摘要:

AbstractA clear understanding of choline metabolism is important in our goal to modify demyelination and remyelination in multiple sclerosis. To develop a technique capable of measuring metabolic changes in the brain, we have studied the incorporation of a phosphonium analogue of choline (P‐choline) in tissue extracts of rats. After feeding adult rats a choline‐deficient diet supplemented with P‐choline, the analogue was not detectable byin vivovolume‐localized1H spectroscopy. However,in vitro31P measurements of brain extracts revealed an 11 % incorporation of P‐choline into phosphatidylcholine. We report that P‐choline incorporates preferentially into the lipid pool over the lipid precursor pool and we provide evidence that the choline peak resolved byin vivo1H spectroscopy is only composed of small molecular weight choline‐contain

ISSN:0740-3194    

DOI:10.1002/mrm.1910330302

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

年代:1995

数据来源: WILEY

摘要:

AbstractChemically selective solid state phosphorus‐31 nuclear magnetic resonance (NMR) imaging of the mineral phase of bone and synthetic calcium phosphate models for bone mineral is demonstrated with microscopy‐scale (about 5 mm field of view) apparatus at 6.0 T magnetic field strength. Pixel‐by‐pixel linear combination of image data from multiple radio frequency (RF) pulse sequences, chosen to develop contrast between chemical constituents of interest in the mineral, generates derived images showing the distribution of individual constituents. The technique combines the noninvasive character of magnetic resonance imaging (MRI) with the ability of solid state NMR spectroscopy to characterize subtle chemical variations in bone mineral, as well as to measure the amount of mineral. These methods are, in principle, extensible to larger dimensional scales suitable for live animal subjects or huma

ISSN:0740-3194    

DOI:10.1002/mrm.1910330303

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

年代:1995

数据来源: WILEY

摘要:

AbstractA hybrid of two localized spectroscopy techniques, chemical shift imaging (CSI) and Hadamard spectroscopic imaging (HSI), is used to obtain an array of 16 x 16 x 4 (3 x3 x 3 cm3voxels) proton‐decoupled phosphorus (31P) spectra of human brain. For equal spatial resolution, this organ's oblate shape requires fewer axial than coronal or sagittal slices. These different spatial requirements are well suited to 1D, 4thorder, transverse HSI in the axial direction, combined with 2D 16 x 16 CSI in the other two orientations. The reduced localization matrix (16 x 16 x 4 over just the brain versus a cubic‐16 x 16 x 16 matrix of equal resolution, over the entire head) may proportionally shorten data acquisition if the voxel size is not signal‐to‐noise limited. In addition, the use of Hadamard encoding can improve the intervoxel spectral is

ISSN:0740-3194    

DOI:10.1002/mrm.1910330304

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

年代:1995

数据来源: WILEY

摘要:

AbstractA new single‐step method is described for estimation of local cerebral blood volume (CBV) from multiple rapidly acquiredT2‐ orT2*‐weighted MRI images after bolus administration of a susceptibility contrast agent. This method improves on existing methods in three ways. First, the method includes the baseline scan intensity value as a fitted parameter. Second, the model is fitted directly to the original scan intensity values instead of to transformed concentration curves, avoiding the propagation of errors that occurs in the transformation. Third, the equations are reparameterized with CBV fitted directly as a model parameter. Parameter estimation methods are compared by Monte Carlo simulation. The direct method described here yields more precise parameter estimates, with smaller mean absolute deviations from the true parameter values, than the existing method when compared using simulated data. Implementation is discussed and numerical evaluation of the Digamma or Psi function is desc

ISSN:0740-3194    

DOI:10.1002/mrm.1910330305

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

年代:1995

数据来源: WILEY

摘要:

AbstractExperiments were performed to determine whether changes inT2*‐weighted MR images during and after hyperoxia differentiate tumors from normal tissue. Mammary adenocarcinomas implanted in the right hind limbs of rats were studied. Gradient echo images were obtained at 2 Tesla with an evolution time of 20 ms and a recycle time of 1 s. Breathing gas was either air or 100% O2. Significant increases in image intensity were observed in tumor centers and rims during hyperoxia while much smaller changes were detected in the surrounding muscle. The relaxation rate (1/T2*) in tumors decreased during hyperoxia by an average of 2.5 ± 1.0 s−1, while in muscle the average change was an increase of 0.6 ± 2.1 s−1. The largest decreases in relaxation rate were detected in non‐necrotic tumor regions with relatively low density of blood vessels. Immediately following hyperoxia significant decreases in intensity were detected in tumors while much smaller decreases were detected in the surroundi

ISSN:0740-3194    

DOI:10.1002/mrm.1910330306

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

年代:1995

数据来源: WILEY

摘要:

