CT 상의 금속 허상물 제거를 위한 효율적인 빔 경화 교정 알고리즘

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·컴퓨터공학부, 2021. 2. 신영길.빔경화는 다색 X선을 사용하고 에너지 의존적인 물질 감쇠 계수를 이용하는 CT 시스템의 특성상 불가피한 현상이며, 이는 특히 금속 영역을 포함하는 프로젝션 상의 값을 오측정하여 결과적으로 CT 영상에 허상물을 유발한다. 금속 허상물 저감화는 CT 영상에 존재하는 이러한 허상물을 제거하고 가려진 실제 정보를 복원하는 과정이다. 영상을 통한 진단과 방사선치료를 위한 계획 수립에 있어서 정확한 CT 영상을 획득하기 위해 금속 허상물의 제거는 필수적이다. 반복적인 재구성에 의한 수치적 방법에 기반을 둔 효과적인 금속 허상물 제거에 관한 최신 연구들이 발표되었으나 무거운 계산량으로 인해 임상 실습에 적용이 어려운 상황이다. 본 논문에서는 이러한 계산적인 이슈를 해결하기 위한 효율적인 빔 경화 추정 모델과 이를 이용한 금속 허상물 저감화 방법을 제안한다. 제안한 모델은 금속 물체의 기하정보와 다색 X선이 물체를 통과하면서 발생하는 빔경화의 물리적인 특성을 반영한다. 모델에 필요한 대부분의 매개변수들은 수치학적인 방법으로 교정 전의 CT 영상과 CT 시스템으로부터 추가적인 최적화 과정 없이 획득한다. 빔경화 허상물과 관련된 매개 변수 중 단 하나만 재구성 이후의 영상 단계에서 선형 최적화를 통해 탐색된다. 또한 제안한 방법으로 교정된 결과 영상에 잔존하는 허상물들을 제거하기 위한 추가적인 두가지 개선 방법을 제시한다. 다수의 시뮬레이션 데이터와 실제 데이터를 사용하여 정성적 및 정량적 비교를 통해 제안 기법의 유효성이 체계적으로 평가되었다. 제안 알고리즘은 정확성 및 견고성 측면에서 유의미한 결과를 보여주었고, 기존의 기법들에 비해 향상된 결과 영상의 품질 뿐만 아니라 임상적으로 적용할만한 빠른 수행 시간을 보여주었다. 이 연구는 CT 영상을 통한 진단과 방사능 치료의 계획 수립을 위한 정확성 향상에 유의미한 의미를 갖는다.Beam hardening in X-ray computed tomography (CT) is an inevitable problem due to the characteristics of CT system that uses polychromatic X-rays and energy-dependent attenuation coefficients of materials. It causes artifacts in CT images as the result of underestimation on the projection data, especially on metal regions. Metal artifact reduction is the process of reducing the artifacts in CT and restoring the actual information hidden by the artifacts. In order to obtain exact CT images for more accurate diagnosis and treatment planning on radiotherapy in clinical fields, it is essential to reduce metal artifacts. State-of-the-art approaches on effectively reducing metal artifact based on numerical methods by iterative reconstruction have been presented. However, it is difficult to be applied in clinical practice due to a heavy computational burden. In this dissertation, we proposes an efficient beam-hardening estimation model and a metal artifact reduction method using this model to address this computational issue. The proposed model reflects the geometric information of metal objects and physical characteristics of beam hardening during the transmission of polychromatic X-ray through a material. Most of the associated parameters are numerically obtained from an initial uncorrected CT image and CT system without additional optimization. Only the unknown parameter related to beam-hardening artifact is fine-tuned by linear optimization, which is performed only in the reconstruction image domain. Two additional refinement methods are presented to reduce residual artifacts in the result image corrected by the proposed metal artifact reduction method. The effectiveness of the proposed method was systematically assessed through qualitative and quantitative comparisons using numerical simulations and real data. The proposed algorithm showed significant results in the aspects of accuracy and robustness. Compared to existing methods, it showed improved image quality as well as fast execution time that is clinically applicable. This work may have significant implications in improving the accuracy of diagnosis and treatment planning for radiotheraphy through CT imaging.Chapter 1 Introduction 1 1.1 Background and motivation 1 1.2 Scope and aim 5 1.3 Main contribution 6 1.4 Contents organization 8 Chapter 2 Related Works 9 2.1 CT physics 9 2.1.1 Fundamentals of X-ray 10 2.1.2 CT reconstruction algorithms 13 2.2 CT artifacts 18 2.2.1 Physics-based artifacts 19 2.2.2 Patient-based artifacts 21 2.3 Metal artifact reduction 22 2.3.1 Sinogram-completion based MAR 24 2.3.2 Sinogram-correction based MAR 27 2.3.3 Deep-learning based MAR 29 2.4 Summary 31 Chapter 3 Constrained Beam-hardening Estimator for Polychromatic X-ray 33 3.1 Characteristics of polychromatic X-ray 34 3.2 Constrained beam-hardening estimator 35 3.3 Summary 41 Chapter 4 Metal Artifact Reduction with Constrained Beam-hardening Estimator 43 4.1 Metal segmentation 44 4.2 X-ray transmission length 46 4.3 Artifact reduction with CBHE 48 4.3.1 Artifact estimation for a single type of metal 48 4.3.2 Artifact estimation for multiple types of metal 51 4.4 Refinement methods 54 4.4.1 Collaboration with ADN 54 4.4.2 Application of CBHE to bone 57 4.5 Summary 59 Chapter 5 Experimental Results 61 5.1 Data preparation and quantitative measures 62 5.2 Verification on constrained beam-hardening estimator 67 5.2.1 Accuracy 67 5.2.2 Robustness 72 5.3 Performance evaluations 81 5.3.1 Evaluation with simulated phantoms 81 5.3.2 Evaluation with hardware phantoms 86 5.3.3 Evaluation on refinement methods 91 Chapter 6 Conclusion 95 Bibliography 101 초록 115 Acknowledgements 117Docto

