Mathias Hoehn-Berlage

Evolution of regional changes in apparent diffusion coefficient during focal ischemia of rat brain: the relationship of quantitative diffusion NMR imaging to reduction in cerebral blood flow and metabolic disturbances.

Middle cerebral artery occlusion was performed in rats while the animals were inside the nuclear magnetic resonance (NMR) tomograph. Successful occlusion was confirmed by the collapse of amplitude on an electrocorticogram. The ultrafast NMR imaging technique UFLARE was used to measure the apparent diffusion coefficient (ADC) immediately after the induction of cerebral ischemia. ADC values of normal cortex and caudate-putamen were 726 ± 22 × 10−6 mm2/s and 659 ± 17 × 10−6 mm2/s, respectively. Within minutes of occlusion, a large territory with reduced ADC became visible in the ipsilateral hemisphere. Over the 2 h observation period, this area grew continuously. Quantitative analysis of the ADC reduction in this region showed a gradual ADC decrease from the periphery to the core, the lowest ADC value amounting to about 60% of control. Two hours after the onset of occlusion, the regional distribution of ATP and tissue pH were determined with bioluminescence and fluorescence techniques, respectively. There was a depletion of ATP in the core of the ischemic territory (32 ± 20% of the hemisphere) and an area of tissue acidosis (57 ± 19% of the hemisphere) spreading beyond that of ATP depletion. Regional CBF (rCBF) was measured autoradiographically with the iodo[14C]antipyrine method. CBF gradually decreased from the periphery to the ischemic core, where it declined to values as low as 5 ml 100 g−1 min−1. When reductions in CBF and in ADC were matched to the corresponding areas of energy breakdown and of tissue acidosis, the region of energy depletion corresponded to a threshold in rCBF of 18 ± 14 ml 100 g−1 min−1 and to an ADC reduction to 77 ± 3% of control. Tissue acidosis corresponded to a flow value below 31 ± 11 ml 100 g−1 min−1 and to an ADC value below 90 ± 4% of control. Thus, the quantification of ADC in the ischemic territory allows the distinction between a core region with total breakdown of energy metabolism and a corona with normal energy balance but severe tissue acidosis.

Potassium-Induced Cortical Spreading Depressions during Focal Cerebral Ischemia in Rats: Contribution to Lesion Growth Assessed by Diffusion-Weighted NMR and Biochemical Imaging

In focal ischemia of rats, the volume of ischemic lesion correlates with the number of peri-infarct depolarizations. To test the hypothesis that depolarizations accelerate infarct growth, we combined focal ischemia with externally evoked spreading depression (SD) waves. Ischemic brain infarcts were produced in halothane-anaesthetized rats by intraluminal thread occlusion of the middle cerebral artery (MCA). In one group of animals, repeated SDs were evoked at 15-min intervals by microinjections of potassium acetate into the frontal cortex. In another group, the spread of the potassium-evoked depolarizations was prevented by application of the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (MK-801). The volume of ischemic lesion was monitored for 2 h by diffusion-weighted imaging (DWI) and correlated with electrophysiological recordings and biochemical imaging techniques. In untreated rats, each microinjection produced an SD wave and a stepwise rise of the volume and signal intensity of the DWI-visible cortical lesion. The volume of this lesion increased between 15 min and 2 h of MCA occlusion from 19 ± 15% to 66 ± 16% of ipsilateral cortex. In dizocilpine-treated animals, microinjections of potassium did not evoke SDs, nor did the volume and signal intensity of the DWI-visible cortical lesion change. At 15 min after MCA occlusion, the DWI-visible lesion was larger than in untreated animals—43 ± 16% of the ipsilateral cortex; however, after 2 h, it increased only slightly further to 49 ± 21%. Slower lesion growth in the absence of SDs was also reflected by the volume of ATP-depleted tissue, which, after 2 h of MCA occlusion, involved 26 ± 12% of the ipsilateral cortex in treated and 49 ± 9% in untreated animals (p < 0.01). These observations support the hypothesis that peri-infarct depolarizations accelerate cerebral infarct growth.

