MRI tissue characterization of experimental cerebral ischemia in rat

In silico modeling of electric field modulation by transcranial direct current stimulation in stroke patients with skull burr holes: Implications for safe clinical application

BACKGROUND

Transcranial direct current stimulation (tDCS) has emerged as a promising tool for stroke rehabilitation, supported by evidence demonstrating its beneficial effects on post-stroke recovery. However, patients with skull defects, such as burr holes, have been excluded from tDCS due to limited knowledge regarding the effect of skull defects on the electric field.

OBJECTIVE

We investigated the effect of burr holes on the electric field induced by tDCS and identified the electrode location that modulates the electric field.

METHODS

We generated mesh models of the heads of five patients with burr holes and five age-matched control patients who had never undergone brain surgery, based on magnetic resonance imaging. Then we conducted tDCS simulations, with the cathode fixed in one position and the anode in various positions. Regression analysis was employed to investigate the relationship between the electric field at the burr hole and the distance from the burr hole to the anode.

RESULTS

In patients with burr holes, the electric field intensity increased as the anode approached the burr hole, reaching a maximum electric field when the anode covered it, with this pattern remaining consistent across all patient models. Assuming the holes were filled with cerebrospinal fluid, the maximum electric field was 1.20 ± 0.20 V/m (mean ± standard deviation, SD). When the anode was positioned more than 60 mm away from the burr hole, the electric field at the burr hole remained low and constant, with an average value of 0.29 ± 0.04V/m (mean ± SD). In contrast, for all patients without burr holes, the electric field intensity stayed constant regardless of the anode's position, with a maximum amplitude of 0.36 ± 0.04 V/m (mean ± SD). Furthermore, when the burr hole was assumed to be filled with scar tissue, the mean peak electric field was 0.93 ± 0.16 V/m, indicating that the electric field strength varies depending on the conductivity of the tissue filling the burr hole.

CONCLUSION

Based on the simulations, the minimum recommended distance from the burr hole to the anode is 60 mm to prevent unintended stimulation of the brain cortex during tDCS. These findings will contribute to the development of safe and effective tDCS treatments for patients with burr holes.

Optimized high-definition tDCS in patients with skull defects and skull plates

Introduction

Transcranial direct current stimulation (tDCS) has been shown to benefit patients with brain lesions or traumatic brain injury (TBI). These patients usually have skull defects with different sizes and electrical conductivities. There is very little data in the literature that show how to optimally stimulate these patients with the presence of skull defects.

Methods

Here we leveraged high-resolution (1 mm) realistic head models to explore the best montages targeting right beneath the skull defects with different sizes and conductivities. Specifically, open-source software ROAST was used to solve for the lead field on the publicly available MIDA model. Four different skull defects/plates were modeled with the center above the right primary motor cortex: a larger defect (10 cm diameter) modeled as either titanium or acrylic plate, and a smaller defect (2.5 cm diameter) modeled as either acute state filled with cerebrospinal fluid (CSF) or chronic state with scar tissue. Optimized stimulation with maximal intensity was run using ROAST targeting the right primary motor cortex.

Results

We show that optimized high-definition montages can achieve an average of 0.3 V/m higher stimulation intensities at the target compared to un-optimized montages (M1-SO or 4×1). Large skull defects with titanium or acrylic plates significantly reduce the stimulation intensity by about 80%, while small defects with acute (CSF) or chronic (scar) tissues significantly increase the stimulation intensity by about 200%. Furthermore, one can use M1-SO to achieve almost the same stimulation strength as the optimized montage if the skull has a large defect with titanium plate, and there is no significant difference in stimulation intensity between 4×1 montage and the optimized montage for small skull defects with scar tissue.

Discussion

Based on this work, future modeling studies leveraging individual anatomy of skull defects may help guide tDCS practice on patients with skull defects and skull plates.

Individual electric field predicts functional connectivity changes after anodal transcranial direct-current stimulation in chronic stroke

The neuromodulation effect of anodal tDCS is not thoroughly studied, and the heterogeneous profile of stroke individuals with brain lesions would further complicate the stimulation outcomes. This study aimed to investigate the functional changes in sensorimotor areas induced by anodal tDCS and whether individual electric field could predict the functional outcomes. Twenty-five chronic stroke survivors were recruited and divided into tDCS group (n = 12) and sham group (n = 13). Increased functional connectivity (FC) within the surrounding areas of ipsilesional primary motor cortex (M1) was only observed after anodal tDCS. Averaged FC among the ipsilesional sensorimotor regions was observed to be increased after anodal tDCS (t(11) = 2.57, p = 0.026), but not after sham tDCS (t(12) = 0.69, p = 0.50). Partial least square analysis identified positive correlations between electric field (EF) strength normal to the ipsilesional M1 surface and individual FC changes in tDCS group (r = 0.84, p < 0.001) but not in sham group (r = 0.21, p = 0.5). Our results indicated anodal tDCS facilitates the FC within the ipsilesional sensorimotor network in chronic stroke subjects, and individual electric field predicts the functional outcomes.

