Mitigation of artifacts due to isolated acoustic heterogeneities in photoacoustic computed tomography using a variable data truncation-based reconstruction method

Photoacoustic computed tomography (PACT) is an emerging computed imaging modality that exploits optical contrast and ultrasonic detection principles to form images of the absorbed optical energy density within tissue. If the object possesses spatially variant acoustic properties that are unaccounted for by the reconstruction method, the estimated image can contain distortions. While reconstruction methods have recently been developed to compensate for this effect, they generally require the object's acoustic properties to be known a priori. To circumvent the need for detailed information regarding an object's acoustic properties, we previously proposed a half-time reconstruction method for PACT. A half-time reconstruction method estimates the PACT image from a data set that has been temporally truncated to exclude the data components that have been strongly aberrated. However, this method can be improved upon when the approximate sizes and locations of isolated heterogeneous structures, such as bones or gas pockets, are known. To address this, we investigate PACT reconstruction methods that are based on a variable data truncation (VDT) approach. The VDT approach represents a generalization of the half-time approach, in which the degree of temporal truncation for each measurement is determined by the distance between the corresponding ultrasonic transducer location and the nearest known bone or gas void location. Computer-simulated and experimental data are employed to demonstrate the effectiveness of the approach in mitigating artifacts due to acoustic heterogeneities

High-Pitch, Low-Voltage and Low-Iodine-Concentration CT Angiography of Aorta: Assessment of Image Quality and Radiation Dose with Iterative Reconstruction

Objective: To assess the image quality of aorta obtained by dual-source computed tomography angiography (DSCTA), performed with high pitch, low tube voltage, and low iodine concentration contrast medium (CM) with images reconstructed using iterative reconstruction (IR). Methods: One hundred patients randomly allocated to receive one of two types of CM underwent DSCTA with the electrocardiogram-triggered Flash protocol. In the low-iodine group, 50 patients received CM containing 270 mg I/mL and were scanned at low tube voltage (100 kVp). In the high-iodine CM group, 50 patients received CM containing 370 mg I/mL and were scanned at the tube voltage (120 kVp). The filtered back projection (FBP) algorithm was used for reconstruction in both groups. In addition, the IR algorithm was used in the low-iodine group. Image quality of the aorta was analyzed subjectively by a 3-point grading scale and objectively by measuring the CT attenuation in terms of the signal- and contrast-to-noise ratios (SNR and CNR, respectively). Radiation and CM doses were compared.Results: The CT attenuation, subjective image quality assessment, SNR, and CNR of various aortic regions of interest did not differ significantly between two groups. In the low-iodine group, images reconstructed by FBP and IR demonstrated significant differences in image noise, SNR, and CNR (p<0.05). The low-iodine group resulted in 34.3% less radiation (4.4 ± 0.5 mSv) than the high-iodine group (6.7 ± 0.6 mSv), and 27.3% less iodine weight (20.36 ± 2.65 g) than the high-iodine group (28 ± 1.98 g). Observers exhibited excellent agreement on the aortic image quality scores (κ = 0.904). Conclusions: CT images of aorta could be obtained within 2 s by using a DSCT Flash protocol with low tube voltage, IR, and low-iodine-concentration CM. Appropriate contrast enhancement was achieved while maintaining good image quality and decreasing the radiation and iodine doses

Mechanistic and pathological study of the genesis, growth, and rupture of abdominal aortic aneurysms

