摘要:

AbstractThe fast response (FR) is an early reduction in the amplitude of the fMR signal occurring shortly after the onset of stimulation. Owing to its potential advantages in terms of temporal and spatial resolution, it may be of considerable significance in functional experiments. A model for the mechanism of the FR was developed which accounts for the fMRS finding that the amplitude of the dip decreases with increasing echo time. Two computer simulations that confirmed the predictions of the model and showed that it is possible to select optimal echo times for the detection of the FR were performed.

ISSN:0740-3194    

DOI:10.1002/mrm.1910380102

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

年代:1997

数据来源: WILEY

摘要:

AbstractA methodology of MRI data acquisition is introduced which involves lengthening the duration of signal readout period with complementary shortening of the phase encode pulses, and vice versa, in sequences where gradient encoding time is limited by RF pulse spacing. This variable encoding time (VET) methodology can be used to increase spatial resolution or reduce data acquisition time in 2D and 3D FT MRI based on CPMG and gradient echo sequences. Advantages of higher image SNR and different spatial frequency distributions ink‐space are discussed and evaluated in preliminary experiment

ISSN:0740-3194    

DOI:10.1002/mrm.1910380103

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

年代:1997

数据来源: WILEY

摘要:

AbstractFunctional brain imaging of the human cortex is limited by poor contrast to noise ratio (CNR) and image degradation due to subject motion during the acquisition period. The work described here combines the use of closely coupled phased array receiver coils with a stabilization system to address these needs. Several phased array designs are evaluated and compared with the conventional “birdcage” design. Coil performance is reported in terms of relative SNR and fMRI results. Relative improvements of up to 360% are obtained for the occipital region and 180% in the temporal region. More modest gains of 10–30% were obtained for a “dome”‐shaped birdcage volume coil covering the en

ISSN:0740-3194    

DOI:10.1002/mrm.1910380104

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

年代:1997

数据来源: WILEY

摘要:

AbstractThe early stages of atherosclerosis are characterized by the deposition of cholesteryl esters and triglycerides into the arterial wall. In the excised human atherosclerotic plaque these lipids are in a liquid‐like state at body temperature and observable via MRI and NMR spectroscopy. To assess the ability of MRI to quantitatively image the lipids of atherosclerotic plaquein vivo, we have investigated eight New Zealand White rabbits fed atherogenic diets (2 weight (wt)% cholesterol, 1 wt% cholesterol + 6 wt% peanut oil, and 1 wt% cholesterol + 6 wt% corn oil). Postmortem examination indicated that all rabbits developed atherosclerosis in the aorta. Except for one animal, magnetic resonance angiography showed no noticeable obstruction in the aorta. MRI was carried out in an attempt to image atherosclerotic plaque lipids directly, but no signal was detectedin vivo.However, a plaque lipid signal was observed from excised tissue using a small diameter RF coil.1H NMR spectroscopy of the atherosclerotic plaque from excised aortas indicated that the major fraction of plaque lipids in rabbits is not in a liquid state at physiological temperature and are only marginally MRI‐visible compared to human plaque lipid. The differences in the MRI characteristics of rabbit and human plaque are due to differences in the fatty acid profile of the cholesteryl esters, chiefly a decrease of linoleic acid in rabbit lesi

ISSN:0740-3194    

DOI:10.1002/mrm.1910380105

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

年代:1997

数据来源: WILEY

摘要:

AbstractA variety of treatments that modulate tumor oxygen tension are used clinically to improve the outcome of radiotherapy. High resolution, noninvasive measurements of the effects of these treatments would greatly facilitate the development of improved therapies and could guide treatment of cancer patients. Previous work demonstrated that magnetic resonance (MR) gradient echo imaging of the water proton resonance detects changes inT2* andT1in tumors during hyperoxia that may reflect increased tumor oxygenation. This report describes the use of high resolution MR spectroscopic imaging with short repetition time (TR= 0.2 s) to improve the accuracy with which changes inT2* andT1are measured. Mammary adenocarcinomas grown in the hind limbs of rats were studied. Carbogen inhalation was used to induce hyperoxia. A single 2‐mm slice through the center of tumors and underlying muscle was imaged at 4.7 Tesla with in‐plane resolution of approximately 1.2 mm and frequency resolution of 5.8 Hz. The peak integral increased by an average of 6% in tumors during carbogen inhalation suggesting a decrease inT1(n= 8,P<0.001). Peak height increased by an average of 15% in tumors during carbogen inhalation (n= 8,P<0.001). The large difference between increases in peak height and peak integral demonstrates that the width of the water resonance decreased. Assuming a Lorentzian lineshape, an average increase of 12% inT2* was observed in tumors. In muscle, peak integral and peak height increased slightly (about 1.2% and 3%. respectively;P<0.02) during carbogen inhalation but no significant change inT2*was observed. Spectroscopic imaging detects changes in the water proton resonance in tumors during hyperoxia accurately and reproducibly with high signal‐to‐noise ratio and allows clear separation ofT1andT2* effects. Increases inT2* may be due to decreased deoxyhemoglobin in tumor blood vessels (i.e., the BOLD effect) and may provide a clinically useful index of increases in tumor oxyg

ISSN:0740-3194    

DOI:10.1002/mrm.1910380106

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

年代:1997

数据来源: WILEY

摘要:

