디지털 단층합성 X-선 영상의 화질개선을 위한 TV-압축센싱 기반 영상복원기법 연구

제의규,김규석,조효성,김건아,박소영,임현우,박철규,박연옥,Je, Uikyu,Kim, Kyuseok,Cho, Hyosung,Kim, Guna,Park, Soyoung,Lim, Hyunwoo,Park, Chulkyu,Park, Yeonok Korean Society of Medical Physics 2016 의학물리 Vol.27 No.1

본 연구에서는 디지털 단층합성 엑스선 영상의 화질특성을 개선하기 위해 TV-압축센싱 기반 영상복원 기법을 제안한다. 제안된 영상복원 기법의 유효성을 검증하기 위해 우선 관련 영상복원 알고리즘을 구현하였으며, 이를 이용하여 관련 시뮬레이션 및 실험을 함께 수행하였다. 실험을 위해 일반 x-선관($90kV_p$, 6 mAs), CMOS형 평판형 검출기($198{\mu}m$ 픽셀크기)로 구성된 실험장치를 구성하였으며, 제한된 각도 $60^{\circ}$도에서 $2^{\circ}$ 간격으로 총 51장의 투상영상을 획득하고 제안된 알고리즘으로 영상복원을 수행한 후 필터링 역투사법(FBP)을 사용하여 디지털 단층합성 영상을 구현하였다. 본 연구에서 수행된 결과에 의하면, 제안된 영상복원 기법은 일반 엑스선 영상 및 디지털 단층합성 영상의 흐린 영상화질을 선명하게 개선하고 또한 디지털 단층합성 영상의 깊이 분해능을 향상시키는 이점이 있음을 확인함으로써 기존 디지털 단층합성 영상의 화질을 크게 개선할 수 있을 것으로 전망된다. In this work, we considered a compressed-sensing (CS)-based image deblurring scheme with a total-variation (TV) regularization penalty for improving image characteristics in digital tomosynthesis (DTS). We implemented the proposed image deblurring algorithm and performed a systematic simulation to demonstrate its viability. We also performed an experiment by using a table-top setup which consists of an x-ray tube operated at $90kV_p$, 6 mAs and a CMOS-type flat-panel detector having a $198-{\mu}m$ pixel resolution. In the both simulation and experiment, 51 projection images were taken with a tomographic angle range of ${\theta}=60^{\circ}$ and an angle step of ${\Delta}{\theta}=1.2^{\circ}$ and then deblurred by using the proposed deblurring algorithm before performing the common filtered-backprojection (FBP)-based DTS reconstruction. According to our results, the image sharpness of the recovered x-ray images and the reconstructed DTS images were significantly improved and the cross-plane spatial resolution in DTS was also improved by a factor of about 1.4. Thus the proposed deblurring scheme appears to be effective for the blurring problems in both conventional radiography and DTS and is applicable to improve the present image characteristics.

Experimental Study on the Application of a Compressed-sensing (CS) Algorithm to Dental Cone-beam CT (CBCT) for Accurate, Low-dose Image Reconstruction

오지은,조효성,제의규,이민식,김효정,홍대기,박연옥,이선화,조희문,최성일,구양서 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.5

In practical applications of three-dimensional (3D) tomographic imaging, there are often challenges for image reconstruction from insufficient data. In computed tomography (CT), for example, image reconstruction from few views would enable fast scanning with reduced doses to the patient. In this study, we investigated and implemented an efficient reconstruction method based on compressed-sensing (CS) algorithm, which exploits the sparseness of the gradient image with substantially high accuracy, for accurate, low-dose dental cone-beam CT (CBCT) reconstruction. We applied the algorithm to a commercially-available dental CBCT system (Expert7<SUP>TM</SUP>, Vatech Co., Korea) and performed experimental works to demonstrate the algorithm for image reconstruction in insufficient sampling problems. We successfully reconstructed CBCT images with several undersampling data and evaluated the reconstruction quality in terms of the universal-quality index (UQI). Experimental demonstrations of the CS-based reconstruction algorithm appear to show that it can be applicable to current dental CBCT systems for reducing imaging doses and further improving the image quality.

Investigation of Proper Imaging Conditions in the Moving Grid Technique for a Reduction of Grid Line Artifacts

박철규,조효성,제의규,홍대기,이민식,박연옥,Yoonseok Kang,Jinsoo Kim,Nagkun Chung,Jinwon Kim,Jinguk Kim 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.65 No.9

The most critical problem remaining as an obstacle for the successful use of antiscatter gridsin digital X-ray imaging is probably the observation of grid line artifacts such as moir´e patternsand shadows of the grid strips themselves in X-ray images, resulting in a risk of misdiagnosis byphysicians. In this paper, in order to find a practical solution to the problem of grid line artifacts, werevisited the moving grid technique and investigated its proper imaging conditions. We implementeda simple and useful moving-grid analysis code, iTOMTM, and performed systematic simulations fora theoretical analysis. We also performed experiments and compared the results to the simulatedones to demonstrate the effectiveness of the code. According to our simulation and experimentalresults, the grid line artifacts can be effectively reduced when the grid moves with a large velocityor with specific velocities that reduce the coefficient of variation (CV), even with a small velocity. These velocities are determined by using the related parameters such as the grid pitch, the gridstrip width, and the exposure time.

