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R M Weisskoff

Publications and source records attributed to R M Weisskoff.

89 records · Page 5Linked to original sources
PubMed1992

Magnetic resonance imaging mapping of brain function. Human visual cortex.

Magnetic resonance imaging (MRI) studies of human brain activity are described. Task-induced changes in brain cognitive state were measured using high-speed MRI techniques sensitive to changes in cerebral blood volume (CBV), blood flow (CBF), and blood oxygenation. These techniques were used to generate the first functional MRI maps of human task activation, by using a visual stimulus paradigm. The methodology of MRI brain mapping and results from the investigation of the functional organization and frequency response of human primary visual cortex (V1) are presented.

Brain Mapping↗
PubMed1992

Brain parenchyma motion: measurement with cine echo-planar MR imaging.

With echo-planar magnetic resonance (MR) imaging, the authors measured the intrinsic pulsatile motion of brain parenchyma. Phase-sensitive, electrocardiography-gated, two-dimensional cine images were acquired throughout the cardiac cycle by using a spin-echo, blipped echo-planar MR pulse sequence. Transverse and coronal planes were obtained in 14 healthy volunteers. Corrections were made for gross head motion. Brain motion consisted of a rapid displacement in systole, with a slow diastolic recovery. The motion occurred chiefly in the cephalocaudal and lateral directions; the anteroposterior motions were relatively small. Cephalocaudal velocities increase with proximity to the foramen magnum. The lateral motion is mainly a compressive motion of the thalami. Brain parenchymal velocities were as high as 2 mm/sec caudally in the brain stem and 1.5 mm/sec medially in the thalami. Net parenchymal excursions were at most 0.5 mm. Phase-based echo-planar velocity measurements agreed well with echo-planar Fourier velocity zeugmatography measurements and were consistent with reported values. Velocity mapping with echo-planar imaging offers a rapid and flexible method of assessing the pulsation velocities of the human brain.

Adult↗
PubMed1991

Functional mapping of the human visual cortex by magnetic resonance imaging.

Knowledge of regional cerebral hemodynamics has widespread application for both physiological research and clinical assessment because of the well-established interrelation between physiological function, energy metabolism, and localized blood supply. A magnetic resonance technique was developed for quantitative imaging of cerebral hemodynamics, allowing for measurement of regional cerebral blood volume during resting and activated cognitive states. This technique was used to generate the first functional magnetic resonance maps of human task activation, by using a visual stimulus paradigm. During photic stimulation, localized increases in blood volume (32 +/- 10 percent, n = 7 subjects) were detected in the primary visual cortex. Center-of-mass coordinates and linear extents of brain activation within the plane of the calcarine fissure are reported.

Blood Volume↗
PubMed1991

Real-time magnetic resonance imaging of laser heat deposition in tissue.

We applied diffusion-sensitive echo planar (Instascan) imaging to study thermal changes caused by a Nd:YAG laser. Images of phantom materials and normal rabbit brain tissue in vivo, acquired in 150 ms, every 2s, clearly showed the dynamics of temperature-related signal intensity changes in the regions irradiated by the laser.

Animals↗
PubMed1991

Equipment requirements to facilitate contrast-enhanced MR imaging.

During the past decade, MR equipment and imaging techniques have experienced unprecedented development. Significant improvements have been made in image quality, enhancing conventional contrast agent studies. In addition, the development of new applications, such as MR angiography, has expanded the role of existing agents. A major benefit for contrast agent research has been the development of ultrafast MR imaging. This ability to provide information on contrast agent dynamics will aid in the detection and characterization of neoplastic and other disease states. While the gradient-echo techniques may provide adequate temporal resolution for many applications, echo planar imaging will enable the assessment of perfusion for an entire organ, which is critical in the heart or when the location of the pathology is unknown a priori. It is also likely that continued development in MR contrast agents will have a synergistic impact on pulse sequences, e.g., MR diffusion imaging. The growth in functional MR imaging will be based in large measure on the continued interactive development of imaging strategies and magnetopharmaceutials.

Animals↗
PubMed1991

Susceptibility contrast imaging of cerebral blood volume: human experience.

Magnetic resonance (MR) can offer a unique window on the structure/function relationships in the brain, by utilizing the established link between tissue function, metabolism, and hemodynamics. This report focuses on recent applications of MR-based cerebral blood volume (CBV) imaging in humans. Our methodology uses high-speed "single-shot" or echo planar imaging techniques, which provide the necessary temporal resolution for mapping the rapid cerebral transit of contrast agents. These MR CBV mapping techniques have been used to study normal human brain task activation and in the clinical study of patients with brain tumors. In the latter, positron emission tomography imaging was used for functional metabolic and CBV correlation. Susceptibility contrast CBV imaging should allow us to improve our understanding of the relationship between the detailed physiology and morphology of the microvascular bed and functional attributes of the brain. These techniques can be applied to understanding fundamental questions of cognitive neuroscience and can aid in improving diagnostic sensitivity and specificity in various neuropathologies.

Brain↗
PubMed1991

Ultra-fast imaging.