Postprint (published version

저선량 폐암 검진에서 단일에너지 영상을 이용한 비고형성 결절의 측정 반복성

학위논문 (박사)-- 서울대학교 대학원 : 의과대학 의학과, 2019. 2. 이경원.Purpose: To investigate whether monoenergetic images captured with dual-layer spectral CT can improve the repeatability of subsolid nodule measurement, and whether this approach can further reduce the radiation dose of computed tomography (CT) while maintaining its measurement repeatability. Materials and Methods: An anthropomorphic phantom with simulated subsolid nodules at three different levels was repeatedly scanned with both conventional single-energy CT and dual-layer spectral CT. A proxy for the measurement repeatability in the National Lung Screening Trial (proxy for NLST) was calculated with the typical CT protocol used in NLST. Using the dual-layer spectral CT, monoenergetic images of 40 to 110 keV, with an interval of 10 keV, were generated. The average diameter and volume of a total of 15,660 nodules in 870 CT images were measured by using a commercially-available computer-aided detection system. The repeatability coefficient (RC), %RC, and 95% confidence intervals of each image set were calculated and compared. Results: At the same tube voltage and tube current-time product, monoenergetic images resulted in significantly lower RC than the proxy for NLST, indicating that measurement repeatability was enhanced. When the radiation dose was lowered by 30% or 55%, monoenergetic images showed significantly lower RC at high-energy keV than the proxy for NLST. The estimated measurement repeatability from monoenergetic images with 30% or 55% lower radiation dose was comparable to the repeatability from conventional single-energy CT images with standard radiation dose and iterative reconstruction. Conclusion: Monoenergetic images captured by using dual-layer spectral CT can improve the repeatability of subsolid nodule measurement. The use of monoenergetic images would allow lung cancer screening with a lower radiation dose, while maintaining comparable measurement repeatability.목적: 이중층 스펙트럼 CT(computed tomography)로 촬영된 단일에너지 영상이 비고형성 결절 측정의 반복성을 향상시킬 수 있는지, 그리고 이 영상 기법이 측정 반복성을 유지하면서 CT의 방사선량을 추가로 감소시킬 수 있는지 여부를 조사하고자 함. 방법: 세 가지 다른 위치에 가상 비고형성 결절들을 포함하고 있는 인체 모형 팬텀을 기존의 단일에너지 CT와 이중층 스펙트럼 CT를 이용하여 반복적으로 촬영하였다. National Lung Screening Trial (NLST) 에서의 측정 반복성에 대한 대리 표지자는 NLST에서 일반적으로 사용된 조건들을 사용하여 단일 에너지 CT로 촬영하여 계산하였다. 이중층 스펙트럼 CT를 사용하여 10 keV의 간격으로 40 keV에서 110 keV의 다양한 단일에너지 영상이 생성되었다. 이렇게 생성된 870개의 CT 영상에서 총 15,660 개의 결절의 평균 직경과 부피를 시판되는 컴퓨터 보조 검출 시스템을 사용하여 측정하였다. 각각의 영상에 대한 반복계수와 백분율로 표시된 반복계수, 그리고 95 % 신뢰구간을 계산하고 비교했습니다. 결과: 동일한 관전압 및 관전류에서 촬영했을 때 단일 에너지 이미지는 NLST의 대리 표지자보다 훨씬 낮은 반복계수를 나타내어 측정 반복성이 향상되었음을 나타냈다. 방사선량이 30 % 또는 55 % 감소되었을 때에도 고에너지 keV하에서의 단일 에너지 영상은 NLST의 대리 표지자보다 낮은 반복계수를 보였다. 30 % 또는 55 % 더 낮은 방사선량을 보인 단일 에너지 영상으로부터 계산된 측정 반복성은, 표준 방사선 량으로 촬영하여 반복영상재구성 기법으로 재구성한 기존의 단일 에너지 CT 이미지의 반복성에 필적했다. 결론: 이중층 스펙트럼 CT에서 촬영하고 재구성된 단일에너지 영상은 비고형성 결절 측정의 반복성을 향상시킨다. 또한 단일에너지 영상을 사용하면 측정 반복성을 유지하면서 방사선량을 낮추어 폐암 선별 검사를 시행할 수 있게 된다.Abstract i Contents iii List of tables and figures iv List of abbreviations v Introduction 1 Materials and Methods 3 Results 11 Discussion 16 Conclusions 20 References 21 Abstract in Korean 26Docto

Standards for quantitative assessments by coronary computed tomography angiography (CCTA)