A Method to Experimentally Estimate the Conductivity of Chronic Stroke Lesions: A Tool to Individualize Transcranial Electric Stimulation

The inconsistent response to transcranial electric stimulation in the stroke population is attributed to, among other factors, unknown effects of stroke lesion conductivity on stimulation strength at the targeted brain areas. Volume conduction models are promising tools to determine optimal stimulation settings. However, stroke lesion conductivity is often not considered in these models as a source of inter-subject variability. The goal of this study is to propose a method that combines MRI, EEG, and transcranial stimulation to estimate the conductivity of cortical stroke lesions experimentally. In this simulation study, lesion conductivity was estimated from scalp potentials during transcranial electric stimulation in 12 chronic stroke patients. To do so, first, we determined the stimulation configuration where scalp potentials are maximally affected by the lesion. Then, we calculated scalp potentials in a model with a fixed lesion conductivity and a model with a randomly assigned conductivity. To estimate the lesion conductivity, we minimized the error between the two models by varying the conductivity in the second model. Finally, to reflect realistic experimental conditions, we test the effect rotation of measurement electrode orientation and the effect of the number of electrodes used. We found that the algorithm converged to the correct lesion conductivity value when noise on the electrode positions was absent for all lesions. Conductivity estimation error was below 5% with realistic electrode coregistration errors of 0.1° for lesions larger than 50 ml. Higher lesion conductivities and lesion volumes were associated with smaller estimation errors. In conclusion, this method can experimentally estimate stroke lesion conductivity, improving the accuracy of volume conductor models of stroke patients and potentially leading to more effective transcranial electric stimulation configurations for this population.

Automatic brain extraction and hemisphere segmentation in rat brain MR images after stroke using deformable models

PURPOSE

Experimental ischemic stroke models play an essential role in understanding the mechanisms of cerebral ischemia and evaluating the development of pathological extent. An important precursor to the investigation of ischemic strokes associated with rodents is the brain extraction and hemisphere segmentation in rat brain diffusion-weighted imaging (DWI) and T2-weighted MRI (T2WI) images. Accurate and reliable image segmentation tools for extracting the rat brain and hemispheres in the MR images are critical in subsequent processes, such as lesion identification and injury analysis. This study is an attempt to investigate rat brain extraction and hemisphere segmentation algorithms that are practicable in both DWI and T2WI images.

METHODS

To automatically perform brain extraction, the proposed framework is based on an efficient geometric deformable model. By introducing an additional image force in response to the rat brain characteristics into the skull stripping model, we establish a unique rat brain extraction scheme in DWI and T2WI images. For the subsequent hemisphere segmentation, we develop an efficient brain feature detection algorithm to approximately separate the rat brain. A refinement process is enforced by constructing a gradient vector flow in the proximity of the midsurface, where a parametric active contour is attracted to achieve hemisphere segmentation.

RESULTS

Extensive experiments with 55 DWI and T2WI subjects were executed in comparison with the state-of-the-art methods. Experimental results indicated that our rat brain extraction and hemisphere segmentation schemes outperformed the competitive methods and exhibited high performance both qualitatively and quantitatively. For rat brain extraction, the average Dice scores were 97.13% and 97.42% in DWI and T2WI image volumes, respectively. Rat hemisphere segmentation results based on the Hausdorff distance metric revealed average values of 0.17 and 0.15 mm for DWI and T2WI subjects, respectively.

CONCLUSIONS

We believe that the established frameworks are advantageous to facilitate preclinical stroke investigation and relevant neuroscience research that requires accurate brain extraction and hemisphere segmentation using rat DWI and T2WI images.

Effect of Methionine Diet on Time-Related Metabolic and Histopathological Changes of Rat Hippocampus in the Model of Global Brain Ischemia

Hyperhomocysteinemia (hHcy) represents a strong risk factor for atherosclerosis-associated diseases, like stroke, dementia or Alzheimer's disease. A methionine (Met)-rich diet leads to an elevated level of homocysteine in plasma and might cause pathological alterations across the brain. The hippocampus is being constantly studied for its selective vulnerability linked with neurodegeneration. This study explores metabolic and histo-morphological changes in the rat hippocampus after global ischemia in the hHcy conditions using a combination of proton magnetic resonance spectroscopy and magnetic resonance-volumetry as well as immunohistochemical analysis. After 4 weeks of a Met-enriched diet at a dose of 2 g/kg of animal weight/day, adult male Wistar rats underwent 4-vessel occlusion lasting for 15 min, followed by a reperfusion period varying from 3 to 7 days. Histo-morphological analyses showed that the subsequent ischemia-reperfusion insult (IRI) aggravates the extent of the sole hHcy-induced degeneration of the hippocampal neurons. Decreased volume in the grey matter, extensive changes in the metabolic ratio, deeper alterations in the number and morphology of neurons, astrocytes and their processes were demonstrated in the hippocampus 7 days post-ischemia in the hHcy animals. Our results suggest that the combination of the two risk factors (hHcy and IRI) endorses and exacerbates the rat hippocampal neurodegenerative processes.

Automated segmentation of haematoma and perihaematomal oedema in MRI of acute spontaneous intracerebral haemorrhage

BACKGROUND

Spontaneous intracerebral haemorrhage (SICH) is a common condition with high morbidity and mortality. Segmentation of haematoma and perihaematoma oedema on medical images provides quantitative outcome measures for clinical trials and may provide important markers of prognosis in people with SICH.

