PubMed Health⌕ Search

Biomedical subjects

A Connelly

Publications and source records attributed to A Connelly.

At least 19 recordsLinked to original sources
PubMed2006

Quantification of the shape of fiber tracts.

The fiber tracts generated using diffusion MRI are usually simply displayed and assessed visually for a specific clinical or medical research purpose. This paper proposes computational techniques that can be used to study the shape of the tracts and make interindividual comparisons. These methods make use of fundamental geometric invariants, such as curvatures and torsions, or Fourier descriptors, together with the link of a pair of curves. Intersubject comparisons only require that the starting and ending points of the tracts can be defined and do not require point-by-point correspondences such as obtained using image registration. Principal component analysis-based shape analysis is also investigated. The invariants are tested on simulations and in vivo datasets, and the scale dependence and noise sensitivity of the measures are assessed. The potential for these techniques to be used in neuroscience research and clinical applications is demonstrated.

Algorithms↗
PubMed2005

A rigorous framework for diffusion tensor calculus.

In biological tissue, all eigenvalues of the diffusion tensor are assumed to be positive. Calculations in diffusion tensor MRI generally do not take into account this positive definiteness property of the tensor. Here, the space of positive definite tensors is used to construct a framework for diffusion tensor analysis. The method defines a distance function between a pair of tensors and the associated shortest path (geodesic) joining them. From this distance a method for computing tensor means, a new measure of anisotropy, and a method for tensor interpolation are derived. The method is illustrated using simulated and in vivo data.

Anisotropy↗
PubMed2005

The role of the medial temporal lobe in autistic spectrum disorders.

The neural basis of autistic spectrum disorders (ASDs) is poorly understood. Studies of mnemonic function in ASD suggest a profile of impaired episodic memory with relative preservation of semantic memory (at least in high-functioning individuals). Such a pattern is consistent with developmental hippocampal abnormality. However, imaging evidence for abnormality of the hippocampal formation in ASD is inconsistent. These inconsistencies led us to examine the memory profile of children with ASD and the relationship to structural abnormalities. A cohort of high-functioning individuals with ASD and matched controls completed a comprehensive neuropsychological memory battery and underwent magnetic resonance imaging for the purpose of voxel-based morphometric analyses. Correlations between cognitive/behavioural test scores and quantified results of brain scans were also carried out to further examine the role of the medial temporal lobe in ASD. A selective deficit in episodic memory with relative preservation of semantic memory was found. Voxel-based morphometry revealed bilateral abnormalities in several areas implicated in ASD including the hippocampal formation. A significant correlation was found between parental ratings reflecting autistic symptomatology and the measure of grey matter density in the junction area involving the amygdala, hippocampus and entorhinal cortex. The data reveal a pattern of impaired and relatively preserved mnemonic function that is consistent with a hippocampal abnormality of developmental origin. The structural imaging data highlight abnormalities in several brain regions previously implicated in ASD, including the medial temporal lobes.

Adolescent↗
PubMed2004

Language reorganization in children with early-onset lesions of the left hemisphere: an fMRI study.

It is widely assumed that following extensive damage to the left hemisphere sustained in early childhood, language functions are likely to reorganize and develop in the right hemisphere, especially if the lesion affects the classical Broca's or Wernicke's language areas. In the present study, functional MRI (fMRI) was used to examine language lateralization in 10 children and adolescents with intractable epilepsy who sustained an early lesion in the left hemisphere. Lesions were adjacent to or within anterior language cortex in five patients, while they were remote from both Broca's and Wernicke's areas in the remainder. A lateralization index was calculated on the basis of the number of voxels activated in the left and right inferior frontal gyri when performing a covert verb generation task. Only two patients were right-handed, suggesting a high incidence of functional reorganization for motor control in the remaining patients. Five out of 10 showed bilateral or right language lateralization, but lateralization could not be inferred from the proximity of lesions to classical language areas on an individual basis. Lesions in or near Broca's area were not associated with inter-hemispheric language reorganization in four out of five cases, but with perilesional activation within the damaged left hemisphere. Paradoxically, lesions remote from the classical language areas were associated with non-left language lateralization in four out of five cases. Finally, handedness, age at onset of chronic seizures, and site of EEG abnormality also showed no obvious association with language lateralization. In conclusion, it is difficult to infer intra- versus inter-hemispheric language reorganization on the basis of clinical observations in the presence of early pathology to the left hemisphere.

Adolescent↗
PubMed2003

Anisotropic noise propagation in diffusion tensor MRI sampling schemes.

The subject of this study is the controversial choice of directions in diffusion tensor MRI (DT-MRI); specifically, the numerical algebra related to this choice. In DT-MRI, apparent diffusivities are sampled in six or more directions and a least-squares equation is solved to reconstruct the diffusion tensor. Numerical characteristics of the system are considered, in particular the condition number and normal matrix, and are shown to be dependent on the relative orientation of the tensor with respect to the laboratory frame. As a consequence, noise propagation can be anisotropic. However, the class of icosahedral direction schemes is an exception, and icosahedral directions have the same condition number and normal matrix for direction encoding as the ideal scheme with an infinite number of directions. This normal matrix and its condition number are rotationally invariant. Numerical simulations show that for icosahedral schemes with 30 directions the standard deviation of the fractional anisotropy is both low and nearly independent of fiber orientation. The recommended choice of directions for a DT-MRI experiment is therefore the icosahedral set of directions with the highest number of directions achievable in the available time.

Anisotropy↗
PubMed2002

Limitations and requirements of diffusion tensor fiber tracking: an assessment using simulations.

Diffusion tensor fiber tracking potentially can give information about in vivo brain connectivity. However, this technique is difficult to validate due to the lack of a gold standard. Fiber tracking reliability will depend on the quality of the data and on the robustness of the algorithms used. Information about the effects of various anatomical and image acquisition parameters on fiber tracking reliability may be used in the design of imaging sequences and of tracking algorithms. In this study, tracking was performed on two different simulated models to study the effects on tracking quality of SNR, anisotropy, curvature, fiber cross-section, background anisotropy, step size, and interpolation. Tracking was also performed on volunteer data to assess the relevance of the simulations to real data. Our results show that, in general, tracking with high SNR and high anisotropy using interpolation and a low step size gives the most reliable results. Partial volume effects are shown to have a detrimental effect when the background is anisotropic and when tracking narrow fibers. The results derived from real data show similar trends and thus support the findings of the simulations. These simulations may therefore help to determine which structures can be tracked for a given image quality.

Algorithms↗
PubMed2002

The precision of anatomical normalization in the medial temporal lobe using spatial basis functions.

We investigated the accuracy of spatial basis function normalization using anatomical landmarks to determine how precisely homologous regions are colocalized. We examined precision in terms of: (1) the number of nonlinear basis functions used by the normalization procedure; (2) the degree of (Bayesian) regularization; and (3) the effect of substituting different templates and how this interacted with the number of basis functions. The face validity of spatial normalization was assessed as a function of these parameters, using the colocalization of homologous landmarks in a test sample of 20 normally developing children and 5 children with bilateral hippocampal pathology. Our results suggest that when optimal normalization parameters are used, anatomical landmarks in the medial temporal lobes are colocalized to within a standard deviation of about 1 mm. When suboptimal parameters are used this standard deviation can increase up to 3 mm. Interestingly the optimal parameters are those that provide a rather constrained normalization as opposed to those that optimize intensity matching at the expense of rendering the warps "unlikely." The implications of our results, for users of voxel-based morphometry, are discussed.

Adolescent↗
PubMed2002

A direct test for lateralization of language activation using fMRI: comparison with invasive assessments in children with epilepsy.

This study introduces a direct method of assessing cerebral lateralization for language based on fMRI activation. The method, derived from a voxel-based morphometry study by C. H. Salmond et al. (2000, Hum. Brain Mapping 11, 223-232), bases lateralization on the direct statistical comparison of the magnitude of task-induced activation in homotopic regions of the two hemispheres. Lateralization results obtained with this direct method were compared to those obtained with a widely used method which involves the calculation of a laterality index (LI) based on the number of significantly activated voxels in the inferior frontal gyrus of each hemisphere. In order to compare the validity of the two methods, a covert verb-generation task was performed by eight children with epilepsy whose language lateralization was examined using invasive techniques. Lateralization results derived from fMRI activation showed that the calculation of a LI presented some limitations. Importantly, the LI value was dependent on the activation threshold chosen to calculate that LI. As a consequence, the correlation between the LI and the invasive methods could vary with the chosen threshold. By contrast, the proposed direct method gave some indication of the reliability of the lateralization and provided results that, in all eight children, were consistent with those obtained using invasive techniques. It is suggested that the direct method could be used in future fMRI studies to establish hemispheric lateralization for cognitive functions.

Adolescent↗
PubMed2002

MRI analysis of an inherited speech and language disorder: structural brain abnormalities.

Analyses of brain structure in genetic speech and language disorders provide an opportunity to identify neurobiological phenotypes and further elucidate the neural bases of language and its development. Here we report such investigations in a large family, known as the KE family, half the members of which are affected by a severe disorder of speech and language, which is transmitted as an autosomal-dominant monogenic trait. The structural brain abnormalities associated with this disorder were investigated using two morphometric methods of MRI analysis. A voxel-based morphometric method was used to compare the amounts of grey matter in the brains of three groups of subjects: the affected members of the KE family, the unaffected members and a group of age-matched controls. This method revealed a number of mainly motor- and speech-related brain regions in which the affected family members had significantly different amounts of grey matter compared with the unaffected and control groups, who did not differ from each other. Several of these regions were abnormal bilaterally, including the caudate nucleus, which was of particular interest because this structure was also found to show functional abnormality in a related PET study. We performed a more detailed volumetric analysis of this structure. The results confirmed that the volume of this nucleus was reduced bilaterally in the affected family members compared with both the unaffected members and the group of age-matched controls. This reduction in volume was most evident in the superior portion of the nucleus. The volume of the caudate nucleus was significantly correlated with the performance of affected family members on a test of oral praxis, a test of non-word repetition and the coding subtest of the Wechsler Intelligence Scale. These results thus provide further evidence of a relationship between the abnormal development of this nucleus and the impairments in oromotor control and articulation reported in the KE family.

Adolescent↗
PubMed2002

Magnetic resonance imaging findings within 5 days of status epilepticus in childhood.

The nature of the relationships between status epilepticus, acute hippocampal injury, mesial temporal sclerosis (MTS) and temporal lobe epilepsy remains unclear. The aim of this study was to investigate whether generalized status epilepticus is associated with brain abnormalities, especially in the mesial temporal lobe, within 5 days of the acute event. Such changes may be the first part of a causative pathophysiological sequence relating status epilepticus and MTS. Thirty-five children with a history of status epilepticus, including 21 with a history of prolonged febrile convulsion (PFC), underwent qualitative and quantitative MRI investigations within 5 days of the acute episode. Quantitative assessments of the hippocampus included T(2) relaxometry and hippocampal volumetry. Hippocampal volumes were large in patients with PFC when compared with controls. In addition, T(2) relaxation time was elevated in patients with PFC compared with control subjects during the first 2 days of the acute event. No difference was observed in patients examined 3-5 days after the event. Patients with afebrile status epilepticus had a variety of imaging abnormalities including elevated hippocampal T(2) values, but no evidence of hippocampal enlargement. PFC is associated with hippocampal abnormalities, consistent with hippocampal oedema, whilst non-febrile status epilepticus is not. A systematic longitudinal study is required to characterize the evolution of these abnormalities and to determine whether any patient develops MTS.

Brain Edema↗
PubMed2002

Quantification of perfusion using bolus tracking magnetic resonance imaging in stroke: assumptions, limitations, and potential implications for clinical use.

BACKGROUND: MR techniques have been very powerful in providing indicators of tissue perfusion, particularly in studies of cerebral ischemia. There is considerable interest in performing absolute perfusion measurements, with the aim of improving the characterization of tissue "at risk" of stroke. However, some important caveats relating to absolute measurements need to be taken into account. The purpose of this article is to discuss some of the issues involved and the potential implications for absolute cerebral blood flow measurements in clinical use. SUMMARY OF COMMENT: In bolus tracking MRI, deconvolution of the concentration-time course can in theory provide accurate quantification. However, there are several important assumptions in the tracer kinetic model used, some of which may be invalid in cerebral ischemia. These can introduce significant errors in perfusion quantification. CONCLUSIONS: Although we believe that bolus tracking MRI is a powerful technique for the evaluation of perfusion in cerebral ischemia, interpretation of perfusion maps requires caution; this is particularly true when absolute quantification is attempted. Work is currently under way in a number of centers to address these problems, and with appropriate modeling they may be overcome in the future. In the interim, we believe that it is necessary for users of bolus tracking perfusion data to be aware of the current technical limitations if they are to avoid misinterpretation or overinterpretation of their findings.

Cerebrovascular Circulation↗
PubMed2002

Distributional assumptions in voxel-based morphometry.

In this paper we address the assumptions about the distribution of errors made by voxel-based morphometry. Voxel-based morphometry (VBM) uses the general linear model to construct parametric statistical tests. In order for these statistics to be valid, a small number of assumptions must hold. A key assumption is that the model's error terms are normally distributed. This is usually ensured through the Central Limit Theorem by smoothing the data. However, there is increasing interest in using minimal smoothing (in order to sensitize the analysis to regional differences at a small spatial scale). The validity of such analyses is investigated. In brief, our results indicate that nonnormality in the error terms can be an issue in VBM. However, in balanced designs, provided the data are smoothed with a 4-mm FWHM kernel, nonnormality is sufficiently attenuated to render the tests valid. Unbalanced designs appear to be less robust to violations of normality: a significant number of false positives arise at a smoothing of 4 and 8 mm when comparing a single subject to a group. This is despite the fact that conventional group comparisons appear to be robust, remaining valid even with no smoothing. The implications of the results for researchers using voxel-based morphometry are discussed.

Brain Mapping↗
PubMed2001

MR perfusion imaging in Moyamoya Syndrome: potential implications for clinical evaluation of occlusive cerebrovascular disease.

BACKGROUND AND PURPOSE: Ischemic symptoms in patients with moyamoya syndrome (MMS) are usually due to hemodynamically mediated perfusion failure, and identification of abnormal tissue perfusion in these patients is therefore clinically important. Although dynamic susceptibility contrast (DSC) MRI can be used to study tissue perfusion, there are potential technical problems in MMS. This study investigates the scope and limitations of perfusion MRI in the clinical evaluation of such patients. METHODS: Thirteen patients with bilateral MMS were studied with the use of structural, diffusion, and perfusion MRI. The DSC MRI data were analyzed both visually and by a quantitative regional analysis, and the relationship between perfusion status and clinical symptoms was investigated. RESULTS: Extensive bilateral DSC MRI abnormalities were observed in all the patients. There was a very heterogeneous distribution of bolus arrival time. The areas of abnormality included the major arterial border zones in all cases, although these usually appeared normal on structural and diffusion MRI. Only the most clinically unstable patients had peak width (defined as time to peak minus bolus arrival time) >5 seconds on the quantitative regional analysis. Several technical limitations of perfusion quantification in MMS are described, as well as the implications of these limitations in patients with other forms of occlusive large-vessel disease. CONCLUSIONS: The technical limitations of DSC MRI described in this study are important for the accurate interpretation of perfusion MRI in MMS. Despite these limitations, these preliminary findings suggest that the use of quantitative regional analysis of summary parameters may provide clinically useful information in patients with MMS.

Adolescent↗
PubMed2001

Quantitative magnetic resonance characterization of mesial temporal sclerosis in childhood.

OBJECTIVE: To investigate whether quantitative MR techniques can be used to distinguish between mesial temporal sclerosis in patients with a history of prolonged febrile convulsion and in patients without such a history. METHODS: Quantitative hippocampal T2 relaxometry, hippocampal volumetry, and single voxel (1)H-MRS data were acquired from 16 children who subsequently underwent temporal lobe resections for intractable temporal lobe epilepsy and histologically were shown to have sclerosis of the horn of Ammon. Eight children had a history of prolonged febrile convulsion in early childhood and eight children had other or no associations. RESULTS: Patients with a history of prolonged febrile convulsion had smaller hippocampi (p = 0.02) and prolonged T2 relaxation time (p = 0.03) ipsilateral to the seizure focus when compared with patients without such a history. There was also more side-to-side asymmetry of T2 relaxation time (p = 0.004) and hippocampal volume (p = 0.02) in the patients with a history of prolonged febrile convulsion than in those with other or no associations. No differences between the groups were identified using (1)H-MRS. CONCLUSIONS: These data support the view that there are at least two types of mesial temporal sclerosis. There may be several pathogenetic pathways from initial insult to later mesial temporal sclerosis, and these pathways are, at least in part, dependent on the initial insult.

Adult↗
PubMed2001

Preventive counseling during prenatal care: Pregnancy Risk Assessment Monitoring System (PRAMS).

BACKGROUND: Prenatal care provides an opportunity for counseling about behaviors and experiences that increase the likelihood of adverse maternal and fetal outcomes. OBJECTIVE: To document (1) prevalence of preventive health counseling during prenatal care, (2) prevalence of women in higher need of counseling about specific health concerns, and (3) whether women in higher need for counseling were more likely than women in lower need to have received counseling. METHODS: Analysis of the Pregnancy Risk Assessment Monitoring System (PRAMS), a state-specific, population-based, random sample of postpartum women, was performed by using data from 14 states for births during 1997 or 1998, for a total of 24,620 participants. Outcome measures included report of preventive health counseling during prenatal visits by specific topic as well as behaviors and experiences about cigarette use, alcohol use, breast-feeding, partner violence, and preterm labor. RESULTS: The percentage of women that report preventive counseling during prenatal care is relatively high (> or =75%) for 9 of 13 topics. However, the percentage of women that report counseling is relatively low (<75%) for partner violence, seat belt use, illegal drug use, and human immunodeficiency virus (HIV) risk. Except for counseling about cigarette and alcohol use, women in higher need, compared with women in lower need, for three other health topics were not significantly more likely to receive counseling. CONCLUSIONS: Preventive health counseling for partner violence, seat-belt use, illegal drug use, and risk of HIV could be increased across prenatal settings. Counseling should involve assessment of risks, with focused counseling related to those risks.

Adolescent↗
PubMed2001

The amygdala and temporal lobe simple partial seizures: a prospective and quantitative MRI study.

PURPOSE: To determine whether specific temporal lobe simple partial seizures (SPSs) are associated with an abnormal amygdala T2 (AT2) ipsilateral to the seizure focus in patients with intractable unilateral temporal lobe epilepsy (TLE). AT2 relaxation time mapping is a sensitive method for the detection of abnormal tissue in the amygdala in patients with refractory TLE. The relation between an abnormal AT2 in the epileptic temporal lobe and amygdala seizure onset has not been established. METHODS: Fifty patients with intractable unilateral TLE and concordant data during presurgical evaluation were included. Patients with a foreign-tissue lesion on standard magnetic resonance imaging (MRI) were excluded. All had AT2 mapping. Fifteen types of SPSs were ascertained prospectively, systematically, and blinded to the results of AT2 mapping. The SPSs of patients with a normal AT2 (n = 25) were compared with those of patients with an abnormal AT2 ipsilateral to the seizure focus (n = 25). RESULTS: The group of patients with an abnormal AT2 reported a median of six types of SPSs (range 1-11), in comparison with a median of three types of SPSs (range, 0-7) for the group with a normal AT2 (p<0.01). Déjà vu, a warm sensation, an indescribable strange sensation, a cephalic sensation, and fear were associated with an abnormal AT2. The combination of déjà vu, a cephalic sensation, a warm sensation, a gustatory hallucination, and an indescribable strange sensation discriminated best between the 25 patients with a normal and the 25 patients with an abnormal AT2. CONCLUSIONS: A high number and the types of different SPSs provide clinical evidence for early involvement of the amygdala during seizures in patients with refractory unilateral TLE and an abnormal AT2 in the epileptic temporal lobe

Adolescent↗
PubMed2001

Perfusion magnetic resonance abnormalities in patients with sickle cell disease.

Neurological complications are common in sickle cell disease (SCD). However, it is often difficult to relate the clinical presentation to conventional neuroimaging, because subclinical infarction is common and stroke has been described in the absence of large-vessel disease. We studied 48 patients with SCD aged 4-34 (median 13) years with T2-weighted, diffusion and perfusion magnetic resonance imaging (MRI) and with MR angiography. Forty-four underwent transcranial Doppler (TCD). Abnormalities on perfusion imaging were seen in 25 cases, 24 of whom had been symptomatic. The remaining patient had evidence of executive dysfunction and reduced perfusion in the frontal lobes. The perfusion abnormality was larger than the area of infarction in 9 patients and was seen in an arterial distribution with no infarction in a further 9. In 3 patients with transient ischemic attacks, perfusion abnormalities were demonstrated in the absence of any other neuroimaging abnormalities, and perfusion changes were seen in 3 others despite normal MR angiography and TCD. Perfusion abnormalities are associated with neurological symptoms in patients with SCD, whether or not MRI, MR angiography, and TCD are abnormal. It is likely that this technique will guide management in individual patients.

Adolescent↗