Noninvasive detection of cerebral hypoperfusion and reversible ischemia from reductions in the magnetic resonance imaging relaxation time, T2

Quantitative assessment of the balance between oxygen delivery and consumption in the rat brain after transient ischemia with T2 -BOLD magnetic resonance imaging

The balance between oxygen consumption and delivery in the rat brain after exposure to transient ischemia was quantitatively studied with single-spin echo T2-BOLD (blood oxygenation level-dependent) magnetic resonance imaging at 4.7 T. The rats were exposed to graded common carotid artery occlusions using a modification of the four-vessel model of Pulsinelli. T2, diffusion, and cerebral blood volume were quantified with magnetic resonance imaging, and CBF was measured with the hydrogen clearance method. A transient common carotid artery occlusion below the CBF value of approximately 20 mL x 100 g(-1) x min(-1) was needed to yield a T2 increase of 4.6 +/- 1.2 milliseconds (approximately 9% of cerebral T2) and 6.8 +/- 1.7 milliseconds (approximately 13% of cerebral T2) after 7 and 15 minutes of ischemia, respectively. Increases in CBF of 103 +/- 75% and in cerebral blood volume of 29 +/- 20% were detected in the reperfusion phase. These hemodynamic changes alone could account for only approximately one third of the T2 increase in luxury perfusion, suggesting that a substantial increase in blood oxygen saturation (resulting from reduced oxygen extraction by the brain) is needed to explain the magnetic resonance imaging observation.

Assessment of brain tissue viability in acute ischemic stroke by BOLD MRI

The introduction of new neuroprotective treatment strategies for acute stroke patients has provided a requirement for neuroimaging methods capable of identifying salvageable tissue in acute stroke patients. Substantial positron emission tomography evidence points to the fact that a peri-infarct zone with blood flow of 20-45% of normal, metabolic rate of oxygen of >35% of normal and oxygen extraction ratio (OER) of >0.7 are indices of tissue at risk of infarction, yet with potential for recovery. The sensitivity of T(2) to blood oxygen level dependent (BOLD) effects allows the mismatch between oxygen delivery and consumption in the brain to be imaged. Previous evidence from animal models of cerebral hypoperfusion and ischemic stroke strongly suggest that T(2) BOLD MRI highlights viable and salvageable brain regions. The Hahn-echo T(2) and diffusion show distinct flow thresholds in the rat brain so that the former parameter probes areas with high OER and the latter genuine ischemia. In the flow-compromised tissue showing negative T(2) BOLD, substantial residual perfusion is evident as revealed by bolus-tracking perfusion MRI, in agreement with the idea that tissue metabolic viability must be preserved for expression of BOLD. It is concluded that BOLD MRI may have potential for the assessment of tissue viability in acute ischemic stroke.

Ex vivo MR microimaging of neuronal damage after kainate-induced status epilepticus in rat: correlation with quantitative histology

The present study was designed to investigate whether T(2)-weighted signal changes obtained by microimaging of paraformaldehyde-fixed brain correlate with the histologically quantified damage in a model of status epilepticus (SE) induced by kainic acid in the rat. Animals were killed at several time points up to 8 weeks after a single intraperitoneal kainate (KA) injection (9 mg/kg). Perfusion-fixed brains were embedded in gelatin for MR microimaging at 9.4T. After the MRI analysis, the gelatin was removed and the brains were cryoprotected and processed for quantitative histology. Severity of neuronal damage and gliosis were assessed from thionin-stained serial sections. Correlative analysis of microimaging and histology data was done in the hippocampus, amygdala, parietal rhinal cortex (PaRH), piriform cortex (Pir), and entorhinal cortex. The relative signal intensities in T(2)-weighted images correlate with the severity of neuronal damage in the matched histological sections (correlation coefficients of 0.752-0.826). Our data show that MR microimaging ex vivo detects the degree of neuronal damage and its anatomical distribution after KA-induced SE, thus providing a useful tool for detecting the dynamics of progressive neuronal damage after prolonged seizures.

Frequency and clinical context of decreased apparent diffusion coefficient reversal in the human brain

PURPOSE

To determine the probability that regions of decreased apparent diffusion coefficient (ADC) return to normal without persistent symptoms or T2 change and the settings in which these ADC reversals occur.

