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Experimental Studies |
1 From the Institute for Biodiagnostics, National Research Council Canada, Winnipeg, Manitoba (M.Q., K.L.M., U.I.T.); and Department of Pathology, University of Manitoba, Winnipeg, Canada (M.R.D.B.). Received April 10, 2001; revision requested May 11; revision received August 2; accepted September 7. Supported by grant FRN-15635 from the Canadian Institutes for Health Research. Address correspondence to U.I.T., Institute for Biodiagnostics (West), National Research Council, 3330 Hospital Dr NW, Calgary, Alberta, Canada T2N 4N1 (e-mail: ursula.tuor@nrc.ca).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: One– and 4-week-old rats were randomly assigned to control or transient cerebral hypoxia-ischemia (ie, right carotid artery occlusion plus exposure to 8% oxygen) groups. Hypoxic-ischemic changes compared with normal ipsilateral brain tissue on DW images and the apparent diffusion coefficient of water were measured during and at 1 and 24 hours after hypoxia-ischemia ended. Hypoxic-ischemic changes in extracellular space and ipsilateral versus contralateral differences in Na+-K+–ATPase and cytochrome oxidase activity were measured.
RESULTS: Hyperintensities on DW images obtained during hypoxia-ischemia correlated well (P < .05) with extracellular space reductions, which occurred 15 minutes earlier in the brains of 4-week-old rats than in the brains of 1-week-old rats. Similarly, within 1 hour after hypoxia-ischemia ended, DW image and extracellular space changes normalized. In contrast, Na+-K+–ATPase and cytochrome oxidase activity decreased in some regions during hypoxia-ischemia and remained reduced 1 hour after the end of hypoxia-ischemia. Twenty-four hours after signal intensity normalization, hyperintense areas reappeared on DW images, and Na+-K+–ATPase and cytochrome oxidase activity remained decreased.
CONCLUSION: Signal intensity alterations with diffusion-sensitive MR imaging during and after transient hypoxia-ischemia are closely associated with a corresponding shrinkage and reexpansion of the extracellular space, irrespective of age. Mechanisms other than Na+-K+–ATPase changes may induce the early cell volume changes detected with diffusion-sensitive MR imaging.
© RSNA, 2002
Index terms: Animals • Brain, ischemia, 10.781 • Brain, MR, 10.121411, 10.12144 • Magnetic resonance (MR), diffusion study, 10.121411, 10.12144
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Several different ischemic tissue changes are often proposed as contributors to the alterations in signal intensity on diffusion-sensitive MR images. It is well accepted that during ischemia, biochemical and cellular changes are produced by a compromised energy supply and a reduction in adenosine triphosphate, which are initiated by a reduction in cerebral blood flow, where it is believed that accompanying anatomic cellular changes alter the MR imaging–detectable diffusibility of water (1–3,11,12). A common theory regarding the sequence of events is that once energy stores are insufficient to maintain the function of Na+-K+–adenosine triphosphatase (ATPase), an enzyme that affects cell volume, there is cell swelling and a shift of water from the extracellular space into the intracellular space that results in a decrease in the diffusibility of water (13–15). Direct support of this theory is limited, and this proposed sequence may need to be adjusted as more studies are performed that compare ischemic changes on DW MR images with biochemical and structural changes in tissue. For example, the results of a recent study (16) demonstrated that a change in the tissue diffusibility of water precedes membrane depolarization during transient focal cerebral ischemia.
With regard to the role of Na+-K+– ATPase, its activity has been shown to be reduced when DW MR image signal intensities are increased during focal ischemia in rats and are consistent with decreases in the ADC observed following the inhibition of cerebral Na+-K+– ATPase with ouabain (13,17). Demonstrations of a correlation between changes in cell volume and alterations in signal intensity on diffusion-sensitive MR images during cerebral ischemia are limited to in vitro studies of optic nerve and in vivo studies of cardiac arrest or N-methyl-D-aspartate–induced central nervous system injury (18–20). Little is known about the correlation between these changes during transient ischemia, when there can be a rapid reversal of ischemic DW MR imaging changes. The correlation between ischemic changes on DW images and extracellular space may be inexact, because there are indications that the MR imaging–detectable diffusibility of water within tissue can be influenced by changes in cell organelles and macromolecules, in cell structure, and in intracellular transport (19,21,22).
