Diana Baumann

MRI‐based monitoring of inflammation and tissue damage in acute and chronic relapsing EAE

Experimental autoimmune encephalomyelitis (EAE) is a commonly used animal model that in several respects mimics human multiple sclerosis (MS), and can be used to design or validate new strategies for treatment of this disease. In the present study, different MRI techniques (macrophage tracking based on labeling cells in vivo by ultrasmall particles of iron oxide (USPIO), blood–brain barrier (BBB) breakdown, and magnetization transfer imaging (MTI)), as well as immunohistological staining were used to study the burden of disease in Lewis rats immunized by guinea pig myelin. The resulting imaging data was compared with behavioral readouts. Animals were studied during the acute phase and the first relapse. Activated monocytes were detected during both episodes in the brain stem or cortex. These areas coincided in part with areas of BBB breakdown. Significant changes of the magnetization transfer ratios (MTRs) of up to 35% were observed in areas of USPIO accumulation. This suggests that infiltrating monocytes are the major source of demyelination in EAE, but monocyte infiltration and breakdown of the BBB are temporally or spatially independent inflammatory processes. Magn Reson Med 50:309–314, 2003.

Predictability of FTY720 efficacy in experimental autoimmune encephalomyelitis by in vivo macrophage tracking: Clinical implications for ultrasmall superparamagnetic iron oxide-enhanced magnetic resonance imaging

To examine the efficacy of FTY720 as a new agent to reduce inflammatory activity in an animal model of multiple sclerosis (MS) by in vivo macrophage tracking.

Quantitative assessment of rat kidney function by measuring the clearance of the contrast agent Gd(DOTA) using dynamic MRI.

Magnetic resonance imaging (MRI) has been applied to assess kidney function in normal rats by monitoring the passage of the extracellular contrast agent GdDOTA. High-resolution images have been obtained using either the rapid acquisition with relaxation enhancement (RARE) or the snapshot pulse sequence. The latter was superior in anatomic definition due to the shorter echo delays used. The GdDOTA induced signal enhancements in the various renal structures were theoretically modeled and the results of the regression analysis then used to estimate local tissue concentrations in renal cortex, inner medulla and outer medulla/pelvis. The concentration-time curves in vena cava and renal cortex were similar and distinctly different from the ones in medulla and pelvis. This is reflected in the time-to-peak (TTP) values, which were TTP (blood) = 0.18 +/- 0.03 < TTP (cortex) = 0.26 +/- 0.05 < TTP (outer medulla) = 0.62 +/- 0.03 < TTP (inner medulla/pelvis) = 0.92 +/- 0.16 min. The initial tracer uptake rates depended linearly on the dose of GdDOTA administered, the value of the uptake rate in the cortex being significantly higher than those in the outer and inner medulla, which were identical within error limits. The initial medullar tracer uptake followed a first-order kinetics. The rate constant k(cl) = (dc[medulla]/dt)/c[cortex] = 3.4 +/- 0.5 min(-1) for the transition from cortex (predominantly blood signal) to medulla (predominantly urine) was considered a measure for the renal clearance. Intravenous administration of furosemide at doses 2.5, 5, and 10 mg/kg led to a dose-dependent decrease of k(cl). This reflects the inhibitory effect of the diuretic furosemide on medullary water resorption and thus the dilution of the GdDOTA in urine.

Regional brain activation by bicuculline visualized by functional magnetic resonance imaging. Time-resolved assessment of bicuculline-induced changes in local cerebral blood volume using an intravascular contrast agent

Functional magnetic resonance imaging (fMRI) has been applied to study rat focal brain activation induced by intravenous administration of the GABAA antagonist bicuculline. Using magnetite nanoparticles as a blood pool contrast agent, local changes in cerebral blood volume (CBV) were assessed with high temporal (10 s) and spatial (0.35 × 0.6 mm2) resolutions. Upon infusion of the bicuculline region‐specific increases in CBV have been observed, suggesting CBV to reflect brain activity. During the first 2 min, the signal increases were predominant in the cortex, followed by increases in other brain areas, such as the caudate putamen, thalamus and cerebellum. Ten minutes after the start of infusion, a dominant response was observed in the thalamus, while in the caudate putamen a biphasic response pattern was seen. The magnitude of the signal responses in all brain regions was dependent on the dose of bicuculline and, in general, matched the known distribution of GABAA binding sites. This study suggests that pharmacological fMRI, displaying brain function at the highly specific level of drug–receptor interaction, should foster our understanding of normal and pathological brain function. Copyright

Macrophage labeling by SPIO as an early marker of allograft chronic rejection in a rat model of kidney transplantation.