MATERIALS AND METHODS

Three hundred magnetic resonance (MR) imaging studies were selected at random from a database of 7,147 examinations to determine the probability of a pathologically decreased ADC. In cases with decreased ADC, the clinical history was recorded and, if available, follow-up MR imaging findings were evaluated. Five cases of ADC reversal became known during the same period and were evaluated to determine the initial ADC decrease, clinical outcome, and findings at follow-up imaging.

RESULTS

Findings in 116 of 300 MR imaging studies revealed regions of decreased ADC. In 49 of 116 studies, follow-up MR imaging examinations were performed at least 4 weeks after the onset of symptoms; ADC did not reverse. Five cases of ADC reversal were identified in the same period, giving an estimated 0.2%-0.4% probability of ADC reversal. Clinical settings were venous sinus thrombosis and seizure (n = 3), hemiplegic migraine (n = 1), and hyperacute arterial infarction (n = 1). Both white matter (n = 3) and gray matter (n = 3) regions were involved.

CONCLUSION

Reversal of ADC lesions is rare, occurs in complicated clinical settings, and can involve white or gray matter.

Use of spin echo T(2) BOLD in assessment of cerebral misery perfusion at 1.5 T

Inadequate blood supply relative to metabolic demand, a haemodynamic condition termed as misery perfusion, often occurs in conjunction with acute ischaemic stroke. Misery perfusion results in adaptive changes in cerebral physiology including increased cerebral blood volume (CBV) and oxygen extraction ratio (OER) to secure substrate supply for the brain. It has been suggested that the presence of misery perfusion may be an indication of reversible ischaemia, thus detection of this condition may have clinical impact in acute stroke imaging. The ability of single spin echo T(2) to detect misery perfusion in the rat brain at 1.5 T owing to its sensitivity to blood oxygenation level dependent (BOLD) contrast was studied both theoretically and experimentally. Based on the known physiology of misery perfusion, tissue morphometry and blood relaxation data, T(2) behaviour in misery perfusion was simulated. The interpretation of these computations was experimentally assessed by quantifying T(2) in a rat model for cerebral misery perfusion. CBF was quantified with the H(2) clearance method. A drop of CBF from 58+/-8 to 17+/-3 ml/100 g/min in the parieto-frontal cortex caused shortening of T(2) from 66.9+/-0.4 to 64.6+/-0.5 ms. Under these conditions, no change in diffusion MRI was detected. In contrast, the cortex with CBF of 42+/-7 ml/100 g/min showed no change in T(2). Computer simulations accurately predicted these T(2) responses. The present study shows that the acute drop of CBF by 70% causes a negative BOLD that is readily detectable by T(2) MRI at 1.5 T. Thus BOLD may serve as an index of misery perfusion thus revealing viable tissue with increased OER.

Comparison of diffusion, blood oxygenation, and blood volume changes during global ischemia in rats

Rapid diffusion, blood oxygenation, and blood volume weighted echo planar imaging was used to monitor global cerebral ischemia by cardiac arrest in rats. Serial CBV measurements used intravascular iron oxide contrast media (iron dextran). ADC dropped by 5% within 20 sec of cardiac arrest, then continued to decay slowly until a larger rapid drop after 2 min. After iron oxide injection, the initial 5% drop was not observed. The transverse relaxation rate (R(2), R(*)(2) no iron injection) increased rapidly after cardiac arrest, peaking at about 30 sec, then declining towards baseline. The CBV dropped by about 50% within 20 sec. The initial 5% ADC drop may be a vascular artifact. The rapidity of the CBV-weighted signal drop suggests a flow-mediated contribution to the iron oxide contrast mechanism. Magn Reson Med 45:10-16, 2001.