METHODS

We take advantage of improved contrast seen on magnetic resonance (MR) images of patients with acute and early subacute SICH and introduce an automated algorithm for haematoma and oedema segmentation from these images. To our knowledge, there is no previously proposed segmentation technique for SICH that utilises MR images directly. The method is based on shape and intensity analysis for haematoma segmentation and voxel-wise dynamic thresholding of hyper-intensities for oedema segmentation.

RESULTS

Using Dice scores to measure segmentation overlaps between labellings yielded by the proposed algorithm and five different expert raters on 18 patients, we observe that our technique achieves overlap scores that are very similar to those obtained by pairwise expert rater comparison. A further comparison between the proposed method and a state-of-the-art Deep Learning segmentation on a separate set of 32 manually annotated subjects confirms the proposed method can achieve comparable results with very mild computational burden and in a completely training-free and unsupervised way.

CONCLUSION

Our technique can be a computationally light and effective way to automatically delineate haematoma and oedema extent directly from MR images. Thus, with increasing use of MR images clinically after intracerebral haemorrhage this technique has the potential to inform clinical practice in the future.

Automated Ischemic Lesion Segmentation in MRI Mouse Brain Data after Transient Middle Cerebral Artery Occlusion

Magnetic resonance imaging (MRI) has become increasingly important in ischemic stroke experiments in mice, especially because it enables longitudinal studies. Still, quantitative analysis of MRI data remains challenging mainly because segmentation of mouse brain lesions in MRI data heavily relies on time-consuming manual tracing and thresholding techniques. Therefore, in the present study, a fully automated approach was developed to analyze longitudinal MRI data for quantification of ischemic lesion volume progression in the mouse brain. We present a level-set-based lesion segmentation algorithm that is built using a minimal set of assumptions and requires only one MRI sequence (T2) as input. To validate our algorithm we used a heterogeneous data set consisting of 121 mouse brain scans of various age groups and time points after infarct induction and obtained using different MRI hardware and acquisition parameters. We evaluated the volumetric accuracy and regional overlap of ischemic lesions segmented by our automated method against the ground truth obtained in a semi-automated fashion that includes a highly time-consuming manual correction step. Our method shows good agreement with human observations and is accurate on heterogeneous data, whilst requiring much shorter average execution time. The algorithm developed here was compiled into a toolbox and made publically available, as well as all the data sets.

Motor Cortex Neurostimulation Technologies for Chronic Post-stroke Pain: Implications of Tissue Damage on Stimulation Currents

Background: Central post stroke pain (CPSP) is a highly refractory syndrome that can occur after stroke. Primary motor cortex (M1) brain stimulation using epidural brain stimulation (EBS), transcranial magnetic stimulation (TMS), and transcranial direct current stimulation (tDCS) have been explored as potential therapies for CPSP. These techniques have demonstrated variable clinical efficacy. It is hypothesized that changes in the stimulating currents that are caused by stroke-induced changes in brain tissue conductivity limit the efficacy of these techniques. Methods: We generated MRI-guided finite element models of the current density distributions in the human head and brain with and without chronic focal cortical infarctions during EBS, TMS, and tDCS. We studied the change in the stimulating current density distributions' magnitude, orientation, and maxima locations between the different models. Results: Changes in electrical properties at stroke boundaries altered the distribution of stimulation currents in magnitude, location, and orientation. Current density magnitude alterations were larger for the non-invasive techniques (i.e., tDCS and TMS) than for EBS. Nonetheless, the lesion also altered currents during EBS. The spatial shift of peak current density, relative to the size of the stimulation source, was largest for EBS. Conclusion: In order to maximize therapeutic efficiency, neurostimulation trials need to account for the impact of anatomically disrupted neural tissues on the location, orientation, and magnitude of exogenously applied currents. The relative current-neuronal structure should be considered when planning stimulation treatment, especially across techniques (e.g., using TMS to predict EBS response). We postulate that the effects of altered tissue properties in stroke regions may impact stimulation induced analgesic effects and/or lead to highly variable outcomes during brain stimulation treatments in CPSP.

Magnetic resonance imaging-based cerebral tissue classification reveals distinct spatiotemporal patterns of changes after stroke in non-human primates

Background

Spatial and temporal changes in brain tissue after acute ischemic stroke are still poorly understood. Aims of this study were three-fold: (1) to determine unique temporal magnetic resonance imaging (MRI) patterns at the acute, subacute and chronic stages after stroke in macaques by combining quantitative T₂ and diffusion MRI indices into MRI ‘tissue signatures’, (2) to evaluate temporal differences in these signatures between transient (n = 2) and permanent (n = 2) middle cerebral artery occlusion, and (3) to correlate histopathology findings in the chronic stroke period to the acute and subacute MRI derived tissue signatures.

Results

An improved iterative self-organizing data analysis algorithm was used to combine T₂, apparent diffusion coefficient (ADC), and fractional anisotropy (FA) maps across seven successive timepoints (1, 2, 3, 24, 72, 144, 240 h) which revealed five temporal MRI signatures, that were different from the normal tissue pattern (P < 0.001). The distribution of signatures between brains with permanent and transient occlusions varied significantly between groups (P < 0.001). Qualitative comparisons with histopathology revealed that these signatures represented regions with different histopathology. Two signatures identified areas of progressive injury marked by severe necrosis and the presence of gitter cells. Another signature identified less severe but pronounced neuronal and axonal degeneration, while the other signatures depicted tissue remodeling with vascular proliferation and astrogliosis.

