ISSN:0740-3194    

DOI:10.1002/mrm.1910190202

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe traditional perfusion rate is based on the amount of fluid flowing longitudinally down the capillaries, often given in ml/100 g tissue/min. In principle, fluid flowing radially out of or into the capillaries can be measured as well. Radial and longitudinal perfusion have the same units but independent values. If spins leave and reenter a capillary several times during a transit, radial perfusion can exceed longitudinal perfusion and it probably does. Radial perfusion measures capillary wall function. Progressively increasing the time between two gradient pulses in a proposed radial perfusion imaging sequence allows more magnetization to move into the vasculature to be dephased or removed. Progressive decreases in gradient strength keep diffusion effects constant allowing isolation of the desired perfusion effects. © 1991 Academic Press, Inc

ISSN:0740-3194    

DOI:10.1002/mrm.1910190203

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe purpose of this paper is to present an overview of the diffusive behavior of water in biological tissues. The analysis shows that obstructive barriers would have to occupy very large volume fractions in order to account for the reduction in the diffusion coefficient (D) observed in biological systems. Generally, most models lead to the supposition that a substantial fraction (20–40%) of the cell water is hydration water, or that the diffusion coefficient of the cytoplasmic water is reduced substantially from the free water value. Thus, the conclusion that a substantial fraction of cell water has diffusive properties that are altered by the macromolecules of the cytoplasm seems inescapable. The impact of these findings on MRI remain to be determined. © 1991 Academic Press, I

ISSN:0740-3194    

DOI:10.1002/mrm.1910190204

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

年代:1991

数据来源: WILEY

摘要:

AbstractVariations in microvascular system functions were observed among a number of brain areas. The rates of local blood flow varied 18‐fold among areas and were extremely high in neuroendocrine structures. Marked differences in blood flow were also foundwithinsome brain structures. The volume of radiolabeled blood in perfused parenchymal micro‐vessels ranged from 5 to 70 μl/g and correlated closely with local cerebral blood flow. The hematocrits within parenchymal microvessels were 45–75% of the arterial hematocrit, which indicates that red cells more rapidly traverse brain microvessels than do plasma proteins. The mean transit times of blood through parenchymal microvessels were extremely short and ranged from 0.3 to 0.6 s. © 1991 Academic Pre

ISSN:0740-3194    

DOI:10.1002/mrm.1910190205

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe purpose of this paper is to review the basic principles of diffusion measurement with spin echoes. These principles can be combined with those of MR imaging to generate maps of diffusion coefficients. Diffusion imaging can be extended to imaging of other intravoxel incoherent motions (IVIM), such as blood microcirculation. Some of the technical problems encountered when implementing IVIM imaging are presented. © 1991 Academic Press, Inc

ISSN:0740-3194    

DOI:10.1002/mrm.1910190206

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

年代:1991

数据来源: WILEY

摘要:

AbstractDiffusion and perfusion phenomena under strong gradient fields (∼100 G/cm) are examined in high resolution nuclear magnetic resonance (NMR) imaging and microscopy, where diffusion‐associated signal attenuation predominates overT1andT2relaxation decays. Image contrast based on the diffusion and microcirculation is discussed with experimental results obtained with a 7.0‐T microscopy system. Ultimate resolution limit due to diffusion is investigated in high resolution NMR imaging and microscopy. © 1991 Academic Pres

ISSN:0740-3194    

DOI:10.1002/mrm.1910190207

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe application of stimulated echo acquisition mode (STEAM) sequences for NMR imaging of diffusion is especially suited for spins withT1≫T2as, e.g., encountered in proton NMR studies of biological systems. Molecular self‐diffusion coefficients may be calculated from a set of diffusion‐weighted images acquired with different gradient strengths. A variation of the diffusion time allows the determination of restricted and/or anisotropic diffusion in cellular systems ranging from plants to humans. Problems associated with the presence of unavoidable macroscopic motionsin vivoare demonstrated in diffusion studies of human brain. Motion ghosting in diffusion‐weighted images may be overcome by means of a high‐speed STEAM sequence yielding single‐shot images within subsecond acquisition times. © 1991 Academi

ISSN:0740-3194    

DOI:10.1002/mrm.1910190208

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

年代:1991

数据来源: WILEY

摘要:

AbstractAssuming that the SSFP magnetization response maintains a steady state which is penodic in the presence of diffusion, we can solve for the diffusion effect in such sequences. Formulating a Fourier series decomposition solution to the Bloch‐Torrey equation and imposing the steady‐state condition, analytical expressions describing the signal decay due to diffusion are developed. Magnetization responses for any system and sequence parameters can then be obtained. Also, sensitivity tobfactor changes is quite different than standard diffusion measurement techniques. Assumptions made in the solution are verified via finite difference solutions and simulations of the Bloch‐Torrey equ

ISSN:0740-3194    

DOI:10.1002/mrm.1910190209

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

年代:1991

数据来源: WILEY

摘要:

AbstractUse of the Stejskal‐Tanner sequence for performing diffusion images in the human brain tends to be complicated by the presence of artifacts caused by voluntary or involuntary, sometimes pulsatile, motion. We describe the implementation of the technique of echo‐planar diffusion imaging, which avoids these artifacts and allows reproducible quantitative values of the diffusion coefficient to be measuredin vivo. The effects of perfusion are easily visible in a phantom containing a gel. The results for human brain show a significant “perfusion fraction” in grey matter, consistent with an extracellular, possibly microvascular, volume of about 10%. © 1991 Academic Pr

ISSN:0740-3194    

DOI:10.1002/mrm.1910190210

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

年代:1991

数据来源: WILEY

摘要:

AbstractRecently, several models for determining microcirculatory parameters using magnetic resonance imaging have been proposed. These include the intravoxel incoherent motion (IVIM) model, the intravoxel coherent motion (IVCM) model, and various tracer models. In order to evaluate these models before extension into physiological systems, phantom studies were used to assess model assumptions, measurement uncertainties, and sensitivity to changes in perfusion. Emphasis is placed on the IVIM model, but the techniques discussed could be extended to evaluation of other models as well. An overview of considerations in pulse sequence development, phantom design, and data interpretation is presented for a variety of phantoms ranging in complexity from stationary volumes of fluid and mechanically pumped phantoms to isolated animal kidneys and finally to anin vivoanimal model. © 1991 Academic Press, Inc

ISSN:0740-3194    

DOI:10.1002/mrm.1910190211

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

年代:1991

数据来源: WILEY