Ralph Myers

In-vivo measurement of activated microglia in dementia

BACKGROUND Activated microglia have a key role in the brains immune response to neuronal degeneration. The transition of microglia from the normal resting state to the activated state is associated with an increased expression of receptors known as peripheral benzodiazepine binding sites, which are abundant on cells of mononuclear phagocyte lineage. We used brain imaging to study expression of these sites in healthy individuals and patients with Alzheimers disease. METHODS We studied 15 normal individuals (age 32-80 years), eight patients with Alzheimers disease, and one patient with minimal cognitive impairment. Quantitative in-vivo measurements of glial activation were obtained with positron emission tomography (PET) and carbon-11-labelled (R)-PK11195, a specific ligand for the peripheral benzodiazepine binding site. FINDINGS In normal individuals, regional [11C](R)-PK11195 binding did not significantly change with age, except in the thalamus, where an age-dependent increase was found. By contrast, patients with Alzheimers disease showed significantly increased regional [11C](R)-PK11195 binding in the entorhinal, temporoparietal, and cingulate cortex. INTERPRETATION In-vivo detection of increased [11C](R)-PK11195 binding in Alzheimer-type dementia, including mild and early forms, suggests that microglial activation is an early event in the pathogenesis of the disease.

Three-dimensional maximum probability atlas of the human brain, with particular reference to the temporal lobe

Probabilistic atlases of neuroanatomy are more representative of population anatomy than single brain atlases. They allow anatomical labeling of the results of group studies in stereotaxic space, automated anatomical labeling of individual brain imaging datasets, and the statistical assessment of normal ranges for structure volumes and extents. No such manually constructed atlas is currently available for the frequently studied group of young adults. We studied 20 normal subjects (10 women, median age 31 years) with high‐resolution magnetic resonance imaging (MRI) scanning. Images were nonuniformity corrected and reoriented along both the anterior‐posterior commissure (AC–PC) line horizontally and the midsagittal plane sagittally. Building on our previous work, we have expanded and refined existing algorithms for the subdivision of MRI datasets into anatomical structures. The resulting algorithm is presented in the Appendix . Forty‐nine structures were interactively defined as three‐dimensional volumes‐of‐interest (VOIs). The resulting 20 individual atlases were spatially transformed (normalized) into standard stereotaxic space, using SPM99 software and the MNI/ICBM 152 template. We evaluated volume data for all structures both in native space and after spatial normalization, and used the normalized superimposed atlases to create a maximum probability map in stereotaxic space, which retains quantitative information regarding inter‐subject variability. Its potential applications range from the automatic labeling of new scans to the detection of anatomical abnormalities in patients. Further data can be extracted from the atlas for the detailed analysis of individual structures. Hum. Brain Mapping 19:224–247,2003. ©2003 Wiley‐Liss,Inc.

PK (‘peripheral benzodiazepine’) – binding sites in the CNS indicate early and discrete brain lesions: microautoradiographic detection of [3H]PK 11195 binding to activated microglia

The isoquinoline PK 11195 has been suggested as a marker of glial pathology in the lesioned brain. The aim of the present study is to clarify the precise cellular location of its binding site in the central nervous system. Here, we report that in the facial nucleus after facial nerve axotomy–a lesion causing a retrograde neuronal reaction without nerve cell death while keeping the blood–brain barrier intact–activated microglia are the predominant source of lesion-induced increases of PK 11195 binding. Likewise, increased PK 11195 binding is seen in the gracile nucleus after anterograde neuronal injury following sciatic nerve transection. The peak of PK 11195 binding, using the single isomer R-PK 11195, was observed 4 days after the peripheral nerve lesion, consistent with the well-known time course of microglial activation. Photoemulsion microautoradiography confirmed the restriction of PK 11195 binding to activated microglia. The increase of PK 11195 binding in the facial nucleus seen after selective cell death of facial motoneurons by retrograde suicide transport of toxic ricin, a lesion that is accompanied by the rapid transformation of microglia into phagocytes, was no higher than that seen following axotomy. This suggests that the full transformation of microglia into parenchymal phagocytes is not necessary to reach maximal levels of PK 11195 binding. PK 11195, therefore, is a well-suited marker to detect microglial activation in areas of subtle brain pathology, where neither a disturbance of the blood–brain barrier function nor the presence of macrophages and inflammatory cells indicate an on-going disease process.

