摘要:

AbstractA method is introduced for obtaining proton spectrain vivowith all the advantages of a full water signal. The method, based on F1oversampledJ‐resolved spectroscopy, makes it possible to separate metabolite signals from unwanted baseline artifacts. The dominant water resonance is used as a 2D reference signal for the phase‐sensitive reconstruction of the 2DJ‐resolved metabolite spectra. The powerful specificity of this method is demonstrated with model compound spectra, phantoms, andin vivoexa

ISSN:0740-3194    

DOI:10.1002/mrm.1910400302

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

年代:1998

数据来源: WILEY

摘要:

AbstractUnder ideal conditions, continuous arterial spin labeling (ASL) techniques are higher in SNR than pulsed ASL techniques by a factor of e. Presented here is a direct theoretical and experimental comparison of continuous ASL and pulsed ASL, using versions of both that are amenable to multislice imaging and insensitive to variations in transit times (continuous ASL with a delay before imaging, and QUIPSS II (Quantitative Imaging of Perfusion Using a Single Subtraction–second version)). Perfusion image quality for comparable imaging time was nearly identical for both single‐slice and multislice imaging. The measured raw signal was approximately 25% higher with continuous ASL, but the SNR per unit time was identi

ISSN:0740-3194    

DOI:10.1002/mrm.1910400303

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

年代:1998

数据来源: WILEY

摘要:

AbstractA new, fast, 2D MR imaging technique allowing passive catheter visualization adequate for use as a tool for guiding the movement of a catheter during endovascular procedures is described. This imaging technique samples low spatial frequencies more often than high spatial frequencies; it also uses bothk‐space view sharing and temporal interpolation. Unlike other techniques for passive visualization that exploit magnetic‐susceptibility–induced artifacts, we have adopted a strategy that takes advantage of theT1‐shortening effect of paramagnetic contrast agents, such as Gd‐DTPA and a projection dephaser. This not only permits visualization of the entire catheter length but also minimizes the risk of intravascular heating. Using this method, a temporal frame rate of up to eight images per second and a tip localization accuracy of ± 1mm (root mean square difference) can b

ISSN:0740-3194    

DOI:10.1002/mrm.1910400304

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

年代:1998

数据来源: WILEY

摘要:

AbstractA method to provideB1correction and cerebrospinal fluid (CSF) referencing is developed and applied to spectroscopic imaging of the human brain at 4.1 T using a volume head coil. TheB1image allows rapid determination of the spatially dependentB1that is then used to compensate for theB1sensitivity of the spectroscopic sequence. The reference signal is acquired from CSF located in a lateral ventricular position using a point‐resolved echo spectroscopy (PRESS) acquisition. The CSF spectrum is also corrected forB1dependence. Together withT2andT1corrections, this method is used to provide quantitative values ofN‐acetylaspartate (NAA), creatine (Cr), and choline (Ch). The metabolite concentrations obtained from a spectroscopic imaging slice through the ventricles in seven normal controls are in good agreement with previously published literature values. This method is applied in a patient with secondary progressive multiple sclerosis, showing separate areas of abnormalities in both NAA and

ISSN:0740-3194    

DOI:10.1002/mrm.1910400305

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe effects of mechanical compression on the multiple quantum coherences generated from sodium ions in articular cartilage were investigated. Cartilage samples obtained from bovine patellae were studied during compression at 0.7 MPa (100 psi) for 1 hour. The double quantum filtered spectra showed marked lineshape changes in the compressed samples. Compression did not seem to influence the lineshapes of the single quantum and triple quantum filtered spectra significantly. We found that the residual quadrupolar interaction was reduced in the compressed samples. Changes in the ordering of collagen fibers may be responsible for the observed effect.

ISSN:0740-3194    

DOI:10.1002/mrm.1910400306

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

年代:1998

数据来源: WILEY

摘要:

AbstractMagnetic field homogeneity is of major concern forin vivospectroscopy, and with the increased use of volumetric chemical shift imaging (CSI) techniques, the ability to shim over a large volume of tissue is now one of the primary limiting constraints in performing these studies.In vivoshimming is routinely performed using linear shim correction terms, and although many scanners are also equipped with additional resistive shim supplies that can provide second and third‐order shim fields, they are often not used due to the additional effort and scan time required. The question as to how much improvement can be achieved using additional higher‐order shims compared with the linear shims alone was quantitatively addressed. Performance measures for both intervoxelB0uniformity and intravoxelT2* line broadening were evaluated for 15 normal volunteers. The analysis tools developed in this study, along with the summarized data, can be useful in deciding if a given application warrants the additional time, effort, and expense (if additional hardware needs to be purchased) of Implementing higher‐order shimming routines. For CSI studies of the brain, the use of the higher‐order shims, compared with linear terms alone, yielded approximately 30% greater volume of brain tissue that could be shimmed within typical constraints for intervoxelB0shifts and intravoxelT2* linebroadening. In addition, a regional analysis shows significant improvement in the homogeneity within specific areas of the brain, particularly those near th

