Correlation of the average water diffusion constant with cerebral blood flow and ischemic damage after transient middle cerebral artery occlusion in cats

Characterizing spatiotemporal progression and prediction of infarct lesion volumes in experimental acute ischemia using quantitative perfusion and diffusion imaging

PURPOSE

This study was conducted to explore the diagnostic value of arterial spin labeling (ASL) combined with diffusion weighted imaging (DWI) in characterizing the spatiotemporal progression of infarct lesions in a rabbit middle cerebral artery occlusion (MCAO) model and predicting the acute cerebral infarction (ACI) volume.

MATERIALS AND METHODS

Forty-two male rabbits (2.9 ± 0.2 kg body weight) were used in this experimental study. Animals were initially anesthetized by intravenous injection of uratan. There were seven experimental groups with six rabbits in each group. The apparent diffusion coefficient (ADC) and cerebral blood flow (CBF) thresholds were established in the control group (n = 6), which were sacrificed at 12 h, stained for infarct volume, and imaged at each time point.

RESULTS

The normal ADC and CBF were estimated as 0.90 ± 0.03 × 10-3 mm2/s and 0.68 ± 0.06 mL g-1 min-1, respectively. The viability thresholds of ADC and CBF yielding the lesion volumes (LVs) at 3 h, which best approximated the 2,3,5-triphenyltetrazolium chloride (TTC) infarct volumes at 12 h, were 0.52 ± 0.02 × 10-3 mm2/s (42.2 ± 3% reduction) and 0.33 ± 0.09 mL g-1 min-1 (51.0 ± 11% reduction), respectively. The temporal evolution of the ADC- and CBF-defined LVs showed a significant perfusion/diffusion mismatch up to 1 h (p = 0.001).

CONCLUSION

ADC values and ACI volumes were positively correlated, while CBF was negatively correlated, which is supposed to be a reference for predicting ACI volume.

Magnetic Resonance Imaging Findings of Term and Preterm Hypoxic-Ischemic Encephalopathy: A Review of Relevant Animal Models and Correlation to Human Imaging

Background:

Neonatal hypoxic-ischemic encephalopathy is brain injury caused by decreased perfusion and oxygen delivery that most commonly occurs in the context of delivery complications such as umbilical cord compression or placental abruption. Imaging is a key component for guiding treatment and prediction of prognosis, and the most sensitive clinical imaging modality for the brain injury patterns seen in hypoxic-ischemic encephalopathy is magnetic resonance imaging.

Objective:

The goal of this review is to compare magnetic resonance imaging findings demonstrated in the available animal models of hypoxic-ischemic encephalopathy to those found in preterm (≤ 36 weeks) and term (>36 weeks) human neonates with hypoxic-ischemic encephalopathy, with special attention to the strengths and weaknesses of each model.

Methods:

A structured literature search was performed independently by two authors and the results of the searches were compiled. Animal model, human brain age equivalency, mechanism of injury, and area of brain injury were recorded for comparison to imaging findings in preterm and term human neonates with hypoxic-ischemic encephalopathy.

Conclusion:

Numerous animal models have been developed to better elicit the expected findings that occur after HIE by allowing investigators to control many of the clinical variables that result in injury. Although modeling the same disease process, magnetic resonance imaging findings in the animal models vary with the species and methods used to induce hypoxia and ischemia. The further development of animal models of HIE should include a focus on comparing imaging findings, and not just pathologic findings, to human studies.

Refinement of embolic stroke model in rats: Effect of post-embolization anesthesia duration on arterial blood pressure, cerebral edema and mortality

BACKGROUND

Injection of a clot into the internal carotid artery is an experimental model of ischemic stroke that is considered to closely mimic embolic stroke in humans. In this model, the common carotid artery typically remains temporarily occluded to permit time for stabilization of the clot in the middle cerebral artery. However, the associated lengthening of the anesthesia duration could affect arterial blood pressure and stroke outcome.

NEW METHOD

We refined the model by examining how increasing isoflurane anesthesia duration from 30 to 60 min after clot embolization affects mortality, infarct volume, edema, blood-brain barrier permeability, and the 8-h post-ischemic time course of blood pressure, which has not been reported previously in this model.

RESULTS

We found that arterial pressure increased after discontinuing anesthesia in both embolized groups and that the increase was greater than in the corresponding non-embolized sham-operated rats. At 24 h, the group with 60-min post-ischemia anesthesia exhibited greater brain water content and a greater ipsilateral-to-contralateral ratio of extravasated Evans blue dye. Mortality was greater in the 60-min group, but infarct volume among survivors was not different from that in the 30-min anesthesia group.

COMPARISON WITH EXISTING METHODS

This study refines the embolic stroke model by demonstrating the importance of minimizing the duration of anesthesia after embolization.

CONCLUSIONS

These data indicate that early discontinuation of isoflurane anesthesia after clot embolization permits an earlier hypertensive response that limits edema formation and mortality without significantly affecting infarct volume in survivors, thereby decreasing the required number of animals.

Identification of hyperacute ischemic stroke with a more homogenous nature

Previous reports revealed that middle cerebral artery occlusion (MCAO) models in rats were very diverse in nature, and experimental stroke of a more homogenous nature had not been previously documented. This paper aims to present our novel observations of experimental stroke in rats with similar MRI characteristics after MCAO. Immediately after MCAO, 19 rats were placed into a 4.7 T MRI scanner, and diffusion weighted imaging (DWI) of axial and coronal planes was repeated every 10 minutes up to post-occlusion 115 minutes. Apparent diffusion coefficient (ADC) values of the ischemic lesions were calculated and compared to those of the unaffected contra-lateral hemispheres. Successful MCAO was defined when the whole left MCA territory showed ADC abnormality on DWI. Percentage of hemispheric lesion volume (% HLV), relative ADC value (rADC), and relative DWI signal intensity (rDWI) were serially evaluated for quantitative analysis of ADC-derived lesion characteristics. Successful MCA territorial infarction was induced in nine rats (9/19, 47.4%). In quantitative analysis of ADC-derived lesion characteristics, lesion volumes of seven rats (group 1) were very similar, but larger than those of the other two rats (group 2): % HLV of initial MRI = 45.4 ± 2.5 / 19.1 ± 6.6. rADCs and rDWIs of group 1 showed similar patterns of temporal change, which was different from those of group 2. Using prospective diffusion MRI after MCAO in rats, we identified territorial hyperacute ischemic lesions with similar MRI characteristics. This observation would contribute to the establishment of more homogenous rodent models for ischemic stroke translational research.

