摘要:

AbstractBrain function can be mapped with magnetic resonance (MR) imaging sensitized to regional changes in blood oxygenation due to cortical activation. Several MR imaging methods, including conventional imaging and echo‐planar imaging, have been successfully used for this purpose. The authors investigated spiral k‐space MR imaging, implemented with an unmodified 1.5‐T clinical imager, for imaging of cortical activation. A gradient‐echo, spiral k‐space imaging method was used to measure activation in the primary visual cortex (number sequence task), primary motor cortex (fist‐clenching task), and prefrontal cortex (verbal fluency task). Comparison of conventional and spiral k‐space imaging in the visual and motor cortex, in which signal‐to‐noise ratio, voxel size, and imaging time were matched, showed that artifacts were reduced with the spiral k‐space method, while the area and degree of activation were similar. The number of sections that could be imaged in a fixed time interval was increased by a factor of four with this implementation of spiral k‐space imaging compared with

ISSN:1053-1807    

DOI:10.1002/jmri.1880050112

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractThe authors have developed a method for reducing magnetic resonance (MR) image artifacts caused by planar motion. Segments of k‐space acquired with the subject stationary are detected automatically. Each k‐space segment is Fourier transformed into an image in which rotational and translational displacements are measured manually. Before correction, k‐space is made as Hermitian as allowed by the largest symmetric range of low spatial frequencies acquired with the subject stationary. Segments of k‐space acquired with the subject in different positions are corrected separately. Although translation corrections can be applied effectively to both k‐space and the spatial domain, rotation corrections are applied in the spatial domain to avoid image artifacts. To complement the correction, data corrupted by rotation are replaced by the complex conjugate of data of the opposite kx, and ky, provided that these data have not been corrupted by rotation. The method reduced ghosts and blurring substantially on sagittal head images acquired with a standard spin‐echo pulse sequence while a volunteer subject nodd

ISSN:1053-1807    

DOI:10.1002/jmri.1880050113

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractThe authors developed and experimentally verified expressions that describe the frequency response of prospectively gated phase‐contrast magnetic resonance velocity measurements. Both interleaved and noninterleaved phase‐contrast techniques were evaluated. The primary determinants of the frequency response were (a) the number of interleaved acquisitions (N), (b) the time between acquisitions (ΔT), and (c) the degree of balance between the first moments of the velocity‐encoding gradients. To quantify the last factor, an imbalance parameter (U) was defined. Depending on the chosen implementation andU, deviations from the ideal frequency responses were predicted and observed. The expressions also revealed an advantage of interleaved acquisitions that use a one‐sided gradient configuration: no changes in the frequency response. The effects of concurrently encoding orthogonal velocity components with a Hadamard four‐point scheme wer

ISSN:1053-1807    

DOI:10.1002/jmri.1880050114

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractThe authors investigated eddy current artifacts due to metallic objects within the magnetic resonance imaging field. The problem was simplified by using a circular copper loop as a model for the more complex eddy current pathways present in a metallic implant. With this simple geometry, the authors show that radio‐frequency (RF)‐induced eddy currents in the metal produce a significant local artifact. However, no appreciable artifacts due to the switching magnetic field gradients were observed. A detailed quantitative analysis of the mechanism of RF‐induced eddy current artifact due to the the copper loop was performed. The artifact was demonstrated experimentally to result from perturbations of the transmit and receive sensitivities of the RF coil. Theoretical calculations of these perturbations showed excellent agreement with experimental results. With an understanding of the artifact mechanism, methods for correcting the RF‐induced eddy current artifact were

ISSN:1053-1807    

DOI:10.1002/jmri.1880050115

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractDiastolic function is an important element of overall left ventricular function. The pattern of flow across the mitral valve is commonly used as a measure of diastolic ventricular function. Magnetic resonance (MR) velocity mapping of blood flow across the mitral valve was compared with Doppler echocardiography. Nineteen patients with known coronary artery disease (mean age. 62 years: 11 with previous myocardial infarction) were studied. The mean value of peak early filling velocity (± standard deviation) was 60.1 cm/sec ± 14.3 with the MR method and 59.4 cm/sec ± 13.7 with echocardiography (P= 0.732). The mean difference between the two measurements (95% confidence interval) was −0.8 cm/sec (−5.2 cm/sec. +2.2 cm/sec). The mean value of early deceleration was 4.3 mlsec2± 1.5 with the MR method and 4.0 m/sec2± 1.5 with echocardiography (P = 0.073). The mean Werence was −0.4 cm/sec2(−0.92 cm/eec2. +0.05 cm/sec2). The mean value of peak atriosystolic velocity was 51 cm/sec ± 14.6 with the MR method and 62 cm/sec ± 17.2 with echocardiography (P = 0.002). The mean difference was −11 cm/sec (−18.1 cm/sec, −3.45 cm /sec). Peak atrial filling was consistently lower with the MR method than with echocardiography. Time‐averaged measurements of ventricular fflling with MR velocity mapping are an accurate measure of early diastolic filling but underestimate the velocity of