Thalamic microglial activation in ischemic stroke detected in vivo by PET and [11C]PK11195

Article abstract Using quantitative PET, the authors studied the binding of [11C]PK11195, a marker of activated microglia, in the thalamus of patients with chronic middle cerebral artery infarcts. All patients showed increased [11C]PK11195 binding in the ipsilateral thalamus, indicating the activation of microglia in degenerating projection areas remote from the primary lesion. A persistent increase in [11C]PK11195 binding suggests active, long-term thalamic microstructural changes after corticothalamic connection damage.

Evolution of microglial activation in patients after ischemic stroke: A [11C](R)-PK11195 PET study

We obtained [11C](R)-PK11195 PET scans in six patients at different time points between 3 and 150 days after onset of ischemic stroke in order to measure the time course of microglial activation. Increased [11C](R)-PK11195 binding around the lesion was observed as early as 3 days. Scans at later time points showed ongoing changes in the distribution of the [11C](R)-PK11195 signal, involving the area of the primary lesion and areas distant from the primary lesion site. Our data suggest that [11C](R)-PK11195 PET can be used to investigate both the primary lesion and remote pathological changes following Wallerian degeneration.

[11C](R)-PK11195 positron emission tomography imaging of activated microglia in vivo in Rasmussen’s encephalitis

Article abstract This study was designed to explore the feasibility of PET using [11C](R)-PK11195 as an in vivo marker of activated microglia/brain macrophages for the assessment of neuroinflammation in Rasmussen’s encephalitis (RE). [11C](R)-PK11195 PET was carried out in four normal subjects, two patients with histologically confirmed RE, and three patients with clinically stable hippocampal sclerosis and low seizure frequency. Binding potential maps showing specific binding of [11C](R)-PK11195 were generated for each subject. Regional binding potential values were calculated for anatomically defined regions of interest after coregistration to and spatial transformation into the subjects’ own MRI. In one patient with RE who underwent hemispherectomy, the resected, paraffin-embedded brain tissue was stained with an antibody (CR3/43) that labels activated human microglia. Whereas specific binding of [11C](R)-PK11195 in clinically stable hippocampal sclerosis was similar to that in normal brain, patients with RE showed a focal and diffuse increase in binding throughout the affected hemisphere. In RE, [11C](R)-PK11195 PET can reveal in vivo the characteristic, unilateral pattern known from postmortem neuropathologic study. PET imaging of activated microglia/brain macrophages offers a tool for investigation of a range of brain diseases where neuroinflammation is a component and in which conventional MRI does not unequivocally indicate an inflammatory tissue reaction. [11C](R)-PK11195 PET may help in the choice of appropriate biopsy sites and, further, may allow assessment of the efficacy of antiinflammatory disease–modifying treatment.

Assessment of spatial normalization of PET ligand images using ligand-specific templates.

Recent advances allow robust computation of parametric maps of ligand-receptor binding from PET data sets. Parametric maps may be statistically analyzed at the voxel level, given suitable techniques for both the spatial normalization of image data into a standard space and the application of appropriate statistical tests. The purpose of this study was to spatially normalize parametric maps of [carbonyl-11C]WAY-100635 and [11C]raclopride binding using SPM 96 and ligand-specific templates. Ligand-specific templates were created from integral images taken from healthy subjects. For this, a MRI-based spatial normalization was used: T1-weighted MRI scans were coregistered to the PET integral images, and the spatial normalization of the MRI to the SPM 96 T1 MRI template was applied to the integral images. These integral images were meaned and smoothed to form [carbonyl-11C]WAY-100635 and [11C]raclopride templates. Reliability of spatial normalization using the ligand template method and the previous MRI-based spatial normalization was investigated by using a second set of integral images taken from a different cohort: Landmark coordinates were defined on all spatially normalized integral images. Mean coordinates were found in order to produce an overall (average) landmark for each location. For each image, at each location, the distance from the landmark coordinates to the overall landmark were found. A multivariate analysis of variance was used to examine the effects of observer variance, landmark location, and the method used. Visually acceptable templates were created. While observer variance was not significant, the landmark x method interaction was significant. The ligand template method had significantly smaller distances: Among the landmark locations with this method, the mean distances between individual image landmarks and overall image landmarks ranged from 1. 1 to 4.9 mm. The ligand template method provides a reliable approach for spatial normalization of PET ligand images.

[11C](R)-PK11195 PET imaging of microglial activation in multiple system atrophy

Microglia, the brain’s intrinsic macrophages, bind (R)-PK11195 when activated by neuronal injury. The authors used [11C](R)-PK11195 PET to localize in vivo microglial activation in patients with multiple system atrophy (MSA). Increased [11C](R)-PK11195 binding was primarily found in the dorsolateral prefrontal cortex, putamen, pallidum, pons, and substantia nigra, reflecting the known distribution of neuropathologic changes in MSA. Providing an indicator of disease activity, [11C](R)-PK11195 PET can thus be used to characterize the in vivo neuropathology of MSA.

Macrophage and astrocyte populations in relation to [3H]PK 11195 binding in rat cerebral cortex following a local ischaemic lesion.

PK 11195 is a selective and specific ligand for the peripheral-type benzodiazepine binding site. Its potential for in vivo visualisation of lesioned human brain using positron emission tomography (PET) is currently being assessed. The present study examines the relationship between the temporal development of a local ischaemic lesion with its associated cell populations and the binding of [3H]PK 11195 in rat brain. Unilateral cortical infarcts were induced using the photosensitive dye Rose Bengal. At time intervals from 1 to 7 days after lesioning, the localisation of [3H]PK 11195 binding was determined using in vivo and in vitro autoradiography. Sections adjacent to those used for autoradiography were processed for immunohistochemistry using glial fibrillary acidic protein for astrocytes and ED-1 for macrophages. The results show that the binding of [3H]PK 11195 correlates in both time and spatial localisation with the appearance of macrophages around the lesion. Reactive astrocytes, although present, occupy a separate region in the tissue surrounding the lesion and lie outside the region defined by the [3H]PK 11195 binding. We conclude that the [3H]PK 11195 signal associated with this ischaemic lesion originates primarily from binding to macrophages and that [C]PK 11195 could be used for imaging acute inflammatory response in human brain using PET.

The design and physical characteristics of a small animal positron emission tomograph

A small diameter positron emission tomography, designed specifically for small animal studies, was constructed from existing, commercially available, bismuth germanate (BGO) detectors and electronics. The scanner consists of 16 BGO detector blocks arranged to give a tomograph with a diameter of 115 mm and an axial field of view (FOV) of 50 mm. Each block is cut to produce eight (axial) by seven (radial) individual detector elements. The absence of interplane septa enables the acquisition of 3D data sets consisting of 64 sinograms. A 2D data set of 15 sinograms, consisting of eight direct and seven adjacent cross planes, can be extracted from the 3D data set. Images are reconstructed from the 2D sinograms using a conventional filtered backprojection algorithm. Two methods of normalization were investigated, based on either a rotating 68Ge rod source, or a uniform 68Ge plane source, with a uniform cylindrical 18F phantom. Attenuation of the emitted photons was estimated using a rotating 68Ge rod source. The transaxial resolution of the tomograph was measured as 2.3 mm full width at half maximum (FWHM) and 5.6 mm full width at tenth maximum (FWTM) at the centre of the FOV, degrading to 6.6 mm (radial) and 4.4 mm (tangential) FWHM and 10.4 mm (radial) and 14.4 mm (tangential) FWTM at 40.0 mm from the centre of the FOV. The axial slice width was 4.3 mm FWHM, 10.3 mm FWTM at the centre of the transaxial field of view and 4.4 mm FWHM, 10.6 mm FWTM at 20.0 mm from the centre of the FOV. A scatter fraction of 31.0% was measured at 250-850 keV, for an 18F line source centred in a 60 mm diameter, water-filled phantom, reducing to 20.4% and 13.8% as the lower energy discrimination was increased to 380 keV and 450 keV, respectively. The count rate performance was measured using a noise equivalent count rate method, and the linearity of the dead time correction was confirmed over the count rates encountered during routine scanning. In 2D mode, the absolute sensitivity of the tomograph was measured as 9948 counts s-1 MBq-1 at 250-850 keV, 8284 counts s-1 MBq-1 at 380-850 keV and 6280 counts s-1 MBq-1 at 450-850 keV.