Dynamic functional cerebral blood volume responses to normobaric hyperoxia in acute ischemic stroke

Studies suggest that neuroprotective effects of normobaric oxygen (NBO) therapy in acute stroke are partly mediated by hemodynamic alterations. We investigated cerebral hemodynamic effects of repeated NBO exposures. Serial magnetic resonance imaging (MRI) was performed in Wistar rats subjected to focal ischemic stroke. Normobaric oxygen-induced functional cerebral blood volume (fCBV) responses were analyzed. All rats had diffusion-weighted MRI (DWI) lesions within larger perfusion deficits, with DWI lesion expansion after 3 hours. Functional cerebral blood volume responses to NBO were spatially and temporally heterogeneous. Contralateral healthy tissue responded consistently with vasoconstriction that increased with time. No significant responses were evident in the acute DWI lesion. In hypoperfused regions surrounding the acute DWI lesion, tissue that remained viable until the end of the experiment showed relative preservation of mean fCBV at early time points, with some rats showing increased fCBV (vasodilation); however, these regions later exhibited significantly decreased fCBV (vasoconstriction). Tissue that became DWI abnormal by study-end initially showed marginal fCBV changes that later became moderate fCBV reductions. Our results suggest that a reverse-steal hemodynamic effect may occur in peripheral ischemic zones during NBO treatment of focal stroke. In addition, CBV responses to NBO challenge may have potential as an imaging marker to distinguish ischemic core from salvageable tissues.

Quantitative evaluation of C-arm CT cerebral blood volume in a canine model of ischemic stroke

BACKGROUND AND PURPOSE

Previous studies have shown the feasibility of assessing qualitative CBV measurements in the angiography suite by using FPD-CBCT systems. We have investigated the correlation of FPD-CBCT CBV lesion volumes to the infarct volume.

MATERIALS AND METHODS

Unilateral strokes were created in 7 adult dogs. MR imaging and FPD-CBCT data were obtained after MCA occlusion. FPD-CBCT CBV and ADC maps were generated for all subjects. The animals were sacrificed immediately following the last imaging study to measure infarct volume on histology. The reliability of FPD-CBCT-based lesion volume measurements was compared with those measured histologically by using regression and Bland-Altman analysis.

RESULTS

The best correlation (R(2) = 0.72) between lesion volumes assessed with FPD-CBCT and histology was established with a threshold of mean healthy CBV - 2.5 × SD. These results were inferior to the correlation of lesion volumes measured with ADC and histology (R(2) = 0.99). Bland-Altman analysis showed that the agreement of ADC-derived lesion volumes with histology was superior to the agreement of FPD-CBCT-derived lesion volumes with histology.

CONCLUSIONS

We correlated FPD-CBCT measurements of CBV and MR ADC lesion volumes with histologically assessed infarct volume. As expected, ADC is a very accurate and precise method for determining the extent of infarction. FPD-CBCT CBV lesion volumes are correlated to the size of the infarct. Improvement of FPD-CBCT image quality provides an opportunity to establish quantitative CBV measurement in the angiography suite.

Imaging stroke patients with unclear onset times

Stroke is a leading cause of death and adult morbidity worldwide. By defining stroke symptom onset by the time the patient was last known to be well, many patients whose onsets are unwitnessed are automatically ineligible for thrombolytic therapy. Advanced brain imaging may serve as a substitute witness to estimate stroke onset and duration in those patients who do not have a human witness. This article reviews and compares some of these imaging-based approaches to thrombolysis eligibility, which can potentially expand the use of thrombolytic therapy to a broader population of acute stroke patients.

Multi-parametric neuroimaging reproducibility: a 3-T resource study

Modern MRI image processing methods have yielded quantitative, morphometric, functional, and structural assessments of the human brain. These analyses typically exploit carefully optimized protocols for specific imaging targets. Algorithm investigators have several excellent public data resources to use to test, develop, and optimize their methods. Recently, there has been an increasing focus on combining MRI protocols in multi-parametric studies. Notably, these have included innovative approaches for fusing connectivity inferences with functional and/or anatomical characterizations. Yet, validation of the reproducibility of these interesting and novel methods has been severely hampered by the limited availability of appropriate multi-parametric data. We present an imaging protocol optimized to include state-of-the-art assessment of brain function, structure, micro-architecture, and quantitative parameters within a clinically feasible 60-min protocol on a 3-T MRI scanner. We present scan-rescan reproducibility of these imaging contrasts based on 21 healthy volunteers (11 M/10 F, 22-61 years old). The cortical gray matter, cortical white matter, ventricular cerebrospinal fluid, thalamus, putamen, caudate, cerebellar gray matter, cerebellar white matter, and brainstem were identified with mean volume-wise reproducibility of 3.5%. We tabulate the mean intensity, variability, and reproducibility of each contrast in a region of interest approach, which is essential for prospective study planning and retrospective power analysis considerations. Anatomy was highly consistent on structural acquisition (~1-5% variability), while variation on diffusion and several other quantitative scans was higher (~<10%). Some sequences are particularly variable in specific structures (ASL exhibited variation of 28% in the cerebral white matter) or in thin structures (quantitative T2 varied by up to 73% in the caudate) due, in large part, to variability in automated ROI placement. The richness of the joint distribution of intensities across imaging methods can be best assessed within the context of a particular analysis approach as opposed to a summary table. As such, all imaging data and analysis routines have been made publicly and freely available. This effort provides the neuroimaging community with a resource for optimization of algorithms that exploit the diversity of modern MRI modalities. Additionally, it establishes a baseline for continuing development and optimization of multi-parametric imaging protocols.

Acute-stage diffusion-weighted magnetic resonance imaging for predicting outcome of poor-grade aneurysmal subarachnoid hemorrhage

We investigated the role of acute-stage diffusion-weighted images (DWIs) for predicting outcome of poor-grade subarachnoid hemorrhage (SAH). This study included 38 patients with poor-grade SAH who underwent DWI within 24 h after onset. DWI findings were divided into three groups on the basis of lesion area: none (N), spotty (S, <or=10 mm(2)), or areal (A, >10 mm(2)). We evaluated the correlation between preoperative DWI findings and clinical outcome, and the characteristics of DWI abnormalities. DWI abnormalities were revealed in 81.6% of cases (group S 34.2%; group A 47.3%). All patients in groups N and S and 73.3% of patients in group A were treated radically. For those patients without rerupture, favorable outcomes were achieved in 100% of group N, 53.8% of group S, and 0% of group A. Abnormal lesions on initial DWI, which resulted in permanent lesions, showed a mean apparent diffusion coefficient ratio to the control value of 0.71, which was significantly lower than 0.95 observed in reversible lesions (P<0.01). We recommend radical treatment for even poor-grade SAH as long as the preoperative DWI shows no or only spotty lesions. DWI may provide an objective means to estimate the outcome of poor-grade SAH.

