Kichirou Koshida

Development of a non-linear weighted hybrid cone-beam CT reconstruction for circular trajectories.

We investigated an image reconstruction algorithm to reduce cone-beam artifacts in cone-beam CT. Our new algorithm to reduce such artifacts features: (1) a change in weighting with respect to projection data obtained at different projection angles; (2) distribution of correction coefficients so that they are larger near the center of the detector, while taking individual channel data for the detector into account, and smaller near the edges; (3) three-dimensional back-projection of corrected projection data. These findings confirmed that this algorithm reduces cone-beam artifacts and generates high-quality reconstruction images.

An Experimental Comparison of Flat-Panel Detector Performance for Direct and Indirect Systems (Initial Experiences and Physical Evaluation)

The purpose of this work was to compare direct and indirect detectors in terms of their system linearity, presampled modulation transfer function (MTF), Wiener spectrum (WS), noise equivalent quanta (NEQ), and power spectrum. Measurements were made on two flat-panel detectors, GE Revolution XR/d (indirect) and Shimadzu Safire (direct) radiographic techniques. The system linearity of the systems was measured using a time-scale method. The MTF of the systems was measured using an edge method. The WS of the systems was determined for a variable range of exposure levels by two-dimensional Fourier analysis. The NEQ was assessed from the measured MTF, WS, and estimated ideal signal-to-noise ratios. Power spectrum analyzed the chest phantom within artificial lesions. System linearity was excellent for the direct systems. For the direct system, the MTF was found to be significantly higher than that for the indirect systems. For the direct system, the WS was relatively uniform across all frequencies. In comparison, the indirect system exhibited a drop in the WS at high frequencies. At lower frequencies, the NEQ for the indirect system was noticeably higher than for the direct system. Power spectrum for the direct system was relatively flat and similar to that for white noise. The indirect system exhibited significant reduction at high spatial frequencies. In general, the direct systems exhibit improved image quality over indirect systems at comparable exposure dose.

Evaluation of transmission data of diagnostic X rays through concrete using Monte Carlo simulation

The transmittance of diagnostic X-ray beams through concrete of two different densities were measured. Broad-beam X-ray transmissions were estimated using a Monte Carlo simulation at tube voltages of 50-150 kV and compared with National Council on Radiation Protection and Measurements (NCRP) data. The observed levels were higher than those predicted by the NCRP data; that is, the NCRP data underestimate the leakage radiation from widely used inverter systems. Some of the issues were resolved and new data were proposed.

CT-AEC を用いた低管電圧撮影の被ばくに関する検討

The purpose of our study was to investigate radiation dose for lower tube voltage CT using automatic exposure control (AEC). An acrylic body phantom was used, and volume CT dose indices (CTDIvol) for tube voltages of 80, 100, 120, and 135 kV were investigated with combination of AEC. Average absorbed dose in the abdomen for 100 and 120 kV were also measured using thermoluminescence dosimeters. In addition, we examined noise characteristics under the same absorbed doses. As a result, the exposure dose was not decreased even when the tube voltage was lowered, and the organ absorbed dose value became approximately 30% high. And the noise was increased under the radiographic condition to be an equal absorbed dose. Therefore, radiation dose increases when AEC is used for lower tube voltage CT under the same standard deviation (SD) setting with 120 kV, and the optimization of SD setting is crucial.

SU‐E‐I‐49: Influence of the Phantom Length in Computed Tomography Dose Profile Dosimetry

PURPOSE We measured the computed tomography (CT) dose profile with CT dose phantom for several X-ray beam widths on 64-slice, and 320-slice multi detector-row CT (MDCT) using micro ionization chamber, and assessed the association of the CTDI100 value with CT dose phantom length. METHODS The CT dose profile (CTDP) was measured as Dm using radiation monitor (Model 9015, Radcal, Monrovia, CA) with micro ionization chamber (10X5-0.18, Radcal, Monrovia, CA) which have an active length of 19 mm and a 0.18 cm3 active area on 64-slice MDCT (Aquilion CX, Toshiba Medical Systems, Tochigi, Japan), and 320-slice MDCT (Aquilion One, Toshiba Medical Systems, Tochigi, Japan). The MDCT systems were used with a tube voltage of 120 kV, tube current of 100 mA, rotation time of 1 second, and nominal X-ray beam widths of 4, 12, 32, and 160 mm (4*1 mm, 4*3 mm, 64*0.5 mm, and 320*0.5 mm axial slice acquisitions, respectively). A micro ionization chamber was placed in the center of the single CT dose phantom, and we acquired several axial scans as 5 mm increments of phantom length. Then two, and three coupled dose phantom were placed together to simulate a 300-mm-long, and 450-mm-long phantom. We acquired several axial scans in similar method. RESULTS The observed percent increase of the Dm with extended length from 150 mm to 300 mm of the CT dose phantom was 120 % at range from -60 to 60 mm. However, it was greatly increased toward peripheral area of the DP. With extended length from 300 mm to 450 mm of CT dose phantom, the observed percent increase of the Dm was slightly increased. There was no difference in the form of the CTDP between 300-mm-long phantom and 450-mm-long phantom. CONCLUSION In upward of the 300-mm-long CT dose phantom, there is no difference in the form of the CTDP.

Development of a new three-dimensional image reconstruction algorithm to reduce cone-beam artifacts

We investigated an image reconstruction algorithm to reduce cone-beam artifacts in cone-beam CT. To examine the factors involved in the occurrence of cone-beam artifacts, micro-spheres phantom were arranged longitudinally at different positions and a computer simulation was performed. Due to differences in projection angle, data projected onto the detector surface were projected along trajectories shown as different periodic functions depending on the distance and position from the center of rotation. Therefore, projection along several detector channels based on different projection data resulting from different periodic functions is considered responsible for the increase in cone-beam artifacts associated with an increase in the distance of reconstruction planes from the center of rotation. Our new algorithm to reduce such artifacts features: 1) A change in weighting with respect to projection data obtained at different projection angles. 2) Distribution of correction coefficients so that they are larger near the center of the detector, while taking individual channel data for the detector into account, and smaller near the edges. 3) Three-dimensional back-projection of corrected projection data. The effect of the reduction in cone-beam artifacts of an object located at the edges markedly enhanced reconstruction planes at positions further from the center of rotation.

SU‐D‐217BCD‐02: Online Angular Tube Current Modulation in X‐Ray Computed Tomography: Can Tube Current Be Modulated Appropriately?

PURPOSE In X-ray computed tomography (CT), X-rays are significantly less attenuated in the anteroposterior direction and more in the lateral direction. Therefore, the tube current should be adjusted within one gantry rotation using angular tube current modulation (TCM). The aim of this study was to evaluate whether online angular TCM could reduce radiation dose appropriately. METHODS A 128-detector dual-source CT (SOMATOM Definition Flash; Siemens Healthcare, Erlangen, Germany) and an online TCM system (CARE Dose 4D; Siemens Healthcare) were used. Dose profiles were acquired using the CT Dose Profiler (RTI Electronics, Molndal, Sweden) and an elliptical cylindrical phantom (MHT; Kyoto Kagaku, Kyoto, Japan) for helical CT scans with and without TCM. In addition, absorbed dose distributions within a single section were acquired using an anthropomorphic phantom (RANI 10; The Phantom Laboratory, Salem, NY) and radiophotoluminescent glass dosimeters (RPLDs) (GD- 302M; Chiyoda Technol, Tokyo, Japan) for helical CT scans with and without TCM after placing RPLDs within all holes of one section and pasting them around the section. A graph of each absorbed dose distribution was drawn using graphing software (ORIGIN 8.6; OriginLab, Northampton, MA). RESULTS The acquired dose profiles suggested that online angular TCM could adjust the tube current in near-real time according to the attenuation measured from the previous projection. The profiles gradually stabilized because the tube current was adjusted properly. The absorbed doses were low and the distributions were stable with TCM compared with those without TCM. CONCLUSIONS In X-ray CT, an online angular TCM can reduce radiation dose effectively by adjusting tube current within one gantry rotation in near-real time.

Coherent scatter tomography using a sliding detector system

The distribution of coherent scatter is useful for determining the structure of a material, hence computed tomography applying coherent scatter has been developed by several authors. To obtain the exact distribution of coherent scatter a monochromatic, high flux, and highly parallel X-ray beam is required, and therefore, this technique is suited for a synchrotron radiation source. If a synchrotron radiation source is used, rotating the source around the patient or even rotating the patient is difficult. We propose a method for the estimation of coherent scatter distributions from any point, by moving only the position of the detector along the beam path. We acquired projection data at different positions along the beam path, and applied the maximum likelihood expectation maximization algorithm. Simulations and experiments were performed to confirm the effectiveness of this method. The estimated scatter profiles were in approximate agreement with that of the original data. Although improvement in accuracy of the estimation is necessary, this new method is useful if the X-ray source cannot be rotated around the patient.