摘要:

AbstractTo validate their correlation with tissue iron concentration, proton transverse relaxation measurements have been made at 2.35 T (100 MHz) in 25 samples of excised, frozen, but unfixed human gray matter tissue obtained from the globus pallidus, putamen, caudate, thalamus, and cortex of five postmortem brains free of neurological disease. The iron concentration was independently measured, using atomic absorption spectroscopy. The proton transverse relaxation measurements exploited the interecho time dependence of the apparent transverse relaxation rate,R2app, obtained from a Carr‐Purcell‐Meiboom‐Gill (CPMG) sequence. An empirical semilogarithmic relationship betweenR2appand the interecho time provided a measure of the relaxation enhancement due to iron, namely, a slopep, which demonstrated a significant correlation (r= 0.78,P<0.001) with tissue iron concentration. Moreover, a simple rate difference, δR2app, determined between interecho time values of 6 and 60 ms, was also found to correlate significantly with iron concentration (r= 0.81,P<0.001). Both of the foregoing correlations were better than that ofR2appitself. When the tissue samples were subdivided into brain structure groups, the intergroup differences inpreflected their known differences in iron accumulation and correlated with those of the mean group iron content, determined by atomic absorption spectr

ISSN:0740-3194    

DOI:10.1002/mrm.1910350304

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

年代:1996

数据来源: WILEY

摘要:

AbstractA physiological artifact reduction method based on extracting respiratory motion and cardiac pulsation directly from functional MR data is described. In fast low angle shot (FLASH), respiratory cycles are derived utilizing the phase of the center of a navigator echo, in echo‐planar imaging (EPI) from the phase of the center κ‐space point. Cardiac cycles are determined from projections obtained from the navigator echo (FLASH) and the center κ‐space line (EPI). Because direct extraction of physiological parameters eliminates the need for external monitoring, the method can be more readily implemented. Experimental results illustrate that the technique provides effective compensation for physiology‐related signal fluctuations in functional MRI and performs as well as the retrospective technique using external physiological monitoring. Key words: fMRI; motion artifacts; physiological motion; image p

ISSN:0740-3194    

DOI:10.1002/mrm.1910350305

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

年代:1996

数据来源: WILEY

摘要:

AbstractImage contrast with gradient‐recalled echo sequences (GRE) used for fMRI can have both blood oxygen level‐dependent (BOLD) and inflow components, and the latter is often undesirable. A dual‐echo technique can be used to differentiate these mechanisms, because modulation of signal from inflow is common to both echoes, whereas susceptibility and diffusion‐related signal losses are larger in the second echo. An efficient dual‐echo interleaved spiral sequence was developed for use with a conventional scanner. It uses a κ‐space trajectory that spirals out from the origin while the first echo is collected, then spirals back in while collecting the second echo. Decomposition of the data provides separate images of the inflow and T2*‐weighted components. Results demonstrate the decomposition with phantom experiments and with photic stimulation in nor

ISSN:0740-3194    

DOI:10.1002/mrm.1910350306

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

年代:1996

数据来源: WILEY

摘要:

AbstractNoninvasive magnetic resonance temperature maps that are used to monitor thermal ablation of tissue are described. In magnetic resonance images, thermally induced proton nuclear magnetic resonance frequency shifts, and changes in the longitudinal relaxation time produce both phase and magnitude changes in the MR signal. Temperature maps with improved sensitivity are derived from the complex‐difference nuclear magnetic resonance signal. Bovine muscle specimens were heated with focused ultrasound to model thermal surgery and create a known thermal distribution to test the method. Resulting MR images acquired in 2 s produce temperature maps with 1 mm resolution and 2°C temperature sensitivity. The temperature sensitivity was increased by extending the acquisition to 5 s, by decreasing the receiver bandwidth, and increasing the echo ti

ISSN:0740-3194    

DOI:10.1002/mrm.1910350307

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

年代:1996

数据来源: WILEY

摘要:

AbstractThe potential for using electron paramagnetic resonance (EPR) imaging in biological applications has been limited by the lack of ideal single‐line imaging probes. The commonly used nitroxides exhibit multiple lines, causing either hyper‐fine‐based limitations in the maximum obtainable image resolution or hyperfine‐based artifacts in the reconstructed image. The application of a numerical method, based on forward‐subtraction principles for removing hyperfine artifacts in the measured projections is reported. It is demonstrated by using computer simulations, imaging of phantoms, and imaging of rat hearts, that marked enhancement in image quality and resolution can be obtained by removing the hyper‐fine‐imposed limit on the gradient magnitude and performing postacquisition corrections for removing hyperfine artifacts

ISSN:0740-3194    

DOI:10.1002/mrm.1910350308

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

年代:1996

数据来源: WILEY

摘要:

AbstractIn vivoorex vivoEPR imaging, EPRI, has been established as a powerful technique for determining the spatial distribution of free radicals and other paramagnetic species in living organs and tissues. While instrumentation capable of performing EPR imaging of free radicals in whole tissues and isolated organs has been previously reported, it was not possible to image rapidly moving organs such as the beating heart Therefore instrumentation was developed to enable the performance of gated‐spectroscopy and imaging on isolated beating rat hearts at L‐band. A synchronized pulsing and timing system capable of gated acquisitions of up to 256 images per cycle, with rates of up to 16 Hz was developed. The temporal and spatial accuracy of this instrumentation was verified using a specially designed beating heart‐shaped isovolumic phantom with electromechanically driven sinusoidal motion at a cycle rate of 5 Hz. Gated EPR imaging was performed on a series of isolated rat hearts perfused with nitroxide spin labels. These hearts were paced at a rate of 6 Hz with either 16 or 32 gated images acquired per cardiac contractile cycle. The images enabled visualization of the time‐dependent alterations in the free radical distribution and anatomical structure of the heart that occur during the cardia