Delayed signal detection by diffusion-weighted imaging in brainstem infarction

Diffusion-weighted magnetic resonance imaging (DW MRI) is a sensitive and specific technique for imaging acute hemispheric infarction. Its utility in the diagnosis of acute brainstem infarction has not been well studied. We present 3 cases of brainstem infarction in which DW MRI performed 5 to 15 hours after symptom onset failed to reveal any abnormality. Repeat diffusion-weighted imaging (DWI) 2 to 5 days later did demonstrate an abnormality in the clinically appropriate region in each instance. This suggests that the time course to the development of abnormalities detectable by DWI may be longer in brainstem than in hemispheric infarctions. Therefore, repeat studies after initially negative DWI might be useful in the diagnosis of brainstem infarctions.

Proton transfer ratio, lactate, and intracellular pH in acute cerebral ischemia

The amide proton transfer ratio (APTR) from the asymmetry of the Z-spectrum was determined in rat brain tissue during and after unilateral middle cerebral artery occlusion (MCAo). Cerebral lactate (Lac) as determined by (1)H NMR spectroscopy, water diffusion, and T(1rho) were quantified as well. Lac concentrations were used to estimate intracellular pH (pH(i)) in the brain during the MCA occlusion. A decrease in APTR during occlusion indicated acidification from 7.1 to 6.79 +/- 0.19 (a drop by 0.3 +/- 0.2 pH units), whereas pH(i) computed from Lac concentration was 6.3 +/- 0.2 (a drop by 0.8 +/- 0.2 pH units). Despite the disagreement between the two methods in terms of the size of the change in the absolute pH(i) during ischemia, DeltaAPTR and pH(i) (and Lac concentration) displayed a strong correlation during the MCAo. Diffusion and T(1rho) indicated cytotoxic edema following MCA occlusion; however, APTR returned slowly toward the values determined in the contralateral hemisphere post-ischemia. These data argue that the APTR during ischemia is affected not only by pH(i) but by other physicochemical factors as well, and indicates different aspects of pathology in the post-ischemic brain compared to those that influence water diffusion and T(1rho).

Influence of duration of focal cerebral ischemia and neuronal nitric oxide synthase on translocation of apoptosis-inducing factor to the nucleus

Translocation of apoptosis-inducing factor (AIF) from the mitochondria to the nucleus can play a major role in neuronal death elicited by oxidant stress. The time course of nuclear translocation of AIF after experimental stroke may vary with the severity of injury and may be accelerated by oxidant stress associated with reperfusion and nitric oxide (NO) production. Western immunoblots of AIF on nuclear fractions of ischemic hemisphere of male mice showed no significant increase with 1 h of middle cerebral artery occlusion and no reperfusion, whereas increases were detectable after 6 and 24 h of permanent ischemia. However, as little as 20 min of reperfusion after 1 h of middle cerebral artery occlusion resulted in an increase in nuclear AIF coincident with an increase in poly(ADP-ribose) polymer (PAR) formation. Further nuclear AIF accumulation was seen at 6 and 24 h of reperfusion. In contrast, 20 min of reperfusion after 2 h of occlusion did not increase nuclear AIF. In this case, nuclear AIF became detectable at 6 and 24 h of reperfusion. With brief occlusion of 30 min duration, nuclear AIF remained undetectable at both 20 min and 6 h and became evident only after 24 h of reperfusion. Inhibition of neuronal NO synthase attenuated formation of PAR and nuclear AIF accumulation. Gene deletion of neuronal NO synthase also attenuated nuclear AIF accumulation. Therefore, reperfusion accelerates AIF translocation to the nucleus when focal ischemia is of moderate duration (1 h), but is markedly delayed after brief ischemia (30 min). Nuclear translocation of AIF eventually occurs with prolonged focal ischemia with or without reperfusion. Neuronally-derived NO is a major factor contributing to nuclear AIF accumulation after stroke.

Local Relationships Between Restricted Water Diffusion and Oxygen Consumption in the Ischemic Human Brain

BACKGROUND AND PURPOSE

MR is widely used to depict still ischemic but viable tissue in acute stroke. However, the relationship between the apparent diffusion coefficient (ADC) and energy failure from reduced oxygen supply are unknown in man.

METHODS

Acute carotid-territory stroke patients were studied prospectively with both diffusion tensor-imaging and back-to-back steady-state 15O-PET. Substantial numbers of voxels with oxygen extraction fraction >0.70 (ie, significant ongoing hypoxia) were identified in 3 patients (imaged at 7, 16 and 21 hours after stroke onset). In this voxel population, the quantitative relationships between the ADC and cerebral metabolic rate of oxygen (CMRO2), and ADC and cerebral blood flow (CBF), were assessed.

RESULTS

The ADC remained essentially unchanged until CBF reached values approximately 20 mls/100g per min, beyond which it declined linearly. In contrast, except when severely reduced, the ADC was a poorer predictor of CMRO2. For both CBF and CMRO2, however, the relationship with ADC became steeper with longer times since onset, ie, the same ADC reflected lower perfusion and CMRO2 with elapsed time.

CONCLUSIONS

Despite the small sample and late times from stroke onset, the findings indicate that the degree of restricted water diffusion reliably reflects the severity of oxygen deprivation below the penumbral threshold but is less strongly related to metabolic disruption, which may explain why the ADC does not reliably predict tissue outcome. However, the same degree of diffusion restriction may correspond to greater severity of tissue disruption with elapsing time, which has relevance for stroke therapy. Time elapsed since stroke onset should be taken into account when interpreting ADC declines and in voxel-based infarct prediction models.

Present status of magnetic resonance imaging and spectroscopy in animal stroke models

Magnetic resonance imaging (MRI) is based on a wide variety of physical parameters, which, in principle, can all influence the image contrast conditions. As these diverse variables are validated by independent physiological, metabolic, hemodynamic, and histological techniques, a physiological MRI evolves. This imaging modality has been successfully applied to experimental stroke studies, covering a broad range of raised questions. In the present review, we present an overview of possible physiological criteria to be studied by in vivo MRI and magnetic resonance spectroscopy, and critically analyze the present limits and future potential of the imaging technique for experimental stroke investigations. The documented applications cover the spectrum from morphological-structural details of the lesion to hemodynamic and metabolic alterations, inflammatory reaction, evaluation of thrombolytic treatment, studies on recovery of functional brain activation by functional MRI, and, finally, the most recent applications of exploring stem cells for regenerative therapy.

