321 research outputs found

    Toward time resolved 4D cardiac CT imaging with patient dose reduction: estimating the global heart motion

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    Applications of Rapid Cardiac Micro-CT

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    Mouse models are an important tool in cardiovascular disease research and a non-invasive imaging method is an advantageous way of monitoring disease progression. Cardiac micro-CT is rapid imaging technique capable of quantifying changes in cardiac structure and function in mice. The goal of this thesis was to demonstrate the utility of this technique in monitoring disease progression in a longitudinal study, as well as its capability for evaluating other methods of measuring cardiac function in mice. In a longitudinal study, a mouse model of myocardial infarction was scanned weekly for four weeks; left ventricular volume and ejection fraction were measured from the images. Cardiac micro-CT was capable of tracking small changes in cardiac structure and function, with the MI mice demonstrating a significant increase in volume and a significant decrease in ejection fraction. Both inter- and intra-variability was low, indicating the results were highly reproducible. Contrast agents are essential to evaluating the heart in micro-CT images. A new blood-pool agent was evaluated to determine its suitability for use in cardiac micro-CT studies. The agent produced excellent enhancement for the first 30 minutes post-injection, and had a unique characteristic of enhancing the myocardium, which may prove useful in studies evaluating wall motion. The effect of x-ray dose delivered during a longitudinal micro-CT study was also evaluated. C57BL/6 mice were scanned weekly for six weeks; the total entrance dose delivered over the study was 5.04 Gy. No significant changes to the heart or lungs were detectable on the micro-CT images at six weeks, and the histology performed on myocardial and pulmonary tissue showed no indication of early inflammation at a cellular level. Micro-CT can therefore be used in longitudinal studies without concern of adverse effects. Cardiac micro-CT was used to evaluate conductance catheters, and found that the catheter volumes were drastically underestimated compared to the micro-CT volumes. It was also determined that catheterization has the potential for causing cardiac enlargement; 40% of the mice demonstrated enlarged hearts following the catheterization procedure. Overall, cardiac-gated micro-CT is a rapid and reproducible imaging technique, and is proving to be valuable tool in cardiovascular disease research

    Cardiac Multidetector Computed Tomography: Basic Physics of Image Acquisition and Clinical Applications

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    Cardiac MDCT is here to stay. And, it is more than just imaging coronary arteries. Understanding the differences in and the benefits of one CT scanner from another will help you to optimize the capabilities of the scanner, but requires a basic understanding of the MDCT imaging physics

    Computed Tomography of the Coronary Arteries

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    Non-invasive coronary computed tomography angiography (CCTA) has become an important tool for visualisation of coronary arteries since the introduction of 64-channel detector CCTA in 2004. It has been proved to be especially beneficial for ruling out coronary artery disease (CAD) in selected patient populations, due to the high negative predictive value (NPV). The aim of this thesis was to study some aspects of the introduction, establishment and development of a new method, retrospectively ECG-gated CCTA with 64-channel detector, to evaluate coronary arteries. In study I the diagnostic capacity and limitation of CCTA was compared to that of invasive coronary angiography (ICA) in a newly established CCTA team. CCTA had a very high NPV but the number of non-diagnostic scans was also high. The main limitations were motion artifacts and vessel calcifications, while short experience in reading CCTA did not affect image interpretation. Study II described the learning-curve effect of the interpretation of 100 CCTA and also compared the diagnostic accuracy of both radiologists and radiographers, after a common introduction. The review time for novices was approximately halved during the first 100 cases, with maintained diagnostic accuracy. There was a learning-curve effect in positive predictive value (PPV) for radiologists, but not for the radiographers. However, the diagnostic accuracy of dedicated radiographers indicated that they might be considered as part of the evaluation team. Study III compared the radiation exposure in retrospectively ECG-gated CCTA and ICA in the same population. Both mean estimated effective dose (ED) and organ doses (skin, breast, lung and oesophagus) were higher in CCTA when compared to ICA. The relatively high radiation dose to breast indicates that bismuth shielding should be used in women when performing CCTA. When using the updated tissue weighting factors provided in ICRP 103 the calculated ED from CCTA were significantly higher than those obtained using outdated ICRP 60. In study IV the image quality and radiation doses were compared when decreasing X-ray tube peak kilovoltage (kVp) from 120 to 100 kVp in patients undergoing CCTA. By reduction of tube voltage the radiation dose was almost halved while the diagnostic image quality was kept at a clinically acceptable level. In conclusion, CCTA is increasingly available throughout the world as an alternative to gold standard ICA, especially due to the excellent capability to rule out CAD. Still, retrospectively ECG-gated 64-channel detector CCTA has limitations such as motion artifacts and vessel calcifications. Another limitation is the high radiation doses required for CCTA compared to ICA. By lowering the kVp from traditionally 120 kVp to 100 kVp the radiation dose is halved while retaining diagnostic accuracy. There is a learning curve effect (regarded PPV and review time) of the interpretation of CCTA. However, more than 100 reviewed CCTA cases are necessary to reach a diagnostic accuracy that is acceptable