Relationship between diffusion-weighted MR images, cerebral blood flow, and energy state in experimental brain infarction

The regional evolution of brain infarction was studied in Wistar rats submitted to remotely controlled thread occlusion of the middle cerebral artery. Occlusion was performed in the magnet of an NMR tomography system to allow continuous recording of diffusion-weighted images. After 30 min (n = 6) or 2 h (n = 9), cerebral blood flow was measured by [14C] iodoantipyrine autoradiography while the regional distribution of ATP, glucose, lactate, and pH was imaged using pictorial bioluminescence and fluoroscopic methods. In diffusion-weighted images, the hemispheric lesion area (HLA) at the level of caudate-putamen amounted to 54.2 +/- 10.9% after 30 min and to 67.0 +/- 5.9% after 2 h vascular occlusion. These areas corresponded to the regions exhibiting tissue acidosis (60.8 +/- 9.3% and 70.4 +/- 4.5%), but were clearly larger than those in which ATP was depleted (22.3 +/- 20.8% and 49.6 +/- 12.9% after 30 min and 2 h, respectively). The threshold of blood flow for the increase of signal intensity in diffusion-weighted images increased between 30 min and 2 h occlusion from 34 to 41 ml/100 g per minute, the threshold of acidosis from 40 to 47 ml/100 g per minute, and the threshold for ATP depletion from 13 to 19 ml/100 g per minute. Our study demonstrates that diffusion-weighted imaging detects both the core and the penumbra of the evolving infarction but is not able to differentiate between the two parts. It further shows that the ischemic lesion grows during the initial 2 h of vascular occlusion, and that the size of the infarct core increases more rapidly than that of the penumbra.

Diffusion nuclear magnetic resonance imaging in experimental stroke. Correlation with cerebral metabolites.

BACKGROUND AND PURPOSE Diffusion-weighted nuclear magnetic resonance imaging has been shown to detect early ischemia-related alterations in experimental stroke. This raises the question of whether the observed increase in signal intensity is correlated with changes in cerebral metabolism. After middle cerebral artery occlusion, nuclear magnetic resonance diffusion images were recorded and compared with the regional concentration of cerebral metabolites and with histology of identical planes. METHODS Seven anesthetized Fischer rats were subjected to permanent occlusion of the middle cerebral artery. T1, T2, and diffusion images (b factors ranging from 0 to 1500 s/mm2) were measured in three to five planes after 7 hours. Thereafter, brains were frozen in situ for histology and quantitative bioluminescence imaging of ATP, glucose, lactate, and for fluorescence imaging of tissue pH. RESULTS Seven hours after middle cerebral artery occlusion, the apparent diffusion coefficient was reduced from 615 +/- 97 x 10(-6).mm2.s-1 (contralateral brain) to 359 +/- 42 x 10(-6).mm2.s-1 (ischemic brain; mean +/- SD, P < .01). A precise topical coincidence was demonstrated between changes in nuclear magnetic resonance diffusion images, pattern of histological damage, ATP-depleted areas, and local tissue acidosis, the lesion area amounting to between 24.1% and 27.6% of the hemisphere at the level of the caudate-putamen. The area of elevated brain lactate clearly exceeded the acidic core of the infarct and included the slightly alkaline border zone. CONCLUSIONS The data demonstrate that after 7-hour middle cerebral artery occlusion, the reduction of the apparent diffusion coefficient in nuclear magnetic resonance diffusion images reflects precisely the region of histological injury, breakdown of energy metabolism, and tissue acidosis.

NMR Imaging of the Apparent Diffusion Coefficient (ADC) for the Evaluation of Metabolic Suppression and Recovery after Prolonged Cerebral Ischemia

Adult normothermic cats were submitted to 1-h complete cerebrocirculatory arrest by intrathoracic occlusion of the internal mammary, the innominate, and the subclavian arteries in combination with pharmacologically induced hypotension. After ischemia, recirculation was initiated at different blood pressure levels to manipulate the postischemia resuscitation conditions. The resulting spectrum of postischemic recovery was studied by combining nuclear magnetic resonance imaging of the apparent diffusion coefficient (ADC) with pictorial assays of brain tissue pH, ATP, glucose, and lactate. Before ischemia, the mean ADC (average of seven coronal slices of five cats) was 713 ± 40 × 10−6 mm2/s. After 10-min ischemia, ADC declined to 68% of control and after 50 min slightly further to 63% of control. During recirculation after 1-h ischemia, recovery of ADC varied depending on the initial reperfusion pressure and other systemic variables. In two animals ADC only transiently increased followed by a secondary decline below the postischemic level. In three other animals ADC returned to near control within 1 h of recirculation. The comparison of ADC changes with previously reported changes in extracellular volume revealed a close relationship, supporting the notion that ADC is a function of the intra/extracellular water compartmentation. Recovery of ADC correlated closely with tissue pH and metabolic recovery, studied 3 h after the initiation of recirculation. Animals without recovery of ADC exhibited global depletion of ATP and glucose and severe lactacidosis, whereas animals with recovery of ADC showed replenishment of ATP and glucose to near control and a substantial reversal of lactacidosis. Our data demonstrate that imaging of ADC provides reliable information about the metabolic state of the brain and can be used to monitor, with high temporal and regional resolution, the manifestation and reversal of ischemic brain injury.

Simultaneous Recording of Evoked Potentials and T* 2 -Weighted MR Images During Somatosensory Stimulation of Rat

Somatosensory evoked potentials (SEP) and T  *2 ‐weighted nuclear magnetic resonance (NMR) images were recorded simultaneously during somatosensory stimulation of rat to investigate the relationship between electrical activation of the brain tissue and the signal intensity change in functional NMR imaging. Electrical forepaw stimulation was performed in Wistar rats anesthetized with α‐chloralose. SEPs were recorded with calomel electrodes at stimulation frequencies of 1.5, 3, 4.5, and 6 Hz. At the same time, T  *2 ‐weighted imaging was performed, and the signal intensity increase during stimulation was correlated with the mean amplitude of the SEP. Both the stimulation‐evoked signal intensity increase in T  *2 ‐weighted images and the amplitude of SEPs were dependent on the stimulation frequency, with the largest signals at a stimulation frequency of 1.5 Hz and decreasing activations with increasing frequencies. The feasibility of simultaneous, artifact‐free recordings of T  *2 ‐weighted NMR images and of evoked potentials is proved. Furthermore, the study demonstrates—in the intact brain—the validity of functional magnetic resonance imaging for estimating the intensity of electrocortical activation. Magn Reson Med 41:469–473, 1999.

Reperfusion after Thrombolytic Therapy of Embolic Stroke in the Rat: Magnetic Resonance and Biochemical Imaging:

The effect of thrombolytic therapy was studied in rats submitted to thromboembolic stroke by intracarotid injection of autologous blood clots. Thrombolysis was initiated after 15 minutes with an intracarotid infusion of recombinant tissue-type activator (10 mg/kg body weight). Reperfusion was monitored for 3 hours using serial perfusion- and diffusion magnetic resonance imaging, and the outcome of treatment was quantified by pictorial measurements of ATP, tissue pH, and blood flow. In untreated animals, clot embolism resulted in an immediate decrease in blood flow and a sharp decrease in the apparent diffusion coefficient (ADC) that persisted throughout the observation period. Thrombolysis successfully recanalized the embolized middle cerebral artery origin and led to gradual improvement of blood flow and a slowly progressing reversal of ADC changes in the periphery of the ischemic territory, but only to transient and partial improvement in the center. Three hours after initiation of thrombolysis, the tissue volume with ADC values less than 80% of control was 39 ± 22% as compared to 61 ± 20% of ipsilateral hemisphere in untreated animals (means ± SD, P = .03) and the volume of ATP-depleted brain tissue was 25 ± 31% as compared to 46 ± 29% in untreated animals. Recovery of ischemic brain injury after thromboembolism is incomplete even when therapy is started as early as 15 minutes after clot embolism. Possible explanations for our findings include downstream displacement of clot material, microembolism of the vascular periphery, and events associated with reperfusion injury.

