Search PubMed⌕ Search

Biomedical subjects

P C Lauterbur

Publications and source records attributed to P C Lauterbur.

At least 19 recordsLinked to original sources
PubMed2003

Prolonged exercise induces angiogenesis and increases cerebral blood volume in primary motor cortex of the rat.

Plastic changes in motor cortex capillary structure and function were examined in three separate experiments in adult rats following prolonged exercise. The first two experiments employed T-two-star (T(2)*)-weighted and flow-alternating inversion recovery (FAIR) functional magnetic resonance imaging to assess chronic changes in blood volume and flow as a result of exercise. The third experiment used an antibody against the CD61 integrin expressed on developing capillaries to determine if motor cortex capillaries undergo structural modifications. In experiment 1, T(2)*-weighted images of forelimb regions of motor cortex were obtained following 30 days of either repetitive activity on a running wheel or relative inactivity. The proton signal intensity was markedly reduced in the motor cortex of exercised animals compared with that of controls. This reduction was not attributable to alterations of vascular iron levels. These results are therefore most consistent with increased capillary perfusion or blood volume of forelimb regions of motor cortex. FAIR images acquired during experiment 2 under normocapnic and hypercapnic conditions indicated that resting cerebral blood flow was not altered under normal conditions but was elevated in response to high levels of CO(2), suggesting that prolonged exercise increases the size of a capillary reserve. Finally, the immunohistological data indicated that exercise induces robust growth of capillaries (angiogenesis) within 30 days from the onset of the exercise regimen. Analysis of other regions failed to find any changes in perfusion or capillary structure suggesting that this motor activity-induced plasticity may be specific to motor cortex.These data indicate that capillary growth occurs in motor areas of the cerebral cortex as a robust adaptation to prolonged motor activity. In addition to capillary growth, the vascular system also experiences heightened flow under conditions of activation. These changes are chronic and observable even in the anesthetized animal and are measurable using noninvasive techniques.

Aging↗
PubMed2001

Apparent diffusion tensor measurements in myelin-deficient rat spinal cords.

The apparent diffusion tensor (ADT) was measured in excised and fixed spinal cords from myelin-deficient (md) rats and age-matched controls. These data were used to obtain the principal diffusivities of the ADT, and also the scalar invariant parameters _D (averaged principal diffusivity) and A(sigma) (anisotropy index) for four white matter and two gray matter regions. The results for white matter regions showed that the principal diffusivities were significantly higher for md animals, and while the _D was increased in tissue from md animals, the A(sigma) was found to be decreased. Grey matter _D was measured to be between those of white matter from control and md animals, and the A(sigma) was much smaller than that of white matter from both sets of animals, indicating that diffusion in md white matter is more anisotropic than in gray matter. The results show that while myelination is not a prerequisite for diffusion anisotropy, it does influence the magnitude of the observed anisotropy. Magn Reson Med 45:191-195, 2001.

Animals↗
PubMed1999

Lactate quantitation in a gerbil brain stroke model by GSLIM of multiple-quantum-filtered signals. Generalized spectral localization by imaging.

Quantitative magnetic resonance imaging of lactate using a zero-quantum/double-quantum filter and generalized spectral localization by imaging (GSLIM) was applied to a model of unilateral stroke in gerbil brain. GSLIM lactate images at 4T clearly reveal elevated concentrations of lactate in the ischemic compared with the normal hemisphere 100-175 minutes after unilateral carotid ligation. These results indicate that the technique is capable of studies of brain infarcts, and that application to human ischemic pathology in brain and other tissues may be possible.

Animals↗
PubMed1999

Apparent water diffusion measurements in electrically stimulated neural tissue.

