Seungwan Lee

Development of virtual monochromatic imaging technique with spectral CT based on a photon-counting detector

With the advent of the coherent age the implementation of massive digital signal processors (DSP) co-integrated with high speed AD and DA converters became feasible allowing for the realization of huge flexibility of transponders. Today the implementation of variable transponders is mainly based on variable programming of DSP to support different modulation formats and symbol rates. Modulation formats with high flexibility are required such as pragmatic QAM formats and hybrid modulation formats. Furthermore, we report on an implementable probabilistically shaping technique allowing for adjusting the bitrate. We introduce fundamental characteristics of all modes and describe basic operation principles. The modifications of the operational modes are enabled simply by switching between different formats and symbol rates in the DSP to adjust the transponders spectral efficiency, the bitrate and the maximum transmission distance. A fine granularity in bitrate and in maximum transmission distance can be implemented especially by hybrid formats and by probabilistically shaped formats. Furthermore, latter allow for ~+25% increase of the maximum transmission distance due their operation close to the Shannon limit as a consequence of their 2D Gaussian like signal nature. If the flexibility and programmability of transponders is implemented, it can be utilized to support different strategies for the application. The variability in symbol rate is mainly translated into variability in bitrate and in bandwidth consumption. Contrary the variable spectral efficiency translates into a variation of the maximum transmission reach and of the bitrate. A co-adjustment of both options will lead to a superior flexibility of optical transponders to address all requirements from application, transponder and fiber infrastructure perspective.

Development of respiratory-correlated 4D digital tomosynthesis imaging technique for image-guided radiation therapy

Recently, image-guided radiation therapy (IGRT) with cone-beam computed tomography (CBCT) has been used to precisely identify the location of target lesion. However, the treatment accuracy for respiratory-sensitive regions is still low, and the imaging dose is also relatively high. These issues can be solved by using the respiratory-correlated 4D IGRT with digital tomosynthesis (DT). The purpose of this study was to develop the 4D DT imaging technique for the IGRT and compare image quality between the 3D DT and 4D DT. A DT model was based on a linear accelerator (LINAC) system. In order to simulate the motion of a lesion the sphere defined in a 3D phantom was moved with an irregular pattern. Projections were separately obtained through 3 phases, which were sorted according to the position of the sphere, for simulating the 4D DT imaging. We measured profile, normalized root-mean-square error (NRMSE), noise, contrast-to-noise ratio (CNR) and figure-of-merit (FOM). Noise of 4D DT images was averagely 0.99 times lower than 3D DT images. And, NRMSEs, CNRs, and FOMs of 4D DT images were averagely 1.03, 1.22, and 4.48 times higher than those of 3D DT images, respectively. The results showed that the 4D DT imaging technique accurately determined the position of a moving target and improved image quality compared to the 3D DT imaging technique. These benefits will enable the high-precision IGRT for respiratory-sensitive regions.

Feasibility of material decomposition using non-radioactive Xe for pulmonary function test in spectral x-ray system: a Monte Carlo simulation study

Due to various factors, the number of chronic obstructive pulmonary disease (COPD) patients continues to increase. In addition, the mortality from COPD is increasing because of the difficult in the early detection of COPD. Radiologic and respiratory examinations should be performed simultaneously for improving the diagnostic accuracy of COPD. But a conventional respiratory examination leads to diagnostic inaccuracy and decreases the reproducibility of examinations because there is air leakage between spirometry and mouth. Also, it is difficult to apply for all ages. In this study, we confirmed the possibility of material decomposition for pulmonary function test by combining dual-energy X-ray images obtained from a photon counting detector. Non-radioactive Xe, which appears in X-ray images, was also used. The RMSE of each material in decomposed images was analyzed to quantitatively evaluate of the possibility of material decomposition for pulmonary function test. Results showed that the average RMSE values of PMMA, lung and nonradioactive Xe were 0.005, 0.0199 and 0.0217, respectively, and we observed the high accuracy of material decomposition. Therefore, the diagnosis of COPD can be simplified through the material decomposition imaging using non-radiologic Xe, and the lung function can be evaluated by decomposing the total lung and actual gas exchange areas.

