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

Dynamics of cerebral blood volume during and after middle cerebral artery occlusion in rats - Comparison between ultrafast ultrasound and dynamic susceptibility contrast-enhanced MRI measurements

Tomographic perfusion imaging techniques are integral to translational stroke research paradigms that advance our understanding of the disease. Functional ultrasound (fUS) is an emerging technique that informs on cerebral blood volume (CBV) through ultrasensitive Doppler and flow velocity (CBFv) through ultrafast localization microscopy. It is not known how experimental results compare with a classical CBV-probing technique such as dynamic susceptibility contrast-enhanced perfusion MRI (DSC-MRI). To that end, we assessed hemodynamics based on uUS (n = 6) or DSC-MRI (n = 7) before, during and up to three hours after 90-minute filament-induced middle cerebral artery occlusion (MCAO) in rats. Recanalization was followed by a brief hyperperfusion response, after which CBV and CBFv temporarily normalized but progressively declined after one hour in the lesion territory. DSC-MRI data corroborated the incomplete restoration of CBV after recanalization, which may have been caused by the free-breathing anesthetic regimen. During occlusion, MCAO-induced hypoperfusion was more discrepant between either technique, likely attributable to artefactual signal mechanisms related to slow flow, and processing algorithms employed for either technique. In vivo uUS- and DSC-MRI-derived measures of CBV enable serial whole-brain assessment of post-stroke hemodynamics, but readouts from both techniques need to be interpreted cautiously in situations of very low blood flow.

Hyperperfusion profiles after recanalization differentially associate with outcomes in a rat ischemic stroke model

Futile recanalization hampers prognoses of ischemic stroke after successful mechanical thrombectomy, hypothetically through post-recanalization perfusion deficits, onset-to-groin delays and sex effects. Clinically, acute multiparametric imaging studies remain challenging. We assessed possible relationships between these factors and disease outcome after experimental cerebral ischemia-reperfusion, using translational MRI, behavioral testing and multi-model inference analyses. Male and female rats (N = 60) were subjected to 45-/90-min filament-induced transient middle cerebral artery occlusion. Diffusion, T2- and perfusion-weighted MRI at occlusion, 0.5 h and four days after recanalization, enabled tracking of tissue fate, and relative regional cerebral blood flow (rrCBF) and -volume (rrCBV). Lesion areas were parcellated into core, salvageable tissue and delayed injury, verified by histology. Recanalization resulted in acute-to-subacute lesion volume reductions, most apparently in females (n = 19). Hyperacute normo-to-hyperperfusion in the post-ischemic lesion augmented towards day four, particularly in males (n = 23). Tissue suffering delayed injury contained higher ratios of hypoperfused voxels early after recanalization. Regressed against acute-to-subacute lesion volume change, increased rrCBF associated with lesion growth, but increased rrCBV with lesion reduction. Similar relationships were detected for behavioral outcome. Post-ischemic hyperperfusion may develop differentially in males and females, and can be beneficial or detrimental to disease outcome, depending on which perfusion parameter is used as explanatory variable.

Implications of Post-recanalization Perfusion Deficit After Acute Ischemic Stroke: a Scoping Review of Clinical and Preclinical Imaging Studies

The goal of reperfusion therapy for acute ischemic stroke (AIS) is to restore cerebral blood flow through recanalization of the occluded vessel. Unfortunately, successful recanalization does not always result in favorable clinical outcome. Post-recanalization perfusion deficits (PRPDs), constituted by cerebral hypo- or hyperperfusion, may contribute to lagging patient recovery rates, but its clinical significance remains unclear. This scoping review provides an overview of clinical and preclinical findings on post-ischemic reperfusion, aiming to elucidate the pattern and consequences of PRPD from a translational perspective. The MEDLINE database was searched for quantitative clinical and preclinical studies of AIS reporting PRPD based on cerebral circulation parameters acquired by translational tomographic imaging methods. PRPD and stroke outcome were mapped on a charting table, creating an overview of PRPD after AIS. Twenty-two clinical and twenty-two preclinical studies were included. Post-recanalization hypoperfusion is rarely reported in clinical studies (4/22) but unequivocally associated with detrimental outcome. Post-recanalization hyperperfusion is more commonly reported (18/22 clinical studies) and may be associated with positive or negative outcome. PRPD has been replicated in animal studies, offering mechanistic insights into causes and consequences of PRPD and allowing delineation of possible courses of PRPD. Complex relationships exist between PRPD and stroke outcome. Diversity in methods and lack of standardized definitions in reperfusion studies complicate the characterization of reperfusion patterns. Recommendations are made to advance the understanding of PRPD mechanisms and to further disentangle the relation between PRPD and disease outcome.

Dynamic oxygen-17 MRI with adaptive temporal resolution using golden-means-based 3D radial sampling

PURPOSE

The aim of this study was to develop a high-resolution 3D oxygen-17 (¹⁷ O) MRI method to delineate the kinetics of ¹⁷ O-enriched water (H₂ ¹⁷ O) across the entire mouse brain after a bolus injection via the tail vein.

METHODS

The dynamic ¹⁷ O signal was acquired with a golden-means-based 3D radial sampling scheme. To achieve adequate temporal resolution with preserved spatial resolution, a k-space-weighted view sharing strategy was used in image reconstruction with an adaptive window size tailored to the kinetics of the ¹⁷ O signal. Simulation studies were performed to determine the adequate image reconstruction parameters. The established method was applied to delineating the kinetics of intravenously injected H₂ ¹⁷ O in vivo in the post-stroke mouse brain.

RESULTS

The proposed dynamic ¹⁷ O-MRI method achieved an isotropic resolution of 1.21 mm (0.77 mm nominal) in mouse brain at 9.4T, with the temporal resolution increased gradually from 3 s at the initial phase of rapid signal increase to 15 s at the steady-state. The high spatial resolution enabled the delineation of the heterogeneous H₂ ¹⁷ O uptake and washout kinetics in stroke-affected mouse brain.

CONCLUSION

The current study demonstrated a 3D ¹⁷ O-MRI method for dynamic monitoring of ¹⁷ O signal changes with high spatial and temporal resolution. The method can be utilized to quantify physiological parameters such as cerebral blood flow and blood-brain barrier permeability by tracking injected H₂ ¹⁷ O. It can also be used to measure oxygen consumption rate in ¹⁷ O-oxygen inhalation studies.

Iron Sucrose as MRI Contrast Agent in Ischemic Stroke Model

BACKGROUND

Despite the growing concern about the safety of gadolinium-based contrast agents (GBCAs), they are still the most commonly used. Ferumoxytol, as an off-label alternative MRI contrast agent, cannot be administered by a rapid bolus for dynamic susceptibility contrast perfusion-weighted imaging (DSC-PWI).

PURPOSE

To assess the feasibility of iron sucrose (IS) as a contrast agent for MR angiography (MRA) and DSC-PWI.

STUDY TYPE

Prospective animal model.

ANIMAL MODEL

Thirty-six normal rats (16 for MRA, 20 for biocompability tests) and 36 occlusion of the middle cerebral artery (MCAO) model rats.

FIELD STRENGTH/SEQUENCE

3.0T; head and neck angiography, using a fast spoiled gradient-recalled-echo (FSPGR) sequence and DSC-MRI using echo planar imaging(EPI) sequence.

ASSESSMENT

MRA was performed on normal rats to examine the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of different doses of IS. DSC-PWI was performed on MCAO rats at 0, 24, 48, and 72 hours postreperfusion to investigate the lesion detectability of IS. Arterial spin labeling (ASL) and DSC-PWI enhanced by GBCAs were conducted on MCAO rats as controls.

STATISTICAL TESTS

Kruskal-Wallis test was used to compare qualitative assessment. One-way analysis of variance (ANOVA) was used to compare the parametric data. Pearson's r values were evaluated between relative cerebral blood flow(rCBF)-ASL, rCBF-DSC_IS , and rCBF obtained from DSC-PWI enhanced by GBCA.

RESULTS

The mean SNR and CNR of the common carotid artery at doses of 10 mg Fe/kg of IS were comparable with the standard dose of GBCAs (SNR: 68.04 ± 12.55 vs. 67.72 ± 14.66; CNR: 23.78 ± 7.21vs. 21.63 ± 6.83). In MCAO rat models, rCBF and relative cerebral blood volume (rCBV) of ipsilateral striatum declined (0.72 ± 0.14, 0.86 ± 0.11) with prolonged relative mean transit time (rMTT) and relative time-to-peak (rTTP) (1.27 ± 0.24, 1.07 ± 0.03) following occlusion. Hyperperfusion was observed in all rats at 48 and 72 hours postreperfusion, in 4/6 rats at 24 hours postreperfusion for IS-mediated DSC-PWI.

DATA CONCLUSION

IS may be an effective contrast agent for both MRA and DSC-PWI in ischemic stroke models.

LEVEL OF EVIDENCE

1 TECHNICAL EFFICACY STAGE: 1 J. Magn. Reson. Imaging 2020;52:836-849.

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.

Role of Genetic Variation in Collateral Circulation in the Evolution of Acute Stroke: A Multimodal Magnetic Resonance Imaging Study

BACKGROUND AND PURPOSE

No studies have determined the effect of differences in pial collateral extent (number and diameter), independent of differences in environmental factors and unknown genetic factors, on severity of stroke. We examined ischemic tissue evolution during acute stroke, as measured by magnetic resonance imaging and histology, by comparing 2 congenic mouse strains with otherwise identical genetic backgrounds but with different alleles of the Determinant of collateral extent-1 (Dce1) genetic locus. We also optimized magnetic resonance perfusion and diffusion-deficit thresholds by using histological measures of ischemic tissue.

METHODS

Perfusion, diffusion, and T₂-weighted magnetic resonance imaging were performed on collateral-poor (congenic-Bc) and collateral-rich (congenic-B6) mice at 1, 5, and 24 hours after permanent middle cerebral artery occlusion. Magnetic resonance imaging-derived penumbra and ischemic core volumes were confirmed by histology in a subset of mice at 5 and 24 hours after permanent middle cerebral artery occlusion.

RESULTS

Although perfusion-deficit volumes were similar between strains 1 hour after permanent middle cerebral artery occlusion, diffusion-deficit volumes were 32% smaller in collateral-rich mice. At 5 hours, collateral-rich mice had markedly restored perfusion patterns showing reduced perfusion-deficit volumes, smaller infarct volumes, and smaller perfusion-diffusion mismatch volumes compared with the collateral-poor mice (P<0.05). At 24 hours, collateral-rich mice had 45% smaller T₂-weighted lesion volumes (P<0.005) than collateral-poor mice, with no difference in perfusion-diffusion mismatch volumes because of penumbral death occurring 5 to 24 hours after permanent middle cerebral artery occlusion in collateral-poor mice.

CONCLUSIONS

Variation in collateral extent significantly alters infarct volume expansion, transiently affects perfusion and diffusion magnetic resonance imaging signatures, and impacts salvage of ischemic penumbra after stroke onset.

In vivo evidence for long-term vascular remodeling resulting from chronic cerebral hypoperfusion in mice

We have characterized both acute and long-term vascular and metabolic effects of unilateral common carotid artery occlusion in mice by in vivo magnetic resonance imaging and positron emission tomography. This common carotid artery occlusion model induces chronic cerebral hypoperfusion and is therefore relevant to both preclinical stroke studies, where it serves as a control condition for a commonly used mouse model of ischemic stroke, and neurodegeneration, as chronic hypoperfusion is causative to cognitive decline. By using perfusion magnetic resonance imaging, we demonstrate that under isoflurane anesthesia, cerebral perfusion levels recover gradually over one month. This recovery is paralleled by an increase in lumen diameter and altered tortuosity of the contralateral internal carotid artery at one year post-ligation as derived from magnetic resonance angiography data. Under urethane/α-chloralose anesthesia, no acute perfusion differences are observed, but the vascular response capacity to hypercapnia is found to be compromised. These hemispheric perfusion alterations are confirmed by water [¹⁵O]-H₂O positron emission tomography. Glucose metabolism ([¹⁸F]-FDG positron emission tomography) or white matter organization (diffusion-weighted magnetic resonance imaging) did not show any significant alterations. In conclusion, permanent unilateral common carotid artery occlusion results in acute and long-term vascular remodeling, which may have immediate consequences for animal models of stroke but also vascular dementia.

Imaging of reactive oxygen species in focal ischemic mouse brain using a radical trapping tracer [(3)H]hydromethidine

Background

Reactive oxygen species (ROS) have been implicated in the pathophysiology of the brain after ischemic stroke. In this study, we investigate the generation of brain ROS after transient focal ischemia in mice using a radical trapping radiotracer, [³H]-labeled N-methyl-2,3-diamino-6-phenyl-dihydrophenanthridine ([³H]hydromethidine), which we recently reported as a ROS imaging probe. We also examined the effect of dimethylthiourea (DMTU), a hydroxyl radical scavenger, on brain ROS generation and infarct volume after transient focal ischemia in mice.

Methods

[³H]Hydromethidine was intravenously injected into mice at 1, 2, 5, and 7 h after transient middle cerebral artery occlusion (tMCAO), and then, the brain autoradiogram was acquired at 60 min after tracer injection. Brain infarct volumes at 24 h after tMCAO were assessed by 2,3,5-triphenyltetrazolium chloride staining.

Results

Accumulation of radioactivity was observed in the ipsilateral striatum and cortex at 1 h after tMCAO. The increase of radioactivity was attenuated at 2 h after tMCAO and then became maximized at 5 h. The high accumulation of radioactivity remained until 7 h after tMCAO. DMTU treatment significantly attenuated the accumulation of radioactivity in the ipsilateral hemisphere at 1, 5, and 7 h after tMCAO. Brain infarct volumes were also significantly reduced in DMTU-treated mice at 24 h after tMCAO.

