N. Van Bruggen

Diffusion-weighted imaging studies of cerebral ischemia in gerbils. Potential relevance to energy failure.

Background and Purpose: Diffusion-weighted magnetic resonance imaging has been shown to be particularly suited to the study of the acute phase of cerebral ischemia in animal models. The studies reported in this paper were undertaken to determine whether this technique is sensitive to the known ischemic thresholds for cerebral tissue energy failure and disturbance of membrane ion gradients. Methods: Diffusion-weighted images of the gerbil brain were acquired under two sets of experimental conditions: as a function of cerebral blood flow after controlled graded occlusion of the common carotid arteries (partial ischemia), as a function of time following complete bilateral carotid artery occlusion (severe global ischemia), and on deocclusion after 60 minutes of ischemia. Results: During partial cerebral ischemia, the diffusion-weighted images remained unchanged until the cerebral blood flow was reduced to 15–20 ml · 100 g−1 · min−1 and below, when image intensity increased as the cerebral blood flow was lowered further. This is similar to the critical flow threshold for maintenance of tissue high-energy metabolites and ion homeostasis. After the onset of severe global cerebral ischemia, diffusion-weighted image intensity increased gradually after a delay of approximately 2.5 minutes, consistent with complete loss of tissue adenosine triphosphate and with the time course of increase in extracellular potassium. This hyperintensity decreased on deocclusion following 60 minutes of ischemia. Conclusions: The data suggest that diffusion-weighted imaging is sensitive to the disruption of tissue energy metabolism or a consequence of this disruption. This raises the possibility of imaging energy failure noninvasively. In humans, this could have potential in visualizing brain regions where energy metabolism is impaired, particularly during the acute phase following stroke.

T2- and diffusion-weighted magnetic resonance imaging of a focal ischemic lesion in rat brain.

We sought to evaluate the application of T2 -weighted and diffusion-weighted magnetic resonance imaging techniques in the study of a focal ischemic lesion in the rat brain. Methods Unilateral cortical infarcts were induced using the photosensitive dye rose bengal and 560 nm light irradiation. Magnetic resonance images were recorded from a total of 11 rats at selected intervals from 1.5 hours to several days after induction of the lesion. Parallel experiments were performed in which Evans blue dye was injected into the lesioned animals either immediately after lesion induction (n=11) or hour before the animals were killed (n=11). The second procedure was designed to show regions of blood–brain barrier permeability to plasma proteins at the time of sacrifice, whereas the first procedure showed the accumulation and subsequent dispersion of plasma protein following disruption of the blood–brain barrier. Results Regions of the cortex highlighted by the T2 -weighted images corresponded well to the pattern dye staining seen from the first procedure while the diffusion-weighted images showed visual correspondence with the staining pattern obtained using the second procedure. Conclusions These results illustrate the complementary use of T2-weighted and diffusion-weighted magnetic resonance imaging in discerning the pathophysiology of developing lesions.

Identification of collaterally perfused areas following focal cerebral ischemia in the rat by comparison of gradient echo and diffusion-weighted MRI.

Diffusion-weighted (DW) and gradient echo (GE) magnetic resonance images were acquired before and after occlusion of the middle cerebral artery (MCA) in the rat. Upon occlusion, an increase in DW imaging signal intensity was observed in a core area within the MCA territory, most likely reflecting cytotoxic edema. The signal from GE images, which is sensitive to changes in the absolute amount of deoxyhemoglobin, decreased following ischemia within a region that extended beyond the core area observed with DW imaging. This hypointensity is attributed to increases in blood volume and/or oxygen extraction fraction, which result from a decrease in perfusion pressure in the collaterally perfused area. The evolution of the GE imaging signal intensity from different regions was studied for 3.5 h following the occlusion. In the core area, the GE imaging signal returned towards baseline values after ∼1–2 h, while it remained stable in the surrounding area. This feature may reflect a decrease in hematocrit due to microcirculatory defect and/or a decrease in the oxygen extraction fraction due to ongoing infarction of the tissue and may indicate that tissue recovery is severely compromised. The combined use of DW and GE imaging offers great promise for the noninvasive identification of specific pathological events with high spatial resolution.