Effects of Intra-arterial Urokinase on a Non-human Primate Thromboembolic Stroke Model

One of the most important prognostic factors in the thrombolytic treatment of acute ischemic stroke is to re-canalize. The purpose of this study was to evaluate the effectiveness and safety of urokinase in a primate thromboembolic stroke model. Thromboembolic stroke was accomplished via occlusion of the middle cerebral artery (MCA) obtained by injecting an autologous blood clot into the left internal carotid artery in 21 male cynomolgus monkeys. Animals were randomly assigned to the following treatment groups: Group 1: vehicle (saline), Group 2: urokinase (40,000 IU), Group 3: urokinase (120,000 IU,) over 2 or 6 h via intra-internal carotid catheter starting 1 h after embolization, respectively. In the urokinase-treated groups, neurologic deficits were improved in consciousness and skeletal muscle coordination, but not sensory and motor systems. The infarction size in Group 2 (11.9 +/- 3.9% of the hemisphere) and 3 (7.6 +/- 2.5%) were significantly smaller than that (24.7 +/- 3.5%) in Group 1. However, 2 of 5 animals in Group 3 died. In conclusion, urokinase improved neurologic deficits and reduced cerebral infarction on thromboembolic stroke in the cynomolgus monkey.

Differences in infarct evolution between lipopolysaccharide-induced tolerant and nontolerant conditions to focal cerebral ischemia

OBJECT

Although brain tissue may be protected by previous preconditioning, the temporal evolution of infarcts in such preconditioned brain tissue during focal cerebral ischemia is largely unknown. Therefore, in this study the authors engaged in long-term observation with magnetic resonance (MR) imaging to clarify the difference in lesion evolution between tolerant and nontolerant conditions.

METHODS

Bacterial lipopolysaccharide (LPS; 0.9 mg/kg) was administered intravenously to induce cross-ischemic tolerance. Focal cerebral ischemia was induced 72 hours later in spontaneously hypertensive rats. Serial brain MR images were obtained 6 hours, 24 hours, 4 days, 7 days, and 14 days after ischemia by using a 7.05-tesla unit. Lesion-reducing effects were evident 6 hours after ischemia in the LPS group. Preconditioning with LPS does not merely delay but prevents ischemic cell death by reducing lesion size. Lesion reduction was a sustained effect noted up to 14 days after ischemia. Reduction of local cerebral blood flow (ICBF) in the periinfarct area was significantly inhibited in the LPS group, which was correlated with endothelial nitric oxide synthase (eNOS) expression.

CONCLUSIONS

Significant preservation of ICBF in the periinfarct area, which is relevant to sustained upregulation of eNOS, could be a candidate for the long-term inhibiting effect on infarct evolution in the LPS-induced tolerant state.

Investigation of techniques to quantify in vivo lesion volume based on comparison of water apparent diffusion coefficient (adc) maps with histology in focal cerebral ischemia of rats

Stroke lesion-volume estimates derived from calculated water apparent diffusion coefficient (ADC) maps provide a quantitative surrogate end-point for investigating the efficacy of drug treatment or studying the temporal evolution of cerebral ischemia. Methodology is described for estimating ischemic lesion volumes in a rat model of permanent middle cerebral artery occlusion (MCAO) based on absolute and percent-reduction threshold values of the water ADC at 3 h post-MCAO. Volume estimates derived from average ADC (ADC(av)) maps were compared with those derived from post-mortem histological sections. Optimum ADC thresholds were established as those that provided the best correlation and one-to-one correspondence between ADC- and histologically derived lesion-volume estimates. At 3 h post-MCAO, an absolute-ADC(av) threshold of 47 x 10(-5) mm(2)/s (corresponding to a 33% reduction in ADC(av) based on a contralateral hemisphere comparison) provided the most accurate estimate of percent hemispheric lesion volume (%HLV). Experimental and data analysis issues for improving and validating the usefulness of DWI as a surrogate endpoint for the quantification of ischemic lesion volume are discussed.

Prediction of tissue survival after stroke based on changes in the apparent diffusion of water (cytotoxic edema)

This study tested the hypothesis that the estimate of the apparent diffusion coefficient (ADC; an index of cytotoxic edema) of water is a reliable pathophysiological index of the viability of ischemic brain tissue. CBF, the partition volume of water (PVW; an index of vasogenic edema), cerebral metabolic rate of oxygen (CMRO2), and of glucose (CMR(glc)) were measured before and after permanent middle cerebral artery occlusion (MCAO) or reperfusion with positron emission tomography (PET) in pigs. Then, the ADC was measured by diffusion-weighted magnetic resonance imaging (DW-MRI) and was compared with physiological variables obtained by PET and with histological findings. Both after permanent MCAO and reperfusion, the ADC was significantly correlated to the CMRO2 and CMR(glc). The sequential decrease of ADC was also correlated to the sequential reduction of CMRO2 in relation to the residual CBF after permanent MCAO. The infarction coincided with a threshold CMRO2 of 50% of the contralateral value and a threshold ADC of 75% of the contralateral value, respectively. The PVW was decreased in relation to the magnitude of residual CBF after MCAO, whereas it was transiently increased in the severely ischemic lesions below 75% of the contralateral ADC value after reperfusion. Thus, the ADC is a reliable pathophysiological index which allows therapy to be tailored to the condition of individual patients in clinical practice.

Pixel-by-pixel spatiotemporal progression of focal ischemia derived using quantitative perfusion and diffusion imaging

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 x 10(-3) mm(2)/s and CBF was 0.7 +/- 0.3 mL x g(-1) x min(-1) (mean +/- SD, n=5). 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 x 10(-3) mm(2)/s (30% +/- 2% reduction) and 0.30 +/- 0.09 mL x g(-1) x 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 = 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 x 10(-3) mm(2)/s (corresponding CBF 0.35 +/- 0.04 mL x g(-1) x min(-1)), surprisingly similar to the TTC-derived thresholds. Together, these results illustrate an analysis approach to systemically track the pixel-by-pixel spatiotemporal progression of acute ischemic brain injury.

Three-dimensional white matter tractography by diffusion tensor imaging in ischaemic stroke involving the corticospinal tract

Diffusion tensor MR imaging (DTI) provides information on diffusion anisotropy, which can be expressed with three-dimensional (3D) white matter tractography. We used 3D white matter tractography to show the corticospinal tract in eight patients with acute or early subacute ischaemic stroke involving the posterior limb of the internal capsule or corona radiata and to assess involvement of the tract. Infarcts and the tract were shown simultaneously, providing information on their spatial relationships. In five of the eight patients, 3D fibre tract maps showed the corticospinal tract in close proximity to the infarct but not to pass through it. All these patients recovered well, with maximum improvement from the lowest score on manual muscle testing (MMT) up to the full score through rehabilitation. In the other three patients the corticospinal tract was shown running through the infarct; reduction in MMT did not necessarily improve favourably or last longer, other than in one patient. As 3D white matter tractography can show spatial relationships between the corticospinal tract and an infarct, it might be helpful in prognosis of gross motor function.