Conclusions

These results indicated that [³H]hydromethidine is a useful radiotracer for detecting in vivo brain ROS generation such as hydroxyl radical after ischemic injury.

Evaluation of EPI distortion correction methods for quantitative MRI of the brain at high magnetic field

High field MRI has been applied to high-resolution structural and functional imaging of the brain. Echo planar imaging (EPI) is an ultrafast acquisition technique widely used in diffusion imaging, functional MRI and perfusion imaging. However, it suffers from geometric and intensity distortions caused by static magnetic field inhomogeneity, which is worse at higher field strengths. Such susceptibility artifacts are particularly severe in relation to the small size of the mouse brain. In this study we compared different distortion correction methods, including nonlinear registration, field map-based, and reversed phase-encoding-based approaches, on quantitative imaging of T1 and perfusion in the mouse brain acquired by spin-echo EPI with inversion recovery and pseudo-continuous arterial spin labeling, respectively, at 7 T. Our results showed that the 3D reversed phase-encoding correction outperformed other methods in terms of geometric fidelity, and that conventional field map-based correction could be improved by combination with affine transformation to reduce the bias in the field map. Both methods improved quantification with smaller fitting error and regional variation. These approaches offer robust correction of EPI distortions at high field strengths and hence could lead to more accurate co-registration and quantification of imaging biomarkers in both clinical and preclinical applications.

Reduced brain edema and infarct volume in aquaporin-4 deficient mice after transient focal cerebral ischemia

Aquaporin-4 (AQP4) is a water channel expressed in astrocyte end-feet lining the blood-brain barrier. AQP4 deletion in mice is associated with improved outcomes in global cerebral ischemia produced by transient carotid artery occlusion, and focal cerebral ischemia produced by permanent middle cerebral artery occlusion (MCAO). Here, we investigated the consequences of 1-h transient MCAO produced by intraluminal suture blockade followed by 23 h of reperfusion. In nine AQP4(+/+) and nine AQP4(-/-) mice, infarct volume was significantly reduced by an average of 39 ± 4% at 24h in AQP4(-/-) mice, cerebral hemispheric edema was reduced by 23 ± 3%, and Evans Blue extravasation was reduced by 31 ± 2% (mean ± SEM). Diffusion-weighted magnetic resonance imaging showed greatest reduction in apparent diffusion coefficient around the occlusion site after reperfusion, with remarkably lesser reduction in AQP4(-/-) mice. The reduced infarct volume in AQP4(-/-) mice following transient MCAO supports the potential utility of therapeutic AQP4 inhibition in stroke.

Functional real-time optoacoustic imaging of middle cerebral artery occlusion in mice

BACKGROUND AND PURPOSE

Longitudinal functional imaging studies of stroke are key in identifying the disease progression and possible therapeutic interventions. Here we investigate the applicability of real-time functional optoacoustic imaging for monitoring of stroke progression in the whole brain of living animals.

MATERIALS AND METHODS

The middle cerebral artery occlusion (MCAO) was used to model stroke in mice, which were imaged preoperatively and the occlusion was kept in place for 60 minutes, after which optoacoustic scans were taken at several time points.

RESULTS

Post ischemia an asymmetry of deoxygenated hemoglobin in the brain was observed as a region of hypoxia in the hemisphere affected by the ischemic event. Furthermore, we were able to visualize the penumbra in-vivo as a localized hemodynamically-compromised area adjacent to the region of stroke-induced perfusion deficit.

CONCLUSION

The intrinsic sensitivity of the new imaging approach to functional blood parameters, in combination with real time operation and high spatial resolution in deep living tissues, may see it become a valuable and unique tool in the development and monitoring of treatments aimed at suspending the spread of an infarct area.

Spatial and temporal MRI profile of ischemic tissue after the acute stages of a permanent mouse model of stroke

OBJECT

To characterize the progression of injured tissue resulting from a permanent focal cerebral ischemia after the acute phase, Magnetic Resonance Imaging (MRI) monitoring was performed on adult male C57BL/6J mice in the subacute stages, and correlated to histological analyses.

MATERIAL AND METHODS

Lesions were induced by electrocoagulation of the middle cerebral artery. Serial MRI measurements and weighted-images (T2, T1, T2* and Diffusion Tensor Imaging) were performed on a 9.4T scanner. Histological data (Cresyl-Violet staining and laminin-, Iba1- and GFAP-immunostainings) were obtained 1 and 2 weeks after the stroke.

RESULTS

Two days after stroke, tissues assumed to correspond to the infarct core, were detected as a hyperintensity signal area in T2-weighted images. One week later, low-intensity signal areas appeared. Longitudinal MRI study showed that these areas remained present over the following week, and was mainly linked to a drop of the T2 relaxation time value in the corresponding tissues. Correlation with histological data and immuno-histochemistry showed that these areas corresponded to microglial cells.

CONCLUSION

The present data provide, for the first time detailed MRI parameters of microglial cells dynamics, allowing its non-invasive monitoring during the chronic stages of a stroke. This could be particularly interesting in regards to emerging anti-inflammatory stroke therapies.

Determination of the brain-blood partition coefficient for water in mice using MRI

Cerebral blood flow (CBF) quantification is a valuable tool in stroke research. Mice are of special interest because of the potential of genetic engineering. Magnetic resonance imaging (MRI) provides repetitive, noninvasive CBF quantification. Many MRI techniques require the knowledge of the brain-blood partition coefficient (BBPC) for water. Adopting an MRI protocol described by Roberts et al (1996) in humans, we determined the BBPC for water in 129S6/SvEv mice from proton density measurements of brain and blood, calibrated with deuterium oxide/water phantoms. The average BBPC for water was 0.89 ± 0.03 mL/g, with little regional variation within the mouse brain.

Cerebral blood flow MRI in mice using the cardiac-spin-labeling technique

Continuous arterial spin labeling MRI with a separate neck labeling coil provides a highly sensitive method to image cerebral blood flow (CBF). In mice, however, this has not been possible because the proximity of the neck coil to the brain uses the neck coil to significantly saturate the brain signal. To overcome this limitation the cardiac spin labeling (CSL) technique is introduced in which the labeling coil is placed at the heart position. To demonstrate its utility, CSL CBF was applied to image quantitative basal CBF and hypercapnia-induced CBF changes. This approach provides a practical means to image CBF with high sensitivity in small animals, compares favorably to existing mouse CBF imaging techniques, and could broaden CBF applications in mice where many brain disease and transgenic models are widely available.

Overexpression of mitochondrial Hsp70/Hsp75 in rat brain protects mitochondria, reduces oxidative stress, and protects from focal ischemia

Mitochondria are known to be central to the cell's response to ischemia, because of their role in energy generation, in free radical generation, and in the regulation of apoptosis. Heat shock protein 75 (Hsp75/Grp75/mortalin/TRAP1) is a member of the HSP70 chaperone family, which is targeted to mitochondria. Overexpression of Hsp75 was achieved in rat brain by DNA transfection, and expression was observed in both astrocytes and neurons. Rats were subjected to 100 mins middle cerebral artery occlusion followed by assessment of infarct volume, neurological score, mitochondrial function, and levels of oxidative stress at 24 h reperfusion. Overexpression of Hsp75 reduced infarct area from 44.6%+/-21.1% to 25.7%+/-12.1% and improved neurological outcome significantly. This was associated with improved mitochondrial function as shown by protection of complex IV activity, marked reduction of free radical generation detected by hydroethidine fluorescence, reduction of lipid peroxidation detected by 4-hydroxy-2-nonenol immunoreactivity, and increased preservation of ATP levels. This suggests that targeting mitochondria for protection may be a useful strategy to reduce ischemic brain injury.

The pre-ischaemic neuroprotective effects of the polyamine analogues BU43b and BU36b in permanent and transient focal cerebral ischaemia models in mice

The present study investigated the neuroprotective potential of two novel polyamine analogues, BU43b and BU36b, when administered 30 min prior to cerebral ischaemia. Neuroprotection in a permanent and a transient focal cerebral ischaemia mouse model (induced by intraluminal middle cerebral artery occlusion (MCAO)) was investigated using a range of histological and behavioural assessments. In the permanent ischaemia model, BU43b reduced oedema and showed a trend towards reduction in %HLV (percentage hemisphere lesion volume) when administered at a dose of 30 mg/kg i.p. Following transient ischaemia, treatment with BU43b decreased the %HLV and reduced oedema when administered at 30 mg/kg. BU43b also improved the locomotor activity (LMA) in MCAO mice at both 20 mg/kg and 30 mg/kg doses. BU36b was less effective than BU43b in both the permanent and the transient models, with its most pronounced effect being a trend towards reduction in oedema in both models. These results demonstrate that BU43b administered 30 min before ischaemia provided a good level of neuroprotection in the two models of cerebral ischaemia used and may have potential as a neuroprotective treatment for stroke.

Early T1- and T2-weighted MRI signatures of transient and permanent middle cerebral artery occlusion in a murine stroke model studied at 9.4T

Early reperfusion following stroke results in reduced tissue injury. Paradoxically, restoration of blood flow under certain conditions may also cause delayed neuronal damage (reperfusion injury). The interrelationship of changes in T1, T2 and diffusion weighted images of tissue water were studied in mouse models of permanent and transient focal cerebral ischemia. A sham surgery or either permanent or transient (30 min) middle cerebral artery occlusion (MCAO) were induced in 14 mice. Magnetic resonance (MR) images of the brain were acquired including: T2 maps, T1 maps and diffusion weighted spin-echo images to produce apparent diffusion coefficient of water apparent diffusion coefficient (ADC) maps. Images were collected on average 90 min after MCAO in both the transient and permanent ischemia groups. Scans were repeated at 24h post-occlusion in mice with transient ischemia. Permanent MCAO resulted in decreases in ADC and no significant change in T2 acutely following MCAO. There were increases in T1 compared to sham controls within the ischemic region in mice following either transient or permanent MCAO (P<0.001). In contrast to permanent MCAO, there were increases in T2 (P<0.001) in the infarct area present in the reperfusion phase within 90 min of transient MCAO. There was considerable infarct growth at 24h (P<0.001). This study demonstrates that following either type of occlusion there are early increases in T1 suggesting an elevated water content in the stroke lesion, while only following transient MCAO are there early increases in T2, indicative of early vasogenic oedema with breakdown of the blood-brain barrier.

Magnetic resonance imaging for detection and analysis of mouse phenotypes

With the enormous and growing number of experimental and genetic mouse models of human disease, there is a need for efficient means of characterizing abnormalities in mouse anatomy and physiology. Adaptation of magnetic resonance imaging (MRI) to the scale of the mouse promises to address this challenge and make major contributions to biomedical research by non-invasive assessment in the mouse. MRI is already emerging as an enabling technology providing informative and meaningful measures in a range of mouse models. In this review, recent progress in both in vivo and post mortem imaging is reported. Challenges unique to mouse MRI are also identified. In particular, the needs for high-throughput imaging and comparative anatomical analyses in large biological studies are described and current efforts at handling these issues are presented.

Murine orthostatic response during prolonged vertical studies: Effect on cerebral blood flow measured by arterial spin-labeled MRI

High-field MRI scanners are, in principle, well suited for mouse studies; however, many high-field magnets employ a vertical design that may influence the physiological state of the rodent. The purpose of this study was to investigate the orthostatic response of cerebral blood flow (CBF) in mice during a prolonged MR experiment in the vertical position. Arterial spin-labeled (ASL) MRI was performed at 4.7-Tesla with a 15-cm gradient insert that allowed horizontal and vertical CBF measurements to be obtained with the same scanner. For mice in the head-up (HU) vertical position, CBF decreased by approximately 40% compared to the horizontal position, although blood pressure did not differ. Furthermore, CBF values for vertically positioned mice treated with phenylephrine remained constant while blood pressure increased. These results support the conclusion that cerebral autoregulation was intact, albeit at a lower level. Since CBF recovers to near horizontal values by volume loading with saline, it appears that a decrease in central venous pressure (CVP) leading to an increase in sympathetic tone may be a contributing mechanism for lowered CBF. This suggests that using an HU vertical position for MRI in mice may have broader implications, especially for studies that rely on CBF (such as BOLD and fMRI).

Comparison of FAIR perfusion kinetics with DSC-MRI and functional histology in a model of transient ischemia

Flow-sensitive alternating inversion recovery (FAIR) is a noninvasive method for perfusion imaging. It has been shown that the FAIR signal may depend on hemodynamic parameters other than perfusion, the most important one being transit delays of labeled spins to the observed tissue. These parameters are expected to change with ischemia. The goal of this study was to assess the effect of these changes on the interpretation of FAIR results in the case of altered perfusion. This was investigated in a rat model of transient cerebral ischemia. It was shown that the ratio of FAIR signal in the infarct compared to the contralateral side was lower at short inflow times, which suggests that transit times affected the effective FAIR signal. The FAIR results were compared with those from functional histology and dynamic susceptibility contrast MRI, and the findings indicated that the altered kinetics of the FAIR signal were related to reduced and delayed inflow in the infarct region--not to a decrease in the number of functional vessels.

