T. A. Carpenter

Voxel-based morphometry in the R6/2 transgenic mouse reveals differences between genotypes not seen with manual 2D morphometry.

The R6/2 mouse is the most common mouse model used for Huntingtons disease (HD), a fatal, inherited neurodegenerative CAG disorder characterized by marked brain atrophy. We scanned 47 R6/2 transgenic and 42 wildtype (WT) ex vivo mouse brains at 18 weeks of age using high resolution, three-dimensional magnetic resonance imaging (MRI) for automated voxel-based morphometry (VBM) analysis. We found differences between genotypes in specific brain structures. Many of these changes were bilateral and were found in regions known to be involved in the behavioral deficits present in both R6/2 mice and HD patients. In particular, changes were evident in the basal ganglia, hippocampus, cortex and hypothalamus. In the striatum, changes were heterogenous and reminiscent of striosomal distribution. Changes were also seen in the cerebellum, as might be expected in a mouse carrying a repeat length typical of juvenile onset HD. Many of these changes were not detected by manual 2D morphometry from the same MR images. These data indicate that VBM will be a valuable technique for in vivo measurement of developing pathology in HD transgenic mice, and may be particularly useful for correlating histologically undetectable changes with behavioral deficits.

Development of a Combined microPET®-MR System

As evidenced by the success of PET-CT, there are many benefits from combining imaging modalities into a single scanner. The combination of PET and MR offers potential advantages over PET-CT, including improved soft tissue contrast, access to the multiplicity of contrast mechanisms available to MR, simultaneous imaging and fast MR sequences for motion correction. In addition, PET-MR is more suitable than PET-CT for cancer screening due to the elimination of the radiation dose from CT. A key issue associated with combining PET and MR is the fact that the performance of the photomultiplier tubes (PMTs) used in conventional PET detectors is degraded in the magnetic field required for MR. Two approaches have been adopted to circumvent that issue: retention of conventional, magnetic field-sensitive PMT-based PET detectors by modification of other features of the MR or PET system, or the use of new, magnetic field-insensitive devices in the PET detectors including avalanche photo-diodes (APDs) and silicon photomultipliers (SiPMs). Taking the former approach, we are assembling a modified microPET® Focus 120 within a gap in a novel, 1T superconducting magnet. The PMTs are located in a low magnetic field (~30mT) through a combination of magnet design and the use of fiber optic ‘bundles’. Two main features of the modified PET system have been tested, namely the effect of using long fiber optic bundles in the PET detector, and the impact of magnetic field upon the performance of the position sensitive PMTs. The design of a modified microPET®-MR system for small animal imaging is completed, and assembly and testing is underway.

Analysis of acute traumatic axonal injury using diffusion tensor imaging

Traumatic axonal injury (TAI) contributes significantly to mortality and morbidity following traumatic brain injury (TBI), but is poorly characterized by conventional imaging techniques. Diffusion tensor imaging (DTI) may provide better detection as well as insights into the mechanisms of white matter injury. DTI data from 33 patients with moderate-to-severe TBI, acquired at a median of 32 h postinjury, were compared with data from 28 age-matched controls. The global burden of whole brain white matter injury (GBWMI) was quantified by measuring the proportion of voxels that lay below a critical fractional anisotropy (FA) threshold, identified from control data. Mechanisms of change in FA maps were explored using an Eigenvalue analysis of the diffusion tensor. When compared with controls, patients showed significantly reduced mean FA (p < 0.001) and increased apparent diffusion coefficient (ADC; p = 0.017). GBWMI was significantly greater in patients than in controls (p < 0.01), but did not distinguish patients with obvious white matter lesions seen on structural imaging. It predicted classification of DTI images as head injury with a high degree of accuracy. Eigenvalue analysis showed that reductions in FA were predominantly the result of increases in radial diffusivity (p < 0.001). DTI may help quantify the overall burden of white matter injury in TBI and provide insights into underlying pathophysiology. Eigenvalue analysis suggests that the early imaging changes seen in white matter are consistent with axonal swelling rather than axonal truncation. This technique holds promise for examining disease progression, and may help define therapeutic windows for the treatment of diffuse brain injury.

Altered functional connectivity in the motor network after traumatic brain injury

Background: A large proportion of survivors of traumatic brain injury (TBI) have persistent cognitive impairments, the profile of which does not always correspond to the size and location of injuries. One possible explanation could be that TBI-induced damage extends beyond obvious lesion sites to affect remote brain networks. We explored this hypothesis in the context of a simple and well-characterized network, the motor network. The aim of this cross-sectional study was to establish the residual integrity of the motor network as an important proof of principle of abnormal connectivity in TBI. Methods: fMRI data were obtained from 12 right-handed patients and 9 healthy controls while they performed the finger-thumb opposition task with the right hand. We used both conventional and psychophysiologic interaction (PPI) analyses to examine the integrity of functional connections from brain regions we found to be activated in the paradigm we used. Results: As expected, the analysis showed significant activations of the left primary motor cortex (M1), right cerebellum (Ce), and bilateral supplementary motor area (SMA) in controls. However, only the activation of M1 survived robust statistical thresholding in patients. In controls, the PPI analysis revealed that left M1, SMA, and right Ce positively interacted with the left frontal cortex and negatively interacted with the right supramarginal gyrus. In patients, we observed no negative interaction and reduced interhemispheric interactions from these seed regions. Conclusions: These observations suggest that patients display compromised activation and connectivity patterns during the finger-thumb opposition task, which may imply functional reorganization of motor networks following TBI.