    Computed Tomography Imaging of the Coronary Arteries

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    Automated Selection of the Optimal Cardiac Phase for Single-Beat Coronary CT Angiography Reconstruction

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    This thesis investigates an automated algorithm for selecting the optimal cardiac phase for CCTA reconstruction. Reconstructing a low-motion cardiac phase improves coronary artery visualization in coronary CT angiography (CCTA) exams. Currently, standard end-systole and/or mid-diastole default phases are prescribed or alternatively, quiescent phases are determined by the user. As manual selection may be time-consuming and standard locations may be suboptimal due to patient variability, an automated method is investigated. An automated algorithm was developed to select the optimal phase based on quantitative image quality (IQ) metrics. For each reconstructed slice at each reconstructed phase, an image quality metric was calculated based on measures of circularity and edge strength of through-plane vessels. The image quality metric was aggregated across slices, while a metric of vessel-location consistency was used to ignore slices that did not contain through-plane vessels. A binary metric based on the edge strength of in-plane vessels was calculated to determine if IQ of in-plane vessels was acceptable. The algorithm performance was evaluated using two observer studies. Fourteen single-beat CCTA exams (Revolution CT, GE Healthcare) reconstructed at 2% intervals were evaluated for best systolic (1), diastolic (6), or systolic and diastolic phases (7) by three readers and the algorithm. Inter-reader (RR) and reader-algorithm (RA) agreement was calculated using the mean absolute difference (MAD) and concordance correlation coefficient (CCC). A reader-consensus best phase was determined and compared to the algorithm selected phase. In cases where the algorithm and consensus best phases differed by more than 2%, IQ was scored by three readers using a 5pt Likert scale. There was no significant difference between RR and RA agreement for either MAD or CCC metrics (p\u3e0.2). The algorithm phase was within 2% of the consensus phase in 71% of cases. There was no significant difference (p\u3e0.2) between the IQ of the algorithm phase (4.06±0.73) and the consensus phase (4.11±0.76). The proposed algorithm was statistically equivalent to a reader in selecting an optimal cardiac phase for CCTA exams. When reader and algorithm phases differed by \u3e2%, IQ was statistically equivalent

    Evaluation of Motion Artifact Metrics for Coronary CT Angiography

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    Purpose This study quantified the performance of coronary artery motion artifact metrics relative to human observer ratings. Motion artifact metrics have been used as part of motion correction and best‐phase selection algorithms for Coronary Computed Tomography Angiography (CCTA). However, the lack of ground truth makes it difficult to validate how well the metrics quantify the level of motion artifact. This study investigated five motion artifact metrics, including two novel metrics, using a dynamic phantom, clinical CCTA images, and an observer study that provided ground‐truth motion artifact scores from a series of pairwise comparisons. Method Five motion artifact metrics were calculated for the coronary artery regions on both phantom and clinical CCTA images: positivity, entropy, normalized circularity, Fold Overlap Ratio (FOR), and Low‐Intensity Region Score (LIRS). CT images were acquired of a dynamic cardiac phantom that simulated cardiac motion and contained six iodine‐filled vessels of varying diameter and with regions of soft plaque and calcifications. Scans were repeated with different gantry start angles. Images were reconstructed at five phases of the motion cycle. Clinical images were acquired from 14 CCTA exams with patient heart rates ranging from 52 to 82 bpm. The vessel and shading artifacts were manually segmented by three readers and combined to create ground‐truth artifact regions. Motion artifact levels were also assessed by readers using a pairwise comparison method to establish a ground‐truth reader score. The Kendall\u27s Tau coefficients were calculated to evaluate the statistical agreement in ranking between the motion artifacts metrics and reader scores. Linear regression between the reader scores and the metrics was also performed. Results On phantom images, the Kendall\u27s Tau coefficients of the five motion artifact metrics were 0.50 (normalized circularity), 0.35 (entropy), 0.82 (positivity), 0.77 (FOR), 0.77(LIRS), where higher Kendall\u27s Tau signifies higher agreement. The FOR, LIRS, and transformed positivity (the fourth root of the positivity) were further evaluated in the study of clinical images. The Kendall\u27s Tau coefficients of the selected metrics were 0.59 (FOR), 0.53 (LIRS), and 0.21 (Transformed positivity). In the study of clinical data, a Motion Artifact Score, defined as the product of FOR and LIRS metrics, further improved agreement with reader scores, with a Kendall\u27s Tau coefficient of 0.65. Conclusion The metrics of FOR, LIRS, and the product of the two metrics provided the highest agreement in motion artifact ranking when compared to the readers, and the highest linear correlation to the reader scores. The validated motion artifact metrics may be useful for developing and evaluating methods to reduce motion in Coronary Computed Tomography Angiography (CCTA) images