The effect of stimulation on diffusion characteristics of electrically stimulated neural tissue was examined. Bullfrog nerves and spinal cords were excised and stimulated electrically during magnetic resonance imaging, and the apparent diffusion coefficients (ADCs) of water parallel and perpendicular to the long axes of the specimens were mapped with and without stimulation of the tissue. Electrophysiological recordings were used to determine the viability of the tissue after each experiment. No stimulus-related ADC changes were observed in either the peripheral nervous system or the central nervous system tissue. These experiments may help to define further the nature of previously reported ADC changes in stimulated neural tissue in situ.

Animals↗
PubMed1998

A fast spin echo technique with circular sampling.

This paper presents a fast spin echo (FSE) imaging method that employs circular sampling of k-space. The technique has been implemented on a 2 Tesla imaging system and validated on both phantoms and living animals. Experimental studies have shown that circular sampling can produce artifact-free FSE images without the need of phase correction. Although not fully explored, preliminary results also show that circular sampling may have advantages over the conventional rectilinear FSE in signal-to-noise ratio and imaging efficiency. A major disadvantage is the increased sensitivity to off-resonance effects. The authors expect that the FSE technique with circular sampling will find its applications in magnetic resonance microscopy, neuro-functional imaging, and real-time dynamic studies.

Animals↗
PubMed1997

Quantitative lactate-specific MR imaging and 1H spectroscopy of skeletal muscle at macroscopic and microscopic resolutions using a zero-quantum/double-quantum coherence filter and SLIM/GSLIM localization.

Quantitative lactate imaging and spectroscopy were performed on phantoms and on electrically stimulated, excised frog skeletal muscle at macroscopic and microscopic resolutions. Lactate selectivity was achieved by use of a zero-quantum/double-quantum coherence (ZQC/DQC) lactate filter, which suppressed all signals besides lactate, including water and lipid, to below noise level. Three-dimensional lactate data sets were acquired in 1-3 h; one of these spatial dimensions was frequency-encoded and the other two were phase-encoded. High-resolution images were reconstructed using the spectral localization by imaging (SLIM) and generalized SLIM (GSLIM) techniques. Lactate quantitation was achieved by employing an external lactate concentration standard and was verified by comparison to quantitative STEAM-localized and nonlocalized spectra that used total creatine as an internal concentration reference. Additionally, quantitatively accurate behavior of the SLIM and GSLIM techniques as applied to data sets of low signal-to-noise ratio and to macroscopically heterogeneous objects was verified using simulations and real muscle lactate data sets with known heterogeneity.

Animals↗
PubMed1997

A comparison of RIGR and SVD dynamic imaging methods.

Several constrained imaging methods have recently been proposed for dynamic imaging applications. This paper compares two of these methods: the Reduced-encoding Imaging by Generalized-series Reconstruction (RIGR) and Singular Value Decomposition (SVD) methods. RIGR utilizes a priori data for optimal image reconstruction whereas the SVD method seeks to optimize data acquisition. However, this study shows that the existing SVD encoding method tends to bias the data acquisition scheme toward reproducing the known features in the reference image. This characteristic of the SVD encoding method reduces its capability to capture new image features and makes it less suitable than RIGR for dynamic imaging applications.

Image Processing, Computer-Assisted↗
PubMed1997

A multiple echo pulse sequence for diffusion tensor imaging and its application in excised rat spinal cords.

A new imaging sequence for rapid determination of the apparent self-diffusion tensor of water was developed and tested on fixed excised rat spinal cords. To reduce the time required to determine the tensor, the sequence utilized a new single-shot approach with multiple spin echoes. An assumption of cylindrical symmetry in the sample was made, thus requiring the measurement of only four of the six unique elements of the tensor. This assumption was found experimentally to be valid, and the results obtained using the new sequence were found to be quantitatively the same as results obtained using a standard spin-echo sequence.

Animals↗
PubMed1997

Diffusion measurement in phantoms and tissues using SLIM localization.

