ISSN:0899-9457    

DOI:10.1002/ima.1850060202

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractA central issue in magnetic resonance imaging of human brain function using blood oxygenation level‐dependent (BOLD) contrast is the accurate interpretation of the signal changes that are observed. Using a method that incorporates repeated phase rotation and convolution with a smoothing function to simulate spin diffusion in the presence of magnetic field perturbers, the dependencies of the absolute and relative changes in transverse relaxation rates (δR2* and δR2) on biophysical and physiologic parameters were explored. First we introduce the modeling methodology. Then we simulate δR2* and δR2as physiologic and biophysical parameters are modulated within the ranges that they vary across subjects and voxels in the brain. The simulations demonstrate that the δR2* and δR2values that occur with activation‐induced changes in blood oxygenation depend most strongly on the resting state blood volume and field strength. The δR2*/δR2ratios depend most strongly on the vessel radius and spin diffusion

ISSN:0899-9457    

DOI:10.1002/ima.1850060203

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractBrain functional magnetic resonance imaging (FMRI) is possible because of local changes in blood flow and oxygenation levels. Understanding the role of each mechanism is important for interpreting FMRI results as well as for the design of the experiment itself. In this review, we address the role of flow and blood oxygen level dependence (BOLD) and how they can be used in conjunction with each other to enhance the BOLD effect. We also discuss the role of intra/extravascular signal changes in the presence of a vessel for the imaging situation and show how the ratio of these two contributions is likely to change as the blood volume fraction changes. Using the BOLD model itself, along with 3‐D phase imaging, we show that blood oxygenation level in veins in vivo isY= 0.544 ± 0.029 as measured in 14 vessels in five subjects. Finally, we comment on the use of phase images themselves as a means to discriminate tissues with different levels of blood such as gray matter, white matter, and cerebrospinal flu

ISSN:0899-9457    

DOI:10.1002/ima.1850060204

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractIn this article, we have described a tailored radio frequency gradient echo (TRFGE) technique which is applied to functional imaging. The TRFGE technique is insensitive to the inflow effect, which often complicates the analysis of functional magnetic resonance imaging (FMRI) data, and is also insensitive to nonfunctional backgrounds such as static tissues. The susceptibility effect measurement obtained from the TRFGE technique therefore provides a unique form of functional imaging, namely, susceptibility‐only functional imaging. To examine TRFGE functional imaging systematically, we designed an experiment to determine the flow effect and susceptibility‐dependent signal change by varying the imaging parameters including flip angle α, repetition time TR, and echo time TE. The results obtained were then compared with the data obtained with conventional gradient‐echo (CGE) functional MR imaging. The experimental results shown were all obtained using human volunteers with a 2.0‐T whole‐body

ISSN:0899-9457    

DOI:10.1002/ima.1850060205

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractWe explored a number of theoretical and practical approaches used to understand and optimize contrast to noise of the versatileT1based perfusion model. The interplay between tissueT1and bloodT1was investigated. We also provided a succinct evaluation of the several popularT1based methods currently applied to measure flow and flow change.

ISSN:0899-9457    

DOI:10.1002/ima.1850060206

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractMagnetic resonance imaging has recently been developed as a tool to image brain function based on regional changes in blood oxygenation. In this article, we examine the methodologic considerations for functional brain mapping using the spiral k‐space acquisition technique. Gradient design and image reconstruction methods as well as considerations for the selection of imaging parameters are presented. Several sources of image artifacts, along with artifact compensation strategies, are examined. Functional maps of activation in primary motor cortex are usd to demonstrate the effect of image parameter selection and of artifact compensation procedures. These results demonstrate that the spiral k‐space acquisition technique is a rapid, flexible, and robust method for imaging brain activ

ISSN:0899-9457    

DOI:10.1002/ima.1850060207

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractSeveral sequences, including conventional gradient‐echo sequence (e.g., FLASH), gradient‐echo echo‐planar imaging (EPI), T2* prepared ultrafast gradient echo technique (TurboFLASH), and T2*‐weighted spiral sequence, are currently used for T2*‐weighted functional magnetic resonance imaging. This paper presents a comparison of FLASH, gradient‐echo EPI, and T2*‐prepared TurboFLASH. Experimental results have demonstrated that these sequences have different susceptibility to physiologic fluctuation, which is likely the dominant source of noise. EPI images exhibit the lowest relative fluctuation, while images obtained by FLASH and TurboFLASH, respectively, display similar relative fluctuation. Functional maps generated using these sequences are comparable but exhibit different sensitivity to activation from large vessels. Consequently, interpretation of activation results from various maps should take into account the difference in

ISSN:0899-9457    

DOI:10.1002/ima.1850060208

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractThis article describes a novel MRI technique as well as its variant, which use prior knowledge to acquire and reconstruct the principal components of an image to reduce scanning time. Examples in the application of this technique are given. The implications of these results and the potential in the application of the functional imaging are discussed.

ISSN:0899-9457    

DOI:10.1002/ima.1850060209

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractLocalized proton spectroscopy using the PRESS sequence was performed to measure the free induction decay of the water signal in a 2 × 2 × 2 = ml voxel located in the visual cortex after a short light pulse. The experiment was run continuously with a repetition time of 400 ms; the onset of the light pulse was triggered by the spectrometer. The FIDs were measured as a function of the echo time (TE) and the length of the stimulus (TS). The time course showed three different phases: An initial response was observed 400 ms after the onset of the stimulus, which was demonstrated to be mainly a change in the apparent spin density and/or T1with a minor contribution of T2*. No significant change in T2could be measured. Changes in the ionic cell environment and/or the subsequent increase in cell size upon stimulation are discussed as a potential mechanism for the observed signal respons

ISSN:0899-9457    

DOI:10.1002/ima.1850060210

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractWe examined the EPI time course in functional MR imaging at 4 T from cortical gray matter, local draining veins, and venous sinuses. We found a small temporal shift between the initial hyperoxygenation blood oxygen level‐dependent (BOLD) response in tissue and the larger draining veins. However, the fluctuations in the tissue time course showed strong similarity with the fluctuations in the local veins when shifted by the appropriate lag, suggesting that a large amount of this apparent “noise” is hemodynamic in nature and not related to motion. The tissue BOLD time course was observed to display a biphasic time dependence (first negative, then positive) in response to photic stimulation when substantial intersubject averaging was performed. Other areas, including those where draining veins are visible, showed only a monophasic (positive) signal change, reaching a maximum of about 6% after the onset of the visual stimulus. These time constants are in very good agreement with published data obtained with intrinsic optical mapping techniques, in which a deoxygenation phase has been shown to occur in functionally specific cortical columns, followed by an increase in blood volume which is more distributed in nature. We can use the time lag between oxygen demand and flow increase to estimate stimulus induced local tissue oxygen extraction increases of abou

ISSN:0899-9457    

DOI:10.1002/ima.1850060211

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY