Temporal MRI assessment of intracerebral hemorrhage in rats.

BACKGROUND AND PURPOSE: MRI was used to evaluate the effects of experimental intracerebral hemorrhage (ICH) on brain tissue injury and recovery. METHODS: Primary ICH was induced in rats (n=6) by direct infusion of autologous blood into the striatum. The evolution of ICH damage was assessed by MRI estimates of T(2) and T(1sat) relaxation times, cerebral blood flow, vascular permeability, and susceptibility-weighted imaging before surgery (baseline) and at 2 hours and 1, 7, and 14 days post-ICH. Behavioral testing was done before and at 1, 7, and 14 days post-ICH. Animals were euthanized for histology at 14 days. RESULTS: The MRI appearance of the hemorrhage and surrounding regions changed in a consistent manner over time. Two primary regions of interest were identified based on T(2) values. These included a core, corresponding to the bulk of the hemorrhage, and an adjacent rim; both varied with time. The core was associated with significantly lower cerebral blood flow values at all post-ICH time points, whereas cerebral blood flow varied in the rim. Increases in vascular permeability were noted at 1, 7, and 14 days. Changes in T(1sat) were similar to those of T(2). MRI and histological estimates of tissue loss were well correlated and showed approximately 9% hemispheric tissue loss. CONCLUSIONS: Although the cerebral blood flow changes observed with this ICH model may not exactly mimic the clinical situation, our results suggest that the evolution of ICH injury can be accurately characterized with MRI. These methods may be useful to evaluate therapeutic interventions after experimental ICH and eventually in humans.

MRI estimation of contrast agent concentration in tissue using a neural network approach.

Using an MRI T(1) by multiple readout pulses (TOMROP) image set, an adaptive neural network (ANN) was trained to directly estimate the concentration of a contrast agent (CA), gadolinium-bovine serum albumin (Gd-BSA), in tissue. In nine rats implanted with a 9L cerebral tumor, MRI acquisition of TOMROP inversion-recovery data was followed by quantitative autoradiography (QAR) using radioiodinated serum albumin (RISA). QAR autoradiograms were used as a training set for the ANN. Precontrast and 25 min postcontrast TOMROP image sets were shown to the ANN in the form of a physical feature set related to 24 inversion-recovery images; QAR autoradiograms at 30 min after injection of RISA were taken as the training standard for the network. After training and optimization, the ANN produced a map of Gd-BSA concentration [g-moles/liter]. The prediction by the ANN of CA concentration at 25 min after injection was well correlated (r = 0.82, P < 0.0001) with the corresponding autoradiogram's measure of CA concentration.

Step-down infusions of Gd-DTPA yield greater contrast-enhanced magnetic resonance images of BBB damage in acute stroke than bolus injections.

A rat model of transient suture occlusion of one middle cerebral artery (MCA) was used to create a unilateral reperfused cerebral ischemic infarct with blood-brain barrier (BBB) opening. Opening of the BBB was visualized and quantitated by magnetic resonance (MR) contrast enhancement with a Look-Locker T(1)-weighted sequence either following an intravenous bolus injection (n=7) or during a step-down infusion (n=7) of gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA). Blood levels of Gd-DTPA after either input were monitored via changes in sagittal sinus relaxation rate. Blood-to-brain influx constants (K(i)) were calculated by Patlak plots. On the basis of the MRI parameters and lesion size, the ischemic injury was determined to be similar in the two groups. The bolus injection input produced a sharp rise in blood levels of Gd-DTPA that declined quickly, whereas the step-down infusion led to a sharp rise that was maintained relatively constant for the period of imaging. Visual contrast enhancement and signal-to-noise (S/N) ratios were better with the step-down method (S/N=1.8) than with bolus injection (S/N=1.3). The K(i) values were not significantly different between the two groups (P>.05) and were around 0.005 ml/(g min). The major reason for the better imaging of BBB opening by the step-down infusion was the higher amounts of Gd-DTPA in plasma and tissue during most of the experimental period. These results suggest that step-down MR contrast agent (MRCA) administration schedule may be more advantageous for detection and delineation of acute BBB injury than the usually used bolus injections.

Angiogenesis and improved cerebral blood flow in the ischemic boundary area detected by MRI after administration of sildenafil to rats with embolic stroke.

