摘要:

AbstractThe biophysical mechanism(s) underlying diffusion‐weighted MRI contrast following brain injury remains to be elucidated. Although it is generally accepted that water apparent diffusion coefficient (ADC) decreases after brain injury, it is unknown whether this is associated with a decrease in intracellular or extracellular water displacement, or both. To address this question, 2‐[19F]luoro‐2‐deoxyglucose‐6‐phosphate (2FDG‐6P) was employed as a compartment‐specific marker in normal and globally ischemic rat brain. Through judicious choice of routes of administration, 2FDG‐6P was confined to the intra‐ or extracellular space. There was no statistical difference between intra‐ and extracellular 2FDG‐6P ADCs in normal or in globally ischemic brain (P>0.16), suggesting that water ADCs in both compartments are similar. However, ischemia did result in a 40% ADC decrease in both compartments (P<0.001). Assuming that 2FDGbP reflects water motion, this study shows that water ADC decreases in both spaces after ischemia, with the reduction of intracellular water motion being the primary source of diff

ISSN:0740-3194    

DOI:10.1002/mrm.1910400102

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe problem of handling data that falls on a nonequally spaced grid occurs in numerous fields of science, ranging from radio‐astronomy to medical imaging. In MRI, this condition arises when sampling under time‐varying gradients in sequences such as echo‐planar imaging (EPI), spiral scans, or radial scans. The technique currently being used to interpolate the nonuniform samples onto a Cartesian grid is called the gridding algorithm. In this paper, a new method for uniform resampling is presented that is both optimal and efficient. It is first shown that the resampling problem can be formulated as a problem of solving a set of linear equationsAx=b, wherexandbare vectors of the uniform and nonuniform samples, respectively, andAis a matrix of the sinc interpolation coefficients. In a procedure called Uniform Re‐Sampling (URS), this set of equations is given an optimal solution using the pseudoinverse matrix which is computed using singular value decomposition (SVD). In large problems, this solution is neither practical nor computationally efficient. Another method is presented, called the Block Uniform Re‐Sampling (BURS) algorithm, which decomposes the problem into solving a small set of linear equations for each uniform grid point. These equations are a subset of the original equationsAx=band are once again solved using SVD. The final result is both optimal and computationally efficient. The results of the new method are compared with those obtained using the conventional gridding algorithm via si

ISSN:0740-3194    

DOI:10.1002/mrm.1910400103

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

年代:1998

数据来源: WILEY

摘要:

AbstractArterial‐phase three‐dimensional (3D) contrast‐enhanced MR angiograms of the carotid and vertebral arteries from their origins through the carotid bifurcations were obtained in 20 patients using acquisition times over 30 sec by using an MR fluoroscopy‐triggered pulse sequence with elliptical centric view order. The typical pixel size was 0.8 mm (x) × 1.6 mm (y) × 1.5 mm (z), and 32–48 coronal slices were acquired. The fluoroscopic monitoring of bolus arrival was effective in 18 of the 20 cases; two failures were attributed directly to a poor choice of RF coil. To exploit peak arterial‐to‐venous contrast, the central 3D views were acquired first in the most compact time period possible for the given TR. For the 18 successfully triggered cases, arterial‐phase 3D images were obtained with excellent venous suppression as demonstrated by an average internal jugular vein to common carotid signal enhancement ratio of

ISSN:0740-3194    

DOI:10.1002/mrm.1910400104

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

年代:1998

数据来源: WILEY

摘要:

AbstractIt is shown that the effect of pH changes can be measured in proton NMR spectra through the pH sensitivity of the signal intensities of metabolite protons exchanging with water. To observe this phenomenon, pulse sequences must be used that can sensitively observe these exchangeable protons under physiological conditions, which is achieved by avoiding magnetization transfer signal losses due to water saturation for solvent suppression purposes. These methods provide an order‐of‐magnitude enhancement of many signals between 5 and 10 ppm, containing both N‐bound protons as well as aromatic C‐H protons coupled to them, the intensity of which is influenced by exchange‐relayed saturation. As a first application, the effects of pH change on these resonances are studied exvivo(perfused cells) andin vivo(c

ISSN:0740-3194    

DOI:10.1002/mrm.1910400105

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe purpose of this work was to develop high‐resolution cardiac magnetic resonance imaging techniques for thein vivomouse model for quantification of myocardial function and mass. Eight male mice were investigated on a 7‐Tesla MRI scanner. High‐quality images in multiple short axis slices (in‐plane resolution 117 μm2, slice thickness 1 mm) were acquired with an ECG‐gated cine sequence. Left ventricular end‐diastolic and end‐systolic volumes and mass were calculated from segmented slice volumes. There was precise agreement of left ventricular mass determined exvivoand by MRI. lntraobserver (5%) and interobserver (5%) variability ofin vivoMR measure