MR imaging of ischemic penumbra

Cerebral ischemic stroke is one of the most fatal diseases despite current advances in medical science. Recent demonstration of efficacy using intravenous and intra-arterial thrombolysis demands therapeutic intervention tailored to the physiologic state of the individual tissue and stratification of patients according to the potential risks for therapies. In such an era, the role of the neuroimaging becomes increasingly important to evaluate the extent and location of tissues at risk of infarction (ischemic penumbra), to distinguish it from unsalvageable infarcted tissues or doomed hemorrhagic parenchyma. In this review, we present briefly the current role and limitation of computed tomography and conventional magnetic resonance imaging (MRI). We also present the possible applications of advanced MR techniques, such as diffusion and perfusion imaging, concentrating on the delineation or detection of ischemic penumbra.

Poly(ADP-ribose) polymerase impairs early and long-term experimental stroke recovery

BACKGROUND AND PURPOSE

Poly(ADP-ribose) polymerase (PARP-1; Enzyme Commission 2.4.30) is a nuclear DNA repair enzyme that mediates early neuronal ischemic injury. Using novel 3-dimensional, fast spin-echo-based diffusion-weighted imaging, we compared acute (21 hours) and long-term (3 days) ischemic volume after middle cerebral artery (MCA) occlusion in PARP-1-null mutants (PARP-/-) versus genetically matched wild-type mice (WT mice). PARP-/- mice were also treated with viral transfection of wild-type PARP-1 to determine whether protection from MCA occlusion is lost with restoration of the gene product.

METHODS

Halothane-anesthetized mice were treated with reversible MCA occlusion via intraluminal suture technique. Ischemic volumes were delineated by diffusion-weighted imaging with high spatial and temporal resolution during MCA occlusion and reperfusion. Recombinant Sindbis virus carrying beta-galactosidase (lacZ) or PARP-1 was injected into ipsilateral striatum, then animals underwent MCA occlusion 3 days later. Infarction volume was measured at 22 hours of reperfusion (2,3,5-triphenyltetrazolium chloride histology).

RESULTS

Reduction in regional water apparent diffusion coefficient (ADC) during occlusion or secondary ADC decline during reperfusion was not different between groups. Ischemic volume was smaller early in occlusion in PARP-/- versus WT mice and remained less at 21 hours of reperfusion. Ischemic volume then increased from 1 to 2 days in all mice, then stabilized without further change. Ischemic damage was smaller in PARP-/- than in WT mice at 3 days. Transfection of PARP-1 into PARP-/- mice increased stroke damage relative to lacZ-injected PARP-/- and increased damage to that of the WT mice. Intraischemic laser-Doppler flowmetry and physiological variables were not different among groups.

CONCLUSIONS

PARP-1 deficiency provides both early and prolonged protection from experimental focal stroke. The mechanism is not linked to preservation of ADC and mitigation of secondary energy depletion during early reperfusion.

Late reversal of cerebral perfusion and water diffusion after transient focal ischemia in rats

Region-specific cerebral blood flow (CBF) and the apparent diffusion coefficient (ADC) of tissue water in the rat brain were quantified by high-field magnetic resonance imaging at 9.4 T in the rat suture occlusion model. Cerebral blood flow and ADC were compared during the short- (4.5 hours) and long-term (up to 6 days) reperfusion after 80 minutes of transient middle cerebral artery occlusion, and correlated with the histology analysis. On occlusion, average CBF fell from approximately 100 to less than 50 mL x 100 g(-1) x min(-1) in the cortex, and to less than 20 mL x 100 g(-1) x min(-1) in the caudate putamen (CP). Corresponding ADC values decreased from (6.98 +/- 0.82) x 10(-4) to (5.49 +/- 0.54) x 10(-4) mm2/s in the cortex, and from (7.16 +/- 0.58) x 10(-4) to (4.86 +/- 0.62) x 10(-4) mm2/s in the CP. On average, CBF recovered to approximately 50% of baseline in the first 24 hours of reperfusion. After 2 to 4 days, a strong hyperperfusion in the ipsilateral cortex and CP, up to approximately 300 mL x 100 g(-1) x min(-1), was observed. The ADC ratio in the ipsilateral and contralateral CP was also inverted in the late reperfusion period. Histology revealed more severe tissue damage at the late stage of reperfusion than at 4.5 hours. Significant reversal of CBF and ADC during the late reperfusion period may reflect the impairment of autoregulation in the ischemic regions. Vascular factors may play an important role in the infarct development after 80-minute focal ischemia.

Severe ADC decreases do not predict irreversible tissue damage in humans

BACKGROUND AND PURPOSE

A mismatch between diffusion- and perfusion-weighted MRI is thought to define tissue at risk of infarction. This concept is based on the assumption that diffusion slowing of and decreases in the apparent diffusion coefficient (ADC) serve as indicator of tissue proceeding to infarction. We tested this hypothesis.

METHODS

MRI (diffusion weighted, perfusion weighted, MRA, T2 weighted) was performed in 15 patients with acute stroke within 2.9+/-0.8 hours (mean+/-SD) of onset and on days 1 and 7. After intraindividual realignment of the ADC maps, the development of ADC range volumes and ADC values was determined.

RESULTS

An increase (354%, group A1) in the total ADC-based lesion volume below a threshold of < 80% occurred in 4 patients on day 1, persisting on day 7 with a pronounced increase of ADC range volumes with low ADC values. An increase in total ADC-based lesion volume (201%, group A2) followed by a secondary drop to day 7 was found in 7 patients. A significant reduction in total ADC-based lesion volume (14%, group B) was found in 4 patients. ADC-based lesion volume increase was associated with persistent vessel occlusion in group A, whereas recanalization in group B resulted in ADC volume decrease. ADC normalization was observed independently from the degree of the initial ADC decrease on days 1 and 7 in group B.

CONCLUSIONS

In line with results from animal experiments, ADC decreases do not reliably indicate tissue infarction Even severely decreased ADC values may normalize in human stroke, and it seems likely that ADC normalization depends on the duration and severity of ischemia rather than the absolute value.

Application of magnetic resonance to animal models of cerebral ischemia

The present review has been compiled to highlight the role of magnetic resonance imaging (MRI) and MR spectroscopy (MRS) for the investigation of cerebral ischemia in the animal experimental field of basic research. We have focused on stroke investigations analyzing the pathomechanisms of the disease evolution and on new advances in both nuclear MR (NMR) methodology or genetic engineering of transgenic animals for the study of complex molecular relationships and causes of the disease. Furthermore, we have tried to include metabolic and genetic aspects, as well as the application of functional imaging, for the investigation of the disturbance or restitution of functional brain activation under pathological conditions as relates to controlled animal experiments.