ISSN:0740-3194    

DOI:10.1002/mrm.1910350309

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

年代:1996

数据来源: WILEY

摘要:

AbstractThe question of whether the apparent diffusion coefficient (ADC) of intracellular water changes after brain injury was addressed by using133Cs as an indicator to report on the state of the intracellular environment. Cesium is an NMR‐detect‐able potassium analog that accumulates in the intracellular space and is detectable in rat brain after being added to the animal's diet. The ADC of cesium was measured before and after the death of the rat. The cesium ADC fell from 0.91 ± 0.05 × 10−3mm2/s (mean ± SEM,n= 5) in the alive rat to 0.71 ± 0.05 × 10−3mm2/s within 20 min (the best time resolution of the experiment) of the death of the animal and stayed at this value for at least 3 h (P<0.001). Assuming that the ADC of cesium reflects motion in the intracellular environment, these results support the idea that there are changes associated with cell injury that would cause a reduction in the ADC of intracellular water. Hence, one factor contributing to the decrease in water ADC after brain injury is a change in the ADC of intrace

ISSN:0740-3194    

DOI:10.1002/mrm.1910350310

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

年代:1996

数据来源: WILEY

摘要:

AbstractMultiple‐quantum filtered (MQF) NMR offers the possibility of monitoring intracellular (IC) Na content in the absence of shift reagents (SR), provided that (i) the contribution from IC Na to the MQF spectrum is substantial and responds to a change in IC Na content, and (ii) the amplitude of the extracellular (EC) MQF component remains constant during a change in IC Na content. The validity and basis for these conditions were examined in isolated perfused rat hearts using SR‐aided and SR‐free triple‐quantum filtered (TQF)23NaNMR. Despite a myocardial Na content that was only ˜1/70 that of EC Na. IC Na contributed to over 25% of the total TQF spectrum acquired in the absence of SR. Transverse relaxation times (T2) were approximately twice as long for EC compared to IC Na, despite SR‐induced relaxation ofT2for the former pool. However, the efficiency of generation of the TQF signal was similar for IC and EC Na, indicating that a much greater percentage of IC relative to EC Na exhibits TQ coherence. During constant perfusion with ouabain (0.2 mM for 25 min) or with a hypoxic and aglycemic solution (50 min), the amplitude of the IC TQF spectrum increased by ˜330% and ˜280%, respectively. In contrast, the amplitude of the EC TQF spectra remained essentially constant for both interventions. The amplitude for IC Na increased ˜250% relative to baseline during no‐flow ischemia (60 min), whereas the amplitude of the EC TQF spectra decreased by ˜33% before stabilizing. In SR‐free experiments, the TQF spectral amplitude increased ˜2‐fold during the constant perfusion interventions, but did not change significantly during no‐flow ischemia. These data suggest that the change in the TQF spectral amplitude during constant perfusion interventions is from IC Na, and that TQF techniques in the absence of SR may be useful in monitoring IC Na during these interventions. The fall in the amplitude of the EC TQF spectral amplitude during no‐flow ischemia complicates the use of TQF techniques without SR

ISSN:0740-3194    

DOI:10.1002/mrm.1910350311

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

年代:1996

数据来源: WILEY

摘要:

AbstractThis paper concerns the spatial and intensity transformations that are required to adjust for the confounding effects of subject movement during functional MRI (fMRI) activation studies. An approach is presented that models, and removes, movement‐related artifacts from fMRI time‐series. This approach is predicated on the observation that movement‐related effects are extanteven after perfect realignment.Movement‐related effects can be divided into those that are a function of position of the object in the frame of reference of the scanner and those that are due tomovement in previous scans.This second component depends on the history of excitation experienced by spins in a small volume and consequent differences in local saturation. The spin excitation history thus will itself be a function of previous positions, suggesting an autoregression‐moving average model for the effects of previous displacements on the current signal. A model is described as well as the adjustments for movement‐related components that ensue. The empirical analyses suggest that (in extreme situations) over 90% of fMRI signal can be attributed to movement, and that this artifactual component can be successfu

ISSN:0740-3194    

DOI:10.1002/mrm.1910350312

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

年代:1996

数据来源: WILEY

摘要:

AbstractMultislice proton MR spectroscopic images (SI) of the brain were quantitated, using the phantom replacement technique. In 16 normal volunteers, ranging in age from 5 to 74 years, average “whole brain” concentrations of choline (Cho), creatine (Cr), andN‐acetylaspartate (NAA) were found to be 2.4 ± 0.4, 7.9 ± 1.3, and 11.8 ± 1.0 (mM, mean ± SD), respectively. These values are in good general agreement with those previously determined by single‐voxel localization techniques. Cortical gray matter was found to have lower Cho and NAA levels, compared to those of white matter, corpus callosum, and basal ganglia. Cho was also found to increase significantly with age in several locations. Quantitative multislice proton SI is feasible in the clinical environment, and regional and age‐dependent variations occur that must be accounted for when evaluating spectra from pathologic

ISSN:0740-3194    

DOI:10.1002/mrm.1910350313

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

年代:1996

数据来源: WILEY