摘要:

AbstractThe natural abundance13C NMR intensity of the glycogen C1 resonance was measured in the surgically exposed liver of rabbitsin vivo(n= 17) byintegrationfrom 98 to 104 ppm and compared double blindedly to the subsequent biochemical measurement. Coil loading was measured each time from a reference sphere at the coil center and the NMR Intensity was normalized accordingly. For quantification, the normalized NMR intensity was calibrated using aqueous glycogen solutions ranging from 110 to 1100 μmol glucosyl units/g (n = 14). Anin vivorange from 110 to 800 pmol glucosyl unit/g wet weight was measured with a highly linear correlation with concentration (r = 0.85, P<0.001). Thein vivoNMR concentration was 0.95 ± 0.05 (mean ± standard error,n= 17) of the concomitant enzymatic measurement of glycogen content. We conclude that the13C NMR signal of liver glycogen C1 is essentially 100% visiblein vivoand that natural abundance13C NMR spectroscopy can provide reliable noninvasive estimates ofin vivoglycogen content over the physiological range of liver glycogen concentrations when using adequate localization and Integration procedur

ISSN:0740-3194    

DOI:10.1002/mrm.1910310602

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

年代:1994

数据来源: WILEY

摘要:

AbstractAutomated formation of MR spectroscopic images (MRSI) is necessary before routine application of these methods is possible forin vivostudies; however, this task is complicated by the presence of spatially dependent instrumental distortions and the complex nature of the MR spectrum. A data processing method is presented for completely automated formation ofin vivoproton spectroscopic images, and applied for analysis of human brain metabolites. This procedure uses the water reference deconvolution method (G. A. Morris,J. Magn. Reson. 80,547(1988)) to correct for line shape distortions caused by instrumental and sample characteristics, followed by parametric spectral analysis. Results for automated image formation were found to compare favorably with operator dependent spectral integration methods. While the water reference deconvolution processing was found to provide good correction of spatially dependent resonance frequency shifts, it was found to be susceptible to errors for correction of line shape distortions. These occur due to differences between the water reference and the metabolite distributions.

ISSN:0740-3194    

DOI:10.1002/mrm.1910310603

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

年代:1994

数据来源: WILEY

摘要:

AbstractSimultaneous multislice proton spectroscopic imaging (SI) is presented using a pulse sequence with multifrequency selective RF excitation and Hadamard encoding in the slice direction, and conventional Fourier phase encoding in the in‐plane directions. Double‐echo data acquisition is used to increase the spectral information of the experiment. Tests on a phantom demonstrate the quality of the slice selection. Results ofin vivomeasurements on the healthy rat brain show that spectra with a high signal‐to‐noise ratio can be acquired from four slices within 32 min. The measurements were performed at 4.7 T using a field of view of 32 × 32 mm2, a slice thickness of 3 mm, and a voxel size of 12 μl. The proposed method is a useful alternative to sequential multislice SI and 3D SI. Furthermore, it is possible to combine sequential and simultaneous multi

ISSN:0740-3194    

DOI:10.1002/mrm.1910310604

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

年代:1994

数据来源: WILEY

摘要:

AbstractMicroscopic susceptibility variations invariably increase apparent transverse relaxation rates. In this paper, we present comparisons between Monte Carlo simulations and experiments with polystyrene microspheres to demonstrate that this enhanced relaxation can be explained quantitatively for both spin echo and gradient echo imaging experiments. The spheres used (1 to 30 μ), and degree of susceptibility variation (caused by 0‐12 mMDy‐DTPA) covered a wide range of biologically relevant compartment sizes and contrast agent concentrations. These results show that several regimes of behavior exist, and that contrast dependence is quite different in these regimes. For a given susceptibility, δχ, a small range of particle sizes show peak transverse relaxation. For the range of susceptibilities found in the first pass of a clinical IV contrast agent bolus, this size range is 5 to 10 μ, or roughly capillary sized compartments. In both our simulations and experiments, smaller spheres showed quadratic relaxation versus concentration curves, and larger particles showed sublinear behavior. For particles corresponding to the peak relaxivity, the relaxation‐concentration curves were linear. In addition, we demonstrated that increasing the diffusion coefficient can increase, decrease, or, paradoxically, leave unaffected the apparent relaxation rate. The regime for which the diffusion coefficient is relatively unimportant corresponds to the region of peak relaxivity. By using the Bloch‐Torrey equation to produce scaling rules, the specific Monte Carlo simulations were extended to more general cases. We use these scaling rules to demonstrate why we often find that susceptibility‐induced relaxation rates vary approximately linearly with concentration of i

ISSN:0740-3194    

DOI:10.1002/mrm.1910310605

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

年代:1994

数据来源: WILEY

摘要:

AbstractAn analysis is presented of proton NMRT2relaxation times measured for aqueous solutions In simple bundles of hollow fibers. The relaxation times are calculated with a two‐compartment diffusive exchange model using the known relaxation times of the aqueous solutions and the fiber geometry. When the relaxation time outside the fibers is short (∼1 ms), three or more relaxation components are observed from this two compartment system, in agreement with the calculation. The amplitude and relaxation times of the third component are consistent with those of a diffusion‐mediated mode, as suggested theoretically by Brownstein and Tarr (Phys. Rev. A19, 2446 (1979)). The possible contribution of such modes to the multicomponent relaxation observed in tissues is disc

ISSN:0740-3194    

DOI:10.1002/mrm.1910310606

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

年代:1994

数据来源: WILEY

摘要:

AbstractUsing surgically implanted RF coils at 300 MHz, three‐dimensional microscopic MR images of rat liver were obtainedin vivoto follow the development of pathology induced by bromobenzene exposure. Formalin fixed specimens of liver from these animals were also imaged usingin vitroMR microscopy, followed by conventional optical microscopy. All MR images were acquired using a spin‐warp pulse sequence withTR= 950 ms andTE= 23 ms. Thein vivoimages were reconstructed as 2562× 32 arrays with a voxel size of (50 μm)2× 219 pm, while thein vitroimages were reconstructed as 2562× 128 arrays, giving an isotropic resolution at (39 μm)2. Based on results from six animals, we have found in all animals exposed to bromobenzene, image intensity decreased in specific hepatic tissue regions. These regions were well correlated to low signal intensity areas observed inin vitroMR images at higher resolution. Conventional optical microscopy indicated that the low signal intensity regions corresponded to areas of necrosis. The decrease in signal intensity is consistent with increased local diffusion coefficients as a result of necrosis. This study demonstrates that MR microscopy with implanted RF coils can be successfully used to follow tissue pathological changes in living

ISSN:0740-3194    

DOI:10.1002/mrm.1910310607

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

年代:1994

数据来源: WILEY

摘要:

AbstractDeep lying soft tissue tumors may be treated by a nonincisional surgical procedure executed inside an MR imaging system using a thermal effect delivered by a focused ultrasound transducer. A prototype system is constructed to assess MRI thermal monitoring and the localization of the heat zone in muscle. The temperature distribution of the focal spot is imaged with MRI while mechanically moving the transducer with an hydraulic 3‐axis positioner. Acoustic power is applied with a spherical shell transducer using 1‐ to 10‐s duration pulses at frequencies of 1.5 MHz to selectively coagulate tissue at 60‐70°C. The procedure is monitored with a series of fast second gradient echo,T1‐weighted, temperature sensitive MR sequences. Acquisitions are optimized for high temperature sensitive images that yield the thermal diffusivity, heat flow time constant and the focal spot size in muscle. MR temperature maps of muscle provide localization and dosimetry both in the focal region and

ISSN:0740-3194    

DOI:10.1002/mrm.1910310608

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

年代:1994

数据来源: WILEY

摘要:

AbstractWe have used a volume‐selective NMR pulse sequence to measure the diffusion coefficient (D) inside a selected cuboid volume. The sequence is a variant of the stimulated echo method for volume‐selective spectroscopy, and allows sequential measurement ofDin any of three orthogonal directions and for a range of diffusion times. The echo attenuation from whichDis calculated is free from the cross terms that can arise because of interactions between volume selection and diffusion‐sensitizing gradients. Anisotropic and diffusion‐time‐dependent behavior is observed in brain structures known to have axon fibers aligned primarily in one

ISSN:0740-3194    

DOI:10.1002/mrm.1910310609

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe effect of different spatial‐encoding (k‐space) sampling distributions are evaluated for magnetic resonance spectroscopic imaging (MRSI) using Fourier reconstruction. Previously, most MRSI studies have used square or cubick‐space functions, symmetrically distributed. These studies examine the conventionalk‐space distribution with spherical distribution, and 1/2k‐space acquisition, using computer simulation studies of the MRSI acquisition for three spatial dimensions and experimental results. Results compare the spatial response function, Gibbs ringing effects, and signal contamination for different spatial‐encoding distribution functions. Results indicate that spherical encoding, in comparison with cubic encoding, results in a modest improvement of the re sponse function with approximately equivalent spatial resolution for the same acquisition time. For spin‐echo acquired data, reduced acquisition times can readily be obtained using 1/2k‐space methods, with a concomitant reduction in signal

ISSN:0740-3194    

DOI:10.1002/mrm.1910310610

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

年代:1994

数据来源: WILEY

摘要:

AbstractThree different dual‐axis quadrupole gradient coils for quantitative high resolution MR imaging of small animals, phantoms and specimens were designed and built using printed circuit board technology. Numerical optimization of the conductor positions was used to increase the volume of 0.4% gradient uniformity by up to a factor of four. In one coil, the volume of 5% gradient uniformity occupied 88% and 83% of the overall diameter and length of the coil, respectively. A systematic error of 0.5% in the wire placement was shown to cause a reduction in the volume of 0.4% gradient uniformity by a factor of two, though the region of 5% gradient uniformity was not significantly affected. Heat transfer calculations were used to determine maximum peak and root‐mean‐squared currents that could safely be applied to the

ISSN:0740-3194    

DOI:10.1002/mrm.1910310611

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

年代:1994

数据来源: WILEY