제21대 총선을 중심으로

학위논문 (석사) -- 서울대학교 대학원 : 행정대학원 행정학과(행정학전공), 2020. 8. 금현섭.본 연구는 국회의원 총선거의 당선 결정 요인을 밝히려는 시도에서 시작됐다. 그 중에서도 2020년 4월 15일 실시된 21대 총선이 주요한 연구 대상이며, 더불어민주당(253명)과 미래통합당(236명)의 지역구 후보자 489명을 대상으로 분석했다. 그동안 한국 정치에서는 정당 내 소수의 실력자가 비민주적이고 불투명한 방식으로 국회의원 선거에 나설 후보자를 공천하는 게 문제로 지적됐다. 본 연구는 그러한 방식보다 경선을 통한 방식이 과정의 민주성뿐 아니라 결과의 효과성도 담보할 수 있을 것으로 가정했다. 실제 253개 지역구의 선거 결과를 분석해 경선을 통한 후보자 공천은 전략(우선)공천, 단수공천 등 비경선 방식보다 당선 가능성이나 득표율 제고 측면에서 유리하다는 결론을 얻었다. 경선의 보너스 효과(primary bonus)가 존재했던 것이다. 또한 21대 총선에서는 여당 후보자인지 아닌지가 선거 결과에 상당히 큰 영향을 끼쳤다. 코로나바이러스감염증-19(COVID-19) 사태 속에서 실시된 이번 선거는 정권 안정론이 정권 심판론을 압도해 여당 프리미엄이 크게 작용했다. 반면 현직 국회의원이 현재 자신의 지역구에 출마할 때 얻는 이른바 현직 효과(incumbency advantage)는 나타나지 않았다. 그리고 후보자 수와 당선 가능성은 큰 상관이 없는 것으로 분석됐다. 사실상 더불어민주당과 미래통합당의 양자 대결 구도로 총선이 치러졌기 때문에 다른 정당 후보자의 숫자는 큰 의미가 없었던 것이다.This study investigates the process and outcome of party nominations in National Assembly election in the Republic of Korea. Especially this study reviews how the two main parties - the ruling Democratic Party (DP) and the main opposition United Future Party (UFP) - nominated their candidates running for the 21st general election. And identifies the factors to the electoral victories of individual candidates. This study assumes that the primary effect and ruling party premium and incumbency advantage are factors affecting election. Most of all, focus on the primary effect. In other words, this study analyzes intensively on bottom-up style candidate selection. And estimates the relationship between types of candidate selections and their political outcomes. The findings of this study can be summarized into a few consequences. First, the bonus effect of the primary exist. That is to say, bottom-up style of candidate selection system helps candidates earn more votes and their winning chances. Second, ruling party premium is strong. Many DP candidates won election for the reason that they are member of the ruling party. Third, incumbency advantage was not existing. There are limits to this study. There was no third party threatening the two main parties. And the election was held in the midst of Coronavirus disease 2019 (COVID-19). For that reason, there were other factors as well, but they were not analyzed.제 1 장 서론 1 제 1 절 연구의 배경과 목적 1 제 2 절 연구의 대상과 방법 5 1. 연구의 대상 5 2. 연구의 방법 7 제 2 장 이론적 논의 및 선행연구의 검토 9 제 1 절 민주성과 효율성 9 제 2 절 규범적 연구 10 1. 