A modified rat model of middle cerebral artery thread occlusion under electrophysiological control for magnetic resonance investigations.

Previous magnetic resonance (MR) investigations of middle cerebral artery (MCA) occlusion in rats were limited by the lack of early post-occlusion MR measurements and/or electrophysiological monitoring. Therefore, we have developed a technique which allows to perform MCA occlusion inside the magnet under simultaneous recording of EEG and direct current (DC) potentials for monitoring the ischemic insult. Rats underwent intraluminal thread occlusion of the right MCA inside the MR tomograph via a catheter extension device, while EEG and DC potentials were recorded by non-magnetic graphite electrodes. The thread was slowly advanced until electrophysiological changes appeared. Diffusion-weighted MR images (DWI) were obtained before and repeatedly after MCA occlusion for up to 7 h. Thereafter, rat brains were frozen in situ or fixed by transcardiac perfusion and investigated by biochemical and histological techniques. In 15 of 18 animals (83%), MCA thread insertion caused immediate EEG changes and a negative DC potential shift at 4.4 +/- 1.8 min (mean +/- SD) after occlusion. In all animals with electrophysiological changes, signal intensity of DWI began to increase within the MCA territory at 12-14 min post-occlusion (the end of the first measurement), and continued to rise throughout the observation period. Ischemia was confirmed by demonstrating focal areas of energy depletion on ATP images. In the animals without electrophysiological changes, DWI or biochemical alterations were absent or confined to the central part of caudate-putamen. The histological lesion area of successfully occluded animals amounted to 70.1 +/- 5.8% of the ipsilateral hemisphere at the level of caudate-putamen.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrafast Perfusion-Weighted MRI of Functional Brain Activation in Rats During Forepaw Stimulation: Comparison with T*2-Weighted MRI

A fast version of the arterial spin tagging technique for the detection of cerebral perfusion is presented. Based on adiabatic spin inversion in combination with snapshot FLASH imaging, our technique allows the recording of perfusion changes with a temporal resolution of about 3 s. Differences of cerebral perfusion dependent on the choice of anesthesia were observed in rat brain. Furthermore, with this arterial spin tagging method we demonstrated perfusion increases in the somatosensory cortex of anaesthetized rats during forepaw stimulation. Comparison of the activated areas in the T*2‐weighted BOLD images and the perfusion‐weighted images showed good spatial correspondence, but the sensitivity to the functional activation was more than ten times higher in the perfusion technique.

Quantitative Diffusion MR Imaging of Cerebral Tumor and Edema

The detection of brain tumors using standard techniques of qualitative, relaxation-weighted magnetic resonance imaging (MRI) requires the application of contrast agents. We investigated whether or not it is possible to use diffusion-weighted MRI to localize tumors without contrast enhancement. Three different experimental rat brain tumors were studied: F98 glioma, RN6 Schwannoma and E376 neuroblastoma. We found a marked hypointensity in the region of the tumor and edema in heavily diffusion-weighted images, which corresponded well with the histological presentation. Quantitative maps of the apparent diffusion coefficient (ADC) allowed a better localization of the tumor than that obtained by regional presentation of T2 times, particularly under conditions in which peritumoral edema was absent. The ADC differences of the three tumor types were statistically not significant. Based upon regions-of-interest evaluations, tumor could be distinguished from peritumoral edema and normal brain tissue. However, a sharp demarcation between tumor and peritumoral edema was not possible, and this is attributed to a similar enlargement of interstitial space. It was concluded that diffusion-weighted MRI possesses a high potential for the detection of brain tumors but does not allow precise demarcation of the tumor border.