Application of a Dual-resolution Voxellation Scheme to Small ROI Reconstruction in Iterative CBCT for the Reduction of Computational Cost

이민식,조효성,제의규,홍대기,박연옥,박철규,조희문,최성일,구양서 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.65 No.9

In iterative methods for cone-beam computed tomography (CBCT) reconstruction, the use of ahuge system matrix is the primary computational bottleneck and is still an obstacle to the morewidespread use of these methods in practice. In this paper, to put iterative methods to practicalapplications, we propose a pragmatic idea, the-so-called dual-resolution voxellation scheme, for asmall region-of-interest (ROI) reconstruction in CBCT in which voxels outside the ROI are binnedwith a double resolution such as 2×2×2, 4×4×4, 8×8×8, 16×16×16, etc., and the voxel size withinthe ROI remains unchanged. In some situations of medical diagnosis, physicians are interestedonly in a small ROI containing a target diagnosis from the examined structure. We implementedan efficient compressed-sensing (CS)-based reconstruction algorithm with the proposed voxellationscheme incorporated and performed both simulation and experimental works to investigate theimaging characteristics. Our results indicate that the proposed voxellation scheme seems to beeffective in reducing the computational cost considerably for a small ROI reconstruction in iterativeCBCT, with the image quality inside the ROI not being noticeably impaired.

Experimental Study for Phase-contrast X-ray Imaging Based on a Single Antiscatter Grid and a Polychromatic X-ray Source

박연옥,조효성,임현우,제의규,박철규,조희문,김규석,김건아,박소영,Park, Yeonok,Cho, Hyosung,Lim, Hyunwoo,Je, Uikyu,Park, Chulkyu,Cho, Heemoon,Kim, Kyuseok,Kim, Guna,Park, Soyoung Korean Society of Medical Physics 2015 의학물리 Vol.26 No.4

본 연구에서는 단일 비산란 그리드 및 다색광 x-선원을 이용하여 위상대조 x-선 영상을 용이하게 구현할 수 있는 새로운 방법을 제안한다. 제안된 신기법의 개념 입증을 위해 집속형 선형 그리드(200 lines/inch 선 밀도), 마이크로 초점 x-선관(${\sim}5{\mu}m$ 초점크기), CMOS형 평판형 검출기($48{\mu}m$ 픽셀 크기)로 실험장치를 구성하였으며, 한 번의 x-선 촬영($90kV_p$, 0.1 mAs)으로 감약대조 x-선 영상과 향상된 가시성을 지진 산란 x-선 영상 및 차분 위상대조 x-선 영상을 Fourier변조복원 기법을 적용하여 성공적으로 분리 획득하였다. 더 나아가, 감약대조 x-선 영상과 산란 x-선 영상을 합성함으로써 일반 감약대조 x-선 영상에서는 명확하게 볼 수 없는 샘플의 미세 구조를 보다 선명하게 나타냄을 확인하였다. 본 논문에서 제안한 단일 비산란 그리드 기반 위상대조 x-선 영상화 기법은 실험 구성 및 절차가 단순하고 새로운 대조도에 기반한 산란 및 위상대조 x-선 영상을 동시에 제공하기 때문에 차세대 x-선 영상화 신기법으로 다양한 응용분야에서 용이하게 적용될 수 있을 것으로 전망한다. In this work, we performed a proof-of-concept experiment for phase-contrast x-ray imaging (PCXI) based on a single antiscatter grid and a polychromatic x-ray source. We established a table-top setup which consists of a focused-linear grid having a strip density of 200 lines/inch, a microfocus x-ray tube having a focal-spot size of about $5{\mu}m$, and a CMOS-type flat-panel detector having a pixel size of $48{\mu}m$. By using our prototype PCXI system and the Fourier demodulation technique, we successfully obtained attenuation, scattering, and differential phase-contrast images of improved visibility from the raw images of several selected samples at x-ray tube conditions of $90kV_p$ and 0.1 mAs. Further, fusion image (e.g., the attenuation+the scattering) may have an advantage in displaying details of the sample's structures that are not clearly visible in the conventional attenuation image. Our experimental results indicate that single-grid-based approach seems a useful method for PCXI with great simplicity and minimal requirements on the setup alignment.

