State of the art: iterative CT reconstruction techniques

Owing to recent advances in computing power, iterative reconstruction (IR) algorithms have become a clinically viable option in computed tomographic (CT) imaging. Substantial evidence is accumulating about the advantages of IR algorithms over established analytical methods, such as filtered back projection. IR improves image quality through cyclic image processing. Although all available solutions share the common mechanism of artifact reduction and/or potential for radiation dose savings, chiefly due to image noise suppression, the magnitude of these effects depends on the specific IR algorithm. In the first section of this contribution, the technical bases of IR are briefly reviewed and the currently available algorithms released by the major CT manufacturers are described. In the second part, the current status of their clinical implementation is surveyed. Regardless of the applied IR algorithm, the available evidence attests to the substantial potential of IR algorithms for overcoming traditional limitations in CT imaging

Blooming Artifact Reduction in Coronary Artery Calcification by A New De-blooming Algorithm: Initial Study

The aim of this study was to investigate the use of de-blooming algorithm in coronary CT angiography (CCTA) for optimal evaluation of calcified plaques. Calcified plaques were simulated on a coronary vessel phantom and a cardiac motion phantom. Two convolution kernels, standard (STND) and high-definition standard (HD STND), were used for imaging reconstruction. A dedicated de-blooming algorithm was used for imaging processing. We found a smaller bias towards measurement of stenosis using the deblooming algorithm (STND: bias 24.6% vs 15.0%, range 10.2% to 39.0% vs 4.0% to 25.9%; HD STND: bias 17.9% vs 11.0%, range 8.9% to 30.6% vs 0.5% to 21.5%). With use of de-blooming algorithm, specificity for diagnosing significant stenosis increased from 45.8% to 75.0% (STND), from 62.5% to 83.3% (HD STND); while positive predictive value (PPV) increased from 69.8% to 83.3% (STND), from 76.9% to 88.2% (HD STND). In the patient group, reduction in calcification volume was 48.1 ± 10.3%, reduction in coronary diameter stenosis over calcified plaque was 52.4 ± 24.2%. Our results suggest that the novel de-blooming algorithm could effectively decrease the blooming artifacts caused by coronary calcified plaques, and consequently improve diagnostic accuracy of CCTA in assessing coronary stenosis

Artificial Intelligence (Enhanced Super-Resolution Generative Adversarial Network) for Calcium Deblooming in Coronary Computed Tomography Angiography: A Feasibility Study

Background: The presence of heavy calcification in the coronary artery always presents a challenge for coronary computed tomography angiography (CCTA) in assessing the degree of coronary stenosis due to blooming artifacts associated with calcified plaques. Our study purpose was to use an advanced artificial intelligence (enhanced super-resolution generative adversarial network [ESRGAN]) model to suppress the blooming artifact in CCTA and determine its effect on improving the diagnostic performance of CCTA in calcified plaques. Methods: A total of 184 calcified plaques from 50 patients who underwent both CCTA and invasive coronary angiography (ICA) were analysed with measurements of coronary lumen on the original CCTA, and three sets of ESRGAN-processed images including ESRGAN-high-resolution (ESRGAN-HR), ESRGAN-average and ESRGAN-median with ICA as the reference method for determining sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV). Results: ESRGAN-processed images improved the specificity and PPV at all three coronary arteries (LAD-left anterior descending, LCx-left circumflex and RCA-right coronary artery) compared to original CCTA with ESRGAN-median resulting in the highest values being 41.0% (95% confidence interval [CI]: 30%, 52.7%) and 26.9% (95% CI: 22.9%, 31.4%) at LAD; 41.7% (95% CI: 22.1%, 63.4%) and 36.4% (95% CI: 28.9%, 44.5%) at LCx; 55% (95% CI: 38.5%, 70.7%) and 47.1% (95% CI: 38.7%, 55.6%) at RCA; while corresponding values for original CCTA were 21.8% (95% CI: 13.2%, 32.6%) and 22.8% (95% CI: 20.8%, 24.9%); 12.5% (95% CI: 2.6%, 32.4%) and 27.6% (95% CI: 24.7%, 30.7%); 17.5% (95% CI: 7.3%, 32.8%) and 32.7% (95% CI: 29.6%, 35.9%) at LAD, LCx and RCA, respectively. There was no significant effect on sensitivity and NPV between the original CCTA and ESRGAN-processed images at all three coronary arteries. The area under the receiver operating characteristic curve was the highest with ESRGAN-median images at the RCA level with values being 0.76 (95% CI: 0.64, 0.89), 0.81 (95% CI: 0.69, 0.93), 0.82 (95% CI: 0.71, 0.94) and 0.86 (95% CI: 0.76, 0.96) corresponding to original CCTA and ESRGAN-HR, average and median images, respectively. Conclusions: This feasibility study shows the potential value of ESRGAN-processed images in improving the diagnostic value of CCTA for patients with calcified plaques