Conclusion

These exploratory results demonstrate the potential of temporally and spatially combined voxel-based methods to generate tissue signatures that may correlate with distinct histopathological features. The identification of distinct ischemic MRI signatures associated with specific tissue fates may further aid in assessing and monitoring the efficacy of novel pharmaceutical treatments for stroke in a pre-clinical and clinical setting.

EEG patterns from acute to chronic stroke phases in focal cerebral ischemic rats: correlations with functional recovery

Monitoring the neural activities from the ischemic penumbra provides critical information on neurological recovery after stroke. The purpose of this study is to evaluate the temporal alterations of neural activities using electroencephalography (EEG) from the acute phase to the chronic phase, and to compare EEG with the degree of post-stroke motor function recovery in a rat model of focal ischemic stroke. Male Sprague-Dawley rats were subjected to 90 min transient middle cerebral artery occlusion surgery followed by reperfusion for seven days (n = 58). The EEG signals were recorded at the pre-stroke phase (0 h), acute phase (3, 6 h), subacute phase (12, 24, 48, 72 h) and chronic phase (96, 120, 144, 168 h) (n = 8). This study analyzed post-stroke seizures and polymorphic delta activities (PDAs) and calculated quantitative EEG parameters such as the alpha-to-delta ratio (ADR). The ADR represented the ratio between alpha power and delta power, which indicated how fast the EEG activities were. Forelimb and hindlimb motor functions were measured by De Ryck's test and the beam walking test, respectively. In the acute phase, delta power increased fourfold with the occurrence of PDAs, and the histological staining showed that the infarct was limited to the striatum and secondary sensory cortex. In the subacute phase, the alpha power reduced to 50% of the baseline, and the infarct progressed to the forelimb cortical region. ADRs reduced from 0.23 ± 0.09 to 0.04 ± 0.01 at 3 h in the acute phase and gradually recovered to 0.22 ± 0.08 at 168 h in the chronic phase. In the comparison of correlations between the EEG parameters and the limb motor function from the acute phase to the chronic phase, ADRs were found to have the highest correlation coefficients with the beam walking test (r = 0.9524, p < 0.05) and De Ryck's test (r = 0.8077, p < 0.05). This study measured EEG activities after focal cerebral ischemia and showed that functional recovery was closely correlated with the neural activities in the penumbra. Longitudinal EEG monitoring at different phases after a stroke can provide information on the neural activities, which are well correlated with the motor function recovery.

Medical image analysis methods in MR/CT-imaged acute-subacute ischemic stroke lesion: Segmentation, prediction and insights into dynamic evolution simulation models. A critical appraisal

Over the last 15 years, basic thresholding techniques in combination with standard statistical correlation-based data analysis tools have been widely used to investigate different aspects of evolution of acute or subacute to late stage ischemic stroke in both human and animal data. Yet, a wave of biology-dependent and imaging-dependent issues is still untackled pointing towards the key question: "how does an ischemic stroke evolve?" Paving the way for potential answers to this question, both magnetic resonance (MRI) and CT (computed tomography) images have been used to visualize the lesion extent, either with or without spatial distinction between dead and salvageable tissue. Combining diffusion and perfusion imaging modalities may provide the possibility of predicting further tissue recovery or eventual necrosis. Going beyond these basic thresholding techniques, in this critical appraisal, we explore different semi-automatic or fully automatic 2D/3D medical image analysis methods and mathematical models applied to human, animal (rats/rodents) and/or synthetic ischemic stroke to tackle one of the following three problems: (1) segmentation of infarcted and/or salvageable (also called penumbral) tissue, (2) prediction of final ischemic tissue fate (death or recovery) and (3) dynamic simulation of the lesion core and/or penumbra evolution. To highlight the key features in the reviewed segmentation and prediction methods, we propose a common categorization pattern. We also emphasize some key aspects of the methods such as the imaging modalities required to build and test the presented approach, the number of patients/animals or synthetic samples, the use of external user interaction and the methods of assessment (clinical or imaging-based). Furthermore, we investigate how any key difficulties, posed by the evolution of stroke such as swelling or reperfusion, were detected (or not) by each method. In the absence of any imaging-based macroscopic dynamic model applied to ischemic stroke, we have insights into relevant microscopic dynamic models simulating the evolution of brain ischemia in the hope to further promising and challenging 4D imaging-based dynamic models. By depicting the major pitfalls and the advanced aspects of the different reviewed methods, we present an overall critique of their performances and concluded our discussion by suggesting some recommendations for future research work focusing on one or more of the three addressed problems.

A comparison of VLSM and VBM in a cohort of patients with post-stroke aphasia

Studies attempting to map post-stroke cognitive or motor symptoms to lesion location have been available in the literature for over 150 years. In the last two decades, two computational techniques have been developed to identify the lesion sites associated with behavioural impairments. Voxel Based Morphometry (VBM) has now been used extensively for this purpose in many different patient populations. More recently, Voxel-based Lesion Symptom Mapping (VLSM) was developed specifically for the purpose of identifying lesion–symptom relationships in stroke patients, and has been used extensively to study, among others functions, language, motor abilities and attention. However, no studies have compared the results of these two techniques so far. In this study we compared VLSM and VBM in a cohort of 20 patients with chronic post-stroke aphasia. Comparison of the two techniques showed overlap in regions previously found to be relevant for the tasks used, suggesting that using both techniques and looking for overlaps between them can increase the reliability of the results obtained. However, overall VBM and VLSM provided only partially concordant results and the differences between the two techniques are discussed.