Interrelations of T(1) and diffusion of water in acute cerebral ischemia of the rat

Interrelation of T(1) and diffusion of water was studied in rat models of acute global and focal cerebral ischemia. Cortical T(1), as quantified with an inversion recovery method, increased by 4-7% within a few minutes of global ischemia at 4.7 and 9.4 T, but a significantly smaller change was detected at 1.5 T. The initial T(1) change occurred within seconds of cardiac arrest, much earlier than the extensive diffusion drop after 1-2 min. Thus, the initial increase in T(1) upon acute cerebral ischemia is directly caused by cessation of blood flow. In transient middle cerebral artery occlusion (MCAO), prolonged T(1) relaxation was detected within 10 min, with a subsequent increase during the course of ischemia. Spin density did not change during the first hour, showing that T(1) increase was not caused by net accumulation of water. Interestingly, partial recovery of T(1) upon release of MCAO, occurring independent of long-term tissue outcome, was observed only in concert with diffusion recovery.

Acute changes in MRI diffusion, perfusion, T(1), and T(2) in a rat model of oligemia produced by partial occlusion of the middle cerebral artery

Oligemic regions, in which the cerebral blood flow is reduced without impaired energy metabolism, have the potential to evolve toward infarction and remain a target for therapy. The aim of this study was to investigate this oligemic region using various MRI parameters in a rat model of focal oligemia. This model has been designed specifically for remote-controlled occlusion from outside an MRI scanner. Wistar rats underwent remote partial MCAO using an undersize 0.2 mm nylon monofilament with a bullet-shaped tip. Cerebral blood flow (CBF(ASL)), using an arterial spin labeling technique, the apparent diffusion coefficient of water (ADC), and the relaxation times T(1) and T(2) were acquired using an 8.5 T vertical magnet. Following occlusion there was a decrease in CBF(ASL) to 35 +/- 5% of baseline throughout the middle cerebral artery territory. During the entire period of the study there were no observed changes in the ADC. On occlusion, T(2) rapidly decreased in both cortex and basal ganglia and then normalized to the preocclusion values. T(1) values rapidly increased (within approximately 7 min) on occlusion. In conclusion, this study demonstrates the feasibility of partially occluding the middle cerebral artery to produce a large area of oligemia within the MRI scanner. In this region of oligemic flow we detect a rapid increase in T(1) and decrease in T(2). These changes occur before the onset of vasogenic edema. We attribute the acute change in T(2) to increased amounts of deoxyhemoglobin; the mechanisms underlying the change in T(1) require further investigation.

Early detection of irreversible cerebral ischemia in the rat using dispersion of the magnetic resonance imaging relaxation time, T1rho

The impact of brain imaging on the assessment of tissue status is likely to increase with the advent of treatment methods for acute cerebral ischemia. Multimodal magnetic resonance imaging (MRI) demonstrates potential for selecting stroke therapy patients by identifying the presence of acute ischemia, delineating the perfusion defect, and excluding hemorrhage. Yet, the identification of tissue subject to reversible or irreversible ischemia has proven to be difficult. Here, the authors show that T1 relaxation time in the rotating frame, so-called T1rho, serves as a sensitive MRI indicator of cerebral ischemia in the rat. The T1rho prolongs within minutes after a drop in the CBF of less than 22 mL 100 g(-1) min(-1). Dependence of T1rho on spin-lock amplitude, termed as T1rho dispersion, increases by approximately 20% on middle cerebral artery (MCA) occlusion, comparable with the magnitude of diffusion reduction. The T1rho dispersion change dynamically increases to be 38% +/- 10% by the first 60 minutes of ischemia in the brain region destined to develop infarction. Following reperfusion after 45 minutes of MCA occlusion, the tissue with elevated T1rho dispersion (yet normal diffusion) develops severe histologically verified neuronal damage; thus, the former parameter unveils an irreversible condition earlier than currently available MRI methods. The T1rho dispersion as a novel MRI index of cerebral ischemia may be useful in determination of the therapeutic window for acute ischemic stroke.

Apparent diffusion coefficient and MR relaxation during osmotic manipulation in isolated turtle cerebellum

The apparent diffusion coefficient (ADC) and relaxation times of water were measured by magnetic resonance imaging (MRI) in the isolated turtle cerebellum during osmotic cell volume manipulation. The aim was to study effects of cell volume changes, a factor in ischemia and spreading depression, in isolation from considerations of blood flow and metabolism. Cerebella were superfused at 12-14 degrees C with solutions ranging from 50-200% normal osmolarity. Hypotonic solutions, which are known to cause cell swelling, led to reductions of ADC and increases of T(2), while hypertonic solutions had the opposite effect. This supports the concept that ADC varies with the extracellular space fraction and, combined with published data on extracellular ion diffusion, is consistent with fast or slow exchange models with effective diffusion coefficients that are approximately 1.7 times lower in intracellular than in extracellular space. Spin-spin relaxation can be affected by osmotic disturbance, though such changes are not seen in all pathologies that cause cell swelling.