이중 에너지 단층 촬영에서의 구현 방법에 따른 가상 단색 재구성 영상의 비교 평가 및 블루밍 인공물 감소에 대한 효과 분석

학위논문(석사)--서울대학교 대학원 :의과대학 의학과,2019. 8. 이활.서론 이중 에너지 컴퓨터 단층 촬영 (DECT)에서 구현하는 가상 재구성 단색영상의 경우 바탕 물질을 구분하고 더 나아가 물질의 K-edge를 시각화함으로서 물질 차별화에 도움을 줄 수 있을 것으로 기대되는 기술이다. 또한 높은 keV 수준으로 재구성된 가상 단색영상의 경우 석회화된 물질에서 발생하는 블루밍 인공물을 감소시킴으로서 심혈관 영상에서 혈관 내경을 평가하는 데에 도움을 줄 수 있는 것으로 믿어지고 있다. 하지만 현재까지는 주요 3가지 이중 에너지 구현방식에 따라서 물질의 구분하고 K-edge를 구현할 수 있는지 여부를 연구한 논문은 없으며, 또한 정확한 측정을 통해 높은 keV의 가상 단색구성 영상이 통상적인 다색구성 영상의 window를 단순히 넓히는 것보다 블루밍 인공물을 감소시킨다는 것을 밝힌 연구 또한 없다. 따라서 본 연구의 목적은 가상 단색구성 영상을 통해 얻은 스펙트럴 곡선으로 DECT의 주요 3가지 구현방식에 따라서 K-edge를 시각화하고 물질을 구분할 수 있는지를 우선 평가하고, 더 나아가 혈관 석회화 팬텀을 통하여 높은 keV 가상 단색 재구성 영상에서 블루밍 인공물 감소여부를 평가한다. 방법 서로 다른 2개의 팬텀을 제작하여 이중 에너지의 구현 방식이 다른 세 종류의 DECT 기기를 통해 촬영한다. 이 세가지 구현방식은 (a) 2개의 튜브를 통한 순차 스캔, (b) 빠른 X 선관 전위 스위칭, 그리고 (c) 상이한 에너지 레벨의 광자를 흡수하는 다층 검출기의 사용과 같다. 이러한 팬텀 1 스캔 영상을 가상 단색영상으로 재구성하여 물질에 따른 스펙트럼 감쇠 곡선을 구성하였다. 그리고 팬텀 2 스캔 영상을 사용한 full width thirty percent maximum 방법의 측정을 통하여 블루밍 인공물로 인해 발생하는 오차를 가상 단색영상과 통상적인 다색영상에서 비교하였다. 결과 팬텀 1 촬영 영상의 가상 단색 재구성에 의해 얻어진 스펙트럼 감쇠 곡선은 구현 방법이나 재료 (칼슘, 요오드 및 가돌리늄)에 관계없이 K-edge를 시각화하지 못하였다. 그러나 감쇠곡선의 기울기 상수 β를 활용한 물질의 식별은 3 가지 방법 모두에서 가능하였고, 기울기 상수 β 값은 다층 검출기 방법에서 재료 내 일관성이 가장 뚜렷하였다. 또한 팬텀 2 촬영영상에서 높은 kVp 영상과 높은 keV 가상 단색 구성영상은 측정오류를 감소시키지 못하였으나 작은 FOV의 다색 및 가상 단색 재구성 영상은 큰 FOV 영상보다 통계적으로 유의하게 블루밍 인공물을 감소시켰다 (P<0.05). 결론 가상 단색영상 재구성으로 그린 스펙트럼 감쇄 곡선은 구현 방법이나 재료에 관계없이 K 에지를 시각화하지 못하였다. 높은 kVp 영상과 높은 keV 가상 단색 구성영상은 블루밍 인공물을 감소시키는 데에 실패하였으나 작은 FOV를 갖는 영상의 경우 기존 영상에 비해 블루밍 인공물을 유의하게 감소시키는 것으로 나타났다Introduction Virtual monochromatic images reconstructed by dual energy computed tomography (DECT) are expected to be useful in the discrimination of basic materials and may possibly visualize the K-edge of gadolinium, therefore enhancing material discrimination. Furthermore, virtual monochromatic images reconstructed at high keV levels are believed to reduce blooming artifact from calcified materials and aid in assessing vascular luminal patency at cardiovascular CT images. However, no previous study compared the three main dual energy implementation methods in discriminating and visualizing the K-edge of basic materials nor provided the objective measurement which proves the superiority of high keV virtual monochromatic images over simply widening the window width in conventional polychromatic images. Therefore, the purpose of this study is to compare the major three methods of DECT implementation in the perspective of K-edge visualization and material discrimination through spectral attenuation curves in virtual monochromatic reconstruction images. And furthermore, we analyzed high keV