In current clinical practice, qualitative or semi-quantitative measures are primarily used to report coronary artery disease on cardiac CT. With advancements in cardiac CT technology and automated post-processing tools, quantitative measures of coronary disease severity have become more broadly available. Quantitative coronary CT angiography has great potential value for clinical management of patients, but also for research. This document aims to provide definitions and standards for the performance and reporting of quantitative measures of coronary artery disease by cardiac CT.</p

Stress-strain analysis of aortic aneurysms

Tato práce se zabývá problematikou aneurysmat břišní aorty a možností využít konečnoprvkovou deformačně-napěťovou analýzu těchto aneurysmat ke stanovení rizika ruptury. První část práce je věnována úvodu do problematiky, popisu kardiovaskulární soustavy člověka s důrazem na abdominální aortu, anatomii, fyziologii a patologii stěny tepny s důrazem na procesy vedoucí ke vzniku aneurysmatu. Dále se práce věnuje rizikovým faktorům přispívajících ke vzniku aneurysmat spolu s analýzou současných klinických postupů ke stanovení rizika ruptury spolu se srovnáním navrhovaného kritéria maximálního napětí. Dominantní část této disertace je věnována identifikaci faktorů ovlivňujících napjatost a deformaci stěny aneurysmatu spolu s návrhem nových postupů, prezentací vlastních poznatků vedoucích ke zpřesnění určení rizika ruptury pomocí deformačně- napěťové analýzy a metody konečných prvků. Nejprve je analyzován vliv geometrie, vedoucí k závěru, že je nezbytné používání individuálních geometrií pacienta. Dále je pozornost zaměřena na odbočující tepny, které ve stěně působí jako koncentrátor napětí a mohou tedy ovlivňovat napjatost v ní. Jako další podstatný faktor byl identifikován vliv nezatížené geometrie a bylo napsáno makro pro její nalezení, které bylo opět zahrnuto jako standardní součást do výpočtového modelu. Mechanické vlastnosti jak stěny aneurysmatu, tak intraluminálního trombu jsou experimentálně testovány pomocí dvouosých zkoušek. Také je zde analyzován vliv modelu materiálu, kde je ukázáno, že srovnávání maximálních napětí u jednotlivých modelů materiálu není vhodné díky zcela rozdílným gradientům napětí ve stěně aneurysmatu. Dále je zdůrazněna potřeba znalosti distribuce kolagenních vláken ve stěně a navržen program k jejímu získání. Intraluminální trombus je analyzován ve dvou souvislostech. Jednak je ukázán vliv jeho ruptury na napětí ve stěně a jednak je analyzován vliv jeho poroelastické struktury na totéž. Posledním identifikovaným podstatným faktorem je zbytková napjatost ve stěně. Její významnost je demonstrována na několika aneurysmatech a i tato je zahrnuta jako integrální součást do našeho výpočtového modelu.Na závěr jsou pak navrženy další možné směry výzkumu.This thesis deals with abdominal aortic aneurysms and the possibility of using finite element method in assessment of their rupture risk. First part of the thesis is dedicated to an introduction into the problem, description of human cardiovascular system where the abdominal aorta, its anatomy, physiology and pathology is emphasized. There Processes leading to formationing of abdominal aortic aneurysms are also discussed. Risk factors contributing to creation of aneurysms are discussed next. Finally, an analysis of current clinical criteria which determine rupture risk of an abdominal aortic aneurysm is presented and compared with the new maximum stress criterion being currently in development. Main part of the thesis deals with the identification of relevant factors which affect stress and deformation of aneurysmal wall. This is connected with proposals of new approaches leading to predicting the rupture risk more accurately by using finite element stress-strain analysis. The impact of geometry is analyzed first with the conclusion that patient-specific geometry is a crucial input in the computational model. Therefore its routine reconstruction has been managed. Attention is then paid to the branching arteries which were neglected so far although they cause a stress concentration in arterial wall. The necessity of knowing the unloaded geometry of aneurysm is then emphasized. Therefore a macro has been written in order to be able to find the unloaded geometry for any patient-specific geometry of aneurysm. Mechanical properties of both aneurysmal wall and intraluminal thrombus were also experimentally tested and their results were fitted by an isotropic material model. The effect of the material model itself has been also investigated by comparing whole stress fields of several aneurysms. It has been shown that different models predict completely different stresses due to different stress gradients in the aneurysmal wall. The necessity of known collagen fiber distribution in arterial wall is also emphasized. A special program is then presented enabling us to obtain this information. Effect of intraluminal thrombus on the computed wall stress is analyzed in two perspectives. First the effect of its failure on wall stress is shown and also the impact of its poroelastic structure is analyzed. Finally the residual stresses were identified as an important factor influencing the computed wall stress in aneurysmal wall and they were included into patient-specific finite element analysis of aneurysms. Further possible regions of investigation are mentioned as the last part of the thesis.