To dynamically investigate the long-term response of an ischemic lesion in rat brain to the administration of sildenafil, male Wistar rats subjected to embolic stroke were treated with sildenafil (n=11) or saline (n=10) at a dose of 10 mg/kg administered subcutaneously 24-h after stroke and daily for an additional 6 days. Magnetic resonance images were acquired and functional performance was measured in all animals at 1 day, 2 days and weekly for 6 weeks post-stroke. All rats were sacrificed 6 weeks after stroke and endothelial barrier antigen immunostaining was employed for morphological analysis and quantification of cerebral vessels. Map-ISODATA was computed from T(1), T(2) and T(1sat) maps. ISODATA derived tissue signatures characterize the degree of ischemic injury. Based on the map-ISODATA calculated at 6 weeks, the ischemic lesion for each animal was divided into two specific regions, the ischemic boundary and ischemic core. The temporal profiles of cerebral blood flow (CBF) and tissue signature were retrospectively tracked in these two regions and were compared with histological evaluation and functional outcome. After 1 week of sildenafil treatment, the ischemic lesion exhibited two significantly different regions, with higher CBF level and correspondingly, lower tissue signature value in the boundary region than in the core region. Sildenafil treatment did not significantly reduce the lesion size, but did enhance angiogenesis. Functional performance was significantly increased after sildenafil treatment compared with the control group. Administration of sildenafil to rats with embolic stroke enhances angiogenesis and selectively increases the CBF level in the ischemic boundary, and improves neurological functional recovery compared to saline-treated rats.

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.

Quantitation and localization of blood-to-brain influx by magnetic resonance imaging and quantitative autoradiography in a model of transient focal ischemia.

The ability of gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) enhanced MRI to localize and quantitate blood-brain barrier (BBB) opening was evaluated against quantitative autoradiographic (QAR) imaging of (14)C-alpha-aminoisobutyric acid (AIB) distribution. The blood-to-brain transfer constant (K(i)) for Gd-DTPA was determined by MRI in rats after 3 h of focal cerebral ischemia plus 2.5 h of reperfusion (n = 9), and that of AIB was determined by QAR shortly thereafter. Tissue regions of interest (ROIs) for Gd-DTPA leakage were identified by ISODATA segmentation of pre- and post-Gd-DTPA Look-Locker (L-L) T(1) maps. Patlak plots were constructed using time course of blood and tissue T(1) changes induced by Gd for estimating K(i). Among the nine rats, 14 sizable regions of AIB uptake were found; 13 were also identified by ISODATA segmentation. Although the 13 MRI-ROIs spatially approximated those of AIB uptake, the segmentation sometimes missed small areas of lesser AIB uptake that did not extend through more than 60% of the 2.0-mm-thick slice. Mean K(i)'s of AIB were highly correlated with those of Gd-DTPA across the 13 regions; the group means (+/-SD) were similar for the two tracers (7.1 +/- 3.3 x 10(-3) and 6.8 +/- 3.5 x 10(-3) ml.g(-1) . min(-1), respectively). In most instances, Gd-DTPA MRI accurately localized areas of BBB opening.

Acute blood-brain barrier opening in experimentally induced focal cerebral ischemia is preferentially identified by quantitative magnetization transfer imaging.

Pathologic changes in brain tissue during and after stroke may lead to injury of the blood-brain barrier (BBB) and subsequent hemorrhagic transformation (HT). In a rat model of HT, the apparent diffusion coefficient of water, cerebral blood flow, relaxation times, T(1) and T(2), and magnetization transfer (MT) related parameters (T(1sat), K(for) and the MT ratio) were repetitively measured during 3 h of focal ischemia and 2 h of reperfusion (n = 8). Areas of BBB opening were identified by sequential assay of the transcapillary influx of Gd-diethylenetriaminepentaacetic acid (Gd-DTPA) by MRI and (14)C-alpha-aminoisobutyric acid (AIB) by quantitative autoradiography. Ischemia-injured regions of interest were identified from the MRI data and divided into those with and without BBB opening. Of the several MRI parameters measured, the T(1sat) in the caudate-putamen and preoptic area during ischemia and the first 2 h of reperfusion correlated best with the regional pattern of BBB opening observed thereafter. These data suggest that an ipsilateral/contralateral T(1sat) ratio > 1.6 demarcates leakage of small molecules such as Gd-DTPA and AIB across the BBB. As to clinical relevance, the quantitation of MT parameters in acute stroke may enable the early detection of areas of BBB opening and potential HT.