AbstractAlthough dynamic imaging is presently used for various applications, it is still limited by the temporal resolution. In this paper, we present a new technique that uses a random phase‐encoding strategy to facilitate faster and smoother update of images and to improve the temporal resolution in dynamic studies. The technique was implemented on a conventional clinical scanner and demonstrated with variousin vivostudies. Technical details, simulations, and experimental results are described. Images from experimental studies indicate that the new technique is robust in generating dynamic images and can be potentially utilized for clinical application

ISSN:0740-3194    

DOI:10.1002/mrm.1910330307

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

年代:1995

数据来源: WILEY

摘要:

AbstractPhase‐related unsteady (pulsatile) flow effects in MRI have been studied by means of linear response theory. These flow effects can be described in the frequency domain: the influence of the gradients on the phase shift is described by a transfer function, the spectrum of the gradient being the determining factor. An analysis of this transfer function is shown to provide information about the process of flow encoding: instant of encoding, induced distortions and how they are related to the gradient waveform. The connection with the traditional description in terms of the gradient moment expansion has also been investigated and clarified. This approach was applied to study the response of two time‐resolved flow quantification techniques (Fourier flow method and phase mapping) by analyzing their amplitude and phase transfer functions. By simulation it is shown that a better interpretation of the measured velocity waveform is obtained and that Fourier analysis in combination with a correction by the inverse transfer function results in an accurate reconstruction of the velocity waveform stud

ISSN:0740-3194    

DOI:10.1002/mrm.1910330308

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

年代:1995

数据来源: WILEY

摘要:

AbstractRF current density imaging (RF‐CDI) is a new MRI technique for imaging the Larmor frequency current density parallel toB0in electrolytic media. To extend the use of RF‐CDI to biological tissue for generating conductivity contrast, the sensitivity must be increased and the data requirements reduced. A rotating frame approach, in which a largeB1field is applied simultaneously as a rotary echo with RF current, is proposed to meet these requirements. Rotating frame magnetic fields are encoded in the phase of an MRI image. Trials have now been performed with this sequence in a three‐compartment cylindrical phantom containing doped water or mineral oil for detecting displacement, conduction and fringe field currents. In a postmortem rat study, 85.56 MHz RF currents injected by implanted electrodes created tissue dependent contrast because of the electrical properties of tissue. A sensitivity and artifact analysis was also performed. The sensitivity of this method is determined by the maximum RF pulse duration. SAR limits pose an upper bound on this time andB1, whereas the avoidance of phase artifacts imposes a lower bound

ISSN:0740-3194    

DOI:10.1002/mrm.1910330309

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

年代:1995

数据来源: WILEY

摘要:

AbstractWhen measuring perfusion by arterial spin labeling, saturation of tissue macromolecular spins during arterial spin labeling greatly decreases tissue water magnetization, reducing the sensitivity of the technique. In this work, a theory has been developed for perfusion measurement by arterial spin labeling without saturation of macromolecular spins. A two‐coil system was used to achieve arterial spin labeling without saturation of brain tissue macromolecular spins for NMR measurement of rat cerebral perfusion. The effects of crossrelaxation on the measurement of perfusion have been studied in the absence of macromolecular spin saturation, and it is demonstrated that at 4.7 Tesla, perfusion is underestimated by approximately 17% when the effect of cross‐relaxation is neglected in the calculation of perfusion. However, assuming water to be a freely diffusable tracer, the effect of cross‐relaxation is predicted to be flow independent, and it can, thus, be accounted for in the calculation of perfusion. The theory and experiments are presented to estimate tissue perfusion, magnetization transfer rate constants, and spin‐lattice relaxation times of water and macromolecular spins in ra

ISSN:0740-3194    

DOI:10.1002/mrm.1910330310

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

年代:1995

数据来源: WILEY

摘要:

AbstractThe DUFIS sequence can make ultra‐fast images (˜10 ms) without the use of rapidly switched gradients. The RF excitation sequence is spatially selective so that only a small fraction of the magnetization in each imaging pixel is used which produces a poor imaging signal to noise ratio (SNR). We have developed several alternative RF sequences that use RF pulses with multiple phases, and also with just 0° and 180° phases to excite almost all the magnetization in a pixel and greatly improve the SNR. The optimization of these pulse sequences (now called OUFIS) have been conducted both analytically and by numerical searches, with various linear and nonlinear models. Both theoretical and computational methods used in the optimizations are described in detail. Preliminary experimental results are briefly presented with several possible applications of the OUFIS excitation sequences sugge

ISSN:0740-3194    

DOI:10.1002/mrm.1910330311

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

年代:1995

数据来源: WILEY