3-D fracture tracing for X-ray computed tomography data

X-ray computed tomography (CT) is a nondestructive imaging method that shows differences in X-ray attenuation, which is a proxy for density. Sensitivity to density differences makes CT a good choice for imaging open natural fractures because of the significant density contrast between air and rock. The image data, however, are prone to artifacts and blurring, which makes taking accurate measurements challenging. As a first step to addressing this problem, a tool was created to quantify the spatial resolution of CT data with a point-spread function (PSF). The PSF tool permits accurate measurement of fine-scale features in CT data – critical for measuring fractures, which are often thinner in one dimension than the PSF size, in turn influencing measurement of aperture. The difference between the measurements given by the PSF method and a simple threshold value pick is shown to demonstrate a non-trivial improvement in accuracy. In addition, a 3-D fracture network tracing algorithm was developed, for which the PSF is a necessary input, to characterize accurately the network’s attributes, such as fracture orientations, apertures, and roughnesses. To date, only single, isolated, relatively flat fractures have been characterized thoroughly in 3-D for CT data, and most numerical modeling has been conducted on 2D subsets. Additionally, research involving fracture networks is currently limited to simplified numerical models and simulations. This new work extends CT fracture characterization to the majority of fractured materials with 3-D networks, thus providing a source of real data for studying the difference between fracture networks and single fractures. A sample of Packsaddle schist, with a large number of thin fractures and complex, anastomosing bifurcations, was selected as a testing ground for the network tracing algorithms being developed. Preliminary analysis verifies the method’s efficacy.Geological Science

Quantifying Airway Dilatation in the Lungs from Computed Tomography

Non CF bronchiectasis and idiopathic pulmonary fibrosis (IPF) are pulmonary diseases characterised by the abnormal and permanent dilatation of the airways. Computed tomography (CT) is used in clinical practice to diagnose and monitor patients with the disease. Currently, analysis of the scans is performed by manual inspection and there is no established computerised method to quantify the enlargement of airways. I developed a pipeline to quantify the cross-sectional area for a given airway track. Using an airway segmentation, my proposed algorithm measures the area at contiguous intervals along the airway arclength from the Carina to the most distal point visible on CT. I showed the use of the data generated from the pipeline in two applications. First, I proposed a novel tapering measure as the gradient of a linear regression between a logarithmic area against the arclength. The measurement was applied to airways affected by bronchiectasis. Second, I used Bayesian Changepoint Detection (BCD) with the area measurements to locate the progression of IPF along the airway track. The proposed pipeline was applied to a set of clinically acquired scans. I show a statistical difference (p = 3.4×10−4 ) in the tapering measurement between bronchiectatic (n = 53) and controlled (n = 39) airways. In addition, I report a statistical difference (p = 7.2×10−3 ) in the change in measurement between airways remaining healthy (n = 14) and airways that have become bronchiectatic (n = 5). I show the tapering measurement is reproducible independent to voxel size, CT reconstruction, and radiation dose. Using BCD, I show on simulated data (n = 14) my proposed method can detect the progression of IPF within 2.5mm. Finally, using results from BCD, I present a novel measure of IPF progression as the percentage volume change in the diseased region of the airways