The measurement of diffusion and perfusion in biological systems using magnetic resonance imaging

The aim of this review is to describe two recent developments in the use of magnetic resonance imaging (MRI) in the study of biological systems: diffusion and perfusion MRI. Diffusion MRI measures the molecular mobility of water in tissue, while perfusion MRI measures the rate at which blood is delivered to tissue. Therefore, both these techniques measure quantities which have direct physiological relevance. It is shown that diffusion in biological systems is a complex phenomenon, influenced directly by tissue microstructure, and that its measurement can provide a large amount of information about the organization of this structure in normal and diseased tissue. Perfusion reflects the delivery of essential nutrients to tissue, and so is directly related to its status. The concepts behind the techniques are explained, and the theoretical models that are used to convert MRI data to quantitative physical parameters are outlined. Examples of current applications of diffusion and perfusion MRI are given. In particular, the use of the techniques to study the pathophysiology of cerebral ischaemia/stroke is described. It is hoped that the biophysical insights provided by this approach will help to define the mechanisms of cell damage and allow evaluation of therapies aimed at reducing this damage.

Neuroserpin reduces cerebral infarct volume and protects neurons from ischemia-induced apoptosis

Neuroserpin, a recently identified inhibitor of tissue-type plasminogen activator (tPA), is primarily localized to neurons within the central nervous system, where it is thought to regulate tPA activity. In the present study neuroserpin expression and its potential therapeutic benefits were examined in a rat model of stroke. Neuroserpin expression increased in neurons surrounding the ischemic core (ischemic penumbra) within 6 hours of occlusion of the middle cerebral artery and remained elevated during the first week after the ischemic insult. Injection of neuroserpin directly into the brain immediately after infarct reduced stroke volume by 64% at 72 hours compared with control animals. In untreated animals both tPA and urokinase-type plasminogen activator (uPA) activity was significantly increased within the region of infarct by 6 hours after reperfusion. Activity of tPA then decreased to control levels by 72 hours, whereas uPA activity continued to rise and was dramatically increased by 72 hours. Both tPA and uPA activity were significantly reduced in neuroserpin-treated animals. Immunohistochemical staining of basement membrane laminin with a monoclonal antibody directed toward a cryptic epitope suggested that proteolysis of the basement membrane occurred as early as 10 minutes after reperfusion and that intracerebral administration of neuroserpin significantly reduced this proteolysis. Neuroserpin also decreased apoptotic cell counts in the ischemic penumbra by more than 50%. Thus, neuroserpin may be a naturally occurring neuroprotective proteinase inhibitor, whose therapeutic administration decreases stroke volume most likely by inhibiting proteinase activity and subsequent apoptosis associated with focal cerebral ischemia/reperfusion.

Neuroserpin reduces cerebral infarct volume and protects neurons from ischemia-induced apoptosis

Neuroserpin, a recently identified inhibitor of tissue-type plasminogen activator (tPA), is primarily localized to neurons within the central nervous system, where it is thought to regulate tPA activity. In the present study neuroserpin expression and its potential therapeutic benefits were examined in a rat model of stroke. Neuroserpin expression increased in neurons surrounding the ischemic core (ischemic penumbra) within 6 hours of occlusion of the middle cerebral artery and remained elevated during the first week after the ischemic insult. Injection of neuroserpin directly into the brain immediately after infarct reduced stroke volume by 64% at 72 hours compared with control animals. In untreated animals both tPA and urokinase-type plasminogen activator (uPA) activity was significantly increased within the region of infarct by 6 hours after reperfusion. Activity of tPA then decreased to control levels by 72 hours, whereas uPA activity continued to rise and was dramatically increased by 72 hours. Both tPA and uPA activity were significantly reduced in neuroserpin-treated animals. Immunohistochemical staining of basement membrane laminin with a monoclonal antibody directed toward a cryptic epitope suggested that proteolysis of the basement membrane occurred as early as 10 minutes after reperfusion and that intracerebral administration of neuroserpin significantly reduced this proteolysis. Neuroserpin also decreased apoptotic cell counts in the ischemic penumbra by more than 50%. Thus, neuroserpin may be a naturally occurring neuroprotective proteinase inhibitor, whose therapeutic administration decreases stroke volume most likely by inhibiting proteinase activity and subsequent apoptosis associated with focal cerebral ischemia/reperfusion.