A new approach to efficient localized diffusion measurements has been developed and evaluated on phantoms and isolated tissues. The combination of a diffusion-sensitive pulse sequence with SLIM (spectral localization by imaging) makes efficient and accurate localized water and metabolite diffusion measurements possible with a substantial improvement in spatial or time resolution compared to standard methods. Phantom experiments showed that diffusion of substances present in relatively low concentration within small compartments can be measured accurately by this method, suggesting potential applications for diffusion measurements of metabolites in vivo. Experiments on excised rat uterine horns demonstrated the ability of this method to measure localized diffusion of water within irregularly shaped regions of biological samples. Accurate diffusion measurements were achieved in the localized regions with acquisition times less than would have been required by standard diffusion imaging methods.

Animals↗
PubMed1996

Fast dynamic imaging using two reference images.

This paper presents a fast dynamic imaging method which is characterized by the acquisition of two high-resolution reference images and a sequence of low-resolution dynamic data sets. Image reconstruction is accomplished using a generalized series based algorithm. Experimental results demonstrate that dynamic images with high temporal resolution can be obtained while maintaining excellent spatial resolution. This method will be useful for a variety of dynamic imaging applications including contrast-enhanced dynamic imaging and functional brain studies.

Algorithms↗
PubMed1995

Design and analysis of microcoils for NMR microscopy.

The signal-to-noise ratio achieved in a nuclear magnetic-resonance microscopy experiment is directly related to the performance of the radiofrequency coil. An accurate determination of coil performance requires that the resistance of the coil be well characterized. Traditional high-frequency electric-circuit models used to describe larger NMR coils are inadequate when the diameter of the conductor is reduced to the dimensions of the electrical skin depth (delta) at the frequency of operation. A more extensive model based on a scaling parameter that includes delta is presented. This model complements other existing circuit models that represent sample losses, ground-loop and parasitic losses, and the signal induced in the RF coil. Experimental verification is accomplished using a series of solenoidal microcoils in 1H NMR microspectroscopy experiments at 4.7 T (200 MHz). This study demonstrates for the first time that a predictable performance enhancement is achieved using microcoils as small as 50 microns in diameter.

Magnetic Resonance Spectroscopy↗
PubMed1994

Dendrimer-based metal chelates: a new class of magnetic resonance imaging contrast agents.

We have developed a new class of magnetic resonance imaging contrast agents with large proton relaxation enhancements and high molecular relaxivities. The reagents are built from the polyamidoamine form of Starburst dendrimers in which free amines have been conjugated to the chelator 2-(4-isothiocyanatobenzyl)-6-methyl-diethylenetriaminepentaacetic acid. The dendrimer gadolinium poly-chelates have enhancement factors, i.e., the ratio of the relaxivity per Gd(III) ion to that of Gd(III)-diethylenetriaminepentaacetic acid, of up to 6. These factors are more than twice those observed for analogous metal-chelate conjugates formed with serum albumins, polylysine, or dextran. One of the dendrimer-metal chelate conjugates has 170 gadolinium ions bound, which greatly exceeds the number bound to other macromolecular agents reported in the literature, and has a molecular relaxivity of 5,800 (mM.s)-1, at 25 MHz, 20 degrees C, and pH of 7.4. We observed that these dendrimer-based agents enhance conventional MR images and 3D time of flight MR angiograms, and that those with molecular weights of 8,508 and 139,000 g/mole have enhancement half lives of 40 +/- 10 and 200 +/- 100 min, much longer than the 24 +/- 4 min measured for Gd(III)-diethylenetriaminepentaacetic acid. Our results suggest that this new and powerful class of contrast agents have the potential for diverse and extensive application in MR imaging.

Animals↗
PubMed1993

Voltage-sensitive magnetic gels as magnetic resonance monitoring agents.

Many polymer gels undergo a volume phase transition in electric fields. We report here the synthesis of a polyelectrolyte gel that incorporates magnetic particles of iron oxide into the polymer network; these particles couple the gel volume transition to the NMR relaxation times of the surrounding water. We find that the water proton relaxation rates (T2(-1)) in aqueous suspensions of particles of the magnetic gel increase significantly in an electric field. Such changes can also be induced by the hyperpolarization of red blood cells in the suspension, presumably as a result of the electric fields at the cell membrane surfaces. These gels may play a part in magnetic resonance imaging analogous to that of voltage-sensitive dyes in optical imaging.

Animals↗
PubMed1993

Three-dimensional NMR microscopy of rat spleen and liver.

Three-dimensional microscopic NMR images of spleen and liver specimens from rats injected with dextran magnetite particles and from controls were obtained at 4.7 T, using a specially designed probe in conjunction with a 3D filtered back projection reconstruction algorithm. All of the images were reconstructed as 64(3) arrays with (25 microns) 3 isotropic voxels. With the aid of the MR contrast agent, the red pulp and marginal zone of the spleen and the portal triad of the liver could be distinguished from the surrounding tissue in T2-weighted images. For mature rat spleen, natural contrast in T2-weighted images was found to distinguish the same features. Histological examinations of the tissues with and without contrast agent were also performed using an optical microscope. Microscopic NMR images, despite their lower resolution, clearly revealed many features seen in the optical images.

Aging↗
PubMed1993

Nuclear magnetic resonance (NMR) imaging of iron oxide-labeled neural transplants.

Nuclear magnetic resonance (NMR) imaging in vivo of adult rat brains was used to observe the fate of iron oxide-labeled intracerebral neural grafts. The host animals received grafts of fetal (E17-E18) rat tissue prepared as cell suspensions and labeled by incubation with reconstituted Sendai viral envelopes containing iron oxide particles. Control studies were performed in animals following surgical trauma alone or transplantation of unlabeled cell suspensions. In vivo NMR imaging (either two-dimensional Fourier transform or three-dimensional Fourier transform) was performed once on each animal between 5 and 60 days postsurgery. The NMR images of the host brains containing the labeled cells showed sufficient anatomical detail for the identification of the major brain structures. The graft sites were seen in the T2-weighted NMR images as dark regions (low-intensity signal) in the cortex. Histochemical staining for ferric iron (prussian blue stain) demonstrated the presence of numerous prussian blue-positive cells in tissue sections corresponding to the dark regions in the NMR images. Surviving prussian blue-positive cells with neuron-like morphology were relatively more numerous 10 days after transplantation than at 1-2 months postgrafting. The NMR images and immunohistochemical and histochemical staining of hosts containing labeled cells suggest cell migration of astrocytes and macrophages up to 2 mm away from the graft sites. The migrating cells were primarily along fiber tracts in the host white matter. Prussian blue deposits were also present within the brains of the control animals, but the NMR images from the same animals did not contain distinct dark regions like those in the images of brains which had received labeled cell transplants. These results demonstrate that NMR imaging can be used to study cell survival and migration following neural grafting procedures.

Animals↗
PubMed1988

SLIM: spectral localization by imaging.

Nonspectroscopic magnetic resonance (MR) imaging often shows that a slice is composed of several compartments, each of which can be assumed to have a spatially homogeneous magnetic resonance spectrum, e.g., a limb composed of fat, muscle, bone marrow, and tumor. We show how to use structural information from such a nonspectroscopic image in order to increase the efficiency of subsequent localized spectroscopic measurements. Specifically, knowledge of the boundaries of N compartments makes it possible to reconstruct compartmental spectra from spectroscopic signals from an entire cross section with N or more different degrees of phase encoding. Experimental studies of a two-compartment phantom show that this method (SLIM) can be used to derive regional hydrogen spectra of a single slice from signals with as few as 2 phase-encoding steps, although Fourier transform chemical-shift imaging requires 64 steps to achieve a result of comparable accuracy. SLIM required only 16 phase-encoding steps to obtain accurate regional single slice spectra in a human limb with three compartments. Spectra of similar quality, obtained by Fourier transform chemical-shift imaging, required 256 to 1024 steps.

Humans↗