monochromatic and conventional polychromatic images to objectively compare the blooming artifacts in a vessel calcification phantom. Methods Two different phantoms were scanned by three DECT vendors with different method of dual energy implementation, which are (a) two temporally sequential scans, (b) rapid switching of X-ray tube potential and (c) multilayer detector absorbing photons at different energy level. Spectral attenuation curves of each basic material of Phantom 1 were obtained by monochromatic reconstruction and compared according to vendor and material. Comparison of blooming artifact between conventional polychromatic and virtual monochromatic images was done by the measurement of Phantom 2 using the full width thirty percent maximum measurement method. Results No peak regarding the K-edge of gadolinium was observed in spectral attenuation curves drawn by virtual monochromatic reconstruction of Phantom 1 images regardless of the implementation method or material (calcium, iodine and gadolinum). Material discrimination was possible in all three methods by the slope constant β, and the multilayer detector method showed highest intra-material consistency. In the study with Phantom 2, high kVp polychromatic and high keV monochromatic reconstruction images did not show reduction in measurement error compared to conventional kVp polychromatic images. However, small FOV proved to significantly decrease the blooming artifacts in polychromatic and monochromatic images Conclusion Spectral attenuation curves drawn by virtual monochromatic images failed to visualize K-edge regardless of the implementation method or material. High kVp images along with high keV monochromatic reconstruction images failed to reduce blooming artifact, but small FOV proved to significantly decrease the blooming artifacts in polychromatic and monochromatic imagesContents Abstract in English --------------------------------------------------------------- 1 Contents -------------------------------------------------------------------------- 5 List of tables and figures -------------------------------------------------------- 6 Introduction --------------------------------------------------------------------- 8 Materials and Methods ------------------------------------------------------- 11 Results ----------------------------------------------------------------------------16 Discussion ------------------------------------------------------------------------24 References -----------------------------------------------------------------------29 Abstract in Korean ------------------------------------------------------------33Maste