Spreading waves of transient and prolonged decreases in water diffusion after subarachnoid hemorrhage in rats

Diffusion-weighted MRI (DWI), which can detect cortical spreading depressions (SDs) as propagating waves of reduced apparent diffusion coefficient (ADC) of water, was used to investigate whether spreading depression occurs after subarachnoid hemorrhage (SAH) induced by endovascular perforation in the rat. Eleven rats underwent SAH while positioned in the magnet. The ADC measurements had a temporal resolution of 12 sec. Transient decreases in ADC to 74 +/- 5% of pre-SAH values were observed in three rats after SAH, which propagated over the cortex with an average speed of 4.2 +/- 0. 6 mm/min, consistent with an SD wave. Furthermore, in all 11 rats, a wavefront of reduced ADC, which did not resolve within the 12 min observation period, spread at a speed of 3.2 +/- 1.7 mm/min in the ipsilateral cortex, and again is consistent with the speed of SD propagation. Therefore, spreading depression-like cellular depolarization is a consequence of acute subarachnoid hemorrhage in rats. Magn Reson Med 44:110-116, 2000.

Quantitative MR microscopy of enzymatically degraded articular cartilage

Structural changes in bovine patellar articular cartilage, induced by component selective enzymatic treatments, were investigated by measuring tissue T(2) relaxation at 9.4 T. This MRI parameter was compared with Young's modulus, a measure of elastic stiffness and loadbearing ability of cartilage tissue. Collagenase was used to digest the collagen network and chondroitinase ABC to remove proteoglycans. Polarized light microscopy and digital densitometry were used to assess enzyme penetration after 44 hr of enzymatic digestion. T(2) relaxation in superficial cartilage increased significantly only in samples treated with collagenase. A statistically significant decrease in Young's modulus was observed in both enzymatically treated sample groups. These results confirm that T(2) of articular cartilage is sensitive to the integrity of collagen in the extracellular matrix. Nonetheless, it does not appear to be an unambiguous indicator of cartilage stiffness, which is significantly impaired in osteoarthrosis.

Temporal changes of the apparent diffusion coefficients of water and metabolites in rats with hemispheric infarction: experimental study of transhemispheric diaschisis in the contralateral hemisphere at 7 tesla

The purpose of the present study was to clarify the temporal changes of the apparent diffusion coefficients (ADCs) of cerebral metabolites during early focal ischemia using stimulated echo acquisition mode with short echo time at a 7 T magnet to assess the pathophysiology of the reduction in diffusion properties observed both in the ischemic cerebral hemisphere and in the contralateral hemisphere. The ADCs of metabolites in the infarcted hemisphere 1 hour and 3 hours after the onset of ischemia decreased with 25% and 29% for choline containing compounds (Cho), 16% and 26% for creatine and phosphocreatine (Cre), and 19% and 19% for N-acetylaspartate (NAA), respectively, compared with the ADC values 2 hours later in the contralateral hemisphere. There were decreases in the ADC of Cho, Cre, and NAA with 21%, 7%, and 18% 8 hours later, respectively, in the noninfarcted hemisphere, which suggested transhemispheric diaschisis in rats with focal cerebral ischemia. The present study proposed that the diffusion characteristics of the brain metabolites might offer new insights into circulatory and metabolic alteration in the cerebral intracellular circumstance.