공천의 민주화 10 2. 상향식 공천의 필요성과 문제점 13 3. 규범적 연구의 한계 19 제 3 절 경험적 연구 19 1. 공천 과정에 대한 분석 20 2. 공천 결과에 대한 분석 22 3. 공천 유형에 따른 의정활동 분석 26 제 3 장 연구의 설계 29 제 1 절 연구 문제 29 1. 정당 내부의 요인 29 2. 정당 외부의 요인 32 3. 후보자 개인의 요인 33 제 2 절 연구의 변수 34 1. 종속변수 34 2. 독립변수 35 3. 통제변수 35 제 3 절 연구의 모형 37 제 4 장 기초 분석 38 제 1 절 공천 기준 38 제 2 절 공천 유형 40 제 3 절 선거 결과 46 제 5 장 심층 분석 55 제 1 절 기술통계 55 제 2 절 가설의 검증 60 1. 정당 내부의 요인 61 2. 정당 외부의 요인 76 3. 후보자 개인의 요인 78 4. 통제변수 80 5. 연구 결과의 요약 81 제 6 장 결론 83 제 1 절 연구의 의의 83 제 2 절 연구의 한계 및 향후 과제 85Maste

Diagnostic Accuracy of Percutaneous Transthoracic Needle Lung Biopsies: A Multicenter Study

OBJECTIVE: To measure the diagnostic accuracy of percutaneous transthoracic needle lung biopsies (PTNBs) on the basis of the intention-to-diagnose principle and identify risk factors for diagnostic failure of PTNBs in a multi-institutional setting. MATERIALS AND METHODS: A total of 9384 initial PTNBs performed in 9239 patients (mean patient age, 65 years [range, 20-99 years]) from January 2010 to December 2014 were included. The accuracy, sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of PTNBs for diagnosis of malignancy were measured. The proportion of diagnostic failures was measured, and their risk factors were identified. RESULTS: The overall accuracy, sensitivity, specificity, PPV, and NPV were 91.1% (95% confidence interval [CI], 90.6-91.7%), 92.5% (95% CI, 91.9-93.1%), 86.5% (95% CI, 85.0-87.9%), 99.2% (95% CI, 99.0-99.4%), and 84.3% (95% CI, 82.7-85.8%), respectively. The proportion of diagnostic failures was 8.9% (831 of 9384; 95% CI, 8.3-9.4%). The independent risk factors for diagnostic failures were lesions ≤ 1 cm in size (adjusted odds ratio [AOR], 1.86; 95% CI, 1.23-2.81), lesion size 1.1-2 cm (1.75; 1.45-2.11), subsolid lesions (1.81; 1.32-2.49), use of fine needle aspiration only (2.43; 1.80-3.28), final diagnosis of benign lesions (2.18; 1.84-2.58), and final diagnosis of lymphomas (10.66; 6.21-18.30). Use of cone-beam CT (AOR, 0.31; 95% CI, 0.13-0.75) and conventional CT-guidance (0.55; 0.32-0.94) reduced diagnostic failures. CONCLUSION: The accuracy of PTNB for diagnosis of malignancy was fairly high in our large-scale multi-institutional cohort. The identified risk factors for diagnostic failure may help reduce diagnostic failure and interpret the biopsy results.ope