We hypothesized that (a) during transient cerebral hypoxia-ischemia, the time course of high-signal-intensity areas observed on diffusion-sensitive MR images corresponds to temporary reductions in extracellular space and to a transient disturbance in the enzymes that influence cell volume and mitochondrial function and (b) considering the known age-related differences in parameters such as extracellular space, cerebral metabolic rate, and onset of DW imaging hyperintensities following hypoxia-ischemia, this correspondence in the brains of neonatal subjects differs from that in the brains of more mature subjects (10,23,24). Thus, the purpose of our study was to investigate the correlation between DW image or ADC changes with alterations in extracellular volume and changes in cytochrome oxidase and Na+-K+–ATPase activity at various times during and up to 24 hours after onset of cerebral hypoxia-ischemia in neonatal and juvenile (ie, 4-week-old) rats.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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A transient episode of cerebral hypoxia-ischemia was produced in the two groups, as described previously (9). In brief, the right common carotid artery was ligated and cut while the animals were under isoflurane-induced anesthesia (3%–4% for induction and 1.5%–2.5% for maintenance). The incision site was infiltrated with 0.25% bupivacaine. In the sham control animals, 13 1-week-old rats and 11 4-week-old rats, the carotid artery was isolated but not ligated. After surgery, the rats were returned to the cage with the mother for 1–2 hours of recovery from anesthesia. Ligation of the carotid artery alone does not result in ischemia because of the collateral blood flow provided by the circle of Willis. Ischemia is produced in the cerebral hemisphere ipsilateral to the occlusion by subjecting the animal to an episode of hypoxia. Different durations of hypoxia for different ages are required to produce comparable cerebral infarctions within the hemisphere ipsilateral to the occlusion without excessive mortality during the hypoxia (23). The brain regions where infarction occurs include the parietal cortex, striatum, thalamus, and hippocampus within the territory of the cerebrum that is supplied by the internal carotid and/or middle cerebral artery (9,23).
Thus, the majority of the rats were exposed to humidified 8% oxygen for either 2 hours (24 of 39 1-week-old rats) or 30 minutes (34 4-week-old rats). In 15 of the 1-week-old animals that were exposed to hypoxia, the period of ischemia was reduced by up to 30 minutes to avoid mortality. Body temperature was maintained at 37.0°–37.5°C during hypoxia by using a circulating water blanket or a heating lamp.
MR Imaging
MR imaging experiments were performed by using a 9.4-T, 21-cm horizontal bore magnet (Magnex Scientific, Yarnton, England) equipped with a console (model MSLX; Bruker, Ettlingen, Germany). The rats were anesthetized with isoflurane (4% for induction, 0.25%–1.00% for maintenance) and placed in a chamber designed to fit the bore of the magnet. The 1-week-old rats were restrained with a foam-lined head holder, and the 4-week-old rats were restrained with ear pins and an incisor bar. While an animal was in the magnet, we monitored its sensitivity to hypoxia by measuring the respiration rate if it was a 4-week-old rat and by using electrocardiography if it was a 1-week-old rat because the small chest movement is difficult to detect in younger animals.