    Motion compensated iterative reconstruction for cardiac X-ray tomography

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    Within this Ph.D. project, three-dimensional reconstruction methods for moving objects (with a focus on the human heart) from cone-beam X-ray projections using iterative reconstruction algorithms were developed and evaluated. This project was carried in collaboration with the Digital Imaging Group of Philips Research Europe – Hamburg. In cardiac cone-beam computed tomography (CT) a large effort is continuously dedicated to increase scanning speed in order to minimize patient or organ motion during acquisition. In particular, motion causes severe artifacts such as blurring and streaks in tomographic images. While for a large class of applications the current scanning speed is sufficient, in cardiac CT image reconstruction improvements are still required. Whereas it is currently feasible to achieve stable image quality in the resting phases of the cardiac cycle, in the phase of fast motion data acquisition is too slow. A variety of algorithms to reduce or compensate for motion artifacts have been proposed in literature. Most of the correction methods address the calculation of consistent projection data belonging to the same motion state (gated CT reconstruction). Even if gated CT leads to better results, not only with respect to the processing time but also regarding the image quality, it is also limited in its temporal and spatial resolution due to the mechanical movement of the gantry. This can lead to motion blurring, especially in the phases of fast cardiac motion during the RR interval. A motion-compensated reconstruction method for CT can be used to improve the resolution of the reconstructed image and to suppress motion blurring. Iterative techniques are a promising approach to solve this problem, since no direct inversion methods are known for arbitrarily moving objects. In this work, we therefore introduced motion compensation into image reconstruction. In order to determine the unknown cardiac motion, 3 different cardiac-motion estimation methodologies were implemented. Visual and quantitative assessment of the method in a number of applications, including: phantoms; cardiac CT reconstructions; Region of Interest (ROI) CT reconstructions of left and right coronaries of several clinical patients, confirmed its potential

    Respiratory organ motion in interventional MRI : tracking, guiding and modeling

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    Respiratory organ motion is one of the major challenges in interventional MRI, particularly in interventions with therapeutic ultrasound in the abdominal region. High-intensity focused ultrasound found an application in interventional MRI for noninvasive treatments of different abnormalities. In order to guide surgical and treatment interventions, organ motion imaging and modeling is commonly required before a treatment start. Accurate tracking of organ motion during various interventional MRI procedures is prerequisite for a successful outcome and safe therapy. In this thesis, an attempt has been made to develop approaches using focused ultrasound which could be used in future clinically for the treatment of abdominal organs, such as the liver and the kidney. Two distinct methods have been presented with its ex vivo and in vivo treatment results. In the first method, an MR-based pencil-beam navigator has been used to track organ motion and provide the motion information for acoustic focal point steering, while in the second approach a hybrid imaging using both ultrasound and magnetic resonance imaging was combined for advanced guiding capabilities. Organ motion modeling and four-dimensional imaging of organ motion is increasingly required before the surgical interventions. However, due to the current safety limitations and hardware restrictions, the MR acquisition of a time-resolved sequence of volumetric images is not possible with high temporal and spatial resolution. A novel multislice acquisition scheme that is based on a two-dimensional navigator, instead of a commonly used pencil-beam navigator, was devised to acquire the data slices and the corresponding navigator simultaneously using a CAIPIRINHA parallel imaging method. The acquisition duration for four-dimensional dataset sampling is reduced compared to the existing approaches, while the image contrast and quality are improved as well. Tracking respiratory organ motion is required in interventional procedures and during MR imaging of moving organs. An MR-based navigator is commonly used, however, it is usually associated with image artifacts, such as signal voids. Spectrally selective navigators can come in handy in cases where the imaging organ is surrounding with an adipose tissue, because it can provide an indirect measure of organ motion. A novel spectrally selective navigator based on a crossed-pair navigator has been developed. Experiments show the advantages of the application of this novel navigator for the volumetric imaging of the liver in vivo, where this navigator was used to gate the gradient-recalled echo sequence

    Coronary CT angiography: Diagnostic value and clinical challenges

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    Coronary computed tomography (CT) angiography has been increasingly used in the diagnosis of coronary artery disease due to improved spatial and temporal resolution with high diagnostic value being reported when compared to invasive coronary angiography. Diagnostic performance of coronary CT angiography has been significantly improved with the technological developments in multislice CT scanners from the early generation of 4-slice CT to the latest 320- slice CT scanners. Despite the promising diagnostic value, coronary CT angiography is still limited in some areas, such as inferior temporal resolution, motion-related artifacts and high false positive results due to severe calcification. The aim of this review is to present an overview of the technical developments of multislice CT and diagnostic value of coronary CT angiography in coronary artery disease based on different generations of multislice CT scanners. Prognostic value of coronary CT angiography in coronary artery disease is also discussed, while limitations and challenges of coronary CT angiography are highlighted
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