The relatively long scan times with currently available technology restrict the range of MRI applications, increase the cost of scanning by limiting throughput, and lead to image artifacts from patient motion during scans. Ultrafast imaging, in several guises, is now poised for introduction into clinical practice. With the Instascan method, a descendant of the echo-planar technique, complete MR images may be obtained hundreds to thousands of times faster than in conventional approaches and now yield spatial resolution and contrast directly comparable to standard MRI. "Single-shot" imaging methods, such as Instascan, are utilized in the study of dynamic processes, in the direct evaluation of motion (as in diffusion sensitive imaging), and in dramatic new applications, including the interactive control of intraparenchymal laser surgery. Improvements to the small flip-angle method, FLASH, have also pushed scan times into the subsecond domain; this method may be implemented on presently available imaging equipment but yields contrast behavior different from the traditional spin-echo techniques and displays signal-to-noise ratios significantly lower than single-shot imaging. Ultimately, incorporating ultra-fast MR imaging techniques into the armamentarium of the radiologist will likely require changes to many aspects of the MRI practice, from expanded involvement with the scan process to management of the increased data load, and may lead to dramatic changes in the scope of the MRI practice.

Humans↗
PubMed1991

Functional studies of the human brain using high-speed magnetic resonance imaging.

Dramatic technical advances in magnetic resonance imaging (MRI or NMR) scanning speed and recent governmental approval for the routine use of NMR contrast agents have yielded techniques for quantitative imaging of cerebral hemodynamics. The technical basis of ultrafast imaging, the methodology of dynamic imaging with contrast agents, and results in normal subjects and patients are presented.

Blood Volume↗
PubMed1990

Sensory stimulation by time-varying magnetic fields.

When two human volunteers were imaged with magnetic field gradient dB/dt of 61 T/s RMS, the subjects reported, to our surprise, feeling muscular twitches synchronous with gradient pulses over repeated experiments. No adverse or sustained effects were seen. Experiments in a canine, intended to assess the safety of MR imaging with dB/dt of up to 66 T/s RMS, failed to induce detectable changes in the electrocardiogram or to show any signs of gross response to gradient pulsing. Although these data are preliminary, and largely anecdotal, they suggest a level above which such stimulation may occur. We believe that this is the first report of direct human stimulation in an MRI device and that determination of the stimulation threshold may have impact on the selection of appropriate operating points for magnetic imaging systems.

Animals↗
PubMedDate not supplied

Measurement of stroke volume and cardiac output within a single breath hold with echo-planar MR imaging.

The measurement of cardiac output and ejection fraction is useful in the treatment of diverse cardiac and cardiopulmonary disease states. Although several techniques are available for accurate measurement of left ventricular parameters, assessment of the right ventricle is less well represented. No single method is overwhelmingly superior, each having different strengths and weaknesses. In the present study, the applicability of an echo-planar magnetic resonance (MR) imaging method in which a complete volumetric measurement of the right and left ventricles may be obtained during 12 heart-beats is demonstrated. This rapidity permits imaging during a 15-second breath hold. The authors show in 12 volunteers that breath-hold echo-planar volume measurements of both ventricles were consistent with results obtained with conventional MR imaging methods.

Adult↗
PubMedDate not supplied

Automated shimming at 1.5 T using echo-planar image frequency maps.

Using echo-planar imaging, we developed an automated image-based procedure to shim the static (B0) field. Our method uses the rapid acquisition capability of echo-planar imaging to collect the required frequency data rapidly, rendering the shim data acquisition time negligible in comparison with the total study time. We address image distortion issues involved in echo-planar imaging acquisition of the data and formulate analytic methods for arriving at an optimal shim for the NMR imaging experiment in a single iteration. We investigated the use of cost functions other than least-squares (Chebychev, high-order numeric) and found that choice between the cost functions we tested was irrelevant to resultant image quality, at least when used in conjunction with low-order shims. With appropriate integration, the method has become routine practice for investigators at our laboratory.

Adult↗
PubMedDate not supplied

Contrast agents in functional MR imaging.

Contrast agents have greatly expanded the role of MR imaging (MRI) to allow assessment of physiologic, or "functional," parameters. Although activation mapping generally does not require contrast agents, other forms of functional MRI, including mapping of cerebral hemodynamics (eg, perfusion imaging), are best done with the use of contrast agents. Serial echo planar images are obtained after bolus injection of lanthanide chelates. Application of susceptibility contrast physics and standard tracer kinetic principles permits generation of relative cerebral blood volume maps. Deconvolution of cerebral blood flow and mean transit time parameters is also possible within technical limitations. By using diffusion and perfusion pulse sequences, an imaging correlate to the ischemic penumbra can be identified. Functional MRI perfusion imaging of intraaxial tumors is analogous to positron emission tomography for delineation of metabolic activity, yet may be even more sensitive to neovascularity and possesses improved image quality. Clinical applications include biopsy site selection and postirradiation follow-up. Further improvements in data analysis and map generation techniques may improve diagnostic accuracy and utility.

Blood-Brain Barrier↗
PubMedDate not supplied

Water diffusion and exchange as they influence contrast enhancement.

