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T B Parrish

Publications and source records attributed to T B Parrish.

28 records · Page 2Linked to original sources
PubMed1999

Neuroanatomic overlap of working memory and spatial attention networks: a functional MRI comparison within subjects.

Frontal and posterior parietal activations have been reported in numerous studies of working memory and visuospatial attention. To directly compare the brain regions engaged by these two cognitive functions, the same set of subjects consecutively participated in tasks of working memory and spatial attention while undergoing functional MRI (fMRI). The working memory task required the subject to maintain an on-line representation of foveally displayed letters against a background of distracters. The spatial attention task required the subject to shift visual attention covertly in response to a centrally presented directional cue. The spatial attention task had no working memory requirement, and the working memory task had no covert spatial attention requirement. Subjects' ability to maintain central fixation was confirmed outside the MRI scanner using infrared oculography. According to cognitive conjunction analysis, the set of activations common to both tasks included the intraparietal sulcus, ventral precentral sulcus, supplementary motor area, frontal eye fields, thalamus, cerebellum, left temporal neocortex, and right insula. Double-subtraction analyses yielded additional activations attributable to verbal working memory in premotor cortex, left inferior prefrontal cortex, right inferior parietal lobule, precuneus, and right cerebellum. Additional activations attributable to covert spatial attention included the occipitotemporal junction and extrastriate cortex. The use of two different tasks in the same set of subjects allowed us to provide an unequivocal demonstration that the neural networks subserving spatial attention and working memory intersect at several frontoparietal sites. These findings support the view that major cognitive domains are represented by partially overlapping large-scale neural networks. The presence of this overlap also suggests that spatial attention and working memory share common cognitive features related to the dynamic shifting of attentional resources.

Adult↗
PubMed1999

A large-scale distributed network for covert spatial attention: further anatomical delineation based on stringent behavioural and cognitive controls.

Functional MRI was used to examine cerebral activations in 12 subjects while they performed a spatial attention task. This study applied more stringent behavioural and cognitive controls than previously used for similar experiments: (i) subjects were included only if they showed evidence of attentional shifts while performing the task in the magnet; (ii) the experimental task and baseline condition were designed to eliminate the contributions of motor output, visual fixation, inhibition of eye movements, working memory and the conditional (no-go) component of responding. Activations were seen in all three hypothesized cortical epicentres forming a network for spatial attention: the lateral premotor cortex (frontal eye fields), the posterior parietal cortex and the cingulate cortex. Subcortical activations were seen in the basal ganglia and the thalamus. Although the task required attention to be equally shifted to the left and to the right, eight of 10 subjects showed a greater area of activation in the right parietal cortex, consistent with the specialization of the right hemisphere for spatial attention. Other areas of significant activation included the posterior temporo-occipital cortex and the anterior insula. The temporo-occipital activation was within a region broadly defined as MT+ (where MT is the middle temporal area) which contains the human equivalent of area MT in the macaque monkey. This temporo-occipital area appears to constitute a major component of the functional network activated by this spatial attention task. Its activation may reflect the 'inferred' shift of the attentional focus across the visual scene.

Adult↗
PubMed1998

Contrast magnetic resonance imaging in the assessment of myocardial viability in patients with stable coronary artery disease and left ventricular dysfunction.

BACKGROUND: The utility of contrast MRI for assessing myocardial viability in stable coronary artery disease (CAD) with left ventricular dysfunction is uncertain. We therefore performed cine and contrast MRI in 24 stable patients with CAD and regional contractile abnormalities and compared MRI findings with rest-redistribution 201Tl imaging and dobutamine echocardiography. METHODS AND RESULTS: Delayed MRI contrast enhancement patterns were examined from 3 to 15 minutes after injection of 0.1 mmol/kg IV gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA). Comparable MRI and 201Tl basal and midventricular short-axis images were subdivided into 6 segments. Segments judged nonviable by quantitative and qualitative assessment of 201Tl scans showed persistent, systematically greater MRI contrast signal intensity than segments judged viable (P</=0.002). Delayed contrast hyperenhancement also occurred in segments judged nonviable by dobutamine echocardiography (P</=0.03). The presence or absence of hyperenhancement correlated most closely with nonviability and viability, respectively, in segments that were akinetic or dyskinetic under resting conditions (83% concordance with 201Tl in both cases). In segments with resting hypokinesis, 58% of segments showing hyperenhancement were judged viable by 201Tl and may have represented an admixture of scar tissue and viable myocardium. CONCLUSIONS: Delayed (by 3 to 15 minutes) hyperenhancement of Gd-DTPA contrast-enhanced MRI images occurs frequently in dysfunctional areas of the left ventricle in patients with stable CAD. Hyperenhancement is associated with nonviability by rest-redistribution 201Tl scintigraphy and dobutamine echocardiography, particularly in regions exhibiting resting akinesis/dyskinesis. The absence of hyperenhancement correlates with radionuclide and echocardiographic determinations of viability, regardless of resting contractile function.

Adult↗
PubMed1998

Trajectories of cholinergic pathways within the cerebral hemispheres of the human brain.

All sectors of the human cerebral cortex receive dense cholinergic input. The origin of this projection is located in the Ch4 cell group of the nucleus basalis of Meynert. However, very little is known about the location of the pathways which link the cholinergic neurons of the nucleus basalis to the human cerebral cortex. This question was addressed in whole-hemisphere sections processed for the visualization of multiple cholinergic markers. Two highly organized and discrete bundles of cholinergic fibres extended from the nucleus basalis to the cerebral cortex and amygdala and were designated as the medial and lateral cholinergic pathways. These bundles contained acetylcholinesterase, choline acetyltransferase and nerve growth factor receptors, confirming their cholinergic nature and origin within the basal forebrain. The medial pathway joined the white matter of the gyrus rectus, curved around the rostrum of the corpus callosum to enter the cingulum and merged with fibres of the lateral pathway within the occipital lobe. It supplied the parolfactory, cingulate, pericingulate and retrosplenial cortices. The lateral pathway was subdivided into a capsular division travelling in the white matter of the external capsule and uncinate fasciculus and a perisylvian division travelling within the claustrum. Branches of the perisylvian division supplied the frontoparietal operculum, insula and superior temporal gyrus. Branches of the capsular division innervated the remaining parts of the frontal, parietal and temporal neocortex. Representation of these cholinergic pathways within a 3D MRI volume helped to identify white matter lesion sites that could interfere with the corticopetal flow of cholinergic pathways.

Adult↗
PubMed1997

Theoretical basis for sodium and potassium MRI of the human heart at 1.5 T.

Knowledge of the extent and location of viable tissue is important to clinical diagnosis. In principle, sodium (23Na) and potassium (39K) MRI could noninvasively provide information about tissue viability. In practice, imaging of these nuclei is difficult because, compared with water protons (1H), 23Na and 39K have lower MR sensitivities (9.2 and 0.051%, respectively), and lower in vivo concentrations (ca. 1000-fold). On the other hand, the relatively short T1 relaxation times of 23Na and 39K (ca. 30 and 10 ms, respectively) suggest that optimized imaging pulse sequences may in part alleviate the weak signal of these nuclei. In this study, numerical simulations of high-speed imaging sequences were developed and used to maximize 23Na and 39K image signal-to-noise ratio (SNR) per unit time within the constraints of existing gradient hardware. The simulation demonstrated that decreasing receiver bandwidth at the expense of echo time (TE) results in a substantial increase in 23Na and 39K image SNR/time despite the short T2 and T2* of these nuclei. Referenced to the available 1H signal on existing 1.5 T scanners, the simulation suggested that it should be possible to acquire three-dimensional 23Na images of the human heart with 7 x 7 x 7 mm resolution and 39K images with 26 x 26 x 26 mm resolution in 30 min. Experimentally in humans at 1.5 T, three-dimensional 23Na images of the heart were acquired in 15 min with 6 x 6 x 12 mm resolution and signal-to-noise ratios of 11 and 7 in the left ventricular cavity and myocardium, respectively, which is very similar to the predicted result. The results demonstrate that by choosing imaging pulse sequence parameters that fully exploit the short relaxation times of 23Na and 39K, potassium MRI is improved but remains impractical, whereas sodium MRI improves to the point where 23Na imaging of the human heart may be clinically feasible on existing 1.5 T scanners.

Heart Ventricles↗
PubMed1997

Contrast-enhanced magnetization transfer MR of the brain: importance of precontrast images.

PURPOSE: To determine the importance of obtaining precontrast T1-weighted magnetization transfer (MT) MR images for better interpretation contrast-enhanced T1-weighted MT images. METHODS: One hundred fifty-five patients referred for MR imaging of the brain were examined prospectively with noncontrast T1-weighted imaging, noncontrast T1-weighted imaging with MT, contrast-enhanced T1-weighted imaging, and contrast-enhanced T1-weighted imaging with MT. In the patients who had abnormally increased signal intensity on postcontrast images (with or without MT), the four imaging sequences were evaluated with regard to number of lesions and lesional signal intensity. For each of the sequences, two experienced neuroradiologists subjectively graded the lesions on a scale of 1 to 4 (4 being the most conspicuous) with regard to abnormally increased signal intensity. RESULTS: Twenty-two of the 155 patients had increased signal intensity on one or more of the postcontrast sequences. Eight of these 22 patients had increased signal intensity of one or more lesions on images without MT. All these lesions were seen better on images obtained with MT. An additional six of the 22 patients had increased signal intensity of one or more lesions on images obtained with MT that was not detected on images obtained without MT. Eight of the 22 patients had no high signal intensity on noncontrast images with or without MT. One of the eight had increased number and conspicuity of lesions on postcontrast MT images. CONCLUSIONS: A significant number of patients had increased signal intensity on noncontrast T1-weighted images with MT that was not seen on noncontrast T1-weighted images without MT. This high signal intensity was also visible on postcontrast MT images, and would have been mistaken for pathologic enhancement if noncontrast MT images had not been available for comparison.

Adult↗
PubMed1996

Myocardial perfusion and function in dogs with moderate coronary stenosis.

MRI studies of first-pass contrast enhancement with polylysine-Gd-DTPA and myocardial tagging using spatial modulation of magnetization (SPAMM) were performed to assess the feasibility of a combined regional myocardial blood flow and 2D deformation exam. Instrumented closed-chest dogs were imaged at a baseline control state (Cntl) followed by two interventions: moderate coronary stenosis (St) achieved by partial occlusion of the left anterior descending (LAD) and moderate coronary stenosis with dobutamine loading (StD). Hypoperfusion of the anterior region (ANT) of the myocardium (LAD distribution) relative to the posterior wall (POS) based on the upslope of the signal intensity time curve from the contrast-enhanced MR images was demonstrated only with dobutamine loading (ANT:POS Cntl = 1.077 +/- 0.15 versus ANT:POS StD = 0.477 +/- 0.11, P < 0.03) and was confirmed with radiolabeled microspheres measurements (ANT:POS Cntl = 1.18 +/- 0.2 ml/min/g versus ANT:POS StD = 0.44 +/- 0.1 ml/min/g; P < 0.002). Significant changes in regional myocardial shortening were only seen in the StD state (P < 0.02); the anterior region showed impaired myocardial shortening with dobutamine loading (P = NS), whereas the nonaffected POS region showed a marked increase in shortening when compared with Cntl (Cntl = 0.964 +/- 0.02 versus StD = 0.884 +/- 0.03; P < 0.001). These results demonstrate that an integrated quantitative assessment of regional myocardial function and semiquantitative assessment of myocardial blood flow can be performed noninvasively with ultrafast MRI.

Animals↗
PubMed1996

Financing special education.

The best available data indicate that approximately 12% of K-12 public education budgets are allocated to special education and that the cost per student is about 2.3 times the cost of regular education. About 8% of special education funds come from the federal government, 56% from state governments, and the remainder from local school districts. However, the division of fiscal responsibility between state and local sources varies considerably from state to state. Fiscal pressures on special education have increased markedly in recent years as a result of increasing overall school populations, increasing proportions of students found eligible for special education services, and increasing fiscal pressures on schools. At least six states have recently adopted, and the federal Department of Education has recently recommended, special education funding based on census counts of total school populations rather than on the number of students identified for special education services. Proponents of census-based funding foresee greater efficiency of services, while opponents fear a loss of services targeted to individualized needs. An additional proposal is that census-based funding be modified to increase funding to jurisdictions with higher rates of poverty. Criteria for evaluating state special education funding formulas are proposed.

Costs and Cost Analysis↗
PubMed1989

Reduction of T2* dephasing in gradient field-echo imaging.

Fast gradient field-echo imaging is becoming more common in morphologic studies but is not as widespread as might be hoped because of its susceptibility to local field inhomogeneities that lead to spin dephasing (reducing T2 to T2*) and geometric distortion (frequency misregistration). These problems are manifested in both the in-plane and section-select directions. The authors show that reversal of many of these adverse effects is possible through the acquisition of the gradient field echo in a three- and four-dimensional mode for a fixed echo time (TE), since phase-encoding leads to the capture of the echo within the sampling window. This echo centering is shown to be equivalent to the reduction of dephasing across a pixel. Improvements are also obtained by reducing TE to as short a value as possible.

Humans↗