Muscle activation changes during body weight support treadmill training after focal cortical ischemia: A rat hindlimb model

The study used a focal ischemia rat hindlimb model to investigate muscle activity changes during a 10-day body weight support (BWS) treadmill training program. The changes being studied included fatigue effects, EMG burst duration in the gait cycle, and the symmetry of muscle activation between affected and unaffected sides. Intramuscular EMG of medial gastrocnemius (MG) and tibialis anterior (TA) muscles in male Sprague Dawley rats at affected side (n=10) and unaffected side (n=10) were recorded during the treadmill running before a middle cerebral artery occlusion/reperfusion (MCAo/r) surgery and poststroke recovery stage. Behavioral test score and bodyweight were recorded at a daily basis after stroke. The mean power frequency (MPF) of the EMG, EMG burst duration in the gait cycle, and symmetry index between two sides were used for analysis. The drop rate of MPF of MG at the unaffected side increased (P<0.05) at poststroke day 2 and it generally decreased along the poststroke training days and almost returned to baseline value at poststroke day 6. Symmetry index of MG and TA showed a large imbalance right after stroke and tended to return to normal. Our findings of the MPF drop after stroke might indicate fatigue effects due to the compensation loading share of the ipsilateral side muscle and the increase of the symmetry index reflects abnormal gait pattern after the onset of stroke. The recovery rate after stroke could be investigated with EMG parameters together with the behavioral score, and both were improved during and after the BWS treadmill training.

Transcranial direct current stimulation in patients with skull defects and skull plates: high-resolution computational FEM study of factors altering cortical current flow

Preliminary positive results of transcranial direct current stimulation (tDCS) in enhancing the effects of cognitive and motor training indicate that this technique might also be beneficial in traumatic brain injury or patients who had decompressive craniectomy for trauma and cerebrovascular disease. One perceived hurdle is the presence of skull defects or skull plates in these patients that would hypothetically alter the intensity and location of current flow through the brain. We aimed to model tDCS using a magnetic resonance imaging (MRI)-derived finite element head model with several conceptualized skull injuries. Cortical electric field (current density) peak intensities and distributions were compared with the healthy (skull intact) case. The factors of electrode position (C3-supraorbital or O1-supraorbital), electrode size skull defect size, skull defect state (acute and chronic) or skull plate (titanium and acrylic) were analyzed. If and how electric current through the brain was modulated by defects was found to depend on a specific combination of factors. For example, the condition that led to largest increase in peak cortical electric field was when one electrode was placed directly over a moderate sized skull defect. In contrast, small defects midway between electrodes did not significantly change cortical currents. As the conductivity of large skull defects/plates was increased (chronic to acute to titanium), current was shunted away from directly underlying cortex and concentrated in cortex underlying the defect perimeter. The predictions of this study are the first step to assess safety of transcranial electrical therapy in subjects with skull injuries and skull plates.

Multiparametric magnetic resonance imaging of brain disorders

Magnetic resonance imaging (MRI) has been shown to improve the diagnosis and management of patients with brain disorders. Multiparametric MRI offers the possibility of noninvasively assessing multiple facets of pathophysiological processes that exist simultaneously, thereby further assisting in patient treatment management. Voxel-based analysis approaches, such as tissue theme mapping, have the benefit over volumetric approaches in being able to identify spatially heterogeneous colocalized changes on multiple parametric MR images that are not readily discernible. Tissue theme maps seem to be a promising tool for integrating the plethora of novel imaging contrasts that are being developed for the noninvasive investigation of the different stages of disease progression into easily interpretable maps of brain injury. We describe here various implementations for combining multiparametric imaging and their merits in the evaluation of brain diseases.

Evaluation of cerebral blood flow changes in focal cerebral ischemia rats by using transcranial Doppler ultrasonography

Ischemic stroke is typically characterized by the disruption of cerebral blood flow. This study aimed to consecutively evaluate the cerebral blood flow changes in a focal ischemia rat model during the occlusion-reperfusion procedure and along the recovery stage after stroke. In 12 Sprague Dawley (SD) rats, a middle cerebral artery occlusion/reperfusion (MCAo/r) surgery was conducted, which combines a permanent occlusion of the right common carotid artery (CCA), external carotid artery (ECA) and a transient occlusion of the right internal carotid artery (ICA) and middle cerebral artery (MCA) with a monofilament introduced from the proximal ICA towards the distal right ICA then removed after 90 min. Blood flow velocity (BFV) from the concerned arteries were measured using ultrasonography (13-4 MHz) at the basal stage before the surgery, after the reperfusion stage and during the post-stroke status. At reperfusion stage and after, BFV increased significantly in the left ICA and in the basilar artery (BA) (starting from post-24 h, p < 0.05 vs. basal). Moreover, BFV were reversed in the distal right ICA and reflow was recorded in the right MCA. Time-average maximum BFV in the right MCA at reperfusion and post-stroke 24-96 h was decreased significantly (p < 0.05 vs. basal). The reversed flow in the right ICA was enabled by the settlement of the collateral supply through the circle of Willis which consisted in higher BFV in the opposite ICA and in the BA still 24 h, although the proximal right ICA remain occluded. Ultrasound measurement of BFV helps to provide information on the redistribution of the blood flow supply after the onset of stroke.