Six rats from each age group were imaged immediately before hypoxia-ischemia onset, during hypoxia-ischemia, and at 1 hour after the hypoxic-ischemic episode. These animals were then sacrificed, and their brains were removed for histologic assessment. Another six rats from each age group were imaged and then sacrificed 24 hours after the hypoxic-ischemic episode. DW MR images were acquired in the animals with a quadrature coil tuned to 400.045 MHz by using a spin-echo MR imaging sequence (1,200/49 [repetition time msec/echo time msec], eight sections, 1.0-mm section thickness for 1-week-old rats; 1.5-mm section thickness for 4-week-old rats) (26). Two diffusion-sensitive gradient pulses of 10-msec duration and 85-mT/m amplitude were applied in the phase-encoding direction 23 msec apart and resulted in a diffusion weighting factor (b) of 1,069 sec/mm2. The field of view was 2 cm, and the data matrix was 256 x 128.
In several additional animals (seven 1-week-old rats and four 4-week-old rats), the data matrix was reduced to 256 x 64 to allow more time for the collection of additional images with b values of 46, 211, 540, and 767 sec/mm2. Two to four images were used to construct an ADC map. After the last image was obtained, the animal was injected with an overdose of pentobarbital sodium (80 mg/kg). The block of brain tissue at the middle portion of the thalamus was removed, frozen in isopentane at -45°C, and stored at -80°C until it was used for future histochemical examination of Na+-K+– ATPase and cytochrome oxidase activity.
The time of hypoxia-ischemia onset and the distribution of signal intensity changes that occurred in the ipsilateral hemisphere were determined by examining the DW MR image findings. In addition, the area of pixels with a higher signal intensity in the ipsilateral hemisphere than in the contralateral hemisphere was measured on sections obtained at the levels of the anterior (striatum), middle (middle thalamus), and posterior (posterior thalamus) regions of the cerebrum. This area was then converted to a percentage of the entire coronal brain section. The signal intensity on DW MR images was documented as a relative ipsilateral versus contralateral ratio measured in the anterior hippocampus or in the entire hemisphere on a section through the middle cerebrum. The ADCs in the ipsilateral and contralateral parietal cortices were calculated. All calculations were performed by an investigator (M.Q.) who was blinded to the group assignments.
Histochemical Analysis
The 1-week-old (six sham control animals, 23 animals exposed to hypoxia-ischemia) and 4-week-old (six sham control animals, 25 animals exposed to hypoxia-ischemia) rats that survived the hypoxic-ischemic episode were used for histochemical analysis of cytochrome oxidase and Na+-K+–ATPase activity during, at 1 hour, or at 24 hours after the hypoxic-ischemic episode. In addition, subgroups of the animals that were killed at 1 and 24 hours after the hypoxic-ischemic episode underwent MR imaging. These animals were decapitated, and their brains were frozen in isopentane at -45°C. The animals exposed to hypoxia-ischemia were decapitated either during the hypoxic-ischemic episode (seven 1-week-old rats, nine 4-week-old rats) or at 1 hour (nine 1-week-old rats, 10 4-week-old rats) or 24 hours (seven 1-week-old rats, six 4-week-old rats) after the episode. The frozen sections (20 µm) from the middle thalamus were mounted on slides, air dried at room temperature for 30 minutes, and then processed for histochemical staining of cytochrome oxidase or Na+-K+–ATPase.
For Na+-K+–ATPase activity analysis, we immersion fixed the sections in 10% formalin before using a one-step lead citrate method for the histochemical detection of Na+-K+–ATPase (27). The experiments were performed initially to determine the optimal incubation times for brain sections from 1- and 4-week-old animals to achieve sufficient signal-to-noise ratios. The slides were then incubated with an enzyme reaction medium—that is, 250 mmol/L of glycine–potassium hydroxide buffer, 4 mmol/L of lead citrate, 10 mmol/L of p-nitrophenyl phosphate, 25% dimethyl sulfoxide, and 2.5 mmol/L of levamisole—at 37°C for 2 hours for the brains of the 1-week-old rats or for 35 minutes for the brains of the 4-week-old rats. The incubation was followed by a reaction with 1% ammonium sulfide for 30 seconds.