AbstractNon‐isotropic contributions to1H MR spectra from human skeletal musclein vivohave recently been observed in the 0‐to 5‐ppm region. One pair of peaks has been identified to be subject to dipolar couplings. The corresponding changes in resonance frequency are related to the orientation of muscle fibers with respect to the external magnetic field and are analogous to the behavior of small molecules dissolved in liquid crystals. Image‐guided localized spectroscopy based on the STEAM method has been applied to verify these phenomena in rat skeletal musclein vivoand to investigate the effect postmortem. Residual dipolar couplings and anisotropic contributions to1H MR spectra of skeletal muscle have been confirmed in animals and at a higher field strength—albeit with a slightly different spectral pattern compared to the human study. The most prominent dipolar doublet due to creatine and/or phosphocreatine vanishes postmortem with a rate similar to the disappearance of phosphocreatine, and is no longer observable 2 h p

ISSN:0740-3194    

DOI:10.1002/mrm.1910380107

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

年代:1997

数据来源: WILEY

摘要:

AbstractQuantitative NMR measurements of perfusion using magnetic labeling of arterial water have been demonstrated previously in several different highly perfused organs. The success of these previous experiments suggested that arterial labeling may be of use in measuring perfusion in skeletal muscle, where resting perfusion is very low and where increased perfusion after exercise is transient. In the experiments described in this paper, adiabatic inversion of arterial water has been used to make single‐voxel measurements of perfusion in the lower hind limb of rats. At rest, the NMR results were quantified to yield a perfusion rate of about 13.8 ml/100g/min. After perturbation due to ischemic exercise, large relative changes in the NMR signal were observed. The peak change of about 2.5% of the NMR signal occurred shortly after perturbation and was followed by a return to resting levels over a period of about 4 mi

ISSN:0740-3194    

DOI:10.1002/mrm.1910380108

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

年代:1997

数据来源: WILEY

摘要:

AbstractThis paper describes the characteristics of a new oxygen sensitive, paramagnetic material that has some significant advantages for measurements of tissue pO2byin vivoEPR. This paramagnetic component of Welsh coal, termed “gloxy” was found to have valuable EPR features that allow accurate measurement of low oxygen tensionsin vivo; these include large oxygen‐dependent changes in linewidth, a high number of paramagnetic spin centers (resulting in high signal amplitude), and stability in tissue allowing repeated pO2, measurements to be madein vivowith high precision. Renal pO2was measured deep in the medulla region of isolated perfused kidneys and found to be lower than that in the cortex (1.7 ± 0.05 and 7.1 ± 0.3 mm Hg, respectively). The quality of the EPR signal obtained from the renal outer medulla and also from tumors in mice was such that the pO2measurements were obtained with a precision of ±3% of the measured pO2(Kidney: 1.7 ± 0.05 mmHg;Tumor: 1.37 ± 0.04 mmHg).In vitrotests on the viability of cells andin vivostudies using Gloxy demonstrate the stability and inertness of this oxygen‐sensit

ISSN:0740-3194    

DOI:10.1002/mrm.1910380109

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

年代:1997

数据来源: WILEY

摘要:

AbstractThe most widely‐used functional magnetic resonance imaging (fMRI) technique is based on the blood oxygenation level dependent (BOLD) effect, which requires at least partial uncoupling between cerebral blood flow (CBF) and oxygen consumption changes during increased mental activity. To compare BOLD and CBF effects during tasking, BOLD and flowsensitive alternating inversion recovery (FAIR) images were acquired during visual stimulation with red goggles at a frequency of 8 Hz in an inteerleaved fashion. With the FAIR technique, absolute and relative CBF changes were determined. Relative oxygen consumption changes cdan be estimated using the BOLD and relative CBF changes. In gray matter areas in the visual cortex, absolute and relative CBF changes in humans during photic stimulation were 31 ± 11 SD ml/100 g tissue/min and 43 ± 16 SD % (n =12), respectively, while the relative oxygen consumption change was close to zero. These findings agree extremely well with previouse results using positron emission tomography. The BOLD signal change is not linearly correlated with the relative CBF increase across subjects and negatively correlates with the oxygen consumption change. Caution should be exercised when interpreting the BOLD percent change as a quantitative index of the CBF change, especially in inter‐subject compar

ISSN:0740-3194    

DOI:10.1002/mrm.1910380110

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

年代:1997

数据来源: WILEY

摘要:

AbstractThe magnetization in hyperpolarized gas (HP) MRI is generated by laser polarization that is independent of the magnet and imaging process. As a consequence, there is no equilibrium magnetization during the image acquisition. The competing processes of gas inflow and depolarization of the spins lead to large changes in signal as one samplesk‐space. A model is developed of dynamic changes in polarization of hyperpolarized3He during infusion andin vivoimaging of the lung and verified experimentally in a live guinea pig. Projection encoding is used to measure the view‐to‐view variation with temporal resolution<4 ms. Large excitation angles effectively sample the magnetization in the early stages of inflow, highlighting larger airways, while smaller excitation angles produce images of the more distal spaces. The work provides a basis for pulse sequences designed to effectively exploit HP MRI in the

ISSN:0740-3194    

DOI:10.1002/mrm.1910380111

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

年代:1997

数据来源: WILEY