Packed erythrocytes are ideally suited as a model system for the study of water diffusion in biological tissue, because cell size, membrane permeability, and extracellular volume fraction can be varied independently. We used a pulsed-field-gradient spin echo NMR technique to measure the time-dependent diffusion coefficient D(t) in packed erythrocytes. The long-time diffusion constant, Deff, depends sensitively on the extracellular volume fraction. This may explain the drop in Ddf during the early stages of brain ischemia, where just minutes after an ischemic insult the extra-cellular volume in the affected region of the brain is significantly reduced.Using an effective medium formula, we estimate the erythrocyte membrane permeability, in good agreement with measurements on isolated cells. From the short-time behavior of D(t), we determine the surface-to-volume ratio of the cells, -(0.72 ,um)-l.Diffusive transport of water in the presence of permeable membranes is of fundamental biological importance (1). The measured diffusion coefficient, D(t), depends on observation time and is a sensitive function of several physical parameters: membrane permeability, K; the volume fraction of connected extracellular fluid, 4; and local water concentrations. The geometrical arrangement of the membranes is important in determining D(t), and 4 is a parameter that characterizes this arrangement. Samples of erythrocytes (RBCs) are ideally suited for the study of such effects since cell size, K, and 4 can all be independently controlled. The pulsed-field-gradient (PFG) NMR technique (2) has been used to determine the membrane permeability of biological membranes in packed RBCs and some types of tissue (3, 4). To derive K from the effective (long time) diffusion coefficient, Doff, these studies utilized the relation, noted by Crick (5), ample, the second term on the righthand side should be replaced by cl(Ka) if, in the space between membranes, other molecules such as proteins occupy a fractional concentration 1 -c. This effect is in addition to the change in the microscopic diffusion coefficient of water produced by the presence of the proteins.PFG NMR is an ideal tool for measuring the timedependent diffusion coefficient (2). The measurement is nondestructive and does not involve the introduction of chemical or isotopic tracers. The observation time can be varied over several orders of magnitude. The minimum observation time is determined by the minimum length of gradient pulses and the subsequent recovery of the apparatus from eddy current and magnetoacoustic effects and by signalto-noise considerations. The maximum time is determined by the spin-lattice and spin-spin relaxation times of the fluid.We used PFG NMR to study the effects of 4 and K on Doff.Using an effective medium theory (EMT), we calculate K from measured values of Deff. We also measured the time dependence of the diffusion coefficient at short times. Our data are consistent with recent work on porous media with solid grains (8,9) and demonstrate that the ...

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Water diffusion in brain tissue is affected by the presence of barriers to translational motion such as cell membranes and myelin fibers. The measured water apparent diffusion coefficient (ADC) value is therefore frequently anisotropic and varies depending upon the orientation of restricting barriers (such as white matter tracts) relative to the diffusion-sensitive-gradient direction. Anisotropic water diffusion can be specified using indices of diffusion anisotropy [e.g. standard deviation of the individual ADC values, fractional anisotropy (FA), lattice index (LI)], which are derived from measurements of the full diffusion tensor. The rotationally invariant nature of particular diffusion anisotropy indices (e.g. FA, LI) allows orientation-independent comparisons of these parameters between different subjects. Pathophysiological processes (such as cerebral ischemia) that modify the integrity of the tissue microstructure result in significant alterations in tissue anisotropy and make this metric a useful endpoint for characterizing the temporal evolution of the disease. Diffusion-tensor imaging (DTI) studies of both experimental and human stroke suggest that DTI may provide additional information about the evolution of the disease that is not available from diffusion-weighted MRI (DWI) alone. Acute reductions in the average diffusivity [ = ( 1 2 3 )/3 where 1 , 2 , and 3 are the eigenvalues of the diffusion tensor] following the onset of cerebral ischemia are often accompanied by increases in diffusion anisotropy. In the transition from acute to sub-acute and chronic stroke, renormalizes and subsequently increases whereas diffusion anisotropy measures (e.g. FA) decline and remained reduced in chronic infarcts. Overall isotropic ADC changes during infarct evolution have been observed to be greater in white matter (WM) than in gray matter (GM) lesions (although there have been conflicting reports on this issue) and GM lesions tend to renormalize prior to WM lesions as the infarct evolves. Ischemic WM exhibits a significant decrease in diffusion anisotropy (relative to normal WM) during ischemic evolution whereas that of ischemic GM remains statistically unchanged. Furthermore, the percentage decrease in ischemic WM is largely determined by reductions in 1 , the eigenvalue that coincides with the long axis of the WM fiber tract. Variations in unidirectional ADC or over the ischemic time course limit the usefulness of this parameter alone as a predictor of ischemic injury. Consequently, ADC information has been combined with that of other MR parameters (including DTI) to unambiguously stage and predict ischemic brain injury over its entire temporal evolution. Combined and diffusion anisotropy measurements have identified three phases of diffusion abnormality: (1) reduced and elevated anisotropy; (2) reduced and reduced anisotropy; and (3) elevated and reduced anisotropy. However, variations in the differential patterns of and diffusion anisotropy evolution have been obser...