Quantitative evaluation of BBB permeability after embolic stroke in rat using MRI.

We sought to identify magnetic resonance imaging (MRI) parameters that can identify as well as predict disruption of the blood-brain barrier (BBB) after embolic stroke in the rat. Rats subjected to embolic stroke with (n=13) and without (n=13) rt-PA treatment were followed with MRI using quantitative permeability-related parameters, consisting of: transfer constant (K(i)) of Gd- DTPA, the distribution volume (V(p)) of the mobile protons, and the inverse of the apparent forward transfer rate for magnetization transfer (k(inv)), as well as the apparent diffusion coefficient of water (ADC(w)), T2, and cerebral cerebral blood flow (CBF). Tissue progressing to fibrin leakage resulting from BBB disruption and adjacent tissue were then analyzed to identify MRI markers that characterize BBB disruption. Animals were killed after final MRI measurements at 24 h after induction of embolic stroke and cerebral tissues were perfused and stained to detect fibrin leakage. K(i), V(p), and k(inv) were the most sensitive early (2 to 3 h) indices of the cerebral tissue that progresses to fibrin leakage. Cerebral blood flow was not significantly different between ischemic tissue with a compromised and an intact BBB. Our data indicate that compromise of the BBB can be sensitively predicted using a select set of MR parameters.

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.

MRI evaluation of treatment of embolic stroke in rat with intra-arterial and intravenous rt-PA.

Using magnetic resonance imaging (MRI), we investigated treatment of a rat model of embolic stroke with rt-PA via intra-arterial (IA) and intravenous (IV) routes of administration. Rats were treated with rt-PA by either IA (n = 13) or IV (n = 13) routes at 3 h after stroke induction. Diffusion, perfusion, T2, and magnetization transfer MRI were performed prior to and at 1-3 and at 24 h after embolization. The IA treated group exhibited smaller lesion volumes than the IV treated group (p = 0.02). The relative areas with low ADCW and cerebral blood flow (CBF) after IA rt-PA intervention were significantly (p < or = 0.03) smaller than those in the IV treated group at 24 h after embolization. Significant differences (p < 0.02) between IA and IV treated groups in the relative area with high T2 and inverse of the apparent forward transfer rate of magnetization (kINV) in the ipsilateral hemisphere were also detected at 24 h after embolization. The IA treated group exhibited less intracerebral hemorrhage (27%) than the IV treated (64%) groups. Our data suggest that the beneficial effects of IA rt-PA treatment can be detected by changes in CBF, ADCW, T2, and kINV.

Patlak plots of Gd-DTPA MRI data yield blood-brain transfer constants concordant with those of 14C-sucrose in areas of blood-brain opening.

The blood-to-brain transfer rate constant (K(i)) of Gd-DTPA was determined in MRI studies of a rat model of transient cerebral ischemia. The longitudinal relaxation rate, R(1), was estimated using repeated Look-Locker measurements. A model-independent analysis of deltaR(1), the Patlak plot, produced maps of K(i) for Gd-DTPA and the distribution volume of the mobile protons (V(p)) with intravascular-Gd changed R(1)'s. The K(i)'s of Gd-DTPA were estimated in regions of interest with blood-brain barrier (BBB) opening (regions of interest, ROIs) and compared to those of (14)C-sucrose determined shortly thereafter by quantitative autoradiography. The K(i)'s for both Gd-DTPA and sucrose were much higher than normal within the ROIs (n = 7); linear regression of K(i) for Gd-DTPA vs. K(i) for sucrose yielded a slope of 0.43 +/- 0.11 and r(2) = 0.72 (P = 0.01). Thus, K(i) for Gd-DTPA varied in parallel with, but was less than, K(i) for sucrose. In the ROIs, mean V(p) was 0.071 ml g(-1) and much higher than mean vascular volume estimated by dynamic-contrast-enhancement (0.013 ml g(-1)) or mean V(p) in contralateral brain (0.015 ml g(-1)). This elevated V(p) may reflect increased capillary permeability to water. In conclusion, K(i) can be reliably calculated from Gd-DTPA-MRI data by Patlak plots.