Journal of X-Ray Science and Technology - Volume 14, issue 3

Authors: Zhang, Li | Tang, Jie | Xing, Yuxiang | Cheng, Jianping

Article Type: Research Article

Abstract: The resolution of practical CT systems depends on many factors. Reconstruction algorithm is one of the most important factors. In this paper, we investigate the resolution of the standard FDK algorithm, present a formula to analytically compute the in-slice point-spread function in FDK reconstructions, and validate the analytic PSF by numerical simulations. The experimental results show that this process can predict the in-slice point spread function in FDK reconstructions quite well.

Keywords: Computed tomography (CT), FDK algorithm, analytic resolution

Citation: Journal of X-Ray Science and Technology, vol. 14, no. 3, pp. 151-159, 2006

Authors: Samant, Sanjiv S. | Gopal, Arun | Sobczak, David

Article Type: Research Article

Abstract: Current video-based electronic portal imaging devices (VEPIDs) that utilize thin phosphor screens have poor image quality due to low light collection. Expensive flat-panel imagers with the same screen yield higher quality imaging at 2–4 monitor-units (MU) exposures, but are susceptible to radiation damage to the peripheral electronics. A prototype thick scintillation crystal (TSC) VEPID for subMU exposures is presented here. Utilizing a 12 mm thick CsI(Tl) crystal scintillator for greater light production, the detective …quantum efficiency (DQE) is significantly increased for the prototype TSC (17 × 17 cm^{2} field-of-view at isocenter) while spatial resolution is not affected by the increased optical path. A high-resolution plumbicon camera with 12-bit ADC and computer-controlled target integration provides variable exposure imaging. Modulation transfer function (MTF) and DQE measurements along with phantom imaging are presented for the TSC, commercial VEPIDs, flat-panels and film. The DQE for TSC is significantly higher than current VEPIDs, and comparable to flat-panel at low frequency. TSC provides portal imaging superior to other VEPIDs at subMU exposures. The TSC provides high quality portal imaging and with further optimization is expected to be better or comparable to flat-panels. TSC subMU imaging has potential for intra-treatment patient position monitoring, and increased portal verification frequency for patient localization. Show more

Keywords: Portal imaging, high quantum efficiency, electronic portal imaging device (EPID)

Citation: Journal of X-Ray Science and Technology, vol. 14, no. 3, pp. 161-175, 2006

Authors: Tang, Shaojie | Yu, Hengyong | Yan, Hao | Bharkhada, Deepak | Mou, Xuanqin

Article Type: Research Article

Abstract: In the X-ray CT research area, the linear integral projections of mathematical phantoms were extensively used to evaluate the proposed algorithms with a hypothesis of monochromatic model. However, X-ray tube emits a spectrum of energies and hence it becomes difficult to study some important aspects, such as the reconstructed density accuracy of special reconstruction algorithm and beam hardening effects of polychromatic imaging. If the energy spectrum of X-ray tube, geometry of phantom, and material compositions of …all sub-regions are known, the actual projections can be simulated with more accuracy. Motivated by this fact, we propose an improved three-step discrete scheme to simulate the X-ray projections based on physical model. First, CT-numbers of all sub-regions of a phantom are determined in advance. Next, CT-numbers are decomposed into different material compositions under the constraint that sub-regions correspond to different human tissues. Finally, the projections are measured according to the energy spectrum distribution of X-ray tube, using the knowledge of X-ray imaging physics and our strategy to simulate the projecting process. Our scheme requires significantly less storage and computation to achieve a higher accuracy and precision. To demonstrate the feasibility of our scheme, we use the well-known FORBILD head phantom to generate projections and the Feldkamp algorithm for image reconstruction. The anticipated beam hardening effects are clearly seen in images reconstructed from projections simulated using our scheme. This scheme can be utilized in many medical X-ray imaging research aspects, such as algorithm design, performance analysis, distortion calibrations, artifact reduction, etc. Show more

Keywords: X-ray projection, physical imaging model, energy spectrum, material composition, FORBILD phantom, Feldkamp algorithm

Citation: Journal of X-Ray Science and Technology, vol. 14, no. 3, pp. 177-189, 2006

Authors: Zhao, Jun | Jiang, Ming | Zhuang, Tiange | Wang, Ge

Article Type: Research Article

Abstract: For dynamic CT reconstruction, we have conceptualized a triple-source helical cone-beam scanning mode, and formulated a backprojected-filtration (BPF) algorithm for exact image reconstruction. The algorithm uses data from the three inter-helix PI-arcs related to the inter-helix PI-lines and the minimum detection windows defined for the triple-source configuration. The proof of the formula is based on Tuy's inversion formula and the geometric relations of inter-helix PI-lines. Simulation results demonstrate the validity of the …reconstruction algorithm. The BPF algorithm is also extended for the (2N+1)-source helical cone-beam CT. Show more

Keywords: Computed tomography (CT), cone-beam, triple-sources, helical scanning, filtered-backprojection (FBP), backprojected-filtration (BPF)

Citation: Journal of X-Ray Science and Technology, vol. 14, no. 3, pp. 191-206, 2006

Authors: Chen, Wei R. | Bartels, Kenneth E. | Liu, Hong | Nordquist, Robert E.

Article Type: Research Article

Abstract: The impact of laser photothermal interactions on tissue damage and recovery was studied. A Ho:YAG laser (2.12 μm) was used to irradiate rabbit skin with different operating parameters. The thermal effect on the skin tissue was examined by measurement of laser-induced lesions and by tissue histology. The lesion size on the surface, the photocoagulation depth and the thermal damage depth were recorded as functions of irradiation parameters. The lesion width and the photocoagulation depth increased with …the beam energy while the area of thermal damage remained relatively unchanged. At the fluence levels of 453 J/cm^{2} and 680 J/cm^{2} , laser energy damaged the dermis all the way to the panniculus carnosa, resulting in total dermal collagen photocoagulation, but with little damage to the muscle. Post-treatment histological observation showed a rapid tissue healing and skin regeneration process. The thermal damage on rabbit skin began recovery within a short period and new epithelial growth was clearly seen at day 3. By 7 days, most of the photocoagulated zone was separated from the main body and was replaced by a new layer of epithelium. Show more

Keywords: Laser-photothermal interaction, holmium laser, tissue damage, recovery of laser injury

Citation: Journal of X-Ray Science and Technology, vol. 14, no. 3, pp. 207-215, 2006