Diffusion-weighted magnetic resonance imaging detects early neuropathology following four vessel occlusion ischemia in the rat

Early neuropathology following a prolonged duration of four-vessel occlusion (4 VO) ischemia in the rat was charted using magnetic resonance imaging (MRI). Animals received either 30 minutes of 4 VO (N = 6) or sham operation (N = 6) prior to in vivo assessment. Proton density and T(2) and combined T(2)/diffusion-weighted (T(2)/DW) MRI were performed at 6, 24, and 72 hours postocclusion. T(2)/DW imaging was the most effective sequence for delineating between injured and intact tissues, indicating neuropathology in the dorsolateral striatum at 24 hours and in the CA1/CA2 subfields of the hippocampus at 72 hours following ischemia. Apparent diffusion coefficient values were significantly reduced in the striatum (P = 0.03) and hippocampus (P = 0.005) at 24 and 72 hours, respectively. This is the first report, to our knowledge, of T(2)/DW imaging detecting lesions following 4 VO in accord with the known temporal evolution of ischemic brain damage.

Prediction of tissue survival after middle cerebral artery occlusion based on changes in the apparent diffusion of water

OBJECT

In this study the authors tested the hypothesis that the estimate of the apparent diffusion coefficient (ADC) of water is a reliable pathophysiological index of the viability of ischemic brain tissue.

METHODS

Cerebral blood flow (CBF) and the cerebral metabolic rates of oxygen and glucose (CMRO2 and CMRglc, respectively) were measured using positron emission tomography (PET) scanning before and after permanent middle cerebral artery occlusion (MCAO) or reperfusion in pigs. The ADC value, which was measured using diffusion-weighted magnetic resonance (DW MR) imaging was compared with physiological variables obtained by PET scanning and with histological findings. After both permanent MCAO and reperfusion, the decrease in the ADC was significantly correlated with decrease in the CMRO2 and CMRglc. The infarction coincided with a CMRO2 threshold of 50% of the value measured on the contralateral side. Thus, an ADC value of 80% or 75% of the contralateral value reflected the CMRO2 threshold after permanent MCAO or reperfusion, respectively. On DW MR images, lesions with ADC values above 80% of the contralateral value are potentially reversible until 6 hours after MCAO, whereas lesions with ADC values below 75% of the contralateral value are irreversible as early as 2 hours after MCAO.

CONCLUSIONS

The ADC of water provides a reliable pathophysiological index for tailoring therapy to the condition of individual stroke patients in clinical practice.

Apparent diffusion coefficient decreases and magnetic resonance imaging perfusion parameters are associated in ischemic tissue of acute stroke patients

Perfusion-and diffusion-weighted magnetic resonance imaging scans are thought to allow the characterization of tissue at risk of infarction. The authors tested the hypothesis that the apparent diffusion coefficient (ADC) decrease should be associated with the severity of the perfusion deficit in ischemic tissue of acute stroke patients. Perfusion-and diffusion-weighted scans were performed in 11 patients with sudden onset of neurologic deficits within the last 6 hours and T2-weighted magnetic resonance imaging scans were obtained after 6 days. Parameter images of the maximum of the contrast agent concentration, time to peak, relative cerebral blood volume, relative cerebral blood flow, and relative mean transit time were computed from the perfusion-weighted data. A threshold function was used to identify tissue volumes with stepwise ADC decreases. An onionlike distribution of successively decreasing ADC values was found, with the lowest ADC in the center of the ischemic region. Correspondingly, tissue perfusion decreased progressively from the periphery toward the ischemic core. This effect was most pronounced in the time-to-peak maps, with a linear association between ADC decrease and time-to-peak increase. Apparent diffusion coefficient values decreased from the periphery toward the ischemic core, and this distribution of ADC values was strongly associated with the severity of the perfusion deficit.

Usefulness of magnetic resonance-derived quantitative measurements of cerebral blood flow and volume in prediction of infarct growth in hyperacute stroke

BACKGROUND AND PURPOSE

The identification of the tissue at risk for infarction remains challenging in stroke patients. In this study, we evaluated the value of quantitative cerebral blood flow (CBF) and cerebral blood volume (CBV) measurements in the prediction of infarct growth in hyperacute stroke.

METHODS

Fluid-attenuated inversion recovery (FLAIR), diffusion-weighted (DW), and gradient-echo echo-planar perfusion-weighted (PW) sequences were obtained in 66 patients within 6 hours of stroke onset; ischemia was confirmed on follow-up FLAIR images. We delineated the following: (1) the initial infarct on DW images, (2) the area of hemodynamic disturbance on mean transit time (MTT) maps, and (3) the final infarct on follow-up FLAIR images. MTT, CBF, and CBV were calculated in the following areas: area of initial infarct (INF), area of infarct growth (IGR, final minus initial infarct), the hemodynamically disturbed area that remained viable (OLI, hemodynamic disturbance minus final infarct), and all contralateral mirror regions.

RESULTS

Compared with mirror regions, the MTT in abnormal areas was always prolonged. The respective mean+/-SD CBF and CBV values were as follows: for INF, 28+/-16 mL/min per 100 g and 6.9+/-2.7%; for IGR, 36+/-20 mL/min per 100 g and 8.9+/-3.1%; for OLI, 50+/-17 mL/min per 100 g and 11.2+/-3%; and for mirror regions, 64+/-23 mL/min per 100 g and 8.7+/-2.5%. The CBV and CBF values were significantly different between all abnormal areas (except for the CBF between INF and IGR). In the area of DW/PW mismatch, a combined CBF or CBV threshold of 35 or 8.2, respectively, predicted evolution to infarction with a sensitivity of 81% and a specificity of 76%.

CONCLUSIONS

Quantitative measurements of CBF and CBV in hyperacute stroke may help to predict infarct growth and to select the subjects who will benefit from thrombolysis.

Experimental thromboembolic stroke in cynomolgus monkey

To develop an experimental model of thromboembolic stroke without intracranial surgery, an autologous blood clot was delivered to the middle cerebral artery (MCA) via the internal carotid artery in cynomolgus monkeys. Male cynomolgus monkeys, in which a chronic catheter had been earlier implanted in the left internal carotid artery, were used. The clot was flushed into the internal carotid artery under sevofluorane anesthesia. A neurologic deficit score was assigned after MCA embolization. After 24 h, cerebral infarct size and location were determined by the TTC staining method. Cerebral blood flow (CBF) was measured prior to and after MCA embolization, using positron emission tomography (PET). After embolization, long-lasting and profound extensor hypotonia of the contralateral upper and lower limbs, and mild to severe incoordination were observed. Contralateral hemiplegia was observed over the following 24 h. In gross morphologic observation of the brain, the lesions involved mostly the caudate nucleus, putamen, globus pallidus and insular cortex. CBF was maximally reduced in the left MCA territory, but not in the right MCA territory. This model is relevant to thromboembolic stroke in human in neurologic dysfunction and histopathologic brain damage.