Summary:Pixel-by-pixel spatiotemporal progression of focal ischemia (permanent occlusion) in rats was investigated using quantitative perfusion and diffusion magnetic resonance imaging every 30 minutes for 3 hours. The normal left-hemisphere apparent diffusion coefficient (ADC) was 0.76 ± 0.03 × 10 −3 mm 2 /s and CBF was 0.7 ± 0.3 mL · g −1 · min −1 (mean ± SD, n‫.)5ס‬ The ADC and CBF viability thresholds yielding the lesion volumes (LV) at 3 hours that best approximated the 2,3,5-triphenyltetrazolium chloride (TTC) infarct volumes (200 ± 30 mm 3 ) at 24 hours were 0.53 ± 0.02 × 10 −3 mm 2 /s (30% ± 2% reduction) and 0.30 ± 0.09 mL · g −1 · min −1 (57% ± 11% reduction), respectively. Temporal evolution of the ADC-and CBF-defined LV showed a significant "perfusion-diffusion mismatch" up to 2 hours (P < 0.05, n ‫ס‬ 11), a potential therapeutic window. Based on the viability thresholds, three pixel clusters were identified on the CBF-ADC scatterplots: (1) a "normal" cluster with normal CBF and ADC, (2) an "ischemic core" cluster with markedly reduced CBF and ADC, and (3) a "mismatch" cluster with reduced CBF but slightly reduced ADC. These clusters were color-coded and mapped onto the image and CBF-ADC spaces. Lesions grew peripheral and medial to the initial ADC abnormality. In contrast to the CBF distribution, the ADC distribution in the ischemic hemisphere was bimodal; the relatively time-invariant bimodal-ADC minima were 0.57 ± 0.02 × 10 ), surprisingly similar to the TTCderived thresholds. Together, these results illustrate an analysis approach to systemically track the pixel-by-pixel spatiotemporal progression of acute ischemic brain injury.

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Background and Purpose-The early ischemic lesions demonstrated by diffusion-weighted imaging (DWI) are potentially reversible. The purposes of this study were to determine whether resolution of initial DWI lesions is transient or permanent after different brief periods of focal brain ischemia and to evaluate histological outcomes. Methods-Sixteen rats were subjected to 10 minutes (nϭ7) or 30 minutes (nϭ7) of temporary middle cerebral artery occlusion or sham operation (nϭ2). DWI, perfusion-weighted imaging (PWI), and T 2 -weighted imaging (T 2 WI) were performed during occlusion; immediately after reperfusion; and at 0.5, 1.0, 1.5, 12, 24, 48, and 72 hours after reperfusion. After the last MRI study, the brains were fixed, sectioned, stained with hematoxylin and eosin, and evaluated for neuronal necrosis. Results-No MRI or histological abnormalities were observed in the sham-operated rats. In both the 10-minute and 30-minute groups, the perfusion deficits and DWI hyperintensities that occurred during occlusion disappeared shortly after reperfusion. The DWI, PWI, and T 2 WI results remained normal thereafter in the 10-minute group, whereas secondary DWI hyperintensity and T 2 WI abnormalities developed at the 12-hour observation point in the 30-minute group. Histological examinations demonstrated neuronal necrosis in both groups, but the number of necrotic neurons was significantly higher in the 30-minute group (95Ϯ4%) than in the 10-minute group (17Ϯ10%, PϽ0.0001). Conclusions-Transient

Fluorine-19 inversion-recovery, echo-planar imaging (IR-EPI) was used in conjunction with a new PFC emulsion, perfluoro-15-crown-5-ether, to map the spatial distribution of oxygen tension in murine liver, spleen and radiation induced fibrosarcoma (RIF-1) tumors. Intravenously administered PFC emulsions were allowed to sequester in the liver, spleen, and tumor 3 to 7 days prior to imaging experiments. Seven, 64 x 64 IR-EPIs were acquired with successively increasing inversion times (TI). A nonlinear least-squares regression algorithm was used to fit the seven two-dimensional matrices, on a pixel-by-pixel basis, to solve for the relaxation rate, R1, of the sequestered PFC. From in vitro calibration curves, the oxygen tension (pO2) was calculated from the measured R1. Oxygen tension maps were then murine liver and spleen were produced (in 2.5 min) to demonstrate the technique and changes in tissue oxygenation as a function of breathing gas (air and carbogen (95% O2-5% CO2)) are presented. Tissue pO2 maps from RIF-1 tumors (n = 5) were obtained in less than 10 min and changes in tumor pO2 were studied when the breathing gas was switched from air to carbogen. The results from tumor pO2 maps were compared with 19F MR spectroscopy measurements to check for consistency. Histogram analysis yielded an average liver and spleen pO2 of 43 torr and 26 torr for RIF-1 tumors when the animals were breathing air. Statistically significant changes in tumor oxygenation as a function of breathing gas were obtained from both pO2 maps (6 +/- 2 torr, P < 0.05) and 19F MR spectroscopy (13 +/- 3 torr, P < 0.01) as evaluated using the Student's paired t test.

Diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI) can rapidly detect lesions in acute ischemic stroke patients. The PWI volume is typically substantially larger than the DWI volume shortly after onset, that is, a diffusion/ perfusion mismatch. The aims of this study were to follow the evolution of the diffusion/ perfusion mismatch in permanent and 60- minute temporary focal experimental ischemia models in Sprague-Dawley rats using the intraluminal middle cerebral artery occlusion (MCAO) method. DWI and arterial spin-labeled PWI were performed at 30, 60, 90, 120, and 180 minutes after occlusion and lesion volumes (mm(3)) calculated At 24 hours after MCAO, and infarct volume was determined using triphenyltetrazolium chloride staining. In the permanent MCAO group, the lesion volume on the ADC maps was significantly smaller than that on the cerebral blood flow maps through the first 60 minutes after MCAO; but not after 90 minutes of occlusion. With 60 minutes of transient ischemia, the diffusion/perfusion mismatch was similar, but after reperfusion, the lesion volumes on ADC and cerebral blood flow maps became much smaller. There was a significant difference in 24- hour infarct volumes between the permanent and temporary occlusion groups.