摘要:

AbstractHigh field magnetic resonance offers new opportunities because of its high SNR and better spectral resolution for MRI and MRS. However, new problems also emerge at high field. As the field strength increases, the wavelength in the tissue becomes shorter and comparable with the body dimensions. This perturbs the field and also causes standing waves within the patient as a result of the impedance mismatching at the tissue interfaces. Due to the Complexity of the boundary conditions and the solution of Maxwell's equations, an exact analytical calculation for a loaded RF resonator has not been possible. In this paper, we present a birdcage coil simulation study based on a 3D finite element method (FEM) model for the characterization of the field within the tissue. First, the accuracy of the FEM solutions is validated by the 2D analytical solutions at 64 and 223 MHz. In these solutions, the frequency dependence of the conductivity and permittivity is also taken into account. Then, a more realistic 3D model is studied. The results are compared with the experimental measurements. It is shown that the 3D model makes it possible to explore the effects of the end rings in the presence of a tissue sample inside the coil.

ISSN:0740-3194    

DOI:10.1002/mrm.1910370112

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

年代:1997

数据来源: WILEY

摘要:

AbstractThe purpose of this study was the development and testing of a method for unsupervised, automated brain segmentation. Two spin‐echo sequences were used to obtain relaxation rates and proton‐density maps from 1.5 T MR studies, with two axial data sets including the entire brain. Fifty normal subjects (age range, 16 to 76 years) were studied. A Three‐dimensional (3D) spectrum of the tissue voxels was used for automatic segmentation of gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF) and for calculation of their volumes. Accuracy and reproducibility were tested with a three‐compartment phantom simulating GM, WM, and CSF. In the normal subjects, a significant decrease of GM fractional volume and increased CSF volume with age were observed (P<0.0001), with no significant changes in WM. This multi‐spectral segmentation method permits reproducible, operator‐independent volumetric m

ISSN:0740-3194    

DOI:10.1002/mrm.1910370113

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

年代:1997

数据来源: WILEY

摘要:

AbstractSegmentation methods based on dual‐echo MR images are generally prone to significant false lesion classifications. We have minimized these false classifications by (1) improving the lesion‐to‐tissue contrast on MR images by developing a fast spin‐echo sequence that incorporates both cerebrospinal fluid signal attenuation and magnetization transfer contrast and (2) including information from MR flow images. Studies on patients with multiple sclerosis indicate that this dual approach to tissue segmentation reduces the volume of false lesion classifications by an average

ISSN:0740-3194    

DOI:10.1002/mrm.1910370114

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

年代:1997

数据来源: WILEY

摘要:

AbstractAn analytical model of restricted diffusion in bovine optic nerve is presented. The nerve tissue model is composed of two different objects: prolate ellipsoids (axons) and spheres (glial cells) surrounded by partially permeable membranes. The free diffusion coefficients of intracellular and extra cellular water may differ. Analytical formulas for signal loss due to diffusion in the pulsed gradient spin echo (PGSE) experiment for this tissue model are derived. The model is fitted to experimental data for bovine optic nerve. The obtained model parameters are shown to be reasonable. The model describes all of the characteristics of the PGSE data: anisotropy, upward curvature of decay curves, and diffusion time dependence. The validity and sensitivity of the model are also discussed.

ISSN:0740-3194    

DOI:10.1002/mrm.1910370115

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

年代:1997

数据来源: WILEY

摘要:

AbstractRecently, intravascular catheter probes have been developed to increase signal‐to‐noise ratio (SNR) for MR imaging of blood vessels. Miniaturization of these catheter probes without degrading their performances is very critical in imaging small vessels such as coronary arteries. Catheter coils have a loop incorporated in their structure and have limitations in physical dimensions and electromagnetic properties. The use of a loopless intravascular catheter antenna is proposed to overcome these problems. The catheter antenna is essentially a dipole, which makes a very thin diameter possible, and its electronic circuitry can be placed outside the blood vessels without performance degradation. The theoretical foundation for the design and operation of the catheter antenna is presented. Several catheter antennae, as small as 1.5 French, were constructed and tested on phantoms and rabbits with great success. The catheter antenna has a simple structure and is easy to design, implement, and oper

ISSN:0740-3194    

DOI:10.1002/mrm.1910370116

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

年代:1997

数据来源: WILEY

摘要:

AbstractA cerebrospinal fluid (CSF)‐suppressed flow‐attenuated inversion recovery (FLAIR) double‐shot diffusion echo‐planar imaging (EPI) sequence was developed and used, along with a non‐CSF‐suppressed version of the sequence, to determine the extent of the contribution of CSF partial‐volume averaging to the apparent diffusion coefficients (ADCs) of normal human brain in vivo. Regional analysis indicates that cortical gray matter and parenchymal tissues bordering the ventricles are most affected by CSF contamination, leading to elevated ADC values. Only slight differences in gray‐ and white‐matter average ADCs were detected after CSF suppression. The human brain average ADCs calculated from high‐resolution CSF‐sup‐pressed diffusion‐weighted images in these studies are similar to those reported in animals. FLAIR diffusion sequences remove CSF as a source of error in ADC determination and ischemic lesion discrimination in diffusion‐weighte

