of 50 keV.
To perform energy-dispersive x-ray computed tomography (EDCT), we constructed a computer program to amplify the digital values of raw radiograms. The CT scanner consists of an x-ray generator with a 0.1-mm-focus tube, a turntable, a flat panel detector (FPD), and a personal computer (PC). An object on the turntable is irradiated by the x-ray generator, 1.3-magnified 720 radiograms are taken by the FPD, and tomograms are reconstructed using the PC. Utilizing the digital amplifier, the object projections obtained using low-energy photons disappeared with increasing amplification factor at a constant maximum value, and the effective energy increased according to increases in the amplification factor by beam hardening. Using the beam-hardening CT (BHCT) scanner, high-contrast tomography for various objects was performed by controlling effective energy. In particular, fine blood vessels were observed by K-edge CT using iodine media.
To realize novel photon-counting energy-dispersive X-ray computed tomography (CT), we have developed a low-dose CT scanner using a detector consisting of a cerium-doped yttrium aluminum perovskite [YAP(Ce)] crystal and a small photomultiplier tube (PMT). X-ray photons are absorbed by the YAP(Ce) crystal, and negative outputs are produced from the PMT. The PMT outputs are amplified by an inverse voltage-to-voltage amplifier, and the event pulses are counted by the counter with a low threshold energy of 20 keV. First, almost all the photons are counted without the photon-energy dependence, since the scintillation-photon number produced by one X-ray photon is proportional to the photon energy. Second, the energy-dispersive imaging is performed using the self-beam hardening by the object. The maximum photon count of the projection data is determined after the air absorption, and the effective photon energy increases with increasing digital-amplification factor at the constant maximum count. In the triple-energy CT, the X-ray beam diameter was 0.5 mm, and the spatial resolutions were approximately 0.3×0.3 mm2 . The exposure time for DE-CT was 9.8 min at a total rotation angle of 180°.
The linear-plasma flash X-ray generator consists of a high-voltage power supply, a 200-nF high-voltage condenser, a turbomolecular pump, and a demountable flash X-ray tube. In the flash X-ray generator, the condenser is charged up to 50 kV by the power supply, and flash X-rays are then produced by the vacuum discharging. The X-ray tube is a demountable triode with a rod-shaped nickel (Ni) target and a Ni reflector. The Ni-target evaporation leads to the formation of weakly ionized linear plasma, consisting of Ni ions and electrons, around the target. In the plasma, K-series characteristic X-ray photons (K photons) are produced, and bremsstrahlung photons with energies beyond the Ni-K-edge energy are absorbed by the plasma and converted into Ni-K photons. Subsequently, Ni-K photons from the Ni reflector easily penetrate the linear Ni plasma. Thus, intense Ni-K photons (rays) are irradiated from the plasma axial direction by K-ray amplification by spontaneous emission of radiation (KASER).
To realize novel energy-dispersive X-ray computed tomography (CT) and to reduce the incident dose for the object, we have developed a low-dose low-scattering CT scanner with high spatial resolutions using a room-temperature cadmium telluride (CdTe) detector. X-ray photons are absorbed by the CdTe crystal, and the electric charges flowing through the CdTe crystal are amplified using a current-to-voltage and voltage-to-voltage amplifiers. The first-generation CT is accomplished by repeated translations by the detector and rotations of the object, and the effective photon energy increases with increasing amplification factor of the digital amplifier at a constant maximum output voltage of 5.0 V. The tripleenergy computed tomography (TE-CT) is performed utilizing the beam hardening by the object. In the TE-CT, the scattering photon count is reduced using a 0.5-mm-diam pinhole behind the object, and the spatial resolution is improved by a 0.25-mm-diam pinhole. The exposure time for TE-CT was 9.8 min at a total rotation angle of 180°.
Human body mainly consists of muscle, bone and air, and we name penetrating photons the human-body-window (HBW) rays. The HBW spectra were measured using a white power light-emitting diode (LED) and a spectrometer. The photons from the LED penetrate the human body, and reflect, refract and scatter. Therefore, we measured only penetrating HBW spectra from the human body. In the computed tomography (CT), we used a 1.0-mm-diam graphite collimator, two 1.0- mm-diam copper pinholes, and a 1.0-mm-diam aluminum pinhole. The white beam diameter is reduced using the collimator and the first copper pinhole, and the 1.0-mm-diam beam is irradiated to the object. The penetrating HBW photons are selected out using the second copper pinhole behind the object and detected by the photodiode through the aluminum pinhole. HBW-CT is accomplished by repeated translations and rotations of the object. The peak wavelength of the HBW spectra was 610 nm. The translation and rotation steps were 0.25 mm and 1.0º, respectively, and the spatial resolutions were determined as 1.0×1.0 mm2. The scanning time and a total rotation angle for CT were 9.8 min and 180º, respectively.