Anatomical and functional information (renography, perfusion) was obtained by MRI in a life‐supporting transplantation model, in which Lewis rats received kidneys from Fisher 344 donors. Renography and perfusion analyses were carried out with Gd‐DOTA and small particles of iron oxide (SPIO), respectively. Starting 12 weeks posttransplantation, images from grafts of untreated recipients exhibited distinctive signal attenuation in the cortex. Animals treated with cyclosporin (Sandimmune Neoral; Novartis Pharma, Basel, Switzerland) to prevent acute rejection showed a signal attenuation in the cortex at 33 weeks posttransplantation, while kidneys from rats treated additionally with everolimus (Certican; Novartis), a rapamycin derivative, had no changes in anatomical appearance. A significant negative correlation was found between the MRI cortical signal intensity and the histologically determined iron content in macrophages located in the cortex. Renography revealed a significantly reduced functionality of the kidneys of untreated controls 33 weeks after transplantation, while no significant changes in perfusion were observed in any group of rats. These results suggest the feasibility, by labeling macrophages with SPIO, of detecting signs of graft rejection significantly earlier than when changes in function occur. Monitoring early changes associated with chronic rejection can have an impact in preclinical studies by shortening the duration of the experimental period and by facilitating the investigation of novel immunomodulatory therapies for transplantation. Magn Reson Med 49:459–467, 2003.

Bicuculline-induced brain activation in mice detected by functional magnetic resonance imaging

Dynamic measurements of local changes in relative cerebral blood volume (CBVrel) during a pharmacological stimulation paradigm were performed in mice. Using magnetite nanoparticles as an intravascular contrast agent, high‐resolution CBVrel maps were obtained. Intravenous administration of the GABAA antagonist bicuculline prompted increases in local CBVrel as assessed by MRI with a high spatial resolution of 0.2 × 0.2 mm2 and a temporal resolution of 21 s. Signal changes occurred 20–30 s after the onset of drug infusion in the somatosensory and motor cortex, followed by other cortical and subcortical structures. The magnitudes of the CBVrel increases were 18% ± 4%, 46% ± 14%, and 67% ± 7%, as compared to prestimulation values for the cortex, and 9% ± 3%, 25% ± 4%, and 36% ± 7% for the caudate putamen for bicuculline doses of 0.6, 1.25, and 1.5 mg/kg, respectively. On‐line monitoring of transcutaneous carbon dioxide tension PtcCO2 reflecting arterial PaCO2 did not show any alteration during the stimulation paradigm. One of five of the mice receiving the highest bicuculline dose, and three of seven receiving the intermediate dose displayed a different cortical response pattern. After a CBVrel increase of 40% lasting for approximately 1 min, significant CBVrelreductions by 80% have been observed. Subcortical structures did not display this behavior. The present study suggests that this noninvasive approach of functional MRI (fMRI) can be applied to study drug‐induced brain activation by central nervous system (CNS) drugs in mice under normal and pathological situations. Magn Reson Med 46:292–298, 2001.

MRI Analysis of the Changes in Apparent Water Diffusion Coefficient, T2 Relaxation Time, and Cerebral Blood Flow and Volume in the Temporal Evolution of Cerebral Infarction Following Permanent Middle Cerebral Artery Occlusion in Rats

Detailed knowledge of similarities and differences between animal models and human stroke is decisive for selecting clinically effective drugs based on efficacy data obtained preclinically. Differences in the temporal evolution of stroke pathologies between animal models and man have been reported. In view of the importance of this issue for the development of neuroprotective treatments, the temporal evolution of stroke pathologies in the rat permanent middle cerebral artery occlusion (pMCAO) model has been evaluated with magnetic resonance imaging modalities under experimental conditions matching as close as possible those used in humans. Changes in the ipsilateral and contralateral cortex and striatum of cerebral blood flow (CBF) and volume (CBV), apparent diffusion coefficient (ADC), and spin-spin relaxation time (T(2)), as well as total cortical and striatal infarct volumes, calculated from CBF, ADC, and T(2) maps, were determined starting 1 h up to 216 h post-pMCAO. The temporal evolution of the MRI parameters in this rat model was similar to that observed in humans. In particular, the ADC values were decreased for more than 3 days and returned back to baseline between 4 to 8 days, to increase by day 9 only. Thus the stroke pathology in this rat model develops at a similar pace as in stroke patients arguing against a fundamental difference in the mechanisms involved. The infarct volumes however develop differently in this rat model as they invariably increase over the first 48 h, while in humans the evolution of infarct volume is slower and more heterogeneous.