Histochemical analysis of cytochrome oxidase activity was performed as described previously (28,29). In brief, fresh sections were incubated in a reaction medium that consisted of 0.5 mg/mL of diaminobenzidine, 250 µg/mL of cytochrome c, and 4% sucrose in 0.1 mol/L of phosphate-buffered saline at 37°C for 2 hours for the brains of the 1-week-old rats or for 30 minutes for the brains of the 4-week-old rats.
An author (M.Q.) who was blinded to the group assignments used a microcomputer imaging system (MCID; Imaging Research, St Catharines, Ontario, Canada) to measure the relative activity levels of Na+-K+–ATPase reaction and cytochrome oxidase products. Five optical density readings were performed from the region of interest at the level of the middle thalamus, which included the dentate gyrus, fields 1–3 of cornu ammonis (ie, CA1, CA2, CA3), parietal cortex, thalamus, and hypothalamus. Data are presented as mean ratios of enzyme activity in the ipsilateral versus contralateral hemisphere. We also measured the area of reduced reaction product activity in the ipsilateral hemisphere as a percentage of the area in the entire brain section at 24 hours after the hypoxic-ischemic episode.
Extracellular Space Measurements
In 26 additional animals, extracellular space changes in the sham control rats and hypoxic-ischemic rats were assessed by using electron microscopic and impedance techniques. Changes in extracellular space in the superficial cortex were assessed in the 1-week-old rats by using flash-frozen tissue that was processed for embedding while frozen and examined with electron microscopic techniques (30,31). Flash freezing prevents the shrinkage of tissue that is produced by most fixatives. The brains in two sham control rats, in two hypoxic-ischemic rats during hypoxia-ischemia, and in two hypoxic-ischemic rats 1 hour after the hypoxic-ischemic episode were studied. At the appropriate time, the rat was deeply anesthetized with 4% isoflurane, the cranium was peeled away swiftly, and a liquid nitrogen–cooled copper block was quickly and firmly applied to the dorsal surface of the brain. Then, the block and brain were submerged in liquid nitrogen. The thicker skulls of the 4-week-old rats precluded the use of these animals.
Frozen pieces of brain from the ischemic hemisphere were placed in liquid nitrogen–cooled tetrahydrofuran and then stored at -80°C before being placed in 1% osmium tetroxide in acetone at -80°C. After fixation in osmium tetroxide in acetone at -80°C, -30°C, 4°C, and then room temperature, the samples were embedded in an araldite/epoxy-resin embedding solution (Araldite 502 Epoxy Resin; Electron Microscopy Sciences, Fort Washington, Pa) (30). Semithin sections stained with toluidine blue were used to select the pieces that included the optimally fixed pial surface of the brain. Ultrathin sections were cut, placed on copper grids, and then stained with lead citrate for contrast. Photographs of the preserved superficial cortex (ie, layer 1) were printed at final magnifications of x13,770 and x36,747. Two to four prints of each original specimen at each of the two magnifications were used to estimate the extracellular space fraction. A neuropathologist (M.R.D.B.) who was blinded to the group assignments laid a B100 counting grid (32) over the photographs, tabulated the number of points over the extracellular space, and converted the number to a fraction.
In 20 additional animals, electrical impedance changes were measured to assess changes in extracellular space in the cerebrum of the hypoxic-ischemic hemisphere (33). The measurements were performed in five 1-week-old rats from the control and hypoxia-ischemia groups and in five 4-week-old rats from these groups by using a multifrequency impedance analyzer (model 4200; Xitron, San Diego, Calif). The animals were anesthetized with 2.5% isoflurane, and a midline incision in the scalp was made to expose the skull. Four 30-gauge stainless steel electrodes (Grass, West Warwick, RI) were inserted 1.5 mm apart through bur holes in the skull on the side of the brain ipsilateral to the occlusion and fixed in position with dental acrylic. The electrodes were inserted 2 mm into the brain; the two inner electrodes were placed 0.8 and 2.3 mm posterior to the bregma. Electrical impedance was measured from the two inner electrodes after current pulses were passed through the two outer electrodes at various times before, during, and for 1 hour after the hypoxic-ischemic episode. The volume of the extracellular space was determined from the impedance data by using computer software (Hydra Data Acquisition Utility; Xitron Technologies, San Diego, Calif). This software normalized the values to values obtained before the hypoxic-ischemic episode. This analysis was performed by a technician and an author (M.Q.) who were blinded to the group assignments.