Regional Amyloid Burden Differences Evaluated Using Quantitative Cardiac MRI in Patients with Cardiac Amyloidosis

Objective: This study aimed to investigate the regional amyloid burden and myocardial deformation using T1 mapping and strain values in patients with cardiac amyloidosis (CA) according to late gadolinium enhancement (LGE) patterns. Materials and methods: Forty patients with CA were divided into 2 groups per LGE pattern, and 15 healthy subjects were enrolled. Global and regional native T1 and T2 mapping, extracellular volume (ECV), and cardiac magnetic resonance (CMR)-feature tracking strain values were compared in an intergroup and interregional manner. Results: Of the patients with CA, 32 had diffuse global LGE (group 2), and 8 had focal patchy or no LGE (group 1). Global native T1, T2, and ECV were significantly higher in groups 1 and 2 than in the control group (native T1: 1384.4 ms vs. 1466.8 ms vs. 1230.5 ms; T2: 53.8 ms vs. 54.2 ms vs. 48.9 ms; and ECV: 36.9% vs. 51.4% vs. 26.0%, respectively; all, p < 0.001). Basal ECV (53.7%) was significantly higher than the mid and apical ECVs (50.1% and 50.0%, respectively; p < 0.001) in group 2. Basal and mid peak radial strains (PRSs) and peak circumferential strains (PCSs) were significantly lower than the apical PRS and PCS, respectively (PRS, 15.6% vs. 16.7% vs. 26.9%; and PCS, -9.7% vs. -10.9% vs. -15.0%; all, p < 0.001). Basal ECV and basal strain (2-dimensional PRS) in group 2 showed a significant negative correlation (r = -0.623, p < 0.001). Group 1 showed no regional ECV differences (basal, 37.0%; mid, 35.9%; and apical, 38.3%; p = 0.184). Conclusion: Quantitative T1 mapping parameters such as native T1 and ECV may help diagnose early CA. ECV, in particular, can reflect regional differences in the amyloid deposition in patients with advanced CA, and increased basal ECV is related to decreased basal strain. Therefore, quantitative CMR parameters may help diagnose CA and determine its severity in patients with or without LGE.ope

Predictive factors of recurrence after resection of subsolid clinical stage IA lung adenocarcinoma

Background: Ongoing studies are currently investigating the extent of surgical resection required for subsolid cancers. This study aimed to investigate the predictive factors related to recurrence in patients with clinical stage IA subsolid cancer who underwent either lobectomy or sublobar resection. Methods: This was a prospective multicenter observational study conducted in eight qualifying university teaching hospitals between April 2014 and December 2016. A total of 173 patients with subsolid nodules pathologically confirmed to have primary lung adenocarcinoma and stage IA disease were included in the final analysis. All patients underwent lobectomy, segmentectomy, or wedge resection performed by experienced thoracoscopic surgeons at each site. The surgical procedure was chosen based on the decision of the surgeons involved. The primary endpoint was time to recurrence (TTR). Results: The study population was 43.9% (76 of 173) male with a mean age of 60.7 years. During the median follow-up period of 5.01 years, nine patients (5%) experienced disease recurrence. In the multivariable analysis, tumor size (size ≥2 cm) (hazard ratio: 73.717, 95% confidence interval [CI]: 3.635-895.036; p < 0.001) and stage IA3 (hazard ratio: 62.010, 95% CI: 2.837-855.185; p < 0.001) were independent predictors of tumor recurrence. When analyzing the recurrence outcome in patients according to surgical procedure, no significant difference was found in TTR among the three groups (i.e., lobectomy, segmentectomy, and wedge resection; p = 0.99). Conclusions: Patients with radiologically subsolid lung adenocarcinoma measuring <3 cm could be candidates for sublobar resection instead of lobectomy.ope