Magnetic resonance imaging in experimental models of brain disorders

This review gives an overview of the application of magnetic resonance imaging (MRI) in experimental models of brain disorders. MRI is a noninvasive and versatile imaging modality that allows longitudinal and three-dimensional assessment of tissue morphology, metabolism, physiology, and function. MRI can be sensitized to proton density, T1, T2, susceptibility contrast, magnetization transfer, diffusion, perfusion, and flow. The combination of different MRI approaches (e.g., diffusion-weighted MRI, perfusion MRI, functional MRI, cell-specific MRI, and molecular MRI) allows in vivo multiparametric assessment of the pathophysiology, recovery mechanisms, and treatment strategies in experimental models of stroke, brain tumors, multiple sclerosis, neurodegenerative diseases, traumatic brain injury, epilepsy, and other brain disorders. This report reviews established MRI methods as well as promising developments in MRI research that have advanced and continue to improve our understanding of neurologic diseases and that are believed to contribute to the development of recovery improving strategies.

MRI reveals changes in intracellular calcium in ischaemic areas of rabbit brain

Since calcium overload is thought to be important in ischaemic neuronal death, we have used a focal ischaemic model to determine the relationships between changes in intracellular calcium concentration ([Ca(2+)]( i)), apparent diffusion coefficient (ADC), and relative cerebral blood flow (rCBF). Focal cerebral ischaemia was induced in seven groups of six rabbits, by transorbital permanent occlusion of one middle cerebral artery (MCAo). Diffusion- and perfusion-weighted imaging was performed from 0.5 to 36 h after the occlusion. Brains were removed, and slices were taken. These slices were incubated with fluo-3 solution, and the fluorescent intensity (FI) of [Ca(2+)]( i) was viewed by confocal microscopy. There were significant differences in FI of Ca(2+) between the ischaemic caudoputamen and the contralateral region in the seven groups of animals ( F=24.34, P <0.001), while the difference between the ischaemic frontoparietal cortex and the contralateral region was not significant within 1.5 h of occlusion ( F=1.06, P >0.05). Calcium overload occurred prior to an abrupt reduction in ADC in the peripheral ischaemic area. The relative ADC (rADC) and FI (rFI) were negatively correlated in the frontoparietal cortex ( r=-0.9, P <0.001), but not in the caudoputamen ( r=-0.21, P >0.05). Our findings suggest that ADC of the perifocal ischaemic area might reflect the changes in intracellular calcium which occur in early ischaemia. They may also suggest that, once the calcium level is high enough and infarction ensues, changes in ADC may not induce a further rise in calcium concentration.

Microsurgical intraluminal middle cerebral artery occlusion model in rodents

OBJECTIVES

Focal brain ischemia induced in rodents by occlusion of the middle cerebral artery (MCA) is a widely used paradigm of human brain infarct. The objective of this study is to compare the effectiveness and reproducibility of MCA filament occlusion model in rats and mice.

MATERIALS AND METHODS

A total of 140 rodents (69 rats and 71 mice) were operated. Ninety-five animals were subjected to MCA occlusion; the surgical procedure consisted of introducing an uncoated surgical nylon monofilament into the cervical common carotid artery (CCA) and advancing it intracranially to permanently block blood flow into the right MCA. Forty-five sham-occluded rodents underwent CCA ligation. Surgical success, autopsy confirmed success and mortality rate were evaluated. Effective MCA occlusion was confirmed by the evidence of motor neurological deficit, by histopathology, immunohistochemistry (IHC) and reverse transcriptase-polymerase chain reaction (RT-PCR). IHC was performed in a randomly selected number of animals to detect the protein product of monocyte chemoattractant protein-1. The brain tissue in mice was examined by RT-PCR for the expression of macrophage inflammatory protein-1 alpha mRNA.

RESULTS

Surgical success rate was 89% in the rats, significantly lower than that in the mice (100%, P < 0.05). Autopsy confirmed success rate in the rats, 60%, was also significantly different from that in the mice (92.5%, P < 0.001). The operative mortality rate was 4.3% in the rats and 15% in the mice.

CONCLUSION

The present study demonstrates that the microsurgical filament occlusion of the MCA can be more successfully performed in mice. The lower rate of success in rats seems to be as a result of the architecture of the carotid canal in this animal. No previous reports, using a considerable number of animals, have compared the feasibility of intraluminal model in the rat with that in the mouse.

Late secondary ischemic injury in patients receiving intraarterial thrombolysis

Although animal models have demonstrated that late secondary cerebral injury after arterial occlusion and subsequent recanalization may limit the benefit of reperfusion therapy, this phenomenon has not been well characterized in humans. Diffusion-perfusion magnetic resonance imaging studies were performed before treatment, early after treatment, and at day 7 in patients undergoing vessel recanalization with intraarterial thrombolytics. Among 18 patients studied, mean age was 71 (range, 27-94), and median entry National Institutes of Health Stroke Scale score was 13 (range, 6-25). Early after recanalization, partial or complete normalization of diffusion imaging abnormalities occurred in 8 of 18 (44%) patients. Among the eight patients with early diffusion imaging reversal, late secondary injury by day 7 occurred in 5 (63%), and sustained normalization of all reversed tissue occurred in 3 (38%). Pretreatment apparent diffusion coefficient values were lowest in regions experiencing no reversal (mean apparent diffusion coefficient, 608 microm(2)/sec), intermediate in regions with reversal and secondary decline (617 microm(2)/sec), and highest in regions with sustained reversal (663 microm(2)/sec). There was a trend toward less improvement in neurological deficit in patients with secondary injury versus patients with sustained reversal. In the future, late secondary tissue injury may become an important therapeutic target for postreperfusion neuroprotective therapies, with treatment efficacy monitored by serial diffusion magnetic resonance imaging.

Delayed onset of brain edema and mislocalization of aquaporin-4 in dystrophin-null transgenic mice

Cerebral water accumulation was studied during induction of brain edema in dystrophin-null transgenic mice (mdx-betageo) and control mice. Immunofluorescence and immunoelectron microscopic analyses of dystrophin-null brains revealed a dramatic reduction of AQP4 (aquaporin-4) in astroglial end-feet surrounding capillaries (blood-brain barrier) and at the glia limitans (cerebrospinal fluid-brain interface). The AQP4 protein is mislocalized, because immunoblotting showed that the total AQP4 protein abundance was unaltered. Brain edema was induced by i.p. injection of distilled water and 8-deamino-arginine vasopressin. Changes in cerebral water compartments were assessed by diffusion-weighted MRI with determination of the apparent diffusion coefficient (ADC). In dystrophin-null mice and control mice, ADC gradually decreased by 5-6% from baseline levels during the first 35 min, indicating the initial phase of intracellular water accumulation is similar in the two groups. At this point, the control mice sustained an abrupt, rapid decline in ADC to 58% +/- 2.2% of the baseline at 52.5 min, and all of the animals were dead by 56 min. After a consistent delay, the dystrophin-null mice sustained a similar decline in ADC to 55% +/- 3.4% at 66.5 min, when all of the mice were dead. These results demonstrate that dystrophin is necessary for polarized distribution of AQP4 protein in brain where facilitated movements of water occur across the blood-brain barrier and cerebrospinal fluid-brain interface. Moreover, these results predict that interference with the subcellular localization of AQP4 may have therapeutic potential for delaying the onset of impending brain edema.