Graded reduction of cerebral blood flow in rat as detected by the nuclear magnetic resonance relaxation time T2: a theoretical and experimental approach

The ability of transverse nuclear magnetic resonance relaxation time, T2, to reveal acutely reduced CBF was assessed using magnetic resonance imaging (MRI). Graded reduction of CBF was produced in rats using a modification of Pulsinelli's four-vessel occlusion model. The CBF in cerebral cortex was quantified using the hydrogen clearance method, and both T2 and the trace of the diffusion tensor (Dav = 1/3TraceD) in the adjacent cortical tissue were determined as a function of reduced CBF at 4.7 T. A previously published theory, interrelating cerebral hemodynamic parameters, hemoglobin, and oxygen metabolism with T2, was used to estimate the effects of reduced CBF on cerebral T2. The MRI data show that T2 reduces in a U-shape manner as a function of CBF, reaching a level that is 2.5 to 2.8 milliseconds (5% to 6%) below the control value at CBF, between 15% and 60% of normal. This reduction could be estimated by the theory using the literature values of cerebral blood volume, oxygen extraction ratio, and precapillary oxygen extraction during compromised CBF. Dav dropped with two apparent flow thresholds, so that a small 11% to 17% reduction occurred between CBF values of 16% to 45% of normal, followed by a precipitous collapse by more than 20% at CBF below 15% of normal. The current data show that T2 can be used as an indicator of acute hypoperfusion because of its ability to indicate blood oxygenation level-dependent phenomena on reduced CBF.

Determination of oxygen extraction ratios by magnetic resonance imaging

The oxygen extraction ratio (OER) of a tissue describes the interplay between oxygen delivery and consumption and, as such, directly reflects the viability and activity of any organ. It is shown that OER can be quantified using a single magnetic resonance imaging observable, namely the relaxation time T2 of venous blood draining from the tissue. This principle is applied to study local OER changes in the brain on visual stimulation in humans, unambiguously demonstrating a mismatch between changes in blood flow and oxygen metabolism on activation.

Quantitative magnetic resonance imaging assessment of cerebral ischemia in rat using on-resonance T(1) in the rotating frame

Sensitivity of T(1) in the rotating frame (T(1rho)) to both transient cerebral ischemia and cortical hypoperfusion was studied in rats. T(1rho) is believed to probe water in close contact with macromolecules, revealing water spins with restricted rotational mobility relative to bulk water. It is shown that T(1rho) increases within minutes of occlusion, thereby demonstrating it as a new, sensitive indicator of ischemia. After reperfusion at 90 minutes of middle cerebral artery occlusion, T(1rho) remains elevated or increases in tissue destined to neuronal damage but returns to the normal level if no neuronal damage develops within 24 hours. T(1rho) determined during the first 2 hours of reperfusion shows a significant positive correlation with the ultimate neuronal damage score. However, T(1rho) is not affected by acute hypoperfusion. These data show that, by combining three magnetic resonance imaging coefficients, i.e. T(1rho), T(2), and diffusion, viable hypoperfused areas that do not develop neuronal damage within 24 hours can be distinguished correctly from tissue already destined for neuronal damage. Magn Reson Med 42:268-276, 1999.

Imaging of acute cerebral ischemia

Until recently, there was no efficacious treatment for acute cerebral ischemia. As a result, the role of neuroimaging and the radiologist was peripheral in the diagnosis and management of this disease. The demonstration of efficacy using thrombolysis has redefined this role, with the success of intervention becoming increasingly dependent on timely imaging and accurate interpretation. The potential benefits of intervention have only begun to be realized. In this State-of-the-Art review of imaging of acute stroke, the role of imaging in the current and future management of stroke is presented. The role of computed tomography is emphasized in that it is currently the most utilized technique, and its value has been demonstrated in prospective clinical trials. Magnetic resonance techniques are equally emphasized in that they have the potential to provide a single modality evaluation of tissue viability and vessel patency in an increasingly rapid evaluation.

