摘要:

Abstract“Diffusion‐diffraction” experiments on water, yielding “q‐space” plots, were conducted on suspensions of oxygenated (diamagnetic) human erythrocytes. (i) These suspensions displayed diffusion‐diffraction of water; (ii) the shape of theq‐space plots depended on the direction along which the diffusion was measured, thus implying alignment of the cells in the magnetic field of the NMR spectrometer; (iii) the diffusion anisotropy was altered in a predictable way by converting the hemoglobin to a paramagnetic form; (iv) the shapes of theq‐space plots were altered in a predictable way by inhibiting water transport; (v) the pseudo‐first order rate constant characterizing the covalent inhibition of water transport, byp‐chloromercuribenzenesulfonate (p‐CMBS), was measured; and (vi) the cell diameter and intercellular spacing were measured from the positions of the interference minima and maxima in theq‐space plots. The relevance of these findings to NMR‐based histological characterization of tissues, and the implications, for magnetic resonance imaging (MRI), of eryth‐rocyte alignment in the small vessels of the b

ISSN:0740-3194    

DOI:10.1002/mrm.1910370502

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

年代:1997

数据来源: WILEY

摘要:

AbstractWe report acquisition of 3D image‐guided localized proton spectroscopy (1H‐MRS) in the human brain on a standard clinical imager. 3D coverage is achieved with a hybrid of chemical shift imaging (CSI) and transverse Hadamard spectroscopic imaging (HSI). 16 × 16 × 4 arrays of 3.5 and 1 ml voxels were obtained in 27 min. The spatially selective HSI 90° pulses incorporate naturally into a PRESS double spin‐echo sequence to subdivide the VOI into four partitions along its short axis. 2D CSI (16 × 16) is performed along the other long axes. Because the hybrid excites the spins in the entire VOI, a √Nsignal‐to‐noise‐ratio (SNR) gain per given examination time is realized compared with sequentially interleavingN2D slices. A two‐fold gain in sensitivity is demonstrated i

ISSN:0740-3194    

DOI:10.1002/mrm.1910370503

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

年代:1997

数据来源: WILEY

摘要:

AbstractQuantification of NMR visible metabolites by spectral modeling usually assumes a Lorentzian or Gaussian lineshape, despite the fact that experimental lineshapes are neither. To minimize systematic fitting errors, a mixed Lorentzian‐Gaussian (Voigt) lineshape model was developed. When tested with synthetic FIDs, the Voigt lineshape model gave more accurate results (maximum error 2%) than either Lorentzian (maximum error 20%) or Gaussian models (maximum error 12%). The three lineshape models gave substantially different peak areas in anin vitroexperiment, with the Voigt model having a much lower X2(2.1 compared with 5.2 for the Lorentzian model and 6.2 for the Gaussian model). In a group of 10 healthy volunteers, fitting of1H spectra from cerebral white matter gave significantly different peak areas between the methods. Even when area ratios were taken, the Lorentzian model gave higher values (+5% for NAA/choline and +2% for NAA/creatine) than the Voigt lineshape model, whereas the Gaussian model gave lower values (−2% and −1%, respecti

ISSN:0740-3194    

DOI:10.1002/mrm.1910370504

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

年代:1997

数据来源: WILEY

ISSN:0740-3194    

DOI:10.1002/mrm.1910370505

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

年代:1997

数据来源: WILEY

摘要:

AbstractTwo novel methods to minimize peak RF power for high order longitudinal Hadamard encoding are described and demonstrated experimentally. The first method uses the fact that the choice of a reference phase in an inversion process does not affect the final frequency response. In this method, the different single inversion‐band pulses are added together, each with a different reference phase. For a proper phase choice, minimization of the peak RF power is obtained. Scaling laws are defined allowing the use of a given phase‐set in multiple cases. In the second method, single inversion‐band pulses are added together, each partially shifted in time. This results in a significant reduction in peak power with only a moderate increase in pulse length. Theoretical conditions outlining the optimal addition order are defined. Experimental results verify the theoretical conditions and demonstrate that the frequency response is not affected by the peak power minimization process. With the new low peak RF power, longitudinal Hadamard encoding of 8TH (or 16TH) order can be performed in any clinical se