Inter- and intraobserver reliability of five MRI sequences in the evaluation of the final volume of cerebral infarct

PURPOSE

To evaluate the reproducibility of fluid attenuated inversion recovery (FLAIR) and four other magnetic resonance imaging (MRI) sequences in the quantitative assessment of final cerebral infarct volume.

MATERIALS AND METHODS

FLAIR, T1-3D, magnetization transfer ratio (MTR)-map, diffusion-weighted trace (DWI)-trace, and apparent diffusion coefficient (ADC)-map, were acquired and measured in 33 patients 30-45 days after onset of a first-ever ischemic stroke. The infarct area was visually detected and manually delineated two times by two readers separately after images and sequences randomization. The reliability was assessed by using an intraclass correlation coefficient (ICC) and its two-sided 95% confidence interval (95% CI).

RESULTS

DWI-trace had the best reliability, with an ICC of 0.96 (95% CI = 0.93-0.98). FLAIR had an ICC of 0.86 (95% CI = 0.73-0.93), and a much higher volume. T1-3D, MTR-map and ADC-map had lower reliability or excessive volume values equal to 0 in comparison to DWI-trace.

CONCLUSION

DWI-trace performed within 30th and 45th day following onset of acute ischemic stroke was the most reliable sequence for final infarct volume quantification. This sequence should be added to FLAIR evaluation to strengthen the statistical results of the pharmacological trials and reduce their variability.

An objective method for combining multi-parametric MRI datasets to characterize malignant tumors

Medical imaging has made significant contributions to the characterization of malignant tumors. In many cases, however, maps from multiple modalities may be required for more complete tumor mapping. In this manuscript we propose an objective method for combining multiple imaging datasets with the goal of characterizing malignant tumors. We refer to the proposed technique as the percent overlap method (POM). To demonstrate the power and flexibility of the POM analysis, we present four patients with recurrent glioblastoma multiforme. Each patient had multiple magnetic resonance imaging procedures resulting in seven different parameter maps. Chemical shift imaging was used to provide three metabolite ratio maps (Cho:NAA, Cho:Cre, Lac:Cre). A perfusion scan provided regional cerebral blood volume and permeability maps. Diffusion and carbogen-based hypoxia mapping data were also acquired. Composite maps were formed for each patient using POM, then were compared to results from the ISODATA clustering technique. The POM maps of likely recurrent tumor regions were found to be consistent with the ISODATA clustering method. This manuscript presents an objective method for combining parameters from multiple physiologic imaging techniques into a single composite map. The accuracy of the map depends strongly on the sensitivity of the chosen imaging parameters to the disease process at the time of image acquisition. Further validation of this method may be achieved by correlation with histological data.

Transcranial direct current stimulation: a computer-based human model study

OBJECTIVES

Interest in transcranial direct current stimulation (tDCS) in clinical practice has been growing, however, the knowledge about its efficacy and mechanisms of action remains limited. This paper presents a realistic magnetic resonance imaging (MRI)-derived finite element model of currents applied to the human brain during tDCS.

EXPERIMENTAL DESIGN

Current density distributions were analyzed in a healthy human head model with varied electrode montages. For each configuration, we calculated the cortical current density distributions. Analogous studies were completed for three pathological models of cortical infarcts.

PRINCIPAL OBSERVATIONS

The current density magnitude maxima injected in the cortex by 1 mA tDCS ranged from 0.77 to 2.00 mA/cm(2). The pathological models revealed that cortical strokes, relative to the non-pathological solutions, can elevate current density maxima and alter their location.

CONCLUSIONS

These results may guide optimized tDCS for application in normal subjects and patients with focal brain lesions.

Infarct prediction and treatment assessment with MRI-based algorithms in experimental stroke models

There is increasing interest in using algorithms combining multiple magnetic resonance imaging (MRI) modalities to predict tissue infarction in acute human stroke. We developed and tested a voxel-based generalized linear model (GLM) algorithm to predict tissue infarction in an animal stroke model in order to directly compare predicted outcome with the tissue's histologic outcome, and to evaluate the potential for assessing therapeutic efficacy using these multiparametric algorithms. With acute MRI acquired after unilateral embolic stroke in rats (n=8), a GLM was developed and used to predict infarction on a voxel-wise basis for saline (n=6) and recombinant tissue plasminogen activator (rt-PA) treatment (n=7) arms of a trial of delayed thrombolytic therapy in rats. Pretreatment predicted outcome compared with post-treatment histology was highly accurate in saline-treated rats (0.92+/-0.05). Accuracy was significantly reduced (P=0.04) in rt-PA-treated animals (0.86+/-0.08), although no significant difference was detected when comparing histologic lesion volumes. Animals that reperfused had significantly lower (P<0.01) GLM-predicted infarction risk (0.73+/-0.03) than nonreperfused animals (0.81+/-0.05), possibly reflecting less severe initial ischemic injury and therefore tissue likely more amenable to therapy. Our results show that acute MRI-based algorithms can predict tissue infarction with high accuracy in animals not receiving thrombolytic therapy. Furthermore, alterations in disease progression due to treatment were more sensitively monitored with our voxel-based analysis techniques than with volumetric approaches. Our study shows that predictive algorithms are promising metrics for diagnosis, prognosis and therapeutic evaluation after acute stroke that can translate readily from preclinical to clinical settings.