LOGIS (LOcalization of Ground-glass-opacity and pulmonary lesions for mInimal Surgery) registry: Design and Rationale

Background and purpose: An optimal pulmonary localization technique for video-assisted thoracic surgery (VATS) of small lung nodules has not yet been established. The LOcalization of Ground-glass-opacity and pulmonary lesions for mInimal Surgery (LOGIS) registry aims to establish a multicenter database and investigate the usefulness and safety of localization techniques for small pulmonary lesions in individuals undergoing VATS. Methods/Design: The LOGIS registry is a large-scale, multicenter cohort study, aiming to enroll 825 patients at 10 institutions. Based on the inclusion and exclusion criteria, all study participants with pulmonary lesions indicated for VATS will be screened and enrolled at each site. All study participants will undergo preoperative lesion localization by the hook-wire or lipiodol localization methods according to site-specific methods. Within a few hours of marking, thoracoscopic surgery will be done under general anesthesia by experienced thoracoscopic surgeons. The primary endpoints are the success and complication rates of the two localization techniques. Secondary endpoints include procedure duration, recurrence rate, and all-cause mortality. Study participant enrollment will be completed within 2 years. Procedure success rates and incidence of complications will be analyzed based on computed tomography findings. Procedure duration, recurrence rate, and all-cause mortality will be compared between the two techniques. The study will require 5 years for completion, including 6 months of preparation, 3.5 years for recruitment, and 1 year of follow-up endpoint assessment. Discussion: The LOGIS registry, once complete, will provide objective comparative results regarding the usefulness and safety of the lipiodol and hook-wire localization techniques.ope

Dual-Energy CT for Pulmonary Embolism: Current and Evolving Clinical Applications

Pulmonary embolism (PE) is a potentially fatal disease if the diagnosis or treatment is delayed. Currently, multidetector computed tomography (MDCT) is considered the standard imaging method for diagnosing PE. Dual-energy CT (DECT) has the advantages of MDCT and can provide functional information for patients with PE. The aim of this review is to present the potential clinical applications of DECT in PE, focusing on the diagnosis and risk stratification of PE.ope

Analysis of Complications of Percutaneous Transthoracic Needle Biopsy Using CT-Guidance Modalities In a Multicenter Cohort of 10568 Biopsies

OBJECTIVE: To analyze the complications of percutaneous transthoracic needle biopsy using CT-based imaging modalities for needle guidance in comparison with fluoroscopy in a large retrospective cohort. MATERIALS AND METHODS: This study was approved by multiple Institutional Review Boards and the requirement for informed consent was waived. We retrospectively included 10568 biopsies from eight referral hospitals from 2010 through 2014. In univariate and multivariate logistic analyses, 3 CT-based guidance modalities (CT, CT fluoroscopy, and cone-beam CT) were compared with fluoroscopy in terms of the risk of pneumothorax, pneumothorax requiring chest tube insertion, and hemoptysis, with adjustment for other risk factors. RESULTS: Pneumothorax occurred in 2298 of the 10568 biopsies (21.7%). Tube insertion was required after 316 biopsies (3.0%), and hemoptysis occurred in 550 cases (5.2%). In the multivariate analysis, pneumothorax was more frequently detected with CT {odds ratio (OR), 2.752 (95% confidence interval [CI], 2.325-3.258), p < 0.001}, CT fluoroscopy (OR, 1.440 [95% CI, 1.176-1.762], p < 0.001), and cone-beam CT (OR, 2.906 [95% CI, 2.235-3.779], p < 0.001), but no significant relationship was found for pneumothorax requiring chest tube insertion (p = 0.497, p = 0.222, and p = 0.216, respectively). The incidence of hemoptysis was significantly lower under CT (OR, 0.348 [95% CI, 0.247-0.491], p < 0.001), CT fluoroscopy (OR, 0.594 [95% CI, 0.419-0.843], p = 0.004), and cone-beam CT (OR, 0.479 [95% CI, 0.317-0.724], p < 0.001) guidance. CONCLUSION: Hemoptysis occurred less frequently with CT-based guidance modalities in comparison with fluoroscopy. Although pneumothorax requiring chest tube insertion showed a similar incidence, pneumothorax was more frequently detected using CT-based guidance modalities.ope