ISSN:0740-3194    

DOI:10.1002/mrm.1910370506

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

年代:1997

数据来源: WILEY

摘要:

AbstractA new model of compartmentalized relaxation—that which occurs for spins (protons) exchanging between compartments of different relaxation rates—is presented. This model generalizes previous ones by allowing spatially dependent relaxation within compartments. Solutions for the diffusion‐Bloch equations are found via an efficient numerical technique known as the generalized moment expansion, and they agree well with the solutions to the standard two‐site exchange equations (TSEE) for many typical situations. Specific models are developed for liposomes, red blood cells, capillaries, and arteries with respect to applied contrast agents. A parameter derived from tissue characteristics is introduced to predict the nature of the solutions. A new method is proposed for using contrast agents to detect capillaries, which exploits their high surface‐to‐volume ratio relative to the other elements of the

ISSN:0740-3194    

DOI:10.1002/mrm.1910370507

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

年代:1997

数据来源: WILEY

ISSN:0740-3194    

DOI:10.1002/mrm.1910370508

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

年代:1997

数据来源: WILEY

摘要:

AbstractThe hypothesis that increased muscleT2after exercise is caused by increased extracellular fluid volume was tested by comparing the effects of exercise versus external leg negative pressure on muscleT2relaxation in normal human subjects.T2in lower leg muscles was measured by echo‐planar imaging at 63 echo times from 24 to 272 ms, and the relaxation spectrum was calculated by using a non‐negative least squares algorithm.T2relaxation in anterior leg muscle before exercise was characterized by a single component with meanT2= 29.3 ± 0.7 (SE,n= 5). After ankle dorsiflexion exercise, this single component broadened, and meanT2increased to 38.3 ± 0.7 ms. In contrast, after leg negative pressure, which increased the total leg muscle cross‐sectional area by 21% (range 12–32%n= 6), there was a variable appearance of much slower‐relaxing components (60–500 ms). The results suggest that increased extracellular fluid can account for only a minor portion of the increase in muscleT2observed du

ISSN:0740-3194    

DOI:10.1002/mrm.1910370509

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

年代:1997

数据来源: WILEY

摘要:

AbstractCalculation of global cardiac function parameters has been validated using fast, segmentedk‐space, breath‐hold, gradient‐echo, magnetic resonance images. Images of phantoms, experimental animals, normal volunteers, and patients were acquired with a 1.5 T clinical scanner. Humans were imaged using two phased‐array surface coils in multicoil mode. Myo‐cardial contours were extracted using a new interactive, semi‐automated method based on the active contour model method. Images were acquired in the short‐axis orientation, and, using a new imaging and analysis strategy, in rotating plane long‐axis orientations, to provide better definition of the valve planes and the apex, and also to reduce the number of slices (compared with the short‐axis method) required to sample the whole heart. Validation was accomplished through calculation of the volumes of phantoms and left and right ventricular masses of animal hearts. Functional parameters from MRI were compared with those from echocardiograms and radionuclide angiograms in normal volunteers and patie

ISSN:0740-3194    

DOI:10.1002/mrm.1910370510

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

年代:1997

数据来源: WILEY

摘要:

AbstractSignificant degrees of magnetization transfer (MT) have been observed in the magnetic resonance imaging of biological materials by conventional clinical imaging sequences, as well as by sequences specifically designed to enhance MT image contrast. Two aspects of these procedures distinguish them from the classic spectroscopic MT experiments using either so‐called “hard” radiofrequency (RF) pulses of short duration and high power, or continuous wave RF irradiation of low power. First, clinical sequences must make exclusive use of “soft” pulses of intermediate length and power. Second, biological materials are modeled by a two‐spin system involving magnetization transfer between a narrow and a broad homogeneous spectral component. Such materials are a relatively restricted group within the larger family of materials studied with MT by spectroscopy. The current paper addresses these two issues with a theory that gives a new transient analysis of the off‐resonance pulsed MT problem for biological materials. It leads to predictions for optimal magnetization transfer in the context of medical imaging that have been verified by comp

ISSN:0740-3194    

DOI:10.1002/mrm.1910370511

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

年代:1997

数据来源: WILEY