Magnetic Resonance Imaging of Propagating Waves of Spreading Depression in the Anaesthetised Rat

Gradient echo magnetic resonance (MR) imaging was used to demonstrate propagating waves of cortical spreading depression (SD) in the anaesthetised rat. SD was initiated by remote perfusion with 150 mM KCl applied for 0.5–2 min to the left parietal cortex. Gradient echo MR images were obtained every 12–30 s in either a vertical coronal section or a horizontal section including the superficial cortex in plan view. Within 2 min of application of KCl, we observed a zone of increased signal intensity (3–15%) on the MR image, up to 2 mm across, lasting approximately 1 min and propagating away from the site of initiation. The mean velocity for 27 of such waves seen in seven animals was calculated to be 2.79 mm/min, with means (±SD) in individual animals averaging 2.90 ± 0.46 mm/min (n = 7). Increased signal intensity in gradient echo images has been attributed to an increased level of oxygenation within the venous blood. Our results are consistent with this interpretation although other physiological changes during SD may also contribute to the signal changes.

Vigabatrin-induced lesions in the rat brain demonstrated by quantitative magnetic resonance imaging

Rats treated with 250 mg/kg/day vigabatrin showed lesions detected by magnetic resonance imaging (MRI) in the cerebellar white matter in vivo. No lesions were seen in any control animal. As well as these visually apparent lesions, quantitative T2 relaxation time measurements showed a 12 ms increase in cerebellar white matter from 66 +/- 4 ms (SD, n = 5) to 78 +/- 2 ms (SD, n = 7). This region, as expected from previous studies, showed microvacuolation on post-mortem pathology. Additionally, significant increases in T2 relaxation times of 4-9 ms were found in the cerebral cortex, thalamus and hippocampus. Microvacuolation was not detected by post-mortem histopathology in the cerebral cortex or hippocampus, however, immunohistochemical staining for glial fibrillary acidic protein and for macrophages (ED1) showed reactive astrocytes (gliosis) and in more severe cases, microglial proliferation in these regions; such changes were also seen in association with the microvacuoles. No T2 increase was found in the cerebellar grey matter or olfactory bulbs. MRI techniques, including T2 relaxometry, are therefore sensitive for detecting vigabatrin-induced changes, including reactive astrocytosis, microglial proliferation and vacuolation in the rat brain. These results suggest that quantitative MRI should be a useful method for evaluating whether vigabatrin has neuropathological effects when given to patients.

Applications of magnetic resonance spectroscopy and diffusion-weighted imaging to the study of brain biochemistry and pathology

The first practical demonstration that nuclear magnetic resonance (NMR) spectroscopy could be applied to the study of brain biochemistry in vivo came in 1980, with the studies of the rat brain using a surface coil. Since then the technique has been rapidly and extensively developed into a versatile, non-invasive tool for the investigation of various aspects of brain biochemistry, physiology and disease. NMR is non-destructive and can be used to examine a wide variety of samples, ranging from localized regions within the whole brain in humans or animals, through tissue preparations (perfused organ, tissue slices and homogenates), to isolated cells and aqueous solutions, such as tissue extracts. 31P and 1H NMR spectra deriving from endogenous compounds of the brain in situ allow assessment of tissue metabolites and provide information about high-energy phosphates, lactate, certain amino acids, intracellular pH and ionic concentrations. Exogenous substrates or probes labelled with stable isotopes can also be introduced into the brain and used to monitor metabolism. Animal models of brain diseases have given some impetus to rapid progress in clinical NMR spectroscopy and also magnetic imaging techniques. The purpose of this article is to highlight the type of information available from these NMR techniques, and to present this in a neuroscience context, emphasizing the biochemical, physiological and pathological information that can be obtained using these methods.

