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Biomedical subjects

K R Thulborn

Publications and source records attributed to K R Thulborn.

At least 37 records · Page 2Linked to original sources
PubMed1999

An integrated head immobilization system and high-performance RF coil for fMRI of visual paradigms at 1.5 T.

A flexible quadrature radiofrequency coil that maximizes the signal-to-noise ratio over the field of view of the human brain has been integrated into a head immobilization and visor system for fMRI at 1.5 T. Head motion is reduced by the visor that incorporates a head clamp and a simple visual sighting system that provides feedback on head position. This system is demonstrated in serial images by correction of deliberate head motions. The sensitivity at the cortical surface of fMRI using blood oxygenation level dependent contrast is increased significantly above that of the commercial rigid volume RF coil under the same acquisition conditions. This improved performance is demonstrated using visual activation and eye movement paradigms.

Artifacts↗
PubMed1999

Time course of fMRI-activation in language and spatial networks during sentence comprehension.

Functional neuroimaging previously has been considered to provide inadequate temporal resolution to study changes of brain states as a function of cognitive computations; however, we have obtained evidence of differential amounts of brain activity related to high-level cognition (sentence processing) within 1.5 s of stimulus onset. The study used an event-related paradigm with high-speed echoplanar functional magnetic resonance imaging (fMRI) to trace the time course of the brain activation in the temporal and parietal regions as participants comprehended single sentences describing a spatial configuration. Within the first set of images, on average 1 s from when the participant begins to read a sentence, there was significant activation in a key cortical area involved in language comprehension (the left posterior temporal gyrus) and visuospatial processing (the left and right parietal regions). In all three areas, the amount of activation during sentence comprehension was higher for negative sentences than for their affirmative counterparts, which are linguistically less complex. The effect of negation indicates that the activation in these areas is modulated by the difficulty of the linguistic processing. These results suggest a relatively rapid coactivation in both linguistic and spatial cortical regions to support the integration of information from multiple processing streams.

Adult↗
PubMed1999

Functional organization of activation patterns in children: whole brain fMRI imaging during three different cognitive tasks.

1. Patterns of brain activation were measured with whole brain echo-planar functional magnetic resonance imaging (fMRI) at 3.0 Tesla in healthy children (N = 6) and in one child with a left-hemisphere encephalomalacic lesion as sequellae from early stroke. 2. Three cognitive tasks were used: auditory sentence comprehension, verb generation to line drawings, and mental rotation of alphanumeric stimuli. 3. There was evidence for significant bilateral activation in all three cognitive tasks for the healthy children. Their patterns of activation were consistent with previous functional imaging studies with adults. 4. The child with a left-hemisphere stroke showed evidence of homologous organization in the non-damaged hemisphere.

Adult↗
PubMed1999

Clinical rationale for very-high-field (3.0 Tesla) functional magnetic resonance imaging.

The recent development of integrating very-high-field magnets (3.0 T) into clinical scanners was driven by the demonstration that both functional and anatomic information can be derived reliably at high spatial and temporal resolution. As very-high-field magnetic resonance imaging now approaches its entry into the clinical arena with a product by one major scanner manufacturer that is to be considered for Food and Drug Administration clearance, it is timely to consider the clinical applications that are likely to make such new technology significant in medical imaging. The rationale for very high field is based on the mechanism of blood oxygenation level-dependent contrast and the need for reliable functional studies on individual patients. The use of functional magnetic resonance imaging in the setting of presurgical planning is demonstrated in a number of different clinical scenarios. Such cases covering both pediatric and adult patients indicate that 3.0-T functional magnetic resonance imaging has an important role in neuroradiology.

Adolescent↗
PubMed1999

Comprehensive MR imaging protocol for stroke management: tissue sodium concentration as a measure of tissue viability in nonhuman primate studies and in clinical studies.

PURPOSE: To investigate sodium magnetic resonance (MR) imaging for monitoring tissue viability in stroke. MATERIALS AND METHODS: A comprehensive MR imaging protocol used to measure apparent diffusion coefficient and perfusion parameters was extended to include sodium imaging. Tissue sodium concentration was estimated by using a two-compartment model. This protocol lasted less than 45 minutes. These parameters were followed over the first 6 hours in a nonhuman primate model (n = 2) of acute embolic stroke without or with thrombolytic therapy. This protocol was used in patients in whom acute (< 24 hours, n = 11) or nonacute (> or = 24 hours, n = 31) stroke was ultimately confirmed. RESULTS: The animal model showed abnormal diffusion and perfusion parameters in the lesion immediately after embolization, and these remained abnormal for over 6 hours. Tissue sodium concentration increased with time (5.7 mmol/L/h) unless halted with thrombolytic therapy. Regions with sodium concentrations over 70 mmol/L were histochemically verified as being infarcted. In patients in whom stroke older than 6 hours was clinically confirmed, sodium concentrations over 70 mmol/L were found in the appropriate brain regions. CONCLUSION: Tissue sodium concentration provides a sensitive measure of tissue viability that is complementary to the diagnostic role of diffusion and perfusion imaging for ischemic insult.