Potential role of NovoSeven in the prevention of rebleeding following aneurysmal subarachnoid haemorrhage

&NA; Rebleeding following aneurysmal subarachnoid haemorrhage is a major factor contributing to unfavourable outcome. Antifibrinolytic agents reduce the rate of rebleeding but increase the risk of cerebral ischaemia and infarction and hence provide no overall benefit. To address the theoretical concern that recombinant activated factor VII (NovoSeven®, Novo Nordisk A/S, Bagsvaerd, Denmark) might increase the risk of cerebral ischaemia while stabilizing the clot at the site of aneurysmal rupture, an open‐label, dose‐escalation safety study has been developed in collaboration with the UK Spontaneous Intracranial Haemorrhage Group. The trial design includes the recruitment of 15 patients (aged 18 years or over) in good grade with subarachnoid haemorrhage verified by computerized tomography scan or lumbar puncture. Safety evaluation includes clinical observation, monitoring of laboratory variables, positron emission tomography (PET) scanning (rCBF, rOEF, rCMRO2) and transcranial Doppler ultrasound. To date, ten patients have been recruited [NovoSeven® 80 μg/kg single bolus (” = 2), NovoSeven® 80 μg/kg single bolus followed by continuous infusion at 3.5 μg/kg per h (” = 2) or 7 μg/kg per h (” = 1), or control (” = 5)]. Clinical observation, transcranial Doppler ultrasound and PET studies revealed no evidence of cerebral ischaemia in the first nine patients treated with NovoSeven®. The last patient developed middle cerebral artery branch thrombosis contralateral to the aneurysm. The study is currently suspended pending further investigation. Blood Coagul Fibrinolysis 11 (suppl 1):S117‐S120

Use of magnetic resonance imaging for anatomical phenotyping of the R6/2 mouse model of Huntington's disease.

Huntingtons disease (HD) is a fatal, inherited neurodegenerative CAG disorder characterized by marked brain atrophy. We used magnetic resonance imaging (MRI) with manual volumetry for three dimensional (3D) morphological phenotyping of ex vivo brains of R6/2 mice, the most commonly used model of HD. High resolution 3D images were acquired for 18 week old wild-type (WT) and R6/2 mice. Although overall brain volumes were the same between genotypes, decreases in volumes were found in the cortex and striatum of R6/2 mice, with significant volume increases in the lateral ventricles and globus pallidus. There was no change in the volume of the amygdala, internal capsule or hippocampal formation. There was a significant increase in signal intensity in the globus pallidus, amygdala, cortex and striatum in R6/2 mice that may reflect neuronal atrophy. This study clearly shows the potential of MRI for morphological phenotyping of rodent models of HD and other neurological diseases. Having obtained proof-of-principle for the technique using ex vivo tissue, it is now our intention to carry out in vivo measurement of developing pathology in HD transgenic mice, and correlate this with behavioral deficits.

Study of flow and hydrodynamic dispersion in a porous medium using pulsed–field–gradient magnetic resonance

This paper reports on the use of pulsed–field gradient (PFG) magnetic resonance (MR) techniques to obtain displacement and velocity spectra of steady–state, saturated flow through a column packed with glass beads. The displacement spectra obtained by PFG MR correspond to travel–distance probability–density functions (PDF) for initial conditions of a concentration impulse in a column with zero concentration. These spectra show strong dispersion–time dependence, and depart from Gaussian–shaped PDFs for short dispersion times. These data provide estimates of the dispersion–time dependence of transverse and longitudinal dispersion coefficients. The longitudinal dispersion coefficient reaches its long–time behaviour more slowly than the transverse coefficient; long–time values obtained from MR data agree well with those calculated using existing empirical correlations. A model based on three components of apparent velocities and dispersion coefficients is sufficient to describe the time dependence of displacement spectra for water flow through the bead column. The short–distance component arise because of convection–dispersion–diffusion processes within the narrow necks between particles. The long–distance component, on the other hand, represents a macroscopic convection–dispersion process. This study shows that PFG MR flow spectroscopy is a simple but potentially useful method for the investigation of flow and hydrodynamic dispersion in porous media, especially for time–dependent phenomena.