Transgenic mice expressing human copper-zinc superoxide dismutase exhibit attenuated apparent diffusion coefficient reduction during reperfusion following focal cerebral ischemia

Since ADC reduction reflects intracellular edema which is an early indicator of ischemic cellular metabolic stress, we hypothesized that a decrease in ADC as determined by diffusion weighted MR imaging could be attenuated by SOD expression in transgenic mice during reperfusion following focal cerebral ischemia. Diffusion weighted imaging (DWI) was performed to evaluate apparent diffusion coefficient (ADC) reduction by constructing ADC maps with a color scale to localize ADC change in transgenic (Tg) mice expressing human CuZn superoxide dismutase (SOD) and wild type (Wt) mice during 1 h middle cerebral artery occlusion (MCAO) and 1 h reperfusion. Heat shock protein (hsp) 70-kDa mRNA analysis was evaluated as a marker of sublethal cell stress by in situ hybridization after 4 h reperfusion for comparison with Nissl staining of adjacent sections to assess infarction. Sequential ADC maps were prepared in Tg mice with sufficient temporal and spatial resolution to permit comparison with Wt mice. Tg mice showed substantial recovery of the ADC lesion after reperfusion, while Wt mice showed no recovery. There was no difference between Tg and Wt mice in the size or distribution of the ADC lesion during ischemia. The area with strong expression of hsp70 mRNA in the ischemic hemisphere was substantially larger in the Tg mice. Nissl staining showed less damage of brain tissue in Tg mice than Wt mice especially in the cortex after 4 h reperfusion following 1 h MCAO. Results demonstrate that antioxidant effects of human CuZn-SOD reduce cellular edema due to oxidative stress during reperfusion but not during ischemia after 1 h MCAO. Hsp70 could be one of the proteins that mediates protection by SOD against oxidative stress.

Correlation between changes in apparent diffusion coefficient and induction of heat shock protein, cell-specific injury marker expression, and protein synthesis reduction on diffusion-weighted magnetic resonance images after temporary focal cerebral ischemia in rats

OBJECT

The authors investigated the relationship between the time course of apparent diffusion coefficient (ADC) changes and stress protein induction, ischemic neuroglial damage, and cerebral protein synthesis (CPS) after temporary focal cerebral ischemia in rats.

METHODS

In Group I, ADC changes were measured on magnetic resonance (MR) images obtained during the second half of a 1-hour middle cerebral artery (MCA) occlusion, during a 1-hour reperfusion, and after 23 hours of reperfusion in rats. Immunohistochemical studies for heat shock protein (hsp) 70, glial fibrillary acidic protein (GFAP), and neuronal nuclear (NeuN) protein were performed. In Group II, CPS was assessed using autoradiographic studies obtained after occlusion. At 36 minutes of occlusion, MR imaging demonstrated significantly less ADC reduction in the frontoparietal cortex (82 +/- 9% of the contralateral hemisphere) than in the striatum (64 +/- 11%; p < 0.05). After 1 hour of reperfusion, the lesion resolved and the difference between cortex and striatum was no longer evident. After 23 hours of reperfusion, the ADC lesion recurred in striatum (76 +/- 12%) compared with frontoparietal cortex (100 +/- 11%; p < 0.05). Immunohistochemical studies showed hsp 70 expression and an increased GFAP reactivity localized in the frontoparietal cortex of the ischemic hemisphere, along with a significant drop in striatal NeuN immunoreactivity. A trend toward greater reduction in striatal CPS (53 +/- 15%) than in frontoparietal cortex CPS (78 +/- 23%) was also observed.

CONCLUSIONS

Sequential ADC maps correlate with the expression of neuroglial stress and injury markers after temporary focal ischemia in rats, distinguishing the striatum (infarct core) from the cortex (ischemic penumbra). A greater reduction in striatal CPS further supports the conclusion that the striatum is more susceptible to temporary MCA occlusion than the cortex.

Prediction of hemorrhagic transformation after thrombolytic therapy of clot embolism: an MRI investigation in rat brain

BACKGROUND AND PURPOSE

Thrombolytic treatment of stroke carries the risk of hemorrhagic transformation. Therefore, the potential of MRI for prediction of recombinant tissue plasminogen activator (rtPA)-induced bleeding is explored to identify patients in whom rtPA treatment may provoke such complications.

METHODS

Spontaneously hypertensive rats (SHR) (n=9) were submitted to middle cerebral artery (MCA) clot embolism, followed 3 hours later by intra-arterial infusion of 10 mg/kg rtPA. Untreated SHR (n=9) were infused with saline. MRI imaging was performed before treatment and included apparent diffusion coefficient (ADC), T2, and perfusion mapping and contrast enhancement with gadolinium-DTPA. The distribution of intracerebral hemorrhages was studied 3 days later by histological staining.

RESULTS

Clot embolism led to the rapid decline of ADC in the territory of the occluded artery. Tissue lesion volume derived from ADC imaging increased by 155+/-69% in the untreated animals and by 168+/-87% in the treated animals (P=NS), determined on the histological sections after 3 days. This same lesion growth in both groups indicated absence of therapeutic effect after 3-hour treatment delay. Hemorrhagic transformations were significantly more frequent in treated SHR (P<0.05). In untreated rats, hemorrhages were found in the border zone of the ischemic territory; in treated animals, hemorrhagic transformations occurred in the ischemic core region. rtPA-induced hemorrhages were predicted by a disturbance of the blood-brain barrier in 3 of 4 animals before treatment by Gd-DTPA contrast enhancement but not by ADC, T2, or perfusion imaging. The region of contrast enhancement colocalized with subsequent bleeding in these animals.

CONCLUSIONS

The disturbance of blood-brain barrier but not of other MR parameters allows risk assessment for hemorrhagic transformation induced by subsequent thrombolytic treatment.

Dynamic changes of ADC, perfusion, and NMR relaxation parameters in transient focal ischemia of rat brain

The potential of multiparametric MRI parameters for differentiating between reversibly and irreversibly damaged brain tissue was investigated in an experimental model of focal brain ischemia in the rat. The middle cerebral artery (MCA) was occluded by intraluminal suture insertion for 60 or 90 min, followed by 4.5 h of reperfusion. The apparent diffusion coefficient (ADC) of brain water, T(1) and T(2) relaxation times, and CBF(i), an MR-derived index of cerebral perfusion, were repeatedly measured and correlated with the outcome from the ischemic impact. A novel user-independent approach for segmentation of ADC maps into classes of increasing injury was introduced to define regions of interest (ROIs) in which these parameters were evaluated. MCA occlusion led to a graded decline of ADC, which corresponded with both the severity of flow reduction and an increase in T(1) and T(2) relaxation times. Removal of the suture led to a triphasic restitution of blood flow consisting of a fast initial rise, a secondary decline, and final normalization. Postischemic reperfusion led to a rise of ADC irrespective of the duration of ischemia. However, the quality of recovery declined with increasing severity of the ischemic impact. Throughout the observation time, T(1) and T(2) showed a continuous increase, the intensity of which correlated with the severity of ADC decline during ischemia. Particularly with longer ischemia time, elevated T(2) in combination with reduced ADC yielded a lower probability of recovery during recirculation, while intraischemic perfusion information contributed less to the prediction of outcome. In conclusion, the combination of MR parameters at the end of ischemia correlated with the probability of tissue recovery but did not permit reliable differentiation between reversibly and irreversibly damaged tissue.

