MTF and NPS of single-shot dual-energy sandwich detectors

Transcription

1 MTF and NPS of single-shot dual-energy sandwich detectors Junwoo Kim, a Dong Woon Kim, a Hanbean Youn, b,c Ho Kyung Kim a,c a School of Mechanical Engineering, Pusan National University, Busan , South Korea b Department of Radiation Oncology, Pusan National University Yangsan Hospital, Yangsan, Kyungsangnam-do , South Korea c Center for Advanced Medical Engineering, Pusan National University, Busan , South Korea ABSTRACT Single-shot dual-energy x-ray imaging with a sandwich detector can improve lesion conspicuity without motion artifacts to which the conventional dual-shot method is susceptible. However, the sandwich detector should be optimized with respect to, for example, each detector layer thickness and intermediate filter thickness for the reliable use in specific applications. The Fourier metrics, such as the modulation-transfer function (MTF) and the noise-power spectrum (NPS), hence the detective quantum efficiency or the number of noise-equivalent quanta, can be utilized for the optimization work for given detector design parameters. The actual meaning of the MTF and NPS of the sandwich detector is however unclear, and their properties for various imaging conditions and design parameters are still being questioned. In this study, we experimentally present the singleshot dual-energy MTF and NPS characteristics. From the preliminary MTF results, the sandwich detector can emphasize the contrast transfer efficiency at an intermediate band in spatial frequencies, and the degree of emphasis is dependent upon the weighting factor used for image subtraction. This characteristic may explain the unsharp-masking effect in single-shot dual-energy images. To understand the single-shot dual-energy MTF and NPS characteristics, we design quantitative numerical simulation experiments based on the cascaded-systems analysis approach. The detailed results of the Fourier metrics of single-shot dual-energy sandwich detectors and analyses on them will be presented. Keywords: Single-shot dual-energy imaging, sandwich detector, MTF, NPS, DQE, unsharp masking Summary The actual meaning of the modulation-transfer function (MTF) and the noise-power spectrum (NPS) of the sandwich detector is ambiguous, and their properties for various detector design parameters are also being questioned. In this study, the authors measure the single-shot dual-energy MTF and NPS characteristics. The preliminary MTF results show that the sandwich detector can emphasize the contrast transfer efficiency at an intermediate band in spatial frequencies. This characteristic may explain the unsharp-masking effect in single-shot dual-energy images. The authors design quantitative numerical simulation experiments based on the cascadedsystems analysis approach to analyze the measurement results. Topic areas: METR/OSY/DIAG 0.1 Background Supplement As shown in Fig. 1, we previously developed the sandwich detector by stacking two scintillator-based flat-panel detectors and successfully demonstrated its dual-energy imaging capability. 1, 2 It is interesting to note that the single-shot method shows a sharper image than the dual-shot method (compare the bone images in Fig. 1). The reason is probably due to the different spatial resolving powers between the front and rear detectors and the subtraction operation for the two images obtained from the two detectors, as similar to the unsharp masking in digital image processing. hokyung@pusan.ac.kr; phone ; fax