ISSN:0740-3194    

DOI:10.1002/mrm.1910400106

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

年代:1998

数据来源: WILEY

摘要:

AbstractBiological samples have a high dielectric constant that can shorten RF wavelengths by a factor of 8 relative to the vacuum. At high field strengths, finite wavelength effects within larger samples are the dominant cause of RF field nonunifor‐mity. A coil design is presented that can reduce and even eliminate this inhomogeneity; 4‐T images in phantoms and in the head of a normal volunteer are presented, which demonstrate improved homogeneity relative to a standard coil. This coil design should aid in realizing the potential advantages of imaging large samples at high field streng

ISSN:0740-3194    

DOI:10.1002/mrm.1910400107

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

年代:1998

数据来源: WILEY

摘要:

AbstractVariations in the magnetic field in the human brain caused by the processes of swallowing or speaking are measured. In both processes, motion of the pharyngeal muscles, especially the tongue and jaw, alter the susceptibility‐induced magnetic field distribution at the brain slice being imaged. This leads to image warping, compromising the analysis of a time series of images, such as in functional magnetic resonance imaging (fMRI). These dynamic changes are assessed by acquiring a time series of images using a gradient‐echo asymmetric‐spin‐echo sequence (GREASE), a technique in which two images are acquired for each excitation–one during the gradient echo, and one during the latter part of the spin echo. The NMR phase difference between the two images is a measure of the magnetic field distribution. A series of brain images, acquired with this sequence while the subject either swallows or speaks, indicated negative magnetic field changes of up to O.087 ppm in the inferior region of the brain for both speaking and swallowing, and in some speech, additional positive changes of up to O.056 ppm in the frontal region of the brain were

ISSN:0740-3194    

DOI:10.1002/mrm.1910400108

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

年代:1998

数据来源: WILEY

摘要:

AbstractSpatially localized lineshapes of hyperpolarized (HP)3He in guinea pig lungs have been measuredin vivo. Three different axial slice locations, each containing different compositions of airway sizes and orientations, were studied. Gas peaks from major bronchi (2 ppm) and alveoli (‐2 ppm) were distinguished. The gas phase spectra show structural features that are a result of frequency shifts caused by bulk magnetic susceptibility. For a given slice, the spectral lineshapes reflect the airway composition within the slice location, according to theory. The peak assignments given here also agree with previous studies done by Wagshulet at. with HP129Xe. At each of the slice locations, data were acquired during two phases of the breathing cycle, resulting in a relative frequency shift of ∼0.3 ppm in the superior slices. Spectra obtained over a number of breaths show the dynamics of the gas buildup in the lung and provide further evidence supporting the peak assignme

ISSN:0740-3194    

DOI:10.1002/mrm.1910400109

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

年代:1998

数据来源: WILEY

摘要:

AbstractRecent findings using volumetric MRI techniques have revealed that patients with combat‐related and noncombat‐re‐lated posttraumatic stress disorder (PTSD) have reductions in right hippocampal volume. Twenty‐one veterans with PTSD and eight age‐matched control veterans were studied using proton magnetic resonance spectroscopy to test the hypothesis that the N‐acetyl‐L‐aspartic acidkreatine (NAAICr) ratio would be decreased in the right medial temporal lobe structures of patients with PTSD compared to controls. Patients with PTSD displayed significantly lower NAA/Cr ratio for the right medial temporal lobe relative to the left (P⩽ 0.011). Patients with PTSD also had lower NNCr in right medial temporal lobe (P⩽ O.013) and lower choline/Cr in left medial temporal lobe (P⩽ 0.030) compared to control subjects. Because NAA is regarded as an indicator of neuronal density, this finding suggests that the neuronal density of right‐sided medial temporal structures in patients with combat‐rela

ISSN:0740-3194    

DOI:10.1002/mrm.1910400110

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

年代:1998

数据来源: WILEY

摘要:

AbstractJ‐coupled resonances such as lactate exhibit complex behavior in PRESS‐localized spectroscopy. Polarization transfer effects due to nonideal slice profiles can lead to substantial errors in quantification unless recognized. In this work, it is shown how these effects can be predicted in a systematic way given the sequence timing and the RF pulse envelopes. Experimental results are presented to highlight the problems of working at the popular echo time of

ISSN:0740-3194    

DOI:10.1002/mrm.1910400111

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

年代:1998

数据来源: WILEY