Visualisation of changes in regional cerebral blood flow (rCBF) produced by ketamine using long TE gradient-echo sequences: Preliminary results

Autoradiographic studies have shown that low dose ketamine produces increases in regional glucose utilisation and blood flow in the hippocampus, cerebral cortex, and olfactory lobe in the rat brain, probably due to antagonism at the NMDA receptor. Functional MRI using deoxyhaemoglobin contrast can be used to study changes in regional cerebral blood flow (rCBF). Long TE gradient-echo sequences were used to study changes in rCBF produced by low dose ketamine in rats anaesthetised with nitrous oxide, supplemented with either halothane (HAL) or fentanyl/fluanisone/midazolam (FFM) combination. Images from rats in the FFM group showed a 10-14% increase in signal intensity in the hippocampus, cerebral cortex, and olfactory lobe following either a single bolus or a low dose infusion of ketamine (p < .05). These changes were significantly reduced in the HAL group (p < .005). Halothane is known to attenuate the changes in regional glucose utilisation produced by the noncompetitive NMDA antagonist dizocilpine (MK-801), and its effects on ketamine-induced changes in rCBF seen in this study may be due to a similar effect. The potential use of functional MRI in studying the effect of pharmacological interventions on rCBF is discussed.

An investigation of the origins of contrast in NMR spin echo images of plant tissue

The effects that the spatial distribution of water protons and their transverse relaxation times have on the image contrast of spin echo images of courgette was investigated. The T2-weighted image of courgette contains the most anatomical information. The image contrast was explained using a phenomenological theory based on the Bloch equations, which gave an insight into the morphology and microdynamics of water in the plant tissue. The perceived contrast in the spin echo images of courgette, glucose and Sephadex bead solutions can be dramatically altered by keeping all the imaging acquisition parameters constant, such as the recycle and echo time, but reducing the interpulse spacing by introducing a CPMG train of 180 degrees pulses into the middle of the sequence. These changes were interpreted by considering the microenvironment of the water. This work demonstrates that the origin of image contrast in T2-weighted images of plant tissue can be understood using the water proton transverse relaxation theory developed by Hills et al.

Magnetic resonance imaging analysis of cardiac cycle events in diabetic rats: the effect of angiotensin-converting enzyme inhibition

Non‐invasive magnetic resonance imaging (MRI) was used to characterize changes in left and right ventricular cardiac cycles following induction of experimental, streptozotocin (STZ)‐induced, diabetes in male Wistar rats at different ages. The effects of the angiotensin‐converting enzyme (ACE) inhibitor captopril upon such chronic physiological changes were then evaluated, also for the first time. Diabetes was induced at the age of 7 weeks in two experimental groups, of which one group was subsequently maintained on captopril (2 g l−1)‐containing drinking water, and at 10 and 13 weeks in two further groups. The fifth group provided age‐matched controls. All groups (each n = 4 animals) were scanned consistently at 16 weeks, in parallel with timings used in earlier studies that employed this experimental model. Cine magnetic resonance (MR) image acquisition provided transverse sections through both ventricles at twelve time points covering systole and most of diastole. These yielded reconstructions of cardiac anatomy used to derive critical functional indices and their dependence upon time following the triggering electrocardiographic R waves. The left and right ventricular end‐diastolic (EDV), end‐systolic (ESV) and stroke volumes (SV), and ejection fractions (EF) calculated from each, control and experimental, group showed matching values. This confirmed a necessary condition requiring balanced right and left ventricular outputs and further suggested that STZ‐induced diabetes produced physiological changes in both ventricles. Absolute left and right ventricular SVs were significantly altered in all diabetic animals; EDVs and EFs significantly altered in animals diabetic from 7 and 10 but not 13 weeks. When normalized to body weight, left and right ventricular SVs had significantly altered in animals diabetic from 7 and 10 weeks but not 13 weeks. Normalized left ventricular EDVs were also significantly altered in animals diabetic from 7 and 10 weeks. However, normalized right ventricular EDVs were significantly altered only in animals made diabetic from 7 weeks. Diabetic hearts showed major kinetic changes in left and right ventricular contraction (ejection) and relaxation (filling). Both the initial rates of volume change (dV/dt) in both ventricles and the plots of dV/dt values through the cardiac cycle demonstrated more gradual developments of tension during systole and relaxation during diastole. Estimates of the derived left ventricular performance parameters of cardiac output, cardiac power output and stroke work in control animals were comparable with human values when normalized to both body (or cardiac) weight and heart rate. All deteriorated with diabetes. Comparisons of experimental groups diabetic from 7 weeks demonstrated that captopril treatment relieved the alterations in critical volumes, dependence of SV upon EDV, kinetics of systolic contraction and diastolic relaxation and in the derived indicators of ventricular performance. This study represents the first demonstration using non‐invasive MRI of early, chronic changes in diastolic filling and systolic ejection in both the left and the right ventricles and of their amelioration by ACE inhibition following STZ‐induction of diabetes in intact experimental animals.