In vivo determination of absolute cerebral blood volume using hemoglobin as a natural contrast agent: an MRI study using altered arterial carbon dioxide tension

The ability of the magnetic resonance imaging transverse relaxation time, R2 = 1/T2, to quantify cerebral blood volume (CBV) without the need for an exogenous contrast agent was studied in cats (n = 7) under pentobarbital anesthesia. This approach is possible because R2 is directly affected by changes in CBF, CBV, CMRO2, and hematocrit (Hct), a phenomena better known as the blood-oxygenation-level-dependent (BOLD) effect. Changes in CBF and CBV were accomplished by altering the carbon dioxide pressure, PaCO2, over a range from 20 to 140 mm Hg. For each PaCO2 value, R2 in gray and white matter were determined using MRI, and the whole-brain oxygen extraction ratio was obtained from arteriovenous differences (sagittal sinus catheter). Assuming a constant CMRO2, the microvascular CBV was obtained from an exact fit to the BOLD theory for the spin-echo effect. The resulting CBV values at normal PaCO2 and normalized to a common total hemoglobin concentration of 6.88 mmol/L were 42+/-18 microL/g (n = 7) and 29+/-19 microL/g (n = 5) for gray and white matter, respectively, in good agreement with the range of literature values published using independent methodologies. The present study confirms the validity of the spin-echo BOLD theory and, in addition, shows that blood volume can be quantified from the magnetic resonance imaging spin relaxation rate R2 using a regulated carbon dioxide experiment.

Measuring cerebral blood flow using magnetic resonance imaging techniques

Magnetic resonance imaging techniques measuring CBF have developed rapidly in the last decade, resulting in a wide range of available methods. The most successful approaches are based either on dynamic tracking of a bolus of a paramagnetic contrast agent (dynamic susceptibility contrast) or on arterial spin labeling. This review discusses their principles, possible pitfalls, and potential for absolute quantification and outlines clinical and neuroscientific applications.

Enhanced ornithine decarboxylase activity is associated with attenuated rate of damage evolution and reduction of infarct volume in transient middle cerebral artery occlusion in the rat

Ornithine decarboxylase (ODC) transgenic and alpha-difluoromethyl ornithine (DFMO)-treated rats were exposed to transient middle cerebral occlusion (MCAO) to examine the role of intraischaemic ODC-activity on the evolution of ischaemia-reperfusion damage. Magnetic resonance imaging (MRI) data show that the damage develops slower in ODC transgenic than in DFMO-treated rats, which is not caused by a difference in perfusion. Furthermore, infarct volumes are smaller in the former animals one day later. These data support the idea of endogenous neuroprotective action of ODC.

The relationship between magnetic resonance diffusion imaging and autoradiographic markers of cerebral blood flow and hypoxia in an animal stroke model

This study examined the relationship between magnetic resonance diffusion imaging and autoradiographic markers of cerebral blood flow (99mTc-hexamethylpropylene amine oxime) and cerebral hypoxia (125I-iodoazomycin arabinoside) in a rat model of stroke. Middle cerebral artery occlusion in the rat was performed using an intraluminal suture approach. Diffusion, hypoxia, and blood flow maps were acquired 2 hr following occlusion, and were compared with T2 images and histology at 7 hr. Two hours following middle cerebral artery occlusion the lesion distributions from the diffusion maps and hypoxic autoradiographs were similar. The blood flow threshold for increased uptake of the hypoxic marker was approximately 34 +/- 7% of the normal flow. The combination of diffusion or hypoxic images with perfusion maps allowed differentiation between four regions: 1) normal tissue; 2) a region of decreased perfusion but normal diffusion and normal uptake of hypoxic marker; 3) a region of decreased perfusion, decreased diffusion and increased uptake of hypoxic marker; 4) a region of decreased perfusion, decreased diffusion and low uptake of hypoxic marker. The areas for increased uptake of hypoxic marker and decreased diffusion are equivalent, indicating similar blood flow thresholds. Regions of oligaemic misery perfusion, ischaemic misery perfusion and lesion core may be delineated with the combination of diffusion or hypoxic images and perfusion maps.

Imaging developing brain infarction

Continued advances in neuroimaging technology have made it practical to image multiple aspects of evolving brain infarction during the potential window period of therapeutic opportunity in stroke. Recent methodologic developments include computed tomography angiography and perfusion, and the description of quantitative parameters for magnetic resonance blood oxygen level-dependent perfusion imaging. In pathophysiologic studies, metabolism and function in the ischemic focus and the peri-infarct tissue have been further characterized. Clinical studies have focused on the applications of computed tomography and magnetic resonance imaging for prethrombolysis patient selection. These methods have an important role in the evaluation and development of new pharmaceutical agents and will be increasingly used in clinical practice as new therapies become available.