Adult↗
PubMed1999

Plasticity of language-related brain function during recovery from stroke.

BACKGROUND AND PURPOSE: This study was undertaken to correlate functional recovery from aphasia after acute stroke with the temporal evolution of the anatomic, physiological, and functional changes as measured by MRI. METHODS: Blood oxygenation level-dependent contrast and echo-planar MRI were used to map language comprehension in 6 normal adults and in 2 adult patients during recovery from acute stroke presenting with aphasia. Perfusion, diffusion, sodium, and conventional anatomic MRI were used to follow physiological and structural changes. RESULTS: The normal activation pattern for language comprehension showed activation predominately in left-sided Wernicke's and Broca's areas, with laterality ratios of 0.8 and 0.3, respectively. Recovery of the patient confirmed as having a completed stroke affecting Broca's area occurred rapidly with a shift of activation to the homologous region in the right hemisphere within 3 days, with continued rightward lateralization over 6 months. In the second patient, in whom mapping was performed fortuitously before stroke, recovery of a Wernicke's aphasia showed a similar increasing rightward shift in activation recruitment over 9 months after the event. CONCLUSIONS: Recovery of aphasia in adults can occur rapidly and is concomitant with an activation pattern that changes from left to a homologous right hemispheric pattern. Such recovery occurs even when the stroke evolves to completion. Such plasticity must be considered when evaluating stroke interventions based on behavioral and neurological measurements.

Adult↗
PubMed1998

Correction of B1 inhomogeneities using echo-planar imaging of water.

Reliable interpretation of the MR signal intensity over the FOV of an image must consider the spatial heterogeneity of instrumental sensitivity. A major source of such variation is the nonuniformity of the B1 magnetic field of the radiofrequency coil. This heterogeneity can be minimized by coil design but is exaggerated by surface coils, which are used to maximize the signal-to-noise ratio for some applications. This paper describes a rapid method for mapping the B1 field over the sample of interest, using 1H echo-planar imaging, to correct for B1 distortions. The method applies to 1H imaging and has been extended to non-1H imaging by using dual-frequency coils in which the B1 distributions are matched for the 1H frequency and the frequency of interest. The approach is demonstrated in phantoms, animals, and humans and for sodium imaging.

Algorithms↗
PubMed1998

MR detection of hyperacute parenchymal hemorrhage of the brain.

BACKGROUND AND PURPOSE: The detection of hemorrhage in acutely ill patients is crucial to clinical management. The MR features that allow diagnosis of intracerebral hematomas of less than 24 hours' duration are described and the mechanistic basis of these features is investigated. METHODS: The clinical MR features of seven confirmed hyperacute intracerebral hematomas were compared with those of experimentally induced hematomas in a rat model in which detailed analyses of iron metabolism and morphometry were performed. RESULTS: In all patients and all animals, a hypointense rim on T2-weighted spin-echo images that was less marked on T1-weighted spin-echo images was seen surrounding a central isointense or heterogeneous region of hyperacute hematoma. Histologically, the clot showed interdigitation of intact erythrocytes and tissue at the hematoma-tissue interface without significant hemosiderin, ferritin, or phagocytic activity. Biochemically, the iron from the extravasated blood was present only as heme proteins within the first 24 hours. CONCLUSION: The hypointense rim on T2-weighted images, and to a lesser extent on T1-weighted images, is a distinctive feature of hyperacute hematoma. This pattern is consistent with magnetic susceptibility variations of paramagnetic deoxygenated hemoglobin within intact erythrocytes at a microscopically irregular tissue-clot interface. The detection of hemorrhage is important in the management of patients with acute stroke.

Acute Disease↗
PubMed1997

Three-dimensional projection imaging with half the number of projections.

This study demonstrates a sampling scheme for three-dimensional projection imaging with half the number of projections. The angular distribution of projections is designed so that the oversampling of the low spatial frequencies, in tandem with a partial Fourier algorithm, can be used to recreate purposely missing projections. The performance of the sampling scheme and its associated reconstruction algorithm are illustrated with computer-simulated as well as experimental data sets. We find that this technique produces images of comparable quality to the conventional projection imaging scheme. Although a 30% loss of signal-to-noise ratio (SNR) results from its use, the algorithm should prove useful for applications where robustness against motion artifacts and reduced T2* signal loss are desired.

Algorithms↗
PubMed1997

Fast three dimensional sodium imaging.

An efficient scheme for fast three dimensional acquisition of sodium MR images is described. This scheme relies on the use of three dimensional k-space trajectories with constant sample density to achieve significant (60-70%) reductions in total data acquisition time over conventional projection imaging schemes. The performance of this data acquisition scheme is demonstrated with acquisition of sodium data sets on phantoms and normal human volunteers at 1.5 and 3.0 Tesla. The experimental results demonstrate that high quality three dimensional sodium images (0.2 cc voxel size, 10:1 signal-to-noise ratio) can be acquired at clinical field strengths (1.5 Tesla) in under 10 min.

Computer Simulation↗
PubMed1997

Dual-frequency, dual-quadrature, birdcage RF coil design with identical B1 pattern for sodium and proton imaging of the human brain at 1.5 T.

Rapid quantification of tissue metabolites in vivo by MRS or MRI can be achieved using dual-frequency RF coils with identical B1 field distributions at the observation frequencies of the metabolites and tissue water protons. Tissue sodium is used as an example for optimizing the dual-frequency, dual-quadrature RF coils for such measurements in humans. In the setting of sodium imaging, the challenge of dual-quadrature birdcage configurations is to decouple the sodium and proton channels because the fourth harmonic of the sodium frequency is very close to the proton frequency. A generalizable method for effectively decoupling these two RF frequencies is presented in this paper. The method is demonstrated with the design of an EPI compatible, dual-quadrature, double-tuned, 23Na/1H birdcage coil. The performance of the RF probe is reported at 1.5 Tesla in terms of signal-to-noise ratio, B1 homogeneity and image quality.

Brain↗
PubMed1997

Spectrally weighted twisted projection imaging: reducing T2 signal attenuation effects in fast three-dimensional sodium imaging.

A scheme for the reduction of T2 signal attenuation effects in three-dimensional twisted projection imaging is presented. By purposely reducing the sample density at the high spatial frequencies, a considerable reduction in readout time is achieved. The reduction in readout time leads to decreased T2 signal attenuation which translates into improved signal-to-noise ratio (SNR). The SNR improvement is achieved without decreasing the image's resolution since the point spread function depends on the sample weighting as well as the T2 attenuation. The results indicate that SNR improvements of up to 40% can be achieved using the proposed scheme.

Brain↗
PubMed1997

Magnetic resonance imaging of children without sedation: preparation with simulation.

OBJECTIVE: It was hypothesized that a scanner simulator that replicates the magnetic resonance imaging (MRI) environment could be used to prepare pediatric subjects for successful completion of a diagnostic-quality MRI examination without pharmacological sedation. METHOD: Sixteen healthy children, 6 to 17 years of age, were matched for age and sex with 16 psychotropic medication-naive children with obsessive-compulsive disorder. Distress was measured throughout simulation and scanning procedures using heart rate and a self-report distress scale. Ten healthy children, 6 to 17 years of age, also underwent the same actual MRI scanning procedure but did not undergo the simulation scanning procedure. RESULTS: Significant decreases in heart rate and self-reported distress level were observed in all subjects during the simulator session that were maintained to the end of the actual scanner experience. All subjects successfully completed MRI examinations without chemical restraint. Subjects who were not trained in the simulator had higher heart rates and self-reported distress levels in the actual scanner than did simulation-trained subjects. CONCLUSIONS: Simulation without pharmacological sedation successfully prepared pediatric subjects in this pilot study for high-quality MRI studies. Subject preparation may be an alternative procedure to sedation for routine MRI examination in healthy and anxious children 6 years of age and older.

Adolescent↗
PubMed1996

Brain activation modulated by sentence comprehension.

The comprehension of visually presented sentences produces brain activation that increases with the linguistic complexity of the sentence. The volume of neural tissue activated (number of voxels) during sentence comprehension was measured with echo-planar functional magnetic resonance imaging. The modulation of the volume of activation by sentence complexity was observed in a network of four areas: the classical left-hemisphere language areas (the left laterosuperior temporal cortex, or Wernicke's area, and the left inferior frontal gyrus, or Broca's area) and their homologous right-hemisphere areas, although the right areas had much smaller volumes of activation than did the left areas. These findings generally indicate that the amount of neural activity that a given cognitive process engenders is dependent on the computational demand that the task imposes.

Adolescent↗
PubMed1996

Quantitative tissue sodium concentration mapping of normal rat brain.

A quantitative in vivo method for obtaining maps of tissue sodium concentration (TSC) by MRI is compared to the invasive, global 22Na radionuclide dilutional technique in the normal rat brain. The MR method uses a three-dimensional projectional acquisition scheme to minimize signal losses from transverse relaxation. Internal calibration standards are used to convert the signal intensity into TSC after correction for B1 inhomogeneities by using the ratio of 23Na and 1H images obtained with identical B1 distributions and sensitivities at the two frequencies. Over the biological range of concentrations, the TSC, measured as the ratio of MR signals of 23Na and 1H, gives a linear response with concentration. In the normal rat brain, the mean TSC measured using the MRI method (TSC = 45 +/- 4 mM, animals = 5) is not significantly different from the global 22Na radionuclide method (TSC = 49 +/- 6 mM, animals = 7).

Animals↗