Radiation dose reduction using deep learning-based image reconstruction for a low-dose chest computed tomography protocol: a phantom study

Background: The aim of this study was to compare the dose reduction potential and image quality of deep learning-based image reconstruction (DLIR) with those of filtered back-projection (FBP) and iterative reconstruction (IR) and to determine the clinically usable dose of DLIR for low-dose chest computed tomography (LDCT) scans. Methods: Multi-slice computed tomography (CT) scans of a chest phantom were performed with various tube voltages and tube currents, and the images were reconstructed using seven methods to control the amount of noise reduction: FBP, three stages of IR, and three stages of DLIR. For subjective image analysis, four radiologists compared 48 image data sets with reference images and rated on a 5-point scale. For quantitative image analysis, the signal to noise ratio (SNR), contrast to noise ratio (CNR), nodule volume, and nodule diameter were measured. Results: In the subjective analysis, DLIR-Low (0.46 mGy), DLIR-Medium (0.31 mGy), and DLIR-High (0.18 mGy) images showed similar quality to the FBP (2.47 mGy) image. Under the same dose conditions, the SNR and CNR were higher with DLIR-High than with FBP and all the IR methods (all P<0.05). The nodule volume and size with DLIR-High were significantly closer to the real volume than with FBP and all the IR methods (all P<0.001). Conclusions: DLIR can improve the image quality of LDCT compared to FBP and IR. In addition, the appropriate effective dose for LDCT would be 0.24 mGy with DLIR-High. © Quantitative Imaging in Medicine and Surgery. All rights reserved.ope

Utility of CT Radiomics for Prediction of PD-L1 Expression in Advanced Lung Adenocarcinomas

Background: We aimed to assess if quantitative radiomic features can predict programmed death ligand 1 (PD-L1) expression in advanced stage lung adenocarcinoma. Methods: This retrospective study included 153 patients who had advanced stage (>IIIA by TNM classification) lung adenocarcinoma with pretreatment thin section computed tomography (CT) images and PD-L1 expression test results in their pathology reports. Clinicopathological data were collected from electronic medical records. Visual analysis and radiomic feature extraction of the tumor from pretreatment CT were performed. We constructed two models for multivariate logistic regression analysis (one based on clinical variables, and the other based on a combination of clinical variables and radiomic features), and compared c-statistics of the receiver operating characteristic curves of each model to identify the model with the higher predictability. Results: Among 153 patients, 53 patients were classified as PD-L1 positive and 100 patients as PD-L1 negative. There was no significant difference in clinical characteristics or imaging findings on visual analysis between the two groups (P > 0.05 for all). Rad-score by radiomic analysis was higher in the PD-L1 positive group than in the PD-L1 negative group with a statistical significance (-0.378 ± 1.537 vs. -1.171 ± 0.822, P = 0.0008). A prediction model that uses clinical variables and CT radiomic features showed higher performance compared to a prediction model that uses clinical variables only (c-statistic = 0.646 vs. 0.550, P = 0.0299). Conclusions: Quantitative CT radiomic features can predict PD-L1 expression in advanced stage lung adenocarcinoma. A prediction model composed of clinical variables and CT radiomic features may facilitate noninvasive assessment of PD-L1 expression. Key points: Significant findings of the study Quantitative CT radiomic features can help predict PD-L1 expression, whereas none of the qualitative imaging findings is associated with PD-L1 positivity. What this study adds A prediction model composed of clinical variables and CT radiomic features may facilitate noninvasive assessment of PD-L1 expression.ope