ISSN:0740-3194    

DOI:10.1002/mrm.1910370117

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

年代:1997

数据来源: WILEY

摘要:

AbstractProgress in research on hypertension, heart failure, aging, post‐infarct remodeling, and the molecular basis of cardiovascular diseases in general has been greatly facilitated in recent years by the development of specialized small‐mam‐mal models by selective breeding and/or genetic alteration. Routine noninvasive evaluation of cardiac function and perfusion in these animals models, however, is difficult using existing methods. In principle, MRI can be used for this purpose, but in practice this is difficult because of problems related to RF coils, cardiac gating, and imaging pulse sequences. In this article, solutions to these problems are described that have allowed us to use MRI to routinely image the hearts of rats and rabbits. Specifically described are four RF coils, cardiac gating schemes, and an imaging pulse sequence specially designed for cardiac imaging in these animals on a 4.7 T Omega chemical‐shift imaging (CSI) spectrometer. These techniques can be used to obtain, within 2 min, eight double‐oblique short‐axis images of the rat at different cardiac phases with 200 × 400μm in‐plane resolution and a slice thickness of 2 mm. Moreover, myocardial tissue tagging can be performed with tag thicknesses and separations comparable to those used routinely in humans. The technical information is presented in sufficient detail to allow researchers at other sites to reproduce the results. This information should facilitate the use of MRI for the noninvasive examination of cardiac function and perfusion, which can be combined with other established techniques for the study of cardiovascular disease in specialize

ISSN:0740-3194    

DOI:10.1002/mrm.1910370118

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

年代:1997

数据来源: WILEY

摘要:

AbstractA rapid, in‐plane image registration algorithm that accurately estimates and corrects for rotational and translational motion is described. This automated, one‐pass method achieves its computational efficiency by decoupling the estimation of rotation and translation, allowing the application of rapid cross‐correlation and cross‐spectrum techniques for the determination of displacement parameters. k‐space regridding and modulation techniques are used for image correction as alternatives to linear interpolation. The performance of this method was analyzed with simulations and echo‐planar image data from both phantoms and human subjects. The processing time for image registration on a Hewlett‐Packard 735/125 is 7.5 s for a 128 × 128 pixel image and 1.7 s for a 64 × 64 pixel image. Imaging phantom data demonstrate the accuracy of the method (mean rotational error, −0.09°; standard deviation = 0.17°; range, −0.44° to + 0.31°; mean translational error = −0.035 pixels; standard deviation = 0.054 pixels; range, −0.16 to + 0.06 pixels). Registered human functional imaging data demonstrate a significant reduction in motion artifacts such as linear trends in pixel time series and activation artifacts due to stimulus‐correlated motion. The advantages of this technique are its noniterative one‐pass nature, the reduction in image degradation as compared to previous method

ISSN:0740-3194    

DOI:10.1002/mrm.1910370119

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

年代:1997

数据来源: WILEY

摘要:

AbstractA new magnetic resonance imaging strategy is presented for accessing myocardial perfusion. Most previous work has relied on usingT1‐weighted fast gradient‐echo imaging to monitor dynamically the signal changes during the passage of a contrast media bolus. However, the gradient‐echo approach is limited by an inability to image the entire heart with adequate temporal resolution.This paper focuses on a electrocardiogram‐gated multishot echo‐planar imaging sequence, using the simple strategy of using the intrinsicT1weighting produced by a repetition time equal to the heart period. To quantitate the sequence's performance with respect to normal myocardial perfusion, seven volunteers were imaged, each with three different doses of the contrast medium gadolinium diethylenetriamine penta‐acetic acid (Gd‐DTPA). The first‐pass dynamics of the contrast were quantified in 13 regions per heart for each examination.In all volunteers, the complete heart could be covered, with five to seven slices, every two heartbeats. Enhancement was homogeneous throughout the left ventricular myocardium, with an enhancement of approximately 50% for the optimum contrast dose of 0.05

ISSN:0740-3194    

DOI:10.1002/mrm.1910370120

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

年代:1997

数据来源: WILEY

摘要:

AbstractCurrent MR coronary angiography (MRCA) methods use breath‐holding to minimize respiratory motion. A major limitation to this technique is misregistration between imaging slices due to breath‐hold variability. Prospective adaptive correction of image location using real‐time navigator measurement of diaphragm position is a potential method for improving slice registration in breath‐hold MRCA. Ten subjects underwent MRCA using an ECG‐gated, fat‐suppressed, segmented k‐space, gradient‐echo sequence. Transverse and coronal images were acquired using standard breath‐holding with and without prospective navigator correction. Breath‐hold MRCA with prospective navigator correction resulted in a 47% reduction in craniocaudal slice registration error compared to standard breath‐holding (0.9 ± 0.2 mm versus 1.7 2 0.4 mm,P= 0.04). Prospective adaptive navigator correction of image location significantly improves slice registration for breath‐hold MRCA and is a promising motion correction

ISSN:0740-3194    

DOI:10.1002/mrm.1910370121

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

年代:1997

数据来源: WILEY