High b-value q-space analyzed diffusion-weighted MRI: application to multiple sclerosis

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system (CNS) which affects nearly one million people worldwide, leading to a progressive decline of motor and sensory functions, and permanent disability. High b-value diffusion-weighted MR images (b of up to 14000 s/mm(2)) were acquired from the brains of controls and MS patients. These diffusion MR images, in which signal decay is not monoexponential, were analyzed using the q-space approach that emphasizes the diffusion characteristics of the slow-diffusing component. From this analysis, displacement and probability maps were constructed. The computed q-space analyzed MR images that were compared with conventional T(1), T(2) (fluid attenuated inversion recovery (FLAIR)), and diffusion tensor imaging (DTI) images were found to be sensitive to the pathophysiological state of white matter. The indices used to construct this q-space analyzed MR maps, provided a pronounced differentiation between normal tissue and tissues classified as MS plaques by the FLAIR images. More importantly, a pronounced differentiation was also observed between tissues classified by the FLAIR MR images as normal-appearing white matter (NAWM) in the MS brains, which are known to be abnormal, and the respective control tissues. The potential diagnostic capacity of high b-value diffusion q-space analyzed MR images is discussed, and experimental data that explains the consequences of using the q-space approach once the short pulse gradient approximation is violated are presented.

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.

Complementary emerging techniques: high-resolution PET and MRI

Noninvasive imaging technologies provide a unique window on the anatomy, physiology and function of living organisms. Imaging systems and methods have been developed for the study of small animal model systems that offer exciting new possibilities in neuroscience. Advances in magnetic resonance microscopy and positron emission tomography, and their applications in brain imaging, have provided many benefits to neurobiology, ranging from detailed in vivo neuroanatomy to the measurement of specific molecular targets.

Early development of vasogenic edema in experimental cerebral fat embolism in cats: correlation with MRI and electron microscopic findings

RATIONALE AND OBJECTIVES

To evaluate the magnetic resonance imaging and electron microscopic findings of the hyperacute stage of cerebral fat embolism in cats and the time needed for the development of vasogenic edema.

METHODS

Magnetic resonance imaging was performed at 30 minutes (group 1, n = 9) and at 30 minutes and 1, 2, 4, and 6 hours after embolization with triolein (group 2, n = 10). As a control for group 2, the same acquisition was obtained after embolization with polyvinyl alcohol particles (group 3, n = 5). Magnetic resonance images were analyzed qualitatively and quantitatively. Electron microscopic examination was done in all cats.

RESULTS

In group 1, the lesions were iso- or slightly hyperintense on T2-weighted (T2W) and diffusion-weighted (DWIs) images, hypointense on the apparent diffusion coefficient (ADC) map image, and markedly enhanced on the gadolinium-enhanced T1-weighted images (Gd-T1WIs). In group 2 at 30 minutes, the lesions were similar to those in group 1. Thereafter, the lesions became more hyperintense on T2WIs and DWIs and more hypointense on the ADC map image. The lesions were enhanced on Gd-T1WIs at all acquisition times. In group 3, the lesions showed mild hyperintensity on T2WIs at 6 hours but hypointensity on the ADC map image from 30 minutes, with a tendency toward a greater decrease over time. The lesions were not enhanced on Gd-T1WIs at any time point. Electron microscopic findings revealed discontinuity of the capillary endothelial wall, perivascular and interstitial edema, and swelling of glial and neuronal cells in groups 1 and 2. Cellular swelling and interstitial edema were more prominent in group 2. In group 3, interstitial edema was seen; however, discontinuity of the endothelial wall was absent.

CONCLUSIONS

The lesions were hyperintense on T2WIs and DWIs, hypointense on the ADC map image, and enhanced on Gd-T1WIs. On electron microscopy, the lesions showed cytotoxic and vasogenic edema with disruption of the blood-brain barrier. Vasogenic edema seems to develop within 30 minutes in cerebral fat embolism in cats.

The ischaemic penumbra

The concept of an ischaemic penumbra, surrounding a focal cerebral lesion, is now widely accepted, although no universal definition of the 'penumbra' exists. In the present review, we consider the penumbra as that volume of brain tissue at the periphery of a focal, irreversibly damaged area that is threatened by recruitment into necrosis. Implicit to such a definition are several secondary concepts. First, the penumbra is both spatial, in that it surrounds the densely ischaemic core, but it is also temporal, in that its evolution toward infarction is a relatively progressive phenomenon. The pertinent literature is summarized. Second, penumbral tissue is potentially salvageable; the most recent animal studies are reviewed. Third, because electrically silent and pathologically damaged tissues have identical functional characteristics, it is evident that most clinical rating scales, be they neurological, behavioural, or psychological, are poorly adapted to address the problem of the penumbra. Finally, the penumbral tissue is remarkably and intensively 'active': multiple processes of cell death and repair occur and involve molecular mechanisms, electrophysiology and the vasculature.

Magnetic resonance prediction of outcome after thrombolytic treatment

Treatment of clinical stroke with recombinant tissue plasminogen activator (rt-PA) carries the risk of hemorrhagic complications. Hence, predictors of therapeutic outcome with respect to (a) reperfusion and (b) tissue recovery would be very useful to identify potentially salvageable brain tissue. Magnetic resonance (MR) parameters, especially the apparent diffusion coefficient of water (ADC), perfusion-weighted imaging (PWI) and T(2) relaxometry are thought to provide this information. We evaluated the prognostic implications of ADC, PWI and T(2) relaxometry immediately before initiation of thrombolytic treatment in a model of clot embolism in rats. Animals (n = 14) were treated with intraarterial rt-PA (10 mg/kg) at 90 min after embolism. MR imaging was repeatedly performed at 4.7 T before and up to 5.5 h after embolism. ADC was calculated from diffusion-weighted images (b-values: 30, 765, 1500 s/mm(2)), arterial spin tagging was used for PWI, and quantitative T(2) relaxometry was performed with a Carr-Purcell-Meiboom-Gill (CPMG) sequence. A reperfusion index was calculated to assess the quality of thrombolytic recanalization. The decline of ADC at the end of the experiment to below 80% of control was defined as unfavorable outcome. The probability of tissue injury at the end of the experiments increased with the severity of ADC changes before the initiation of treatment (probability of unfavorable outcome: 21%, 44%, 65% for ADC values of 80-90%, 70-80% and <70% of control, respectively). Pretreatment PWI or T(2) relaxometry also correlated with outcome but-alone or in combination with pretreatment ADC maps-did not improve injury prediction over that obtained by ADC alone. Outcome was influenced positively by successful reperfusion the quality of which, however, could not be predicted by pre-treatment MR characteristics. The data demonstrate that ADC mapping performed before the initiation of thrombolytic treatment provides reliable risk assessment of impeding brain injury but due to uncertainties of postischemic reperfusion does not allow precise outcome prediction in individual experiments.