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.

Ischemic penumbra: evidence from functional imaging in man

The ischemic penumbra is defined as tissue with flow within the thresholds for maintenance of function and of morphologic integrity. Penumbra tissue has the potential for recovery and therefore is the target for interventional therapy in acute ischemic stroke. The identification of the penumbra necessitates measuring flow reduced less than the functional threshold and differentiating between morphologic integrity and damage. This can be achieved by multitracer positron emission tomography (PET) and perfusion-weighted (PW) and diffusion-weighted magnetic resonance imaging (DW-MRI) in experimental models, in which the recovery of critically perfused tissue or its conversion to infarction was documented in repeat studies. Neuroimaging modalities applied in patients with acute ischemic stroke--multitracer PET, PW- and DW-MRI, single photon emission computed tomography (SPECT), perfusion, and Xe-enhanced computed tomography (CT)-- often cannot reliably identify penumbra tissue: multitracer studies for the assessment of flow and irreversible metabolic damage usually cannot be performed in the clinical setting; CT and MRI do not reliably detect irreversible damage in the first hours after stroke, and even DW-MRI may be misleading in some cases: determinations of perfusion alone yield a poor estimate of the state of the tissue as long as the time course of changes is not known in individual cases. Therefore, the range of flow values in ischemic tissue found later, either within or outside the infarct, was rather broad. New tracers--for example, receptor ligands or hypoxia markers--might improve the identification of penumbra tissue in the future. Despite these methodologic limitations, the validity of the concept of the penumbra was proven in several therapeutic studies in which thrombolytic treatment reversed critical ischemia and decreased the volume of final infarcts. Such neuroimaging findings might serve as surrogate targets in the selection of other therapeutic strategies for large clinical trials.

Transgenic mice overexpressing truncated trkB neurotrophin receptors in neurons show increased susceptibility to cortical injury after focal cerebral ischemia

It has been suggested that the increased production of endogenous BDNF after brain insults supports the survival of injured neurons and limits the spread of the damage. In order to test this hypothesis experimentally, we have produced transgenic mouse lines that overexpress the dominant-negative truncated splice variant of BDNF receptor trkB (trkB.T1) in postnatal cortical and hippocampal neurons. When these mice were exposed to transient focal cerebral ischemia by occluding the middle cerebral artery for 45 min and the damage was assessed 24 h later, transgenic mice had a significantly larger damage than wild-type littermates in the cerebral cortex (204 +/- 32% of wild-type, P = 0.02), but not in striatum, where the transgene is not expressed. Our results support the notion that endogenously expressed BDNF is neuroprotective and that BDNF signaling may have an important role in preventing brain damage after transient ischemia.

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.

Secondary reduction in the apparent diffusion coefficient of water, increase in cerebral blood volume, and delayed neuronal death after middle cerebral artery occlusion and early reperfusion in the rat

It has been reported recently that very delayed damage can occur as a result of focal cerebral ischemia induced by vascular occlusion of short duration. With use of diffusion-, T2-, and contrast-enhanced dynamic magnetic resonance imaging (MRI) techniques, the occlusion time dependence together with the temporal profile for this delayed response in a rat model of transient focal cortical ischemia have been established. The distal branch of the middle cerebral artery was occluded for 20, 30, 45, or 90 minutes. Twenty minutes of vascular occlusion with reperfusion exhibited no significant mean change in either the apparent diffusion coefficient of water (ADC) or the T2 relaxation time at 6, 24, 48, or 72 hours after reperfusion (P = 0.97 and 0.70, respectively). Ninety minutes of ischemia caused dramatic tissue injury at 6 hours, as indicated by an increase in T2 relaxation times to 135% of the contralateral values (P < 0.01). However, at intermediate periods of ischemia (30 to 45 minutes), complete reversal of the ADC was seen at 6 hours after reperfusion but was followed by a secondary decline over time, such that a 25% reduction in tissue ADC was seen at 24 as compared with 6 hours (P < 0.02). This secondary response was accompanied by an increase in cerebral blood volume (CBV), as shown by contrast-enhanced dynamic MRI (120% of contralateral values; P < 0.001), an increase in T2 relaxation time (132%; P < 0.01), together with clear morphological signs of cell death. By day 18, the mean volume of missing cortical tissue measured with high-resolution MRI in animals occluded for 30 and 45 minutes was 50% smaller than that in 90-minute occluded animals (P < 0.005). These data show that ultimate infarct size is reduced after early reperfusion and is occlusion time dependent. The early tissue recovery that is seen with intermediate occlusion times can be followed by cell death, which has a delayed onset and is accompanied by an increase in CBV.

Diffusion- and perfusion-weighted MR imaging of transient focal cerebral ischaemia in mice

Temporary focal ischaemia was induced in wild-type C57Black/6 mice by thread occlusion of the middle cerebral artery (MCA). Recirculation was started after 60 min and maintained for 24 h, after which the mouse brain was frozen in situ. Development of the cerebral infarct was monitored by diffusion-, perfusion- and T(2)-weighted magnetic resonance imaging (MRI) during ischaemia, during the early reperfusion period of 90 min, and at 24 h after reperfusion. Ischaemia caused a marked reduction of the perfusion signal intensity and of the apparent diffusion coefficient (ADC) of tissue water in the ipsilateral MCA territory. In sham-operated control animals ADC remained unchanged. Hemispheric lesion volume after 1 h MCA occlusion was 53 +/- 6% (n = 6), as defined by an ADC decrease of more than 20%. Recirculation reduced hemispheric lesion volume to only 27 +/- 13%, while there was a trend towards secondary lesion growth at 24 h. Post-ischaemic recovery of perfusion was slow, heterogeneous and incomplete. A region-of-interest analysis showed only partial and transient recovery of the ADC, particularly in the dorsolateral cortex and lateral caudate putamen, which may be explained by inadequate reperfusion in these regions. Detailed MRI studies of cerebral ischaemia and reperfusion may now also be performed in the transgenic mice.

Identification of major ischemic change. Diffusion-weighted imaging versus computed tomography

BACKGROUND AND PURPOSE

Thrombolytic therapy is not recommended in patients with CT changes of recent major infarction, which has been defined as reduced attenuation or cerebral edema involving >33% of the middle cerebral artery territory (European Cooperative Acute Stroke Study [ECASS] criteria). Diffusion-weighted imaging (DWI) is more sensitive than CT in detecting acute ischemia, and the combination of DWI, MR perfusion imaging, and MR angiography provides additional information from a single examination. We sought to determine whether DWI could identify the presence and extent of major ischemia as well as CT in hyperacute stroke patients.