ISSN:0740-3194    

DOI:10.1002/mrm.1910400307

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

年代:1998

数据来源: WILEY

摘要:

AbstractRecently, several implementations of arterial spin labeling (ASL) techniques have been developed for producing MRI images sensitive to local tissue perfusion. For quantitation of perfusion, both pulsed and continuous labeling methods potentially suffer from a number of systematic errors. In this study, a general kinetic model for the ASL signal is described that can be used to assess these errors. With appropriate assumptions, the general model reduces to models that have been used previously to analyze ASL data, but the general model also provides a way to analyze the errors that result if these assumptions are not accurate. The model was used for an initial assessment of systematic errors due to the effects of variable transit delays from the tagging band to the imaging voxel, the effects of capillary/tissue exchange of water on the relaxation of the tag, and the effects of incomplete water extraction. In preliminary experiments with a human subject, the model provided a good description of pulsed ASL data during a simple sensorimotor activation task.

ISSN:0740-3194    

DOI:10.1002/mrm.1910400308

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

年代:1998

数据来源: WILEY

摘要:

AbstractTumor vascular volume fraction and the magnitude of the mean microvascular random flow velocity were measured in an animal tumor model by combining dynamic Gd‐DTPA‐albumin enhanced MRI and diffusion‐weighted MRI in conjunction with a compartmental modeling analysis. The vascular volume fraction maps were obtained from the dynamic Gd‐DTPA‐albumin enhanced MRI measurement. It was found that the vascular volume fraction for Walker 256 tumor was higher within the outgrowing rim and decreased towards the central region. The average value obtained from five animals was 0.062 ± 0.009 ml/g. By using the vascular volume fraction from the Gd‐DTPA‐albumin enhanced MRI measurement, maps of the magnitude of the mean microvascular random flow velocity were obtained from the diffusion‐weighted MRI measurements with the compartmental modeling analysis. The relative extravascular and intravascular contributions to the diffusion‐weighted MRI signal were determined for three tissue groups with different Gd‐DTPA‐albumin enhancement characteristics, and the flow and molecular diffusion‐induced attenuation factors for the intravascular compartment were also compared. The mean microvascular random flow velocity magnitude maps were obtained with an average v

ISSN:0740-3194    

DOI:10.1002/mrm.1910400309

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

年代:1998

数据来源: WILEY

摘要:

AbstractA diffusion‐CPMG hybrid experiment was used to analyze the diffusion characteristics of differentT2relaxation components in bovine optic nerve. Data were collected using a pulsed field gradient (PFG) multi spin echo (MSE) CPMG sequence for parallel and perpendicular axon orientation and four diffusion times. The apparent diffusion coefficient (ADC) was evaluated for two observedT2components as a function of axonal orientation and diffusion time Δ. The shortT2component exhibited minor diffusional anisotropy and larger ADC, whereas the longT2component showed significant anisotropy effects. This is consistent with the hypothesis that the shortT2component is associated with water within the myelin sheath, which is less restricted than axonal water that is limited by cell membrane permeabili

ISSN:0740-3194    

DOI:10.1002/mrm.1910400310

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

年代:1998

数据来源: WILEY

摘要:

AbstractProgrammed cell death or apoptosis was induced in human promyelocytic leukemia (HL‐60) and Chinese hamster ovary (CHO‐K1) cells using several cytotoxic drugs that have different modes of action, including camptothecin, ceramide, chelerythrine, etoposide, farnesol, geranyl geraniol, and hexadecylphosphocholine. The consequent changes in cellular metabolism were monitored using31P MRS measurements on intact cells and cell extracts. Cells undergoing programmed cell death exhibited characteristic changes in the levels of glycolytic and phospholipid metabolites. The most significant changes were increases in the concentration of the glycolytic intermediate, fructose‐l,6‐bisphosphate and in the concentration of CDP‐choline, which is an intermediate in phosphatidylcholine biosynthesis. In HL‐60 cells, the increase in fructose‐l,6‐bisphosphate levels could be explained by depletion of cellular NAD(H) levels. All of the agents used to induce apoptosis caused the accumulation of CDP‐choline. Since the resonances of this compound occur in a relatively well resolved region of tissue spectra, it could provide a marker for apoptosis that would allow the noninvasive detection of the processin vivousing31P

ISSN:0740-3194    

DOI:10.1002/mrm.1910400311

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

年代:1998

数据来源: WILEY