ISSN:1053-1807    

DOI:10.1002/jmri.1880050116

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractThe authors studied the magnetic resonance (MR) imaging appearance of excised human arteries and correlated these findings with those from histologic sections obtained in corresponding planes. Imaging was performed with a 1.5‐T clinical imager modified with an additional gradient insert capable of a 30 mT/m gradient. High‐resolution images of arteries obtained at autopsy indicated that the medial and adventitial layers could be distinguished and that T2‐weighted sequences offered superior contrast. Intimal thickening could be detected at a relatively early stage as a long T2 rim on the luminal surface of the artery. Atherosclerotic plaque was found to have MR properties similar to those of intimal thickening. although necrotic regions within plaque had low signal intensity. Fat suppression sequences did not significantly alter the appearance of atherosclerotic plaque. Calcified plaque produced effects ranging from slight signal loss to signal void. MR imaging findings correlated reliably with the tissue types indicated by histologic ana

ISSN:1053-1807    

DOI:10.1002/jmri.1880050117

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractCharacterization of myocardial stress and strain is necessary for a complete understanding of myocardial function. The precise quantification of regional myocardial strain is complicated by its time‐varying pattern and regional variation resulting from the anisotropy of the myocardium and by complex torsional and shortening motions of the heart during the cardiac cycle. The authors have developed a technique for point‐specific tracking of myocardial motion along all three axes in a constant selected section of myocardium by combining prospective section selection with in‐plane DANTE (delays alternating with nutations for tailored excitation) tissue tagging and phase‐contrast detection of motion perpendicular to the image plane. With this technique, it is possible to determine point‐specific myocardial strain value

ISSN:1053-1807    

DOI:10.1002/jmri.1880050118

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractVisualization of the cardiac valves with standard magnetic resonance (MR) imaging is not adequate because of long acquisition times. Echo‐planar imaging (PI) can, however, be performed with a temporal resolution (30–50 msec) comparable to that of echocardiography. The authors evaluated the feasibility of real‐time imaging of cardiac valve motion with ultrafast MR techniques. Eight healthy volunteers and three patients with mitral stenosis and re‐ gurgitation were studied with a 1.5‐T whole‐body im‐ ager. Two different EPI sequences were assessed: a standard single‐shot gradient‐echo EPI (GEPI) SCquence and a fast imaging technique based on multiple‐shot EPI with interleaved k‐space acquisition (IGEPI). Fat‐suppressed images with an in‐plane resolution of 3.7 × 3.7 mm were obtained equally spaced through the cardiac cycle. Half‐k‐space acquisition was used. Morphologic evaluation was superior with IGEPI, owing to the better intracavitary signal homogeneity (P≤ 0.01). and the mitral valve leaflets were easier to identify on systolic images. IGEPI provided adequate valve

ISSN:1053-1807    

DOI:10.1002/jmri.1880050119

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractThree methods of performing magnetization transfer (MT) MR imaging ere emlyzed: (a) off‐resonance continuous wave, (b) off‐rceonance shaped pulses, end(c)on‐resonance binomial pulses. With two‐pool Bloch‐model simulations., signal levels from “MT active” spin systems were calculated. with reference to direct saturation of “MT inactive” systems. allowing calculation of contrast due to MT. Simulations demonstrate several trends with variation of excitation amplitude and Offret fiequency for the off‐resoname methods and with variation of excitation emplitude end pulse shape „order”︁ for binomial pulses. The simulations show that nominally optimized versions of each of these approaches provide essentially equivalent contrast at a given level of applied MT power, contrary to previous claims. Experiments with an MT‐inactive phentom, with a whole‐body syetem, show results with off‐ rewnence pulses to be in good agreement with simulations. whereas binomiel‐ pulse experiments show an

ISSN:1053-1807    

DOI:10.1002/jmri.1880050120

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

ISSN:1053-1807    

DOI:10.1002/jmri.1880050121

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY