Exploratory Multisite MR Spectroscopic Imaging Shows White Matter Neuroaxonal Loss Associated with Complications of Type 1 Diabetes in Children

BACKGROUND AND PURPOSE

Type 1 diabetes affects over 200,000 children in the United States and is associated with an increased risk of cognitive dysfunction. Prior single-site, single-voxel MRS case reports and studies have identified associations between reduced NAA/Cr, a marker of neuroaxonal loss, and type 1 diabetes. However, NAA/Cr differences among children with various disease complications or across different brain tissues remain unclear. To better understand this phenomenon and the role of MRS in characterizing it, we conducted a multisite pilot study.

MATERIALS AND METHODS

In 25 children, 6-14 years of age, with type 1 diabetes across 3 sites, we acquired T1WI and axial 2D MRSI along with phantom studies to calibrate scanner effects. We quantified tissue-weighted NAA/Cr in WM and deep GM and modeled them against study covariates.

RESULTS

We found that MRSI differentiated WM and deep GM by NAA/Cr on the individual level. On the population level, we found significant negative associations of WM NAA/Cr with chronic hyperglycemia quantified by hemoglobin A1c (P < .005) and a history of diabetic ketoacidosis at disease onset (P < .05). We found a statistical interaction (P < .05) between A1c and ketoacidosis, suggesting that neuroaxonal loss from ketoacidosis may outweigh that from poor glucose control. These associations were not present in deep GM.

CONCLUSIONS

Our pilot study suggests that MRSI differentiates GM and WM by NAA/Cr in this population, disease complications may lead to neuroaxonal loss in WM in children, and deeper investigation is warranted to further untangle how diabetic ketoacidosis and chronic hyperglycemia affect brain health and cognition in type 1 diabetes.

LEARNING 3D WHITE MATTER MICROSTRUCTURE FROM 2D HISTOLOGY

Histological analysis is typically the gold standard for validating measures of tissue microstructure derived from magnetic resonance imaging (MRI) contrasts. However, most histological investigations are inherently 2-dimensional (2D), due to increased field-of-view, higher in-plane resolutions, ease of acquisition, decreased costs, and a large number of available contrasts compared to 3-dimensional (3D) analysis. Because of this, it would be of great interest to be able to learn the 3D tissue microstructure from 2D histology. In this study, we use diffusion MRI (dMRI) of a squirrel monkey brain and corresponding myelin stained sections in combination with a convolution neural network to learn the relationship between the 3D diffusion estimated axonal fiber orientation distributions and the 2D myelin stain. We find that we are able to estimate the 3D fiber distribution with moderate to high angular agreement with the ground truth (median angular correlation coefficients of 0.48 across the unseen slices). This network could be used to validate dMRI neuronal structural measurements in 3D, even if only 2D histology is available for validation. Generalization is possible to transfer this network to human stained sections to infer the 3D fiber distribution at resolutions currently unachievable with dMRI, which would allow diffusion fiber tractography at unprecedented resolutions. We envision the use of similar networks to learn other 3D microstructural measures from an array of potential common 2D histology contrasts.

Does visual subordinate-level categorisation engage the functionally defined fusiform face area?

Functional magnetic resonance imaging was used to compare brain activation associated with basic-level (e.g. bird) and subordinate-level (e.g. eagle) processing for both visual and semantic judgements. We localised the putative face area for 11 subjects, who also performed visual matching judgements for pictures and aurally presented words. The middle fusiform and occipital gyri were recruited for subordinate minus basic visual judgements, reflecting additional perceptual processing. When the face area was localised individually for each subject, analyses in the middle fusiform gyri revealed that subordinate-level processing activated the individuals face area. We propose that what is unique about the way faces engage this region is the focal spatial distribution of the activation rather than the recruitment of the face per se. Eight subjects also performed semantic judgements on aurally presented basic- and subordinate-level words. The parahippocampal gyri were more activated for subordinate-level than basic-level semantic judgements. Finally, the left posterior inferior temporal gyrus was activated for subordinate-level judgements, both visual and semantic, as well as during passive viewing of faces.

Validation of diffusion tensor MRI in the central nervous system using light microscopy: quantitative comparison of fiber properties

Diffusion tensor imaging (DTI) provides an indirect measure of tissue structure on a microscopic scale. To date, DTI is the only imaging method that provides such information in vivo, and has proven to be a valuable tool in both research and clinical settings. In this study, we investigated the relationship between white matter structure and diffusion parameters measured by DTI. We used micrographs from light microscopy of fixed, myelin-stained brain sections as a gold standard for direct comparison with data from DTI. Relationships between microscopic tissue properties observed with light microscopy (fiber orientation, density and coherence) and fiber properties observed by DTI (tensor orientation, diffusivities and fractional anisotropy) were investigated. Agreement between the major eigenvector of the tensor and myelinated fibers was excellent in voxels with high fiber coherence. In addition, increased fiber spread was strongly associated with increased radial diffusivity (p = 6 × 10(-6)) and decreased fractional anisotropy (p = 5 × 10(-8)), and was weakly associated with decreased axial diffusivity (p = 0.07). Increased fiber density was associated with increased fractional anisotropy (p = 0.03), and weakly associated with decreased radial diffusivity (p < 0.06), but not with axial diffusivity (p = 0.97). The mean diffusivity was largely independent of fiber spread (p = 0.24) and fiber density (p = 0.34).

Validation of diffusion tensor MRI in the central nervous system using light microscopy: quantitative comparison of fiber properties

Diffusion tensor imaging (DTI) provides an indirect measure of tissue structure on a microscopic scale. To date, DTI is the only imaging method that provides such information in vivo, and has proven to be a valuable tool in both research and clinical settings. In this study, we investigated the relationship between white matter structure and diffusion parameters measured by DTI. We used micrographs from light microscopy of fixed, myelin-stained brain sections as a gold standard for direct comparison with data from DTI. Relationships between microscopic tissue properties observed with light microscopy (fiber orientation, density and coherence) and fiber properties observed by DTI (tensor orientation, diffusivities and fractional anisotropy) were investigated. Agreement between the major eigenvector of the tensor and myelinated fibers was excellent in voxels with high fiber coherence. In addition, increased fiber spread was strongly associated with increased radial diffusivity (p = 6 × 10(-6)) and decreased fractional anisotropy (p = 5 × 10(-8)), and was weakly associated with decreased axial diffusivity (p = 0.07). Increased fiber density was associated with increased fractional anisotropy (p = 0.03), and weakly associated with decreased radial diffusivity (p < 0.06), but not with axial diffusivity (p = 0.97). The mean diffusivity was largely independent of fiber spread (p = 0.24) and fiber density (p = 0.34).

Streptococcal toxic shock syndrome: a description of 14 cases from North Yorkshire, UK

OBJECTIVE

To analyze the clinical and laboratory features of patients diagnosed with streptococcal toxic shock syndrome (TSS) in North Yorkshire from 1986 to 1999.

METHODS

Records of patients with features satisfying the published criteria for streptococcal TSS were reviewed from laboratory and clinical records made at the time and from the hospital case notes. Isolates of streptococci were analyzed for serotype and genes encoding for the production of streptococcal pyrogenic exotoxins.

RESULTS

Fourteen patients satisfied the entry criteria. In one district, where the data were complete, the annual incidence of detected streptococcal TSS rose from 1.1 to 9.5 cases per million population in the 1990s. TSS was associated with various M serotypes of group A streptococci and various exotoxin genotypes. Two cases (14% of the series) were associated with severe group G streptococcal infection. The fatality rate was 64%, and the mode of time to death was 4 days. Local tissue necrosis occurred in 71% of cases, including necrotizing fasciitis, intrathoracic and intra-abdominal forms. Non-steroidal anti-inflammatory drugs (NSAIDs) had been taken around the time of onset of disease by 92% of the patients with TSS.

CONCLUSIONS

There has been a dramatic increase in the number of detected cases of streptococcal TSS over the 14 years since the first case was recognized here. There was a wide range of invasive forms of infection, a high fatality rate even in fit young adults, and a rapid course from onset to death. There was a high association of TSS with aggressive streptococcal infection producing local tissue necrosis.

Respiratory effects in human functional magnetic resonance imaging due to bulk susceptibility changes

Functional magnetic resonance imaging relies on detecting small changes in the signal in the presence of noise from various sources. It has been shown that periodic variations in the signal at the respiratory frequency occur in the brain and various techniques have been proposed to remove them. However, the precise mechanism by which respiration affects the fMRI signal has not yet been proven. Here, we explore the nature of respiratory signal variations and the artefacts they produce in brain images. Our results demonstrate conclusively that bulk susceptibility variations in the lungs during respiration cause variations in the static magnetic field within the brain tissue. These variations in field strength and homogeneity lead to a shift of the image and a shading of image intensity in the phase encoding direction. These artefacts, if left uncorrected, may lead to the production of spurious activations and/or decreased statistical significance of true activations in fMRI. In addition, these results suggest that respiration effects may not necessarily be well characterized as simple additive noise and that an alternative model based on the physical origins of susceptibility variations may be more appropriate.

Theoretical analysis of the effects of noise on diffusion tensor imaging

A theoretical framework is presented for understanding the effects of noise on estimates of the eigenvalues and eigenvectors of the diffusion tensor at moderate to high signal-to-noise ratios. Image noise produces a random perturbation of the diffusion tensor. Power series solutions to the eigenvalue equation are used to evaluate the effects of the perturbation to second order. It is shown that in anisotropic systems the expectation value of the largest eigenvalue is overestimated and the lowest eigenvalue is underestimated. Hence, diffusion anisotropy is overestimated in general. This result is independent of eigenvalue sorting bias. Furthermore, averaging eigenvalues over a region of interest produces greater bias than averaging tensors prior to diagonalization. Finally, eigenvector noise is shown to depend on the eigenvalue contrast and imposes a theoretical limit on the accuracy of simple fiber tracking schemes. The theoretical results are shown to agree with Monte Carlo simulations.

The relationship of problems in biomedical MRI to the study of porous media

The NMR methods that are used to characterize inanimate porous media measure relaxation times and related phenomena and material transport, fluid displacement and flow. Biological tissues are comprised of multiple small, fluid-filled compartments, such as cells, that restrict the movement of the bulk solvent water and whose constituents influence water proton relaxation times via numerous interactions with macromolecular surfaces. Several of the methods and concepts that have been developed in one field of application are also of great value in the other, and it may be expected that technical developments that have been spurred by biomedical applications of MR imaging will be used in the continuing study of porous media. Some recent specific studies from our laboratory include the development of multiple quantum coherence methods for studies of ordered water in anisotropic macromolecular assemblies, studies of the degree of restriction of water diffusion in cellular systems, multiple selective inversion imaging to depict the ratios of proton pool sizes and rates of magnetization transfer between proton populations, and diffusion tensor imaging to depict tissue anisotropies. These illustrate how approaches to obtain structural information from biological media are also relevant to porous media. For example, the recent development of oscillating gradient spin echo techniques (OGSE), an approach that extends our ability to resolve apparent diffusion changes over different time scales in tissues, has also been used to compute surface to volume measurements in assemblies of pores. Each of the new methods can be adapted to provide spatially resolved quantitative measurements of properties of interest, and these can be efficiently acquired with good accuracy using fast imaging methods such as echo planar imaging. The community of NMR scientists focused on applications to porous media should remain in close communication with those who use MRI to study problems in biomedicine, to their mutual benefits.

Neonatal auditory activation detected by functional magnetic resonance imaging

The objective of this study was to detect auditory cortical activation in non-sedated neonates employing functional magnetic resonance imaging (fMRI). Using echo-planar functional brain imaging, subjects were presented with a frequency-modulated pure tone; the BOLD signal response was mapped in 5 mm-thick slices running parallel to the superior temporal gyrus. Twenty healthy neonates (13 term, 7 preterm) at term and 4 adult control subjects. Blood oxygen level-dependent (BOLD) signal in response to auditory stimulus was detected in all 4 adults and in 14 of the 20 neonates. FMRI studies of adult subjects demonstrated increased signal in the superior temporal regions during auditory stimulation. In contrast, signal decreases were detected during auditory stimulation in 9 of 14 newborns with BOLD response. fMRI can be used to detect brain activation with auditory stimulation in human infants.

Measurements of restricted diffusion using an oscillating gradient spin-echo sequence

An oscillating gradient spin-echo (OGSE) pulse sequence was used to measure the apparent diffusion coefficient (D(app)) of water in the short diffusion time regime in the presence of restrictions. The diffusion coefficients of water in a simple water sample and a water and oil mixture were measured to be the same for different periods of the gradient oscillation, as expected when there are no restriction effects. The D(app) of water in the spaces between closely packed beads was also measured as a function of the gradient oscillation periods in the range 11 to 80 ms. The D(app) of water in restricted systems varies with the period of the gradient oscillation and the dispersion depends on the scale of the restriction. For a sample of packed beads of diameter 9.1 +/- 0.7 microm, the pore surface-to-volume ratio was estimated experimentally by this method to be 1.3 +/- 0.1 microm(-1), corresponding to a mean pore diameter of 6.4 +/- 0.7 microm. A Monte Carlo computer simulation of the NMR OGSE signal from the spins diffusing in a system of compartments was also implemented and the D(app) demonstrated similar behavior with gradient oscillation periods.

A model for susceptibility artefacts from respiration in functional echo-planar magnetic resonance imaging

Respiration causes variations in the signals acquired during magnetic resonance imaging (MRI) and therefore is a significant source of noise in functional brain imaging. A primary component of respiratory noise may arise from variations of bulk susceptibility or air volume in the chest. Here we investigate the nature of the image artefacts that can be caused by such changes. We develop a simple model which attempts to mimic the effects of variations in susceptibility and volume during respiration. Theoretical calculations, computer simulations and imaging experiments with this model show that small variations in susceptibility within the thorax from alterations in the paramagnetism of cavity gas may lead to a shift of the image on the order of 0.1 pixels as well as a shading of the intensity by +/-1%. These effects are observed to be predominant in the phase-encoding direction. They may lead to the production of spurious activations in functional MRI and are likely to be of more importance at higher field strengths.

Regional brain volume abnormalities and long-term cognitive outcome in preterm infants

CONTEXT

Preterm infants have a high prevalence of long-term cognitive and behavioral disturbances. However, it is not known whether the stresses associated with premature birth disrupt regionally specific brain maturation or whether abnormalities in brain structure contribute to cognitive deficits.

OBJECTIVE

To determine whether regional brain volumes differ between term and preterm children and to examine the association of regional brain volumes in prematurely born children with long-term cognitive outcomes.

DESIGN AND SETTING

Case-control study conducted in 1998 and 1999 at 2 US university medical schools.

PARTICIPANTS

A consecutive sample of 25 eight-year-old preterm children recruited from a longitudinal follow-up study of preterm infants and 39 term control children who were recruited from the community and who were comparable with the preterm children in age, sex, maternal education, and minority status.

MAIN OUTCOME MEASURES

Volumes of cortical subdivisions, ventricular system, cerebellum, basal ganglia, corpus callosum, amygdala, and hippocampus, derived from structural magnetic resonance imaging scans and compared between preterm and term children; correlations of regional brain volumes with cognitive measures (at age 8 years) and perinatal variables among preterm children.

RESULTS

Regional cortical volumes were significantly smaller in the preterm children, most prominently in sensorimotor regions (difference: left, 14.6%; right, 14.3% [P<.001 for both]) but also in premotor (left, 11.2%; right, 12.6% [P<.001 for both]), midtemporal (left, 7.4% [P =.01]; right, 10.2% [P<.001]), parieto-occipital (left, 7.9% [P =.01]; right, 7.4% [P =.005]), and subgenual (left, 8.9% [P =.03]; right, 11.7% [P =.01]) cortices. Preterm children's brain volumes were significantly larger (by 105. 7%-271.6%) in the occipital and temporal horns of the ventricles (P<. 001 for all) and smaller in the cerebellum (6.7%; P =.02), basal ganglia (11.4%-13.8%; P</=.005), amygdala (left, 20.2% [P =.001]; right, 30.0% [P<.001]), hippocampus (left, 16.0% [P =.001]; right, 12.0% [P =.007]), and corpus callosum (13.1%-35.2%; P</=.01 for all). Volumes of sensorimotor and midtemporal cortices were associated positively with full-scale, verbal, and performance IQ scores (P<.01 for all).

CONCLUSIONS

Our data indicate that preterm birth is associated with regionally specific, long-term reductions in brain volume and that morphological abnormalities are, in turn, associated with poorer cognitive outcome. JAMA. 2000;284:1939-1947.

Effects of cell volume fraction changes on apparent diffusion in human cells

Diffusion-weighted imaging was used to study the relationship between apparent diffusion coefficient (ADC) and cell volume fraction in cell suspensions and packed arrays. Cell volume fraction was varied by changing extracellular fluid osmolarity (for human glial cells) and by changing cell density (for human glial and red blood cells). In packed arrays of glial cells, ADC increased 10% when cells shrank and decreased 13% when cells swelled. ADC decreased 34% as cell density increased from 0 to 72%. In erythrocyte suspensions, ADC decreased 90% as the cell density increased from 0 to 89%. These results agree with theoretical predictions.

The fusiform "face area" is part of a network that processes faces at the individual level

According to modular models of cortical organization, many areas of the extrastriate cortex are dedicated to object categories. These models often assume an early processing stage for the detection of category membership. Can functional imaging isolate areas responsible for detection of members of a category, such as faces or letters? We consider whether responses in three different areas (two selective for faces and one selective for letters) support category detection. Activity in these areas habituates to the repeated presentation of one exemplar more than to the presentation of different exemplars of the same category, but only for the category for which the area is selective. Thus, these areas appear to play computational roles more complex than detection, processing stimuli at the individual level. Drawing from prior work, we suggest that face-selective areas may be involved in the perception of faces at the individual level, whereas letter-selective regions may be tuning themselves to font information in order to recognize letters more efficiently.

Abnormal ventral temporal cortical activity during face discrimination among individuals with autism and Asperger syndrome

BACKGROUND

Recognition of individual faces is an integral part of both interpersonal interactions and successful functioning within a social group. Therefore, it is of considerable interest that individuals with autism and related conditions have selective deficits in face recognition (sparing nonface object recognition).

METHOD

We used functional magnetic resonance imaging (fMRI) to study face and subordinate-level object perception in 14 high-functioning individuals with autism or Asperger syndrome (the autism group), in comparison with 2 groups of matched normal controls (normal control group ] [NC1] and normal control group 2 [NC2]) (n = 14 for each). Regions of interest (ROIs) were defined in NC1 and then applied in comparisons between NC2 and the autism group. Regions of interest were also defined in NC2 and then applied to comparisons between NC1 and the autism group as a replication study.

RESULTS

In the first set of comparisons, we found significant task x group interactions for the size of activation in the right fusiform gyrus (FG) and right inferior temporal gyri (ITG). Post hoc analyses showed that during face (but not object) discrimination, the autism group had significantly greater activation than controls in the right ITG and less activation of the right FG. The replication study showed again that the autism group used the ITG significantly more for processing faces than the control groups, but for these analyses, the effect was now on the left side. Greater ITG activation was the pattern found in both control groups during object processing.

CONCLUSIONS

Individuals with autism spectrum disorders demonstrate a pattern of brain activity during face discrimination that is consistent with feature-based strategies that are more typical of nonface object perception.

Expertise for cars and birds recruits brain areas involved in face recognition

Expertise with unfamiliar objects ('greebles') recruits face-selective areas in the fusiform gyrus (FFA) and occipital lobe (OFA). Here we extend this finding to other homogeneous categories. Bird and car experts were tested with functional magnetic resonance imaging during tasks with faces, familiar objects, cars and birds. Homogeneous categories activated the FFA more than familiar objects. Moreover, the right FFA and OFA showed significant expertise effects. An independent behavioral test of expertise predicted relative activation in the right FFA for birds versus cars within each group. The results suggest that level of categorization and expertise, rather than superficial properties of objects, determine the specialization of the FFA.

Density matrix simulations of the effects of J coupling in spin echo and fast spin echo imaging

A computer simulation has been used to calculate the effects of J coupling on the amplitudes of echoes produced by CPMG sequences. The program computes the evolution of the density matrix for different pulse intervals and can predict the signals obtainable from spin systems of any size and complexity. Results from the simulation confirm the prediction that a decrease in the effects of J coupling is largely responsible for the bright fat signal seen in fast spin echo imaging at high pulse rates. The effects of J coupling on CPMG echotrains are examined for A3B2 and A3B2C2 spin systems over a wide range of J coupling and chemical shift values and pulse spacings. The effects of J coupling on the point spread function obtained with fast spin echo imaging are also discussed.

Functional magnetic resonance imaging identifies abnormal visual cortical function in patients with occipital lobe epilepsy

PURPOSE

To determine whether functional magnetic resonance imaging (fMRI) can reliably identify lateralized cortical dysfunction in patients with suspected occipital lobe epilepsy.

METHODS

We compared visual cortical function of 10 patients with intractable occipital lobe epilepsy with nine control subjects' fMRI. Visual stimulation by using an alternating checkerboard pattern results in transient increases in the intensity of the proton magnetic resonance signal of water in the occipital lobes during echo-planar imaging. We used these stimulus-dependent changes in signal intensity to construct functional activation maps, which we registered onto anatomic images.

RESULTS

After full-field stimulation, none of the patients with occipital lobe epilepsy had normal activation patterns, whereas eight of the nine control subjects had normal patterns (p = 0.001). Abnormalities consisted of either a markedly asymmetric activation pattern in six of 10 patients (p = 0.04), or a complete absence of activation in four of 10 patients (p = 0.05). The abnormal side of activation was concordant with the side of seizure onset in all six patients with asymmetric activation maps. Half-field stimulation produced less reliable results. Although more patients had abnormal activation maps than did controls with half-field stimulation (p = 0.04), the abnormal side was discordant with the side of seizure onset in three of the five patients who had markedly asymmetric activation patterns.

CONCLUSIONS

These results suggest that fMRI with full-field stimulation is a reliable, noninvasive method for identifying areas of abnormal visual cortical function ipsilateral to the epileptogenic region in patients with occipital lobe epilepsy.

Activation of the middle fusiform 'face area' increases with expertise in recognizing novel objects

Part of the ventral temporal lobe is thought to be critical for face perception, but what determines this specialization remains unknown. We present evidence that expertise recruits the fusiform gyrus 'face area'. Functional magnetic resonance imaging (fMRI) was used to measure changes associated with increasing expertise in brain areas selected for their face preference. Acquisition of expertise with novel objects (greebles) led to increased activation in the right hemisphere face areas for matching of upright greebles as compared to matching inverted greebles. The same areas were also more activated in experts than in novices during passive viewing of greebles. Expertise seems to be one factor that leads to specialization in the face area.

An fMRI study of Stroop word-color interference: evidence for cingulate subregions subserving multiple distributed attentional systems

BACKGROUND

The goal of this study was to model the functional connectivity of the neural systems that subserve attention and impulse control. Proper performance of the Stroop Word-Color Interference Task requires both attention and impulse control.

METHODS

Word-color interference was studied in 34 normal adult subjects using functional magnetic resonance imaging.

RESULTS

Interregional correlation analyses suggested that the anterior cingulate is coupled functionally with multiple regions throughout the cerebrum. A factor analysis of the significant regional activations further emphasized this functional coupling. The cingulate or related mesial frontal cortices loaded on each of the seven factors identified in the factor analysis. Other regions that loaded significantly on these factors have been described previously as belonging to anatomically connected circuits believed to subserve sensory tuning, receptive language, vigilance, working memory, response selection, motor planning, and motor response functions. These seven factors appeared to be oriented topographically within the anterior cingulate, with sensory, working memory, and vigilance functions positioned more rostrally, and response selection, motor planning, and motor response positioned progressively more caudally.

CONCLUSIONS

These findings support a parallel distributed processing model for word-color interference in which portions of the anterior cingulate cortex modify the strengths of multiple neural pathways used to read and name colors. Allocation of attentional resources is thought to modify pathway strengths by reducing cross-talk between information processing modules that subserve the competing demands of reading and color naming. The functional topography of these neural systems observed within the cingulate argues for the presence of multiple attentional subsystems, each contributing to improved task performance. The topography also suggests a role for the cingulate in coordinating and integrating the activity of these multiple attentional subsystems.

Analysis of J coupling-induced fat suppression in DIET imaging

The DIET (or dual interval echo train) sequence, a modification of the fast spin echo (FSE) sequence that selectively reduces signal from fat in MR images, has been investigated. The DIET sequence uses an initial echo spacing longer than that of a conventional FSE sequence, thus allowing J coupling-induced dephasing to take effect. The sequence is evaluated theoretically, and its effectiveness on a hydrocarbon (1-pentene) is demonstrated numerically using density matrix calculations. The sequence is also evaluated experimentally using in vitro solutions and in vivo imaging. The efficacy of the sequence is compared for different lipid chemical structures, field strengths, and pulse sequence parameters.

A functional magnetic resonance imaging study of tic suppression in Tourette syndrome

BACKGROUND

The inability to inhibit unwanted behaviors and impulses produces functional debility in a broad range of neuropsychiatric disorders. A potentially important model of impulse control is volitional tic suppression in Tourette syndrome.

METHODS

Tic suppression was studied in 22 adult subjects with Tourette syndrome by using functional magnetic resonance imaging. Images acquired during periods of voluntary tic suppression were compared with images acquired when subjects allowed the spontaneous expression of their tics. The magnitudes of signal change in the images were then correlated with measures of the severity of tic symptoms.

CONCLUSIONS

Significant changes in signal intensity were seen in the basal ganglia and thalamus and in anatomically connected cortical regions believed to subserve attention-demanding tasks. The magnitudes of regional signal change in the basal ganglia and thalamus correlated inversely with the severity of tic symptoms. These findings suggest that the pathogenesis of tics involves an impaired modulation of neuronal activity in subcortical neural circuits.

Levels of categorization in visual recognition studied using functional magnetic resonance imaging

BACKGROUND

Recent functional neuroimaging results implicate part of the ventral temporal lobe of the brain in face recognition, and have, together with neurophysiological findings, been used as evidence for a face-specific neural module in the brain. Experimental designs, however, have often failed to distinguish between the class of the object used as the stimulus (face or non-face) and the level of categorization at which the stimulus is recognized (the 'basic' level, such as 'bird', at which familiar objects are first recognized, or more subordinate levels - 'sparrow', for example - which require additional perceptual processing). We have used echo-planar functional magnetic resonance imaging to compare brain activation for the matching of non-face objects with subordinate-level and basic-level descriptors.

RESULTS

The additional visual processing required to verify the subordinate level of a picture over its basic level was associated with activation of the fusiform and inferior temporal gyri (FIT) as well as the temporal poles. These areas correspond closely to those previously implicated in the processing of facial images.

CONCLUSIONS

Our results indicate that areas of the ventral visual pathway that have been associated with face recognition are sensitive to manipulations of the categorization level of non-face objects. This idea offers an alternative to the dominant view that FIT may be organized according to conceptual categories, and our results establish the importance of manipulating task requirements when evaluating a 'neural module' hypothesis.

Diffusion-weighted multiple shot echo planar imaging of humans without navigation

A new method is proposed for reducing the artifacts produced in diffusion-weighted imaging. When data are acquired using multiple shot echo planar acquisitions, conventional reconstruction methods produce artifactual images as a consequence of diffusion weighting and small amounts of bulk motion of the subject. If the amount of motion can be determined, it is possible to correct the data before reconstruction, which removes the artifact. A method for estimating the motion from the acquired data has been developed and evaluated. This method assumes that ghost image effects will be minimized when motion has been correctly compensated. By considering the amount of signal in the background of the image, appropriate corrections to the data can be made, and the accuracy of the motion compensation may be estimated. This technique has been evaluated by computer simulation, and its performance has been demonstrated in a phantom and humans with both two- and four-shot echo planar acquisitions and using both "mosaic" and "interleaved" sampling schemes.

Effects of osmotically driven cell volume changes on diffusion-weighted imaging of the rat optic nerve

The apparent diffusion coefficient (ADC) of the rat optic nerve was measured in vitro, using magnetic resonance imaging, to determine the effects of changes in cellular volume fraction on the diffusion of tissue water. Nerve ADC was determined under conditions of cell membrane depolarization and (i) increased intracellular volume, (ii) decreased intracellular volume, and (iii) negligible volume change. Depolarization alone had little affect on ADC, whereas volume changes produced strong, reversible effects. Increased cell volume decreased ADC and vice versa. These results are consistent with the view that changes in the extracellular space are the major source of ADC changes in brain tissue.

A comparison of the pulpal response to freeze-dried bone, calcium hydroxide, and zinc oxide-eugenol in primary teeth in two cynomolgus monkeys

This study assessed the pulp healing response to human freeze-dried bone (FDB) in two cynomolgus monkeys using 36 noncarious primary teeth pulpotomized and randomly assigned to three medicaments. FDB was applied on the pulp stumps and covered with sterile tin foil as experimental group. The two other groups received either calcium hydroxide [Ca(OH)2], or IRM (reinforced ZOE). All teeth were restored with amalgam. One animal was sacrificed at 6 weeks and the other 6 months after treatment. Teeth were extracted and placed in 10% formalin. Histological evaluation indicated that 100% of teeth treated with FDB had vital pulps compared with 75% of the Ca(OH)2 group after 6 weeks. Dentin bridges were present in 87.5% of FDB versus 75% of Ca(OH)2 group. Inflammatory cells were absent or mild in 100% of FDB-treated versus 75% of the Ca(OH)2 group. After 6 months, 83.3% of FDB-treated teeth had vital pulps compared with the Ca(OH)2 group, which showed 100% pulpal necrosis. In FDB-treated pulps, 100% of teeth showed dentin bridges versus 50% of teeth treated with Ca(OH)2. Inflammatory cells were absent or mildly present in 83.3% of FDB-treated teeth while 100% of Ca(OH)2 showed moderate to severe inflammation. IRM-treated teeth all showed pulpal necrosis after 6 months. We concluded that FDB was superior to calcium hydroxide in treating primary pulp dentition in cynomolgus monkeys.

Diffusion-weighted imaging in tissues: theoretical models

Typical diffusion measurements use Stejskal-Tanner pulsed gradient spin echo sequences to provide information about the average diffusion and displacement profiles of particles in a sample. To derive structural information, a measured displacement profile has to be related by means of a model to the physical and geometrical properties of the tissue, such as diffusion coefficients and shapes of semi-permeable membranes of compartments in the system. The behavior of the NMR signal and the measured ADC are greatly affected by the cellular architecture of a tissue, mainly because cellular membranes are relatively impermeable to water. For long diffusion times, and small signal attenuations, ADC is relatively insensitive to how it is measured. In general, however, ADC values are not readily interpreted unless the measuring conditions are specified in detail. For given measuring conditions, ADC depends on intra- and extracellular diffusion coefficients, membrane permeabilities, cell sizes and the cellular volume fraction. If intra- and extracellular T2 relaxation rates are different enough, ADC may also depend on the relaxation properties of the system and the echo time. An improved understanding of the precise influence of these factors has been obtained by detailed consideration of theoretical and computer models that can be related to experimental data in simple systems. Further refinements of such models should advance our understanding of water diffusion in tissues.

Analysis and correction of motion artifacts in diffusion weighted imaging

For diffusion-weighted magnetic resonance imaging and under circumstances where patient movement can be modeled as rigid body motion, it is shown both theoretically and experimentally that translations and rotations produce phase errors which are zero- and first-order, respectively, in position. While a navigator echo can be used to correct the imaging data for arbitrary translations, only when the diffusion gradient is applied in the phase encode direction is there sufficient information to correct for rotations around all axes, and therefore for general rigid body motion. Experiments in test objects and human brain imaging confirm theoretical predictions and demonstrate that appropriate corrections dramatically improve image quality in vivo.

Functional brain imaging at 1.5 T using conventional gradient echo MR imaging techniques

There is considerable interest in the use of MR imaging to study brain function. Recently, it has been demonstrated that small changes in signal intensity occur in the visual cortex in T2*-weighted imaging in response to appropriate visual stimuli. Similar responses to activation have also been recorded in motor cortex as well as frontal lobes. To date most of these studies have been carried out at very high field strength or they have used echo planar imaging. We report our preliminary results showing that the effects of activation of visual and motor areas of the brains of normal volunteers can be recorded using conventional MR imaging methods on a standard 1.5 T clinical scanner. Using gradient-echo imaging sequences, we have been able to map activated visual and motor cortex with high spatial resolution in multiple planes, and are using this technique to examine the relationship between physiological response and stimulus parameters. Signal changes of the order of 2-12% in images with TE = 45 msec, TR = 120 msec, and alpha = 40 degrees, permit excellent depiction of the regions affected.

Factors influencing contrast in fast spin-echo MR imaging

Multi-echo pulse sequences for producing T2-weighted images in much reduced imaging times have recently been developed for routine clinical use. A number of recent articles have described the contrast obtained with fast spin-echo (FSE) sequences and have generally indicated that they depict tissues very similarly to conventional spin-echo (SE) imaging. There are, however, some important differences in contrast between some tissues in FSE images. This work presents a detailed study of the contrast obtained with FSE imaging sequences and examines the image sequence and tissue parameters which influence contrast. The use of multiple refocusing pulses produces several subtle effects not seen in conventional SE imaging sequences, and in this study the precise nature and extent of such effects are described. The relative contributions to image contrast of magnetization transfer, the decoupling of J-modulation effects, the production of stimulated echoes and direct saturation effects, of diffusion and of the effects of the differential attenuation of different spatial frequencies, are each quantified. The mechanisms responsible for the brighter fat signal seen in FSE images, as well as the loss of signal from some other tissues, are explained. Computer simulations, phantom experiments, and clinical images are all used to support the conclusions.