Cytoprotection does not preserve brain functionality in rats during the acute post-stroke phase despite evidence of non-infarction provided by MRI.

In animal models of stroke the promise of a therapy is commonly judged from infarct size measurements, assuming that a reduction in infarct size results in reduction of the functional deficits. We have evaluated the validity of the concept that structural integrity translates into functional integrity during the acute post‐stroke period (24 h). Unilateral permanent middle cerebral artery occlusion (pMCAO) in Fischer F344 rats leads to infarcts comprising the ipsilateral striatum and cortical structures, including the somatosensory cortex. Infarct volumes were assessed using magnetic resonance imaging (MRI) methods (T2, diffusion, perfusion MRI). The functional integrity of the somatosensory cortex was assessed by functional MRI (fMRI) measuring changes in local cerebral blood volume, and by assessing the forelimb grip strength and the beam‐walking performance of the animals. Treatment with the calcium antagonist isradipine (2.5 mg/kg injected s.c. immediately after pMCAO) reduced the total infarct size by more than 40% compared to vehicle‐injected controls. In particular, the ipsilateral somatosensory cortex appeared normal in diffusion‐ and T2‐weighted MRI images. In sham‐operated rats simultaneous electrical stimulation of both forepaws led to similar activation of both somatosensory cortices, while in pMCAO animals given vehicle only the contralateral cortex showed an fMRI response. Similarly, in pMCAO rats treated with isradipine, functional activation following bilateral electrical stimulation was only detected in the contralateral somatosensory cortex despite the normal appearance of the ipsilateral cortex in MRI images. Furthermore, fMRI responses to pharmacological stimulation with bicuculline were virtually absent in the ipsilateral somatosensory cortices both in vehicle‐ and isradipine‐treated rats. Finally there was no significant difference between vehicle‐ and isradipine‐treated animals upon the performance of beam‐walking test or in forelimb grip strength. It is concluded that during the acute (24 h) post‐occlusion period, structural integrity in the somatosensory cortex revealed by MRI does not translate into preservation of function. Copyright

Recovery of function in cytoprotected cerebral cortex in rat stroke model assessed by functional MRI

Functional recovery in cytoprotected somatosensory cortex in a rat stroke model was studied using functional MRI (fMRI). Calcium antagonist treatment (isradipine) following permanent middle cerebral artery occlusion (pMCAO) reduced the infarct volume by 33 ± 9%. The somatosensory cortex representing the forepaws was spared from infarction; however, cerebral blood flow (CBF) was significantly reduced in this area 24 hr following pMCAO. Neural function was assessed at days 1, 2, 5, and 12 following pMCAO by fMRI using electrical stimulation of both forepaws. Vehicle‐treated rats did not show fMRI responses in the infarcted somatosensory cortex throughout the study. Several of the isradipine‐treated animals displayed functional recovery in the cytoprotected cortex at days 5 (3/5 rats) and 12 (5/10). Correlations with fMRI signals showed that normal T2 and ADC values in the respective brain areas are necessary, but not sufficient prerequisites for functionality. Recovery of neural function is associated with normalization of CBF in the cytoprotected brain area. Magn Reson Med 47:759–765, 2002.

Impaired functionality of reperfused brain tissue following short transient focal ischemia in rats.

Functional magnetic resonance imaging (fMRI) has been applied to study the consequences of transient focal ischemia on neuronal excitability in the rat brain. The experimental paradigm consisted of measuring the changes in local cerebral blood volume (CBV) induced by systemic infusion of the GABA(A) antagonist bicuculline after occlusion of the middle cerebral artery (MCA) for durations of 5, 15, 30 and 60 min using the intraluminal thread model. fMRI studies were carried out 60 min after successful reperfusion of the ischemic territory. Bicuculline-induced dynamic changes in local CBV were assessed in three brain regions: Parietal cortex, caudate putamen and thalamus. The measured CBV response was negatively correlated with the ischemia duration. Additionally, the three regions showed different vulnerability to the transient MCA occlusion, caudate being the most susceptible followed by parietal cortex and thalamus. The fMRI signals weakly correlated with basal CBF and CBV following reperfusion. Our results indicate that fMRI is a sensitive method to assess functional integrity of the brain. Activation maps allow to quantitatively assess the functionally compromized territory at an early stage following the ischemic event prior to the manifestation of pathomorphological changes.