The contrast-enhanced magnetic resonance imaging (MRI) signal is rarely a direct measure of contrast concentration; rather it depends on the effect that the contrast agent has on the tissue water magnetization. To correctly interpret such studies, an understanding of the effects of water movement on the magnetic resonance (MR) signal is critical. In this review, we discuss how water diffusion within biological compartments and water exchange between these compartments affect MR signal enhancement and therefore our ability to extract physiologic information. The two primary ways by which contrast agents affect water magnetization are discussed: (1) direct relaxivity and (2) indirect susceptibility effects. For relaxivity agents, for which T1 effects usually dominate, the theory of relaxation enhancement is presented, along with a review of the relevant physiologic time constants for water movement affecting this relaxation enhancement. Experimental issues that impact accurate measurement of the relaxation enhancement are discussed. Finally, the impact of these effects on extracting physiologic information is presented. Susceptibility effects depend on the size and shape of the contrast agent, the size and shape of the compartment in which it resides, as well as the characteristics of the water movement through the resulting magnetic field inhomogeneity. Therefore, modeling of this effect is complex and is the subject of active study. However, since susceptibility effects can be much stronger than relaxivity effects in certain situations, they may be useful even without full quantitation.

Animals↗
PubMedDate not supplied

Contrast-to-noise ratio in functional MRI of relative cerebral blood volume with sprodiamide injection.

The purpose of this study was to investigate the dependence of contrast-to-noise ratio (CNR) on the dose and rate of sprodiamide injection in magnetic resonance relative cerebral blood volume (rCBV) imaging. rCBV maps for 35 normal volunteers were constructed from dynamic MR image sets acquired with echo-planar spin-echo imaging after intravenous injection of sprodiamide. Doses of .1, .2, and .3 mmol/kg, at rates of 2 ml/second and 5 ml/second, were tested. CNRs and blood/volume ratios of gray to white matter were computed. CNR depended on dose (P < .0001) but was independent of injection rate (P < .69). rCBV ratios of gray to white matter were dose independent (P < .38) and rate independent (P < .97). The dependence of CNR on dose, but not injection rate, has practical implications in optimal protocol design. The independence of gray/white ratios supports the theory underlying the generation of rCBV maps.

Adult↗
PubMedDate not supplied

Signal-to-noise analysis of cerebral blood volume maps from dynamic NMR imaging studies.

The use of cerebral blood volume (CBV) maps generated from dynamic MRI studies tracking the bolus passage of paramagnetic contrast agents strongly depends on the signal-to-noise ratio (SNR) of the maps. The authors present a semianalytic model for the noise in CBV maps and introduce analytic and Monte Carlo techniques for determining the effect of experimental parameters and processing strategies upon CBV-SNR. CBV-SNR increases as more points are used to estimate the baseline signal level. For typical injections, maps made with 10 baseline points have 34% more noise than those made with 50 baseline points. For a given peak percentage signal drop, an optimum TE can be chosen that, in general, is less than the baseline T2. However, because CBV-SNR is relatively insensitive to TE around this optimum value, choosing TE approximately equal to T2 does not sacrifice much SNR for typical doses of contrast agent. The TR that maximizes spin-echo CBV-SNR satisfies TR/T1 approximately equal to 1.26, whereas as short a TR as possible should be used to maximize gradient-echo CBV-SNR. In general, CBV-SNR is maximized for a given dose of contrast agent by selecting as short an input bolus duration as possible. For image SNR exceeding 20-30, the gamma-fitting procedure adds little extra noise compared with simple numeric integration. However, for noisier input images, can be the case for high resolution echo-planar images, the covarying parameters of the gamma-variate fit broaden the distribution of the CBV estimate and thereby decrease CBV-SNR. The authors compared the analytic noise predicted by their model with that of actual patient data and found that the analytic model accounts for roughly 70% of the measured variability of CBV within white matter regions of interest.

Blood Volume↗
PubMedDate not supplied

Patterns of lingual tissue deformation associated with bolus containment and propulsion during deglutition as determined by echo-planar MRI.

Disordered lingual function is a common clinical attribute of patients with oropharyngeal dysphagia. To determine physiologic patterns of lingual tissue motion during swallowing, we imaged the actively deforming tongue during water bolus swallows with sequential single-slice sagittal orientation echo-planar imaging. At rest, with the bolus contained in the oral cavity before swallow initiation, the tongue displayed a characteristic curved configuration consisting of a convex surface (anterior to the bolus) in continuity with a concave surface (containing the bolus) and a posterior-located convex surface (comprising the tongue base). With swallow initiation, the previously deformed tongue underwent rapid biphasic displacement: (a) superior displacement of the anterior tongue and deepening of the midposterior-located bolus-containing concavity, resulting in a laterally beveled surface encompassing the bolus; and (b) retrograde displacement of the configured tissue, resulting in clearance of the bolus from the oral cavity to the oropharynx. These findings indicate that deglutitive tongue action can be depicted by echo-planar imaging as a series of deformative surface modifications, which are related to the activity of intrinsic and extrinsic lingual muscles.

Adult↗