Physiologic characterisation of glioblastoma multiforme using MRI-based hypoxia mapping, chemical shift imaging, perfusion and diffusion maps

PURPOSE

A multiparametric, physiologic MRI approach was considered to more completely characterise biopsy-confirmed glioblastoma multiforme (GBM). Chemical shift imaging (CSI) supplied biochemical information in metabolite ratios, while perfusion images provided data on presumed vascularity from regional cerebral blood volume (rCBV) and permeability maps. Diffusion-weighted images were reduced to apparent diffusion coefficient (ADC) maps to evaluate cellularity, and blood oxygen level-dependent imaging was used to create maps of putative hypoxic regions.

METHODS

Six post-treatment GBM patients were scanned at 3-month intervals until recurrence was suggested by conventional MRI parameters, yielding 20 scans for consideration. The percentage of extreme values in each technique that overlapped with other parameters was measured and compared across hemispheres to assess utility.

RESULTS

We found significantly better performance in selecting the diseased hemisphere for overall percent overlap when compared to voxel counts from individual thresholded parameter maps. Parameters were selected on the basis of highest overlap, and corresponding composite overlap maps show increased specificity to likely recurrent regions by reducing the number of falsely positive voxels, and offer insight into relationships between various parameters.

CONCLUSION

In a pilot group of patients, percent overlap appears to be sensitive to recurrent disease. When used to combine multiple parameters, voxels containing overlap can specifically target probable recurrent areas.

Transcranial magnetic stimulation and stroke: A computer-based human model study

This paper explores how transcranial magnetic stimulation (TMS) induced currents in the brain are perturbed by electrical and anatomical changes following a stroke in its chronic stage. Multiple MRI derived finite element head models were constructed and evaluated to address the effects that strokes can have on the induced stimulating TMS currents by comparing stroke models of various sizes and geometries to a healthy head model under a number of stimulation conditions. The TMS induced currents were significantly altered for stimulation proximal to the lesion site in all of the models analyzed. The current density distributions were modified in magnitude, location, and orientation such that the population of neural elements that are stimulated will be correspondingly altered. The current perturbations were minimized for conditions tested where the coil was far removed from the lesion site, including models of stimulation contralateral to the lesioned hemisphere. The present limitations of TMS to the peri-lesional cortex are explored, ultimately concluding that conventional clinical standards for stimulation are unreliable and potentially dangerous predictors of the site and degree of stimulation when TMS is applied proximal to infarction site.

Characterization of cerebral tissue by MRI map ISODATA in embolic stroke in rat

ISODATA using MRI parameter-weighted images has been previously employed to characterize ischemic cell damage after stroke in rats. In an effort to increase the objectivity and to further automate the ISODATA, MRI parameter maps were now employed. Male Wistar rats were subjected to embolic stroke and received treatment via a femoral vein at 4 h post-stroke. The control rats received saline and were sacrificed at 6, 24 and 48 h after stroke, respectively. Treated rats received rtPA alone or were treated with a combination of rtPA and an antibody, 7E3 F(ab')2, against the glycoprotein receptor that binds the platelet to fibrin. These rats were sacrificed at 24, or 48, h post-stroke. T1, T2 and diffusion maps were employed for map ISODATA analysis. H&E histological analysis of coronal sections of tissue was performed and compared with map ISODATA from the corresponding sections. ISODATA signatures were highly correlated (R approximately 0.80, P < 0.0001) with the ischemic cell damage analyzed at 6, 24 and 48 h post-stroke. At 24 and 48 h after stroke, ISODATA lesion sizes were highly correlated (R > 0.97, P < 0.001) with lesion sizes measured histologically. The combination treatment of rtPA and 7E3 F(ab')2 reduced both infarction size (P < 0.002) and average signature (P < 0.03) at 48 h after stroke, compared to saline-treated animals. No significant difference was found between saline and rtPA-alone-treated rats. The map ISODATA successfully provides objective and automated quantitation of the ischemic damage in both size and severity in an embolic stroke model of rat with and without a therapeutic intervention.

Map-ISODATA demarcates regional response to combination rt-PA and 7E3 F(ab')2 treatment of embolic stroke in the rat

PURPOSE

To investigate the ability of map-ISODATA (Iterative Self-Organizing Data Analysis Technique) to classify the different categories of ischemic damage in the lesion and to evaluate a combined (thrombolysis plus antiplatelet) treatment efficacy in an embolic stroke of rat.

MATERIALS AND METHODS

Rats subjected to embolic stroke with (N=12) and without (N=10) rt-PA and 7E3 F(ab')2 treatment (4 hours after embolization) were followed (at 2, 24, and 48 hours post-MCAO) with magnetic resonance imaging (MRI) using T1, T2, and apparent diffusion coefficient of water (ADCw). ISODATA was computed from T1, T2, and ADCw maps. The signatures characterized by the map-ISODATA were compared with histological quantitative evaluation and were employed to demarcate the specific regions in the lesion.

