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C R Meyer

Publications and source records attributed to C R Meyer.

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

Motion correction in fMRI via registration of individual slices into an anatomical volume.

An automated retrospective image registration based on mutual information is adapted to a multislice functional magnetic resonance imaging (fMRI) acquisition protocol to provide accurate motion correction. Motion correction is performed by mapping each slice to an anatomic volume data set acquired in the same fMRI session to accommodate inter-slice head motion. Accuracy of the registration parameters was assessed by registration of simulated MR data of the known truth. The widely used rigid body volume registration approach based on stacked slices from the time series data may hinder statistical accuracy by introducing inaccurate assumptions of no motion between slices for multislice fMRI data. Improved sensitivity and specificity of the fMRI signal from mapping-each-slice-to-volume method is demonstrated in comparison with a stacked-slice correction method by examining functional data from two normal volunteers. The data presented in a standard anatomical coordinate system suggest the reliability of the mapping-each-slice-to-volume method to detect the activation signals consistent between the two subjects.

Algorithms↗
PubMed1999

Semiautomatic registration of volumetric ultrasound scans.

We demonstrate the ability to register easily and accurately volumetric ultrasound scans without significant data preprocessing or user intervention. Two volumetric ultrasound breast scan data sets were acquired from two different patients with breast cancer. Volumetric scan data were acquired by manually sweeping a linear array transducer mounted on a linear slider with a position encoder. The volumetric data set pairs consisted of color flow and/or power mode Doppler data sets acquired serially on the same patients. A previously described semiautomatic registration method based on maximizing mutual information was used to determine the transform between data sets. The results suggest that, even for the deformable breast, three-dimensional full affine transforms can be sufficient to obtain clinically useful registrations; warping may be necessary for increased registration accuracy. In conclusion, mutual information-based automatic registration as implemented on modern workstations is capable of yielding clinically useful registrations in times <35 min.

Breast Neoplasms↗
PubMed1998

Site-directed mutagenesis of a regulatory site of Escherichia coli ADP-glucose pyrophosphorylase: the role of residue 336 in allosteric behavior.

Site-directed mutagenesis was used to probe the role of glycine residue 336 in the regulatory properties of Escherichia coli ADP-glucose pyrophosphorylase. This residue was previously found to be changed from glycine to aspartate in the gene of an Escherichia coli mutant strain. The mutant enzyme had altered kinetic properties, including higher activity in the absence of the activator fructose 1,6-bisphosphate (FBP), higher apparent affinity for FBP and substrates, and lower apparent affinity for the inhibitor AMP. The observed changes in activity were caused by this single mutation, because the aspartate mutant was prepared from the wild-type gene. The kinetic properties of the site-directed mutant are identical to those of the enzyme from the mutant strain. A series of mutants was prepared to explore the effects of charge, size, shape, and hydrophobicity of the amino acid at residue 336 on the enzyme regulatory properties. All of the mutants, except for the lysine and arginine enzymes, were expressed and purified for kinetic analysis. The glycine-336 residue is able to tolerate diverse substitutions without compromise of catalytic activity. A range of allosteric changes was observed, with the most dramatic effects seen with the highly active aspartate enzyme and the low-activity G336Q mutant, which exhibited lower apparent affinities for activator and substrates and higher apparent affinity for inhibitor. The altered allosteric properties of the G336D mutant enzyme were almost completely abolished by substitution of asparagine. Thus, the aspartate negative charge is essential for the altered binding of effectors.

Adenosine Monophosphate↗
PubMed1998

A kinetic study of site-directed mutants of Escherichia coli ADP-glucose pyrophosphorylase: the role of residue 295 in allosteric regulation.

The effects of amino acid substitutions at residue