Contrast-enhanced spectral mammography based on a photon-counting detector: quantitative accuracy and radiation dose

Contrast-enhanced mammography has been used to demonstrate functional information about a breast tumor by injecting contrast agents. However, a conventional technique with a single exposure degrades the efficiency of tumor detection due to structure overlapping. Dual-energy techniques with energy-integrating detectors (EIDs) also cause an increase of radiation dose and an inaccuracy of material decomposition due to the limitations of EIDs. On the other hands, spectral mammography with photon-counting detectors (PCDs) is able to resolve the issues induced by the conventional technique and EIDs using their energy-discrimination capabilities. In this study, the contrast-enhanced spectral mammography based on a PCD was implemented by using a polychromatic dual-energy model, and the proposed technique was compared with the dual-energy technique with an EID in terms of quantitative accuracy and radiation dose. The results showed that the proposed technique improved the quantitative accuracy as well as reduced radiation dose comparing to the dual-energy technique with an EID. The quantitative accuracy of the contrast-enhanced spectral mammography based on a PCD was slightly improved as a function of radiation dose. Therefore, the contrast-enhanced spectral mammography based on a PCD is able to provide useful information for detecting breast tumors and improving diagnostic accuracy.

Reducing radiation dose by application of optimized low-energy x-ray filters to K-edge imaging with a photon counting detector.

K-edge imaging with photon counting x-ray detectors (PCXDs) can improve image quality compared with conventional energy integrating detectors. However, low-energy x-ray photons below the K-edge absorption energy of a target material do not contribute to image formation in the K-edge imaging and are likely to be completely absorbed by an object. In this study, we applied x-ray filters to the K-edge imaging with a PCXD based on cadmium zinc telluride for reducing radiation dose induced by low-energy x-ray photons. We used aluminum (Al) filters with different thicknesses as the low-energy x-ray filters and implemented the iodine K-edge imaging with an energy bin of 34-48 keV at the tube voltages of 50, 70 and 90 kVp. The effects of the low-energy x-ray filters on the K-edge imaging were investigated with respect to signal-difference-to-noise ratio (SDNR), entrance surface air kerma (ESAK) and figure of merit (FOM). The highest value of SDNR was observed in the K-edge imaging with a 2 mm Al filter, and the SDNR decreased as a function of the filter thicknesses. Compared to the K-edge imaging with a 2 mm Al filter, the ESAK was reduced by 66%, 48% and 39% in the K-edge imaging with a 12 mm Al filter for 50 kVp, 70 kVp and 90 kVp, respectively. The FOM values, which took into account the ESAK and SDNR, were maximized for 8, 6 to 8 and 4 mm Al filters at 50 kVp, 70 kVp and 90 kVp, respectively. We concluded that the use of an optimal low-energy filter thickness, which was determined by maximizing the FOM, could significantly reduce radiation dose while maintaining image quality in the K-edge imaging with the PCXD.

MO‐FG‐CAMPUS‐IeP1‐02: Dose Reduction in Contrast‐Enhanced Digital Mammography Using a Photon‐Counting Detector

PURPOSE Photon-counting detectors (PCDs) allow multi-energy X-ray imaging without additional exposures and spectral overlap. This capability results in the improvement of accuracy of material decomposition for dual-energy X-ray imaging and the reduction of radiation dose. In this study, the PCD-based contrast-enhanced dual-energy mammography (CEDM) was compared with the conventional CDEM in terms of radiation dose, image quality and accuracy of material decomposition. METHODS A dual-energy model was designed by using Beer-Lamberts law and rational inverse fitting function for decomposing materials from a polychromatic X-ray source. A cadmium zinc telluride (CZT)-based PCD, which has five energy thresholds, and iodine solutions included in a 3D half-cylindrical phantom, which composed of 50% glandular and 50% adipose tissues, were simulated by using a Monte Carlo simulation tool. The low- and high-energy images were obtained in accordance with the clinical exposure conditions for the conventional CDEM. Energy bins of 20-33 and 34-50 keV were defined from X-ray energy spectra simulated at 50 kVp with different dose levels for implementing the PCD-based CDEM. The dual-energy mammographic techniques were compared by means of absorbed dose, noise property and normalized root-mean-square error (NRMSE). RESULTS Comparing to the conventional CEDM, the iodine solutions were clearly decomposed for the PCD-based CEDM. Although the radiation dose for the PCD-based CDEM was lower than that for the conventional CEDM, the PCD-based CDEM improved the noise property and accuracy of decomposition images. CONCLUSION This study demonstrates that the PCD-based CDEM allows the quantitative material decomposition, and reduces radiation dose in comparison with the conventional CDEM. Therefore, the PCD-based CDEM is able to provide useful information for detecting breast tumor and enhancing diagnostic accuracy in mammography.