Arterial spin tagging perfusion imaging of rat brain: dependency on magnetic field strength

Perfusion-weighted imaging (PWI), using the method of arterial spin tagging, is strongly T(1)-dependent. This translates into a high field dependency of the perfusion signal intensity. In order to determine the expected signal improvement at higher magnetic fields we compared perfusion-weighted images in rat brain at 4.7 T and 7 T. Application of PWI to focal ischemia and functional activation of the brain and the use of two different anesthetics allowed the observation of a wide range of flow values. For all these (patho-)physiological conditions switching from 4.7 T to 7 T resulted in a significant increase of mean perfusion signal intensity by a factor of 2.96. The ratio of signal intensities of homotopic regions in the ipsi- and contralateral hemisphere was field-independent. The relative contribution of a) T(1) relaxation time, b) net magnetization, c) the Q-value of the receiver coils and d) the degree of adiabatic inversion to the signal improvement at higher field strength were discussed. It was shown that the main parameters contributing to the higher signal intensity are the lengthening of T(1) and the higher magnetization at the higher magnetic field.

CO(2) reactivity measured by perfusion MRI during transient focal cerebral ischemia in rats

BACKGROUND AND PURPOSE

CO(2) response was examined in rats undergoing 60 minutes of middle cerebral artery occlusion (MCAO) and 4.5 hours of reperfusion. Because it is not clear whether the vasoreactivity improves during reperfusion in parallel with tissue recovery, CO(2) response was determined spatially resolved, sequentially in the initially ischemic but later recovered areas and in the permanently damaged areas.

METHODS

Apparent diffusion coefficient (ADC) maps were calculated from diffusion-weighted images, whereas CO(2) reactivity maps were determined from the difference in perfusion signal intensity before and after CO(2) stimulation. CO(2) reactivity (administration of 6% CO(2) for 5 minutes) was expressed in % change of perfusion signal intensity/mm Hg of PCO(2) increase. ATP levels of tissue were used as a measure of outcome. The recovered and permanently damaged tissues were differentiated by combined use of end-ischemic ADC map and ATP image at the end of the experiment.

RESULTS

The preischemic (control) CO(2) reactivity of 3.5+/-0.9%/mm Hg decreased dramatically during MCAO in the ischemic hemisphere. During reperfusion, it remained <1%/mm Hg in the region with end-ischemic ADC <80% of the preischemic control value, but showed gradual recovery in the region with end-ischemic ADC >80% of control. Although at the end of the experiment the CO(2) reactivity was significantly higher in the recovered tissue than in the permanently damaged tissue (1.15+/-0.44 and 0.13+/-0.47%/mm Hg, respectively; P:<0.01), it still remained far below the normal control value (P:<0.01).

CONCLUSIONS

The noninvasive perfusion-weighted MR imaging in combination with a CO(2) challenge permits the investigation of the spatially resolved vascular reactivity during a longitudinal study of cerebral ischemia. Our data suggest that severe ischemia is followed by a prolonged disturbance of CO(2) reactivity, despite already normalized energy metabolism.

Current awareness

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of NMR in biomedicine. Each bibliography is divided into 9 sections: 1 Books, Reviews ' Symposia; 2 General; 3 Technology; 4 Brain and Nerves; 5 Neuropathology; 6 Cancer; 7 Cardiac, Vascular and Respiratory Systems; 8 Liver, Kidney and Other Organs; 9 Muscle and Orthopaedic. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted.

Functional differentiation of multiple perilesional zones after focal cerebral ischemia

Transient and permanent focal cerebral ischemia results in a series of typical pathophysiologic events. These consequences evolve in time and space and are not limited to the lesion itself, but they can be observed in perilesional (penumbra) and widespread ipsi- and sometimes contralateral remote areas (diaschisis). The extent of these areas is variable depending on factors such as the type of ischemia, the model, and the functional modality investigated. This review describes some typical alterations attributable to focal cerebral ischemia using the following classification scheme to separate different lesioned and perilesional areas: (1) The lesion core is the brain area with irreversible ischemic damage. (2) The penumbra is a brain region that suffers from ischemia, but in which the ischemic damage is potentially, or at least partially, reversible. (3) Remote brain areas are brain areas that are not directly affected by ischemia. With respect to the etiology, several broad categories of remote changes may be differentiated: (3a) remote changes caused by brain edema; (3b) remote changes caused by waves of spreading depression; (3c) remote changes in projection areas; and (3d) remote changes because of reactive plasticity and systemic effects. The various perilesional areas are not necessarily homogeneous; but a broad differentiation of separate topographic perilesional areas according to their functional state and sequelae allows segregation into several signaling cascades, and may help to understand the functional consequences and adaptive processes after focal brain ischemia.

Evolution of brain infarction after transient focal cerebral ischemia in mice

The evolution of brain infarction after transient focal cerebral ischemia was studied in mice using multiparametric imaging techniques. One-hour focal cerebral ischemia was induced by occluding the middle cerebral artery using the intraluminal filament technique. Cerebral protein synthesis (CPS) and the regional tissue content of adenosine triphosphate (ATP) were measured after recirculation times from 0 hours to 3 days. The observed changes were correlated with the expression of the mRNAs of hsp-70, c-fos, and junB, as well as the distribution of DNA double-strand breaks, visualized by TUNEL. At the end of 1 hour of ischemia, protein synthesis was suppressed in a larger tissue volume than ATP in accordance with the biochemical differentiation between core and penumbra. Hsp70 mRNA was selectively expressed in the cortical penumbra, whereas c-fos and junB mRNAs were increased both in the lateral part of the penumbra and in the ipsilateral cingulate cortex with normal metabolism. During reperfusion after withdrawal of the intraluminal filament, suppression of CPS persisted except in the most peripheral parts of the middle cerebral artery territory, in which it recovered between 6 hours and 3 days. ATP, in contrast, returned to normal levels within 1 hour but secondarily deteriorated from 3 hours on until, between 1 and 3 days, the ATP-depleted area merged with that of suppressed protein synthesis leading to delayed brain infarction. Hsp70 mRNA, but not c-fos and junB, was strongly expressed during reperfusion, peaking at 3 hours after reperfusion. TUNEL-positive cells were detected from 3 hours on, mainly in areas with secondary ATP depletion. These results stress the importance of an early recovery of CPS for the prevention of ischemic injury and suggest that TUNEL is an unspecific response of delayed brain infarction.