2 Figure 1. Single-shot dual-energy images obtained from the sandwich detector. For comparison, double-shot dual-energy images are included. For the reliable use of the sandwich detector for specific imaging applications, and ultimately for the preclinical or clinical use, the detector should be optimally designed considering the scintillator material and thickness in each detector layer, the intermediate filter material and thickness, and so on. The Fourier metrics, such as the modulation-transfer function (MTF) and the noise-power spectrum (NPS), hence the detective quantum efficiency (DQE) or the number of noise-equivalent quanta combined with specific imaging task functions, can be utilized for the optimization work for given detector design parameters. 3 Richard and Siewerdsen derived the DQE of sandwich detectors as follows: 4 DQE DE (u) = MTF2 DE(u) q DE W DE (1) (u), and each term in Eq. 1 may be defined as: τ q DE = q w 2 τ + 1, (2) [ w 2 τmtf 2 F (u) + MTF 2 ] 1/2 MTF DE (u) = R(u) w 2, (3) τ + 1 and W DE(u) = w 2 W F (u) + W R(u), (4) which describes normalized NPS. τ and w respectively represent the fluence transmittance through the front detector including the filter layer and the weighting factor for image subtraction. The subscripts F and R denote the front and rear detectors, respectively. Because τ and/or w scale the MTF and NPS of the front detector over the entire spatial frequency range, the resultant sandwich detector MTF and NPS will be placed between the front and rear detector MTF and NPS curves. Therefore, these MTF, NPS, and DQE models may not address the unsharp masking effect. Moreover, the models assume that the two images obtained from the front and rear detectors are independent to each other, 5 and which may not be reasonable in the single-shot dual-energy imaging. In this study, we present the measured results of MTF and NPS using the sandwich detector, and investigate their properties for various imaging conditions, and detector configurations and operation parameters. To support our observations and analyze the MTF and NPS characteristics, we develop and perform quantitative simulation experiments based on the cascaded-systems analysis. 0.2 Materials and Methods As shown in Fig. 2, the average output signal from the sandwich detector may be given by d DE = ± ln ( dr d w F ) + c, (5)

3 Figure 2. Cascaded model of the sandwich detector. Figure 3. Reconstructed edge-phantom images for various weighting factors (w = ). where c is a constant. In a previous study, 6 we showed the cascaded model describing the signal and noise from each detector layer in the sandwich detector. Because the weighted logarithmic subtraction is an operation to reconstruct dual-energy images, the operation may also be considered into the model as the cascaded model describing a computed tomography or a tomosynthesis system includes the image reconstruction operations. 7, 8 The modeling of the sandwich detector is in progress. Otherwise numerical simulations on the unsharp masking will be performed. On the other hand, we measure the MTF and NPS using edge-phantom and flood-field images obtained from the sandwich detector in terms of various imaging conditions, detector configurations (e.g., scintillator and filter thicknesses), and detector operation parameters (e.g., weighting factor). The properties of the MTF and NPS will be analyzed with the helps from the cascaded model or the numerical simulations. 0.3 Preliminary Results Figure 3 shows edge-phantom images reconstructed with various weighting factors. As the weighting factor, which exerts on the front image, is increased, the edge sharpens. We also note that the glue patterns becomes apparent with increasing weighting factor. (The glue is used to adhere the fragile photodiode array onto the ceramic substrate, and the glue patterns shown in Fig. 3 come from the front image.)

4 Figure 4. The ESFs and MTFs obtained from dual-energy edge images. The edge spread functions (ESFs) extracted from the edge phantom images, as shown in Fig. 3, and the resultant MTFs are plotted in Fig. 4. We note additional signal transition around the inflection point of the ESF, and this signal transition enhances with increasing weighting factor. These phenomena are reflected into the MTFs as the MTF values at an intermediate band in spatial frequencies are enhanced. We suspect these results in the improvement in contrast at high-frequency regions in bone-enhanced dual-energy images. 0.4 Further Study The remained further study before the meeting includes the followings: the expansion of dual-energy MTF and NPS measurements to various cases and the qualitative analysis on them; the development of the cascaded model of the sandwich detector or a numerical simulation platform describing the unsharp-masking effect; the quantitative analysis of the measured MTF and NPS characteristics with the helps from the developed model or simulations. ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2014R1A2A2A ). REFERENCES 1. S. Yun, J. C. Han, D. W. Kim, H. Youn, H. K. Kim, J. Tanguay, and I. A. Cunningham, Feasibility of active sandwich detectors for single-shot dual-energy imaging, J. C. Han, H. K. Kim, D. W. Kim, S. Yun, H. Youn, S. Kam, J. Tanguay, and I. A. Cunningham, Singleshot dual-energy x-ray imaging with a flat-panel sandwich detector for preclinical imaging, Current Applied Physics 14(12), pp , J. H. Siewerdsen and L. E. Antonuk, Dqe and system optimization for indirect-detection flat-panel imagers in diagnostic radiology, 1998.

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