Dynamics of Cerebral Tissue Injury and Perfusion After Temporary Hypoxia-Ischemia in the Rat : Evidence for Region-Specific Sensitivity and Delayed Damage Editorial Comment: Evidence for Region-Specific Sensitivity and Delayed Damage

BACKGROUND AND PURPOSE Selective regional sensitivity and delayed damage in cerebral ischemia provide opportunities for directed and late therapy for stroke. Our aim was to characterize the spatial and temporal profile of ischemia-induced changes in cerebral perfusion and tissue status, with the use of noninvasive MRI techniques, to gain more insight in region-specific vulnerability and delayed damage. METHODS Rats underwent 20 minutes of unilateral cerebral hypoxia-ischemia (HI). We performed combined repetitive quantitative diffusion-weighted, T2-weighted, and dynamic susceptibility contrast-enhanced MRI from before HI to 5 hours after HI. Data were correlated with parallel blood oxygenation level-dependent MRI and laser-Doppler flowmetry. Finally, MRI and histology were done 24 and 72 hours after HI. RESULTS Severe hypoperfusion during HI caused acute reductions of the apparent diffusion coefficient (ADC) of tissue water in the ipsilateral hemisphere. Reperfusion resulted in dynamic perfusion alterations that varied spatially. The ADC recovered completely within 1 hour in the hippocampus (from 0.68 +/- 0.07 to 0.83 +/- 0.09 x 10[-3] mm2/s), cortex (from 0.56 +/- 0.06 to 0.77 +/- 0.07 x 10[-3] mm2/s), and caudate putamen (from 0.58 +/- 0.06 to 0.75 +/- 0.06 x 10[-3] mm2/s) but only partially or not at all in the thalamus (from 0.65 +/- 0.07 to 0.68 +/- 0.12 x 10[-3] mm2/s) and substantia nigra (from 0.80 +/- 0.08 to 0.76 +/- 0.10 x 10[-3] mm2/s). Secondary ADC reductions, accompanied by significant T2 elevations and histological damage, were observed after 24 hours. Initial and secondary ADC decreases were observed invariably in the hippocampus, cortex, and caudate putamen and in approximately 70% of the animals in the thalamus and substantia nigra. CONCLUSIONS Region-specific responses and delayed ischemic damage after transient HI were demonstrated by MRI. Acute reperfusion-induced normalization of ADCs appeared to poorly predict ultimate tissue recovery since secondary, irreversible damage developed eventually.

Investigation of Effects of Vigabatrin with Magnetic Resonance Imaging and Spectroscopy in Vivo

Vigabatrin (gamma vinyl GABA, GVG) is an irreversible inhibitor of gamma-aminobutyric acid (GABA) transaminase, the enzyme that is responsible for degradation of GABA in the CNS. Vigabatrin is effective against partial seizures (Rimmer and Richens, 1984; Mumford and Dam, 1989; Reynolds, 1992). In toxicological studies involving rodents and dogs, high doses of GVG were associated with the development of a site-selective intramyelinic oedema that was accompanied by an astrocytic reaction (Butler et al., 1987; Gibson et al, 1990). A concern has been expressed about whether similar neuropathological changes may develop in human patients receiving long-term GVG therapy. No such changes have been identified in 51 surgical specimens and 13 autopsy cases (Cannon, 1991). Evoked response latencies have been shown to be prolonged in dogs with GVG-associated intramyelinic oedema, and these parameters have not been delayed in GVG-treated patients (Hammond and Wilder, 1985; Tartara et al., 1986). Evoked responses, however, only sample a small part of the neuraxis.

Restoration of Energy Metabolism and Resolution of Oedema Following Profound Ischaemia

Cerebral ischaemia was produced in 2 groups of gerbils by occlusion of the common carotid arteries for 30 minutes, resulting in cerebral oedema. In group 1 cerebral oedema was measured by specific gravity microgravimetry, and in group 2 brain metabolism and blood flow were measured by 31P and 1H NMR spectroscopy and hydrogen clearance respectively. In group 1 the brain water content did not return to control levels by 180 minutes of reperfusion. Energy metabolism, determined by 31P NMR spectroscopy returned to control by 12 minutes, intracellular pH (pHi) by 20 minutes, and lactate, determined by 1H NMR spectroscopy, by 50 minutes. There was a lag of about 10 minutes before lactate began to be cleared from the brain. We suggest that while pHi is low, Na+/H+ exchange will negate the Na+ extrusion driven by the Na+/K+ ATPase. When pHi approaches normal there will be a net extrusion of Na+, taking osmotic water with it, and presumably with passive washout of lactate. This may be the cause of the initial delay in lactate clearance.