RESULTS

The signature described by map-ISODATA is highly correlated with the degree of tissue damage in the lesion and can distinguish the severity of ischemic tissue injury. Based upon map-ISODATA, ischemic lesion area can be divided into three specific regions, each characterized by a distinct evolution of injury and treatment response. The combined treatment significantly reduces the lesion size between 24 and 48 hours and improves the outcome 48 hours post-MCAO compared with the control group.

CONCLUSION

Map-ISODATA provides an accurate means to identify lesion area, to distinguish ischemic damage, and to detect treatment response. 7E3 F(ab')2 extends the rt-PA treatment window to at least four hours after the onset of embolic stroke of rat.

Predicting final infarct size using acute and subacute multiparametric MRI measurements in patients with ischemic stroke

PURPOSE

To identify early MRI characteristics of ischemic stroke that predict final infarct size three months poststroke.

MATERIALS AND METHODS

Multiparametric MRI (multispin echo T2-weighted [T2W] imaging, T1-weighted [T1W] imaging, and diffusion-weighted imaging [DWI]) was performed acutely (<24 hours), subacutely (three to five days), and at three months. MRI was processed using maps of apparent diffusion coefficient (ADC), T2, and a self-organizing data analysis (ISODATA) technique. Analyses began with testing for individual MRI parameter effects, followed by multivariable modeling with assessment of predictive ability (R(2)) on final infarct size.

RESULTS

A total of 45 patients were studied, 15 of whom were treated with tissue plasminogen activator (tPA) before acute MRI. The acute DWI and DWI-ISODATA mismatch lesion size, and the interactions of ADC, T2, and T2W imaging lesion with tPA remained in the final multivariable model (R(2) = 70%). A large acute DWI lesion or DWI < ISODATA lesion independently predicted increase in the final infract size, with predictive ability 68%. Predictive ability increased (R(2) = 83%) when subacute MRI parameters were included along with acute DWI, DWI-ISODATA mismatch, and acute T2W image lesion size by tPA treatment interaction. Subacute DWI > acute DWI lesion size predicted an increased final infarct size (P < 0.01).

CONCLUSION

Acute-phase DWI and DWI-ISODATA mismatch strongly predict the final infarct size. An acute-to-subacute DWI lesion size change further increases the predictive ability of the model.

Multiparametric ISODATA analysis of embolic stroke and rt-PA intervention in rat

To increase the sensitivity of MRI parameters to detect tissue damage of ischemic stroke, an unsupervised analysis method, Iterative Self-Organizing Data Analysis Technique Algorithm (ISODATA), was applied to analyze the temporal evolution of ischemic damage in a focal embolic cerebral ischemia model in rat with and without recombinant tissue plasminogen activator (rt-PA) treatment. Male Wistar rats subjected to embolic stroke were investigated using a 7-T MRI system. Rats were randomized into control (n=9) and treated (n=9) groups. The treated rats received rt-PA via a femoral vein at 4 h after onset of embolic ischemia. ISODATA analysis employed parametric maps or weighted images (T1, T2, and diffusion). ISODATA results with parametric maps are superior to ISODATA with weighted images, and both of them were highly correlated with the infarction size measured from the corresponding histological section. At 24 h after embolic stroke, the average map ISODATA lesion sizes were 37.7+/-7.0 and 39.2+/-5.6 mm2 for the treated and the control group, respectively. Average histological infarction areas were 37.9+/-7.4 mm2 for treated rats and 39.4+/-6.1 mm2 for controls. The R2 values of the linear correlation between map ISODATA and histological data were 0.98 and 0.96 for treated and control rats, respectively. Both histological and map ISODATA data suggest that there is no significant difference in infarction area between non-treated and rt-PA-treated rats when treatment was administered 4 h after the onset of embolic stroke. The ISODATA lesion size analysis was also sensitive to changes of lesion size during acute and subacute stages of stroke. Our data demonstrate that the multiparameter map ISODATA approach provides a more sensitive quantitation of the ischemic lesion at all time points than image ISODATA and single MRI parametric analysis using T1, T2 or ADCw.

Volumetric evaluation of the ischemic lesion size with serial MRI in a transient MCAO model of the rat: comparison of DWI and T1WI

Magnetic resonance imaging (MRI) is applied in many studies on experimental cerebral ischemia in rodents to monitor the temporal evolution of ischemic damage. We report a protocol to evaluate the infarct size after middle cerebral artery occlusion with reperfusion (MCAO/R) in male Wistar rats. Imaging was performed with a 2.35 T scanner and we focused on diffusion-weighted imaging (DWI), T2-weighted imaging (T2WI) and postcontrast T1-weighted imaging (T1WI). We show the detailed procedure of volumetry, the contrast-to-noise ratio (CNR) and the intraindividual variability of infarct and hemispheric volumes at different reperfusion times. The presented method is of low variability if image contrast between ischemic and nonischemic tissue is very high, which is the case not only for all sequences at 8 and 12 h of reperfusion but also for DWI after 3 and 5 h of reperfusion. Furthermore, we describe the so-called mismatch region of lesion sizes depicted on DWI and postcontrast T1WI that suffers from cytotoxic edema but lacks contrast enhancement.