ISSN:0899-3459    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Cardiac magnetic resonance imaging is a rapidly emerging field that has seen tremendous advances in the past decade. Central to the development of effective imaging strategies has been the advent of high-performance gradient hardware and the exploitation of their speed characteristics through specialized pulse sequences well suited for cardiac imaging. These advances have facilitated unprecedented acquisition times that now approach echocardiographic frame rates, while maintaining excellent image quality. This article provides a detailed overview of advanced pulse sequence technology and approaches currently taken to maximize speed performance and image quality. In particular, segmented &kgr;-space techniques that include single-echo and multiecho spoiled gradient-echo imaging as well as steady-state free precession imaging are discussed. Finally, spiral and fast spin-echo techniques are explored. Examples of common applications of these pulse sequences are presented.

ISSN:0899-3459    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Cardiac magnetic resonance imaging is made possible by utilizing an external signal, the patient's own electrocardiogram, to time image acquisition. ECG gated cardiac magnetic resonance examination may be performed using spin-echo or gradient reversal techniques or their newer k-space segmented variants. Spin-echo techniques provide the highest contrast between rapidly moving blood and the cardiac chambers and arteries and veins containing the blood, but are of relatively low temporal resolution. Gradient reversal acquisition, however, provides higher temporal resolution ideal for evaluating changes in myocardial thickening, ventricular wall motion abnormalities, and changes in chamber volume through the cardiac cycle; gradient reversal technique has significantly lower contrast resolution. Careful attention to the details of image acquisition will provide high-quality images of the heart and great arteries from which important morphologic and physiological information may be obtained, aiding in the diagnosis and management of patients with cardiovascular disease.

ISSN:0899-3459    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

One of the strong assets of cardiac magnetic resonance (CMR) is its ability to assess myocardial anatomy, structure, function, flow, and perfusion within a single examination. Quantification of global and regional function from magnetic resonance imaging (MRI) studies was shown to be accurate and reproducible in experimental and clinical research studies. With the advent of high-performance MRI scanners and newly developed pulse sequences, image acquisition times have been reduced considerably in recent years. However, the clinical use of CMR remains limited for various reasons. Among these limitations is that the amount of images obtained in a typical cardiac examination is so large that visual and especially quantitative image analysis is tedious and time consuming. There is an urgent need for optimized dedicated software tools featuring highly automated contour detection and optimized display capabilities to present the quantitative results to the physician in an orderly fashion, thus facilitating clinical decision making. This article focuses on the state of the art in CMR postprocessing techniques for quantitative assessment of global and regional function.

ISSN:0899-3459    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Magnetic resonance imaging with tissue tagging is a noninvasive technique for measuring three-dimensional motion and deformation in the human heart. Tags are regions of tissue whose longitudinal magnetization has been altered before imaging so that they appear dark in subsequent magnetic resonance images. They then move with the underlying tissue and serve as easily identifiable landmarks within the heart for the detailed detection of motion. Many different motion and strain parameters can be determined from tagged magnetic resonance imaging. Strain components that are based on a high density of tag data, such as circumferential and longitudinal shortening, or parameters that are combinations of multiple strain components, have highest measurement precision and tightest normal ranges. The pattern of three-dimensional motion and strain in the heart is important clinically, because it reflects the basic mechanical function of the myocardium at both local and global levels. Localized abnormalities can be detected and quantified if the pattern of deformation in a given heart is compared to the normal range for that region, because normal motion and strain in the left ventricle is spatially heterogeneous. Contraction strains typically are greatest in the anterior and lateral walls and increase toward the apex. The direction of greatest contraction lies along a counter clockwise helix from base to apex (viewed from the base) and approximates the epicardial muscle fiber direction. This fiber geometry also results in long-axis torsion during systole. Ejection is accomplished primarily by radially inward motion of the endocardium and by descent of the base toward the apex during systole.

ISSN:0899-3459    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Magnetic resonance imaging offers the unique opportunity to directly visualize the size and location of myocardial infarcts (MIs) with excellent spatial resolution. Because infarct size is the most important determinant of postinfarct outcome, precise determination of infarct size may be valuable to risk stratify patients after acute MI. In addition, infarct imaging may provide direct information on the amount of irreversibly injured myocardium and thus can be used to identify myocardial viability in dysfunctional regions. Acute infarcts can be recognized as hyperintense signal on T2-weighted spin-echo images. This technique, however, does not identify chronic infarcts and may overestimate infarct size by including area at risk. Also, T2-weighted images often have a low signal-to-noise ratio. Contrast-enhanced perfusion imaging provides better-quality images. Extravascular contrast agents such as (Gd-DTPA) gadolinium diethyletriaminepentaacetic acid identify infarcts as hyperenhanced regions on images acquired late after contrast injection. In addition, these tracers can examine the integrity and permeability of infarct microvasculature on first-pass perfusion images. Necrosis avid tracers and23Na imaging are other new exciting approaches to identify infarcted myocardium acutely after MI. These techniques, are still investigational, and their value for clinical imaging remains to be established.

ISSN:0899-3459    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Magnetic resonance first-pass perfusion imaging has developed considerably over the past decade. It is possible to acquire 7–8 imaging planes every heartbeat at rest and in two heartbeats during stress using high-performance gradients and hybrid echo-planar methods. T1 weighting can be achieved with volumetric saturation pulses or selective “notch” pulses. First-pass studies can be quantified, but it also is possible to directly visualize myocardial perfusion abnormalities as subendocardial defects with less contrast enhancement than surrounding myocardium. It is feasible to detect stress-induced perfusion abnormalities in patients with coronary artery disease. Magnetic resonance imaging (MRI) perfusion abnormalities associated with myocardial infarction have significantly different characteristics from those seen on nuclear methods such as thallium, where the final appearance of images represents a combination of perfusion, viability, and wall thickness. Infarcted myocardium enhances during the first-pass MRI study unless there is microvascular or epicardial obstruction. Microvascular obstruction after myocardial infarction is easily detected and has adverse prognostic significance. Stress-induced perfusion abnormalities are not synonymous with coronary artery disease, as they can be detected in hypertrophic cardiomyopathy. MRI perfusion methods appear promising as long as physicians interpret the results in accordance of the physiology portrayed in the images.

ISSN:0899-3459    

出版商:OVID    

年代:2000

数据来源: OVID

ISSN:0899-3459    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Coronary artery magnetic resonance imaging strategies have tended to focus on the use of a single method performed during either breath-holding or free-breathing for all patients. However, significant variations exist among patients in terms of breath-holding ability and respiratory regularity that make the use of a single technique alone not universally successful. Therefore, it is prudent to make available a number of magnetic resonance imaging methods such that an appropriate respiratory motion reduction strategy can be tailored to suit the patient's respiratory pattern and characteristics. A tailored approach that can draw on different image acquisition techniques for coronary artery imaging is presented. A decision tree is proposed to triage patients into imaging regimes with the greatest probability of success, according to the patient's ability to breath-hold or exhibit steady respiration. Methods include volume free-breathing acquisitions using navigator echoes for respiratory monitoring in the 8-to 10-min scan time range, two-dimensional spiral navigators (2-to 3-min scan time), breath-held multislice and vessel-tracking spirals (16-to 20-second scan time), and real-time imaging approaches incorporating adaptive signal averaging. The development of multiple acquisition strategies substantially improves the opportunities to generate high-quality, diagnostic images of the coronary arteries.

ISSN:0899-3459    

出版商:OVID    

年代:2000

数据来源: OVID