A New Voxellation Scheme for Iterative CBCT Reconstruction with Less Computational Cost: Polar Voxel Design of an Isosceles-triangle Shape

이민식,조효성,홍대기,제의규,박연옥,박철규,조희문,최성일,구양서 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.65 No.7

We propose a new voxellation scheme for iterative cone-beam computed tomography (CBCT)reconstruction with less computational cost. Here, the voxel configuration is designed with anisosceles-triangle shape by using polar coordinates, exploiting the rotational symmetries inherent tothe CBCT acquisition geometry and keeping the uniformity of the voxel size as well. By using polarsymmetries, we can reduce the size of the system matrix by a factor corresponding to the numberof acquired projections, which speeds up the construction of the system matrix and, thus, allowsiterative methods to be applied to practical applications within a reasonable reconstruction time. In this study, we implemented an efficient algorithm to reconstruct the system matrix based uponthe proposed voxellation scheme and incorporated it into a built-in iterative CBCT reconstructionalgorithm based the gradient-projection Barzilai-Borwein (GPBB) method, and we performed systematicsimulation works to investigate the imaging characteristics. Our results indicate that thevoxellation scheme we proposed in study seems to be effective in reducing the computational timeand the memory burden considerably.

Image Reconstruction for Digital Breast Tomosynthesis (DBT) by Using Projection-angle-dependent Filter Functions

박연옥,박철규,조효성,제의규,홍대기,이민식,조희문,최성일,구양서 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.65 No.5

Digital breast tomosynthesis (DBT) is considered in clinics as a standard three-dimensionalimaging modality, allowing the earlier detection of cancer. It typically acquires only 10-30 projectionsover a limited angle range of 15 − 60 with a stationary detector and typically uses acomputationally-efficient filtered-backprojection (FBP) algorithm for image reconstruction. However,a common FBP algorithm yields poor image quality resulting from the loss of average imagevalue and the presence of severe image artifacts due to the elimination of the dc component ofthe image by the ramp filter and to the incomplete data, respectively. As an alternative, iterativereconstruction methods are often used in DBT to overcome these difficulties, even though they arestill computationally expensive. In this study, as a compromise, we considered a projection-angledependentfiltering method in which one-dimensional geometry-adapted filter kernels are computedwith the aid of a conjugate-gradient method and are incorporated into the standard FBP framework. We implemented the proposed algorithm and performed systematic simulation works toinvestigate the imaging characteristics. Our results indicate that the proposed method is superiorto a conventional FBP method for DBT imaging and has a comparable computational cost, whilepreserving good image homogeneity and edge sharpening with no serious image artifacts.

Volumetric Image Reconstruction in a Dental Panoramic Imaging System with a Limited-angle Zigzag Scan Geometry

홍대기,조효성,이선화,오지은,이민식,김효정,제의규,박연옥,최성일,구양서,조희문 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.2

As a continuation of our dental imaging R&D, we have proposed a novel idea that is capable of implementing cost-effective, low-dose, volumetric image reconstruction directly onto a dental panoramic imaging system. In the proposed geometry, a linear-type panoramic detector is rotated 90˚ from the orientation for panoramic imaging and scanned along a limited-angle zigzag trajectory in the axial direction to cover the whole imaging volume thickness. We used an effective reconstruction algorithm based on the total-variation (TV) minimization approach for the proposed geometry and performed systematic simulation work to demonstrate the viability of our proposed approach and its effectiveness for three-dimensional (3D) dental X-ray imaging. We have successfully reconstructed images of substantially high image accuracy from the proposed geometry and evaluated the reconstruction quality by using an image similarity metric, the universal-quality index (UQI). We expect the proposed method to be applicable to developing a cost-effective, low-dose, all-in-one dental X-ray imaging system.

Simulation of Single Grid-based Phase-contrast Digital Tomosynthesis (PC-DTS)

이헌우,임현우,조효성,서창우,박철규,이동연,김규석,김건아,박소영,강석윤,박정은,제의규,오지은 한국물리학회 2018 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.72 No.3

Digital tomosynthesis (DTS) is a geometric tomography technique by a limited-angle scan which has been popularly used in both medical and industrial X-ray imaging applications. However, conventional DTS remains limited by low contrast especially in imaging samples of low atomic number Z such as breast and cartilage tissues. In this work, we applied the recently proposed phase-contrast imaging (PCI) technique, the so-called single grid-based PCI, to DTS in attempt to overcome this limitation. PCI has superior soft-tissue imaging capability while DTS has improved image contrast. Combining the two techniques can therefore considerably improve the X-ray imaging performance. We developed a useful simulation platform for single grid-based phase-contrast DTS reconstruction and performed a systematic simulation using a three-dimensional (3D) numerical breast phantom. In the simulation, an X-ray grid having a lead strip density of 200 lines/inch was used and the DTS scan comprised 41 projections within an angle range of θ = ±40° at an X-ray energy of 25 keV. We successfully reconstructed DTS images of much improved contrast, compared to conventional DTS images, which demonstrates the viability of the proposed approach.