METHODS

Seventeen suspected hemispheric stroke patients were studied with both CT and DWI within 6 hours of symptom onset. None received thrombolytic therapy. The scans were examined separately by 2 neuroradiologists in a blinded fashion for ischemic change and cerebral edema, graded as normal, <33%, or >33% of the MCA territory. Final diagnosis of stroke was determined with the use of standard clinical criteria and T2-weighted imaging at day 90.

RESULTS

Sixteen of 17 patients had a final diagnosis of stroke. Acute ischemic changes were seen in all 16 on DWI (100% sensitivity) and in 12 of 16 on CT (75% sensitivity). DWI identified all 6 patients with major ischemia on CT, with excellent agreement between the 2 imaging techniques (kappa=0.88). One patient eligible for thrombolysis on the ECASS CT criteria had major ischemia on DWI.

CONCLUSIONS

DWI is more sensitive than CT in the identification of acute ischemia and can visualize major ischemia more easily than CT.

Clinical application of perfusion and diffusion MR imaging in acute ischemic stroke

With the advances in new neuroimaging modalities, the role of imaging of acute ischemic stroke has broadened and progressed from making diagnoses to providing valuable information for patient management. The goal of thrombolytic therapy for acute ischemic stroke should be to salvage the ischemic tissue reversibility that can respond to recanalization and avoid reperfusion of the dead (nonviable) tissue. It is essential to have rapid diagnostic modalities that can distinguish viable ischemic tissue from irreversibly damaged tissue, because there is a risk of reperfusion injury such as hemorrhagic complications with early intervention. Although diffusion magnetic resonance (MR) imaging has been reported to have a high sensitivity and specificity for acute ischemia in acute stroke patients without early reperfusion therapy, the capability to differentiate reversible from irreversible ischemia by diffusion MR imaging has not been established. Perfusion MR imaging techniques provide direct information on parenchymal perfusion status (adequacy of the collateral circulation) and may have the potential for providing important information about tissue viability and/or reversibility for selecting appropriate patients for thrombolytic therapy.

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.

Applications of diffusion-perfusion magnetic resonance imaging in acute ischemic stroke

Diffusion-weighted imaging (DWI) and perfusion imaging (PI) are two new magnetic resonance technologies that are becoming increasingly available for evaluation of acute ischemic stroke patients. DWI provides information about the location of acute focal ischemic brain injury at early time points and PI can document the presence of disturbances in microcirculatory perfusion. DWI and PI are now being used in clinical practice and in clinical trials of potential acute stroke therapies to assess their utility. In the future, DWI and PI may aid in the development of effective acute stroke therapies and help identify which stroke patients are most likely to benefit from specific agents.

Orientation-independent diffusion imaging without tensor diagonalization: anisotropy definitions based on physical attributes of the diffusion ellipsoid

Diffusion tensor imaging can provide a complete description of the diffusion process in tissue. However, this description is not unique but is orientation dependent, and, to quantify properly the intrinsic orientation-independent diffusion properties of the tissue, a set of three rotationally invariant quantities is needed. Instead of using the tensor eigenvalues for this, we define a new set consisting of scaled invariants that have the proper magnitude of actual diffusion constants and that are directly related to the physical attributes of the diffusion ellipsoid, namely, its average radius, surface, and volume. Using these three physical invariants, a new family of anisotropy measures is defined that are normalized between zero (isotropic) and one (completely anisotropic). Because rotational invariants are used, this approach does not require tensor diagonalization and eigenvalue determination and is therefore not susceptible to potential artifacts induced during these number manipulations. The relationship between the new anisotropy definitions and existing orientation-independent anisotropy indices obtained from eigenvalues is discussed, after which the new approach is evaluated for a group of healthy volunteers.

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.

Magnetic resonance imaging during cerebral hypoxia-ischemia: T2 increases in 2-week-old but not 4-week-old rats

We investigated whether the changes detectable with magnetic resonance imaging techniques during and after an episode of cerebral hypoxia-ischemia differ in immature and older brain. Diffusion weighted (DW) and T2-weighted (T2W) images were repeatedly acquired before, during, and after an episode of cerebral hypoxia-ischemia (unilateral carotid artery occlusion plus hypoxia) in 2- and 4-wk-old rats lightly anesthetized with isoflurane. Areas of increased brightness were detected in DW images from both 2- and 4-wk-old rats by 10-20 min after the start of hypoxia. These hyperintense areas increased during hypoxia, comprising 60.8+/-4.9% and 30.5+/-2.7% of the brain image at the level of the thalamus in 2-wk-old and 4-wk-old animals, respectively (p < 0.003). Hyperintense areas (e.g. 27.0+/-8.3%) also appeared in T2W images during hypoxia-ischemia in 2-wk-old animals, but these did not occur in 4-wk-old animals (p < 0.02). This observation was reflected in T2, which increased during hypoxia-ischemia in the 2-wk-old but not the 4-wk-old group. By 60 min after the termination of hypoxia-ischemia in either age group, areas of hyperintensity resolved and then reappeared 24 h later on both DW and T2W images. Thus, irrespective of age, magnetic resonance imaging changes during transient hypoxia-ischemia generally recover with a delayed or secondary increase in DW and T2W hyperintensity hours later. Immature brain differs from older brain primarily with respect to some combination of hypoxic/ischemic cellular or biochemical changes, that are detectable as increases in T2 within 2-wk-old but not 4-wk-old animals.

FAIR excluding radiation damping (FAIRER)

It is shown that the flow-sensitive alternating inversion recovery (FAIR) technique is complicated by the effect of radiation damping, leading to problems in calibrating this method on phantoms and to inaccuracies in measured flows. A modified scheme called FAIRER (FAIR excluding radiation damping) is proposed, which suppresses the damping effects by employing very weak magnetic field gradients (0.06 G/cm) during the inversion recovery, spin-echo, and predelay periods. Results on phantoms and in vivo on cat brain are presented that demonstrate that FAIRER effectively solves these problems.

Single-shot diffusion-weighted trace imaging on a clinical scanner

The single-shot trace-weighted acquisition scheme for isotropic diffusion imaging suggested by Mori and van Zijl (Magn. Reson. Med. 33, 41-52 (1995)) was implemented on a clinical scanner in combination with echo-planar imaging. The results on phantoms and seven volunteers show that good-quality multiplanar isotropic diffusion-weighted images can be obtained within seconds and absolute Dav images within 10-40 s, depending on the number of gradient strengths used. However, care has to be taken in using this type of multi-bipolar-gradient sequence to avoid measurement of artifactual diffusion constants due to background gradient interference. It is outlined how to test for this interference and how to optimize for the correct experiment.