Statistical Analyses
All data are presented as means plus or minus SDs and were analyzed by using commercially available software (Statistica; StatSoft, Tulsa, Okla). A paired t test was used to compare the mean signal intensities of the hyperintense areas at DW MR imaging, the mean values of ADCs in the ipsilateral and contralateral hemispheres or the enzyme activity levels, and the mean values of DW image signal intensity ratios following logarithmic transformation. The results of the impedance measurements were not normally distributed, so a comparison of the extracellular space between the sham control animals and the hypoxic-ischemic animals was made by using the Mann-Whitney rank sum test. Differences were considered to be significant at the P less than .05 level.
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RESULTS |
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Na+-K+–ATPase Activity
Age-dependent Na+-K+–ATPase activity was evident in the control animals: A lower level of activity was detected in the brains of the 1-week-old rats compared with the activity level in the brains of the 4-week-old rats, despite the fourfold longer incubation time for brain sections from the 1-week-old animals (Fig 3). In the brains of the 1-week-old rats, a slight decrease in Na+-K+–ATPase activity was observed in the ipsilateral dentate gyrus, CA3, and cortex at the end of the hypoxic-ischemic episode (P < .05) (Figs 3, 4). One hour after the hypoxic-ischemic episode, the mean activity intensity ratios for Na+-K+–ATPase remained similar to the values measured during hypoxia-ischemia (Figs 3, 4). The hemisphere-related differences in Na+-K+–ATPase activity became most evident at 24 hours after the hypoxic-ischemic episode in the dentate gyrus, CA1, CA2, and CA3 regions (P < .05).
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Cytochrome Oxidase Activity
In the 1-week-old rats, there was a trend toward decreased cytochrome oxidase reaction product activity in the ipsilateral dentate gyrus and CA1 during hypoxia-ischemia (Figs 3, 4). At 1 hour after the hypoxic-ischemic episode, this decrease became more pronounced and was followed by a further reduction in the accumulation of cytochrome oxidase reaction product at 24 hours after the hypoxic-ischemic episode (P < .05) in all regions examined except the hypothalamus.
In the 4-week-old rats, there was a decrease in cytochrome oxidase reaction product activity in the ipsilateral dentate gyrus, cortex, and thalamus (P < .05) during hypoxia-ischemia (Figs 3, 4). The decreased cytochrome oxidase activity became more apparent at 1 hour after the hypoxic-ischemic episode, and significant reductions occurred in the ipsilateral dentate gyrus, CA2, CA3, and cortex (P < .05). Similar reductions in cytochrome oxidase activity were still present at 24 hours after the hypoxic-ischemic episode (P < .05).
Extracellular Space
Results of electron microscopy of the superficial layer of the parietal cortex ipsilateral to the occlusion demonstrated marked changes in extracellular space during hypoxia-ischemia in the brains of the 1-week-old rats. In the control animals, the extracellular space was 51% ± 0.06, which decreased to 3% ± 0.01 during hypoxia-ischemia (Fig 5), and this increased to 30% ± 0.21 within 1 hour after the hypoxic-ischemic episode.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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DW imaging has become a well accepted MR technique to assess the changes in the diffusibility of water that are produced by various disease states, particularly stroke. Clinical imaging units have the capability for relatively quick acquisition of trace DW or ADC maps of the human brain by means of echo-planar imaging. At the high magnetic fields used to image small animals, echo-planar imaging is much more difficult to perform owing to a high susceptibility to field inhomogeneities, eddy currents, and instability-related artifacts.