To improve the spatial resolution in near-infrared-ray computed tomography (NIR-CT), a first-generation scanner in the first-living-body window was constructed. The NIR photons are produced from an 850 nm laser module, and penetrating photons from an object are detected using a photodiode (PD). To improve the spatial resolution, we used a 1.0-mm-diam graphite pinhole and a 1.0-mm-diam 5.0-mm-length graphite collimator. To detect the penetrating photons, the pinhole is set behind the object, and the collimator is attached to the PD to improve the spatial resolution. The NIR-CT is accomplished by repeated translations and rotations of the object. The translation is performed by the object moving between the laser and the PD modules. The translation and rotation steps were 0.25 mm and 1.0°, respectively, and the spatial resolutions were determined as 1.0×1.0 mm2. The scanning time and the total rotation angle for CT were 9.8 min and 180°, respectively.
To obtain two kinds of tomograms at two different X-ray energy ranges simultaneously, we have constructed a dualenergy (DE) X-ray photon counter with a room-temperature cadmium telluride (CdTe) detector. X-ray photons are detected using the CdTe detector system, and event pulses from an amplifier module are sent to three comparators simultaneously to determine three threshold energies of 33, 48 and 50 keV. The DE counter has energy-range and - region selectors, and the energy range and region are 33-48 and beyond 50 keV (50-100 keV); the maximum energy corresponds to the tube voltage. We performed DE computed tomography (DE-CT) using four lead pinholes at a tube voltage of 100 kV. In Gd-K-edge CT at a range of 50-100 keV, Gd media were observed at high contrasts. The spatial resolutions were 0.5×0.5 mm2, and the exposure time for DE-CT was 19.6 min at a total rotation angle of 360°. At a tube voltage of 100 kV and a current of 0.22 mA, the count rate was 36 kilocounts per second.
To perform low-dose low-scattering X-ray computed tomography (CT), we have constructed a dual-energy (DE) X-ray photon counter with a high-count-rate detector system and energy-range and -region selectors. The detector system consists of a cerium-doped yttrium aluminum perovskite [YAP(Ce)] crystal, a small photomultiplier tube (PMT), and an inverse amplifier for the PMT with a pulse-width extender. In DE-CT, both the X-ray source and the detector module are fixed, and the object on the turntable oscillates on the translation stage. A line beam for DE-CT is formed using a two lead (Pb) pinholes in front of the object. The scattering-photon count from the object is reduced using a Pb pinhole behind the object. To improve the spatial resolution, a 0.5-mm-diam Pb pinhole is attached to the YAP(Ce)-PMT detector. X-ray photons are detected using the detector system, and the event pulses are input to the two energy selectors. In DE-CT, the tube voltage and the maximum current were 100 kV and 0.60 mA, respectively. The energy range and region for soft and gadolinium-K-edge CT are 20-40 and beyond 50 keV (50-100 keV), respectively. The maximum count rate of DE-CT was 84 kilocounts per second, and the exposure time for tomography was 19.6 min at a total rotation angle of 360°.
To obtain three kinds of tomograms at three different X-ray energy ranges simultaneously, we have constructed a triple-energy (TE) X-ray photon counter with a cooled cadmium telluride (CdTe) detector and three sets of comparators and microcomputers. X-ray photons are detected using the CdTe detector, and the event pulses produced using amplifiers are sent to three comparators simultaneously to regulate three threshold energies of 15, 33 and 50 keV. Using this counter, the energy ranges are 15-33, 33-50 and 50-100 keV; the maximum energy corresponds to the tube voltage. We performed TE computed tomography (TE-CT) at a tube voltage of 100 kV. Using four lead pinholes, three tomograms were obtained simultaneously. Iodine-K-edge CT was carried out utilizing an energy range of 33-50 keV. At a tube voltage of 100 kV and a current of 0.11 mA, the count rate was 21 kilocounts per second (kcps).
In the near-infrared-ray computed tomography (NIR-CT) scanner, NIR rays are produced from a light-emitting diode (LED) and detected using a phototransistor (PT) and an infrared filter. The LED-peak wavelength is 850 nm, and 850- nm-peak NIRs are detected using the filtrated PD. The photocurrents flowing through the PT are converted into voltages using an emitter-follower circuit, and the output voltages are sent to a personal computer through an analog-digital converter. The NIR projection curves for tomography are obtained by repeated translations and rotations of the object, and the translating is conducted in both directions of its movement. The 850-nm NIRs easily penetrated living bodies, and the NIR-CT was performed with changes in the sensitivity at relative sensitivities of 1 and 21.
To measure X-ray spectra with high count rates, we developed a detector consisting of a cerium-doped yttrium aluminum perovskite [YAP(Ce)] crystal and a recent multipixel photon counter (MPPC). Scintillation photons are detected using the MPPC, and the photocurrents flowing through the MPPC are converted into voltages and amplified using a high-speed current-voltage (I-V) amplifier. The MPPC bias voltage was set to a value at the pre-Geiger mode to perform zero-dark counting. The event-pulse widths were approximately 200 ns, and the widths were extend to approximately 1 μs. X-ray spectra were measured using a multichannel analyzer (MCA) for pulse-height analysis. The photon energy was roughly determined by the two-point calibration using tungsten K photons and iodine K fluorescence. Using the YAP(Ce)-MPPC detector, first-generation dual-energy computed tomography was accomplished using iodine and gadolinium contrast media.
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