Dual-Source Photon-Counting Computed Tomography-Part I: Clinical Overview of Cardiac CT and Coronary CT Angiography Applications

The photon-counting detector (PCD) is a new computed tomography detector technology (photon-counting computed tomography, PCCT) that provides substantial benefits for cardiac and coronary artery imaging. Compared with conventional CT, PCCT has multi-energy capability, increased spatial resolution and soft tissue contrast with near-null electronic noise, reduced radiation exposure, and optimization of the use of contrast agents. This new technology promises to overcome several limitations of traditional cardiac and coronary CT angiography (CCT/CCTA) including reduction in blooming artifacts in heavy calcified coronary plaques or beam-hardening artifacts in patients with coronary stents, and a more precise assessment of the degree of stenosis and plaque characteristic thanks to its better spatial resolution. Another potential application of PCCT is the use of a double-contrast agent to characterize myocardial tissue. In this current overview of the existing PCCT literature, we describe the strengths, limitations, recent applications, and promising developments of employing PCCT technology in CCT

First in-human quantitative plaque characterization with ultra-high resolution coronary photon-counting CT angiography

Purpose: To assess the effect of ultra-high-resolution coronary CT angiography (CCTA) with photon-counting detector (PCD) CT on quantitative coronary plaque characterization. Materials and methods: In this IRB-approved study, 22 plaques of 20 patients (7 women; mean age 77 ± 8 years, mean body mass index 26.1 ± 3.6 kg/m2) undergoing electrocardiography (ECG)-gated ultra-high-resolution CCTA with PCD-CT were included. Images were reconstructed with a smooth (Bv40) and a sharp (Bv64) vascular kernel, with quantum iterative reconstruction (strength level 4), and using a slice thickness of 0.6, 0.4, and 0.2 mm, respectively (field-of-view 200 mm × 200 mm, matrix size 512 × 512 pixels). Reconstructions with the Bv40 kernel and slice thickness of 0.6 mm served as the reference standard. After identification of a plaque in coronary arteries with a vessel diameter ≥2 mm, plaque composition was determined using a dedicated, semi-automated plaque quantification software. Total plaque, calcified, fibrotic, and lipid-rich plaque components were quantified in all datasets. Results: Median plaque volume was highest (23.5 mm3, interquartiles 17.9-34.3 mm3) for reconstructions with the reference standard and lowest for ultra-high-resolution reconstructions with a slice thickness of 0.2 mm and the Bv64 kernel (18.1 mm3, interquartiles 14.1-25.8 mm3, p < 0.001). Reconstructions with the reference standard showed largest calcified (85.1%, interquartiles 76.4-91.1%) and smallest lipid-rich plaque components (0.5%, interquartiles 0.0-1.5%). Smallest calcified plaque components (75.2%, interquartiles 69.9-80.8%) and largest lipid-rich components (6.7%, interquartiles 5.1-8.4%) were found for ultra-high-resolution reconstructions with a slice thickness of 0.2 mm and the Bv64 kernel. At an identical slice thickness, volume of calcified components was always lower, and volume of lipid-rich components was always higher for reconstructions with the Bv64 kernel compared with reconstructions with the Bv40 kernel (all, p < 0.001). Conclusion: This patient study indicates significant differences of ultra-high-resolution scanning with PCD-CT on quantitative coronary plaque characterization. Reduced blooming artifacts may allow improved visualization of fibrotic and lipid-rich plaque components with the ultra-high-resolution mode of PCD-CT. Keywords: coronary artery disease; coronary computed tomographic angiography (CCTA); high risk plaque; photon-counting detector CT (PCD-CT); ultra-high-resolution C

Effects of Iterative Reconstruction on the Diagnostic Assesment of Coronary Calcium Scores

Coronary Artery Calcium (CAC) score is a widely used indicator to determine disease severity and predict the risk of severe cardiac events. However, radiation dose associated with coronary CT scanning for CAC scoring raises concerns, especially for asymptomatic patients. Iterative Reconstruction (IR) technique represents a recently developed image processing approach for reduction of image noise and radiation dose, while improving diagnostic image quality. Despite these advantages over conventional filtered back projection technique, effects of IR techniques on CAC scores remain unclear. This review article aims to provide an overview of clinical applications of IR techniques in coronary CT angiography with a focus on the effects of different IR techniques on CAC score assessment

Inter-observer agreement of the Coronary Artery Disease Reporting and Data System (CAD-RADS^{TM}) in patients with stable chest pain

Purpose: To assess inter-observer variability of the Coronary Artery Disease - Reporting and Data System (CAD-RADS) for classifying the degree of coronary artery stenosis in patients with stable chest pain. Material and methods: A prospective study was conducted upon 96 patients with coronary artery disease, who underwent coronary computed tomography angiography (CTA). The images were classified using the CAD-RAD system according to the degree of stenosis, the presence of a modifier: graft (G), stent (S), vulnerable plaque (V), or non-diagnostic (n) and the associated coronary anomalies, and non-coronary cardiac and extra-cardiac findings. Image analysis was performed by two reviewers. Inter-observer agreement was assessed. Results: There was excellent inter-observer agreement for CAD-RADS (k = 0.862), at 88.5%. There was excellent agreement for CAD-RADS 0 (k = 1.0), CAD-RADS 1 (k = 0.92), CAD-RADS 3 (k = 0.808), CAD-RADS 4 (k = 0.826), and CAD-RADS 5 (k = 0.833) and good agreement for CAD-RADS 2 (k = 0.76). There was excellent agreement for modifier G (k = 1.0) and modifier S (k = 1.0), good agreement for modifier N (k = 0.79), and moderate agreement for modifier V (k = 0.59). There was excellent agreement for associated coronary artery anomalies (k = 0.845), non-coronary cardiac findings (k = 0.857), and extra-cardiac findings (k = 0.81). Conclusions: There is inter-observer agreement of CAD-RADS in categorising the degree of coronary arteries stenosis, and the modifier of the system and associated cardiac and extra-cardiac findings