In the present study, a spin-echo sequence was used to acquire optimal DW MR images with a 9.4-T system and a gradient pulse applied on the phase-encoding axis. The temporal resolution of this MR technique (eg, 5 minutes) was sufficient to enable us to closely follow the changes on DW images that occurred during and after the hypoxic-ischemic episodes. The corresponding ADC maps that were acquired in some of the animals had a poorer temporal resolution, but they corroborated the results obtained by measuring the left-right signal intensity changes on the DW images.
Although the most accurate measurements would have been obtained by acquiring the DW images in three different orthogonal directions to obtain a trace ADC map, to keep the acquisition time at one-third of that required for trace measurements, the gradient pulses were applied in only one direction. Water diffusion in tissue can depend on the direction of the gradient pulses; however, this anisotropy is greatest in white matter, where the translational movement of water in the tracts is restricted in certain directions. Because the anisotropy in gray matter is minimal (2) and the measurements in the present study were made largely in gray matter, the changes observed were considered to represent overall DW MR imaging or ADC changes. This determination is supported by the results of a recent study (21) in which the trace ADC changes measured in a comparable 3-week-old rat model of cerebral hypoxia-ischemia were reported to be similar to those observed in the present study.
The extracellular space shrinkage and cell swelling that result from reduced energy substrate availability are generally considered to be responsible for the decreased diffusibility of water during periods of permanent cerebral ischemia (1–3,34). However, few studies have had results that demonstrated a correlation between cerebral ischemic changes in ADC and extracellular space in vivo, other than one study (18) that was performed in rats following cardiac arrest. One reason for the paucity of information may be the difficulty in measuring extracellular space with traditional techniques, such as electron microscopy, microdialysis, and iontophoresis, or with extracellular space indicator techniques, which have artifacts associated with fixation, interference from nonselective ions, and limited access of the indicators to the tissue because of the blood-brain barrier (35). The present ultrastructural evaluation of flash-frozen tissue with freeze substitution methods prevented the occurrence of extracellular and intracellular volume artifacts related to aldehyde fixation.
The impedance technique used to assess extracellular volume by means of the application of a small current at low frequency within the tissue enabled us to avoid disturbing the extracellular volume. Both the extracellular measurements made with electron microscopic techniques and those made with impedance techniques demonstrated a reduction in extracellular space during hypoxia-ischemia, with some recovery during the 1st hour afterward. The repeated measurements of extracellular space that were possible with the impedance technique demonstrated a very close correlation between the DW imaging and extracellular space changes. Thus, our study results strongly support the belief that the ADC changes during and immediately after transient cerebral ischemia are closely related to alterations in extracellular space in the brains of both immature and more mature subjects.
The DW image changes reported in the present study were similar to those in a previous investigation (10): The onset of changes in extracellular space in the brains of the immature rats was delayed compared with that in the brains of the juvenile animals. The reason for this is not clear, but it is possible that the larger extracellular space in the brains of neonatal subjects, compared with that in the brains of older subjects, has a higher buffering capacity against ion imbalances or the release and accumulation of glutamate or oxidative free radicals (18,24,36). Alternatively, the lower demand for glucose and more flexible use of energy sources in the brains of neonatal subjects protect against major disturbances in energy status during cerebral hypoxia-ischemia and thereby delay the onset of adenosine triphosphate depletion and the associated changes in cell volume (25).
A disturbance in Na+-K+–ATPase function during hypoxia or ischemia is thought to trigger cell swelling and movement of water from the extracellular space into the intracellular space (13–15). A decrease in ADC has been observed following the intracerebral administration of ouabain, an inhibitor of Na+-K+–ATPase (17). Results of the present study indicate that there is some correlation between the changes in Na+-K+–ATPase activity measured histochemically and the DW image changes observed during hypoxia-ischemia. During hypoxia-ischemia, however, the reductions in Na+-K+–ATPase activity are not as widespread or diffuse as the changes in DW image signal intensity, and following hypoxia-ischemia, the decreases in Na+-K+–ATPase remain despite a recovery of DW image changes. The regional susceptibility probably reflects regional differences in the severity of ischemia and the contribution to injury mediated by regional differences in receptors such as glutamate receptors in the developing brain (37). The reduced Na+-K+–ATPase activity could reflect a number of changes, including internalization, conformational changes, or an actual decrease in the enzyme level. The results obtained soon after a hypoxic-ischemic episode, however, suggest that the recovery of extracellular volume changes is independent of Na+-K+–ATPase activity, and other factors that influence cell volume control, such as Na+/H+ and Cl-/HCO3- exchange transport, amino acid level, and membrane permeability, may be involved in the reexpansion of the extracellular space (38).
The histochemical results of this study indicate that ischemic tissue injury persists, despite the normalization of DW image changes 1 hour after the end of hypoxia-ischemia. In addition to Na+-K+–ATPase activity decreases, there were persistent reductions in cytochrome oxidase activity, the final enzyme complex associated with oxidative phosphorylation in the mitochondrial respiratory chain. This reduction was probably severe enough to adversely affect mitochondrial function: Results of other studies (39,40) have shown that in this model of cerebral hypoxia-ischemia, adenosine triphosphate levels do not recover completely, but rather they remain below control levels at reoxygenation. Considering the reported neuronal cell death and astrocytic cell swelling at 1 hour after a hypoxic-ischemic episode in 3-week-old animals, the reductions in cytochrome oxidase and Na+-K+–ATPase activity probably reflect irreversible tissue damage (21). By 24 hours after a hypoxic-ischemic episode, DW image hyperintensities correspond well to areas of reduced histochemical staining for cytochrome oxidase and Na+-K+–ATPase; these results indicate that DW image changes reflect tissue damage well at this later time.
Practical application: The advent of thrombolytic treatment for ischemic stroke has resulted in an urgent need for the early diagnosis of cerebral ischemia and the accurate assessment of the effects of treatment. Successful thrombolytic treatment is equivalent to an insult to transient cerebral ischemia. DW imaging has been promoted as the MR diagnostic technique for the assessment of cerebral ischemia without good knowledge of the tissue changes involved. Results of the present study indicate that a reduction in the ADC or an increase in DW image signal intensity in the acute stages following ischemic insult in patients is related to decreased extracellular space and cellular edema. Reperfusion of tissue—for example, as a result of effective thrombolytic therapy—may be accompanied by a rapid reversal of DW image signal intensity changes that are related to the return of the extracellular space changes to normal. However, the recovery of diffusion-sensitive image changes following transient hypoxia-ischemia can also be associated with continued tissue injury, including reductions in both Na+-K+–ATPase activity and mitochondrial function. Thus, the appearance of "normal" diffusion-sensitive images within the 1st few hours following a transient ischemic insult should be interpreted as representative of healthy tissue with caution.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, U.I.T.; study concepts, all authors; study design, U.I.T., M.Q., K.L.M.; literature research, U.I.T., M.Q.; experimental studies, M.Q., K.L.M.; data acquisition, M.Q., K.L.M., M.R.D.B.; data analysis/interpretation, M.Q., U.I.T., M.R.D.B.; statistical analysis, M.Q.; manuscript preparation, M.Q., U.I.T., M.R.D.B.; manuscript definition of intellectual content, M.Q., U.I.T., M.R.D.B.; manuscript editing, U.I.T., M.Q., manuscript revision/review, all authors; manuscript final version approval, U.I.T.
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] or cortex [
]) during hypoxia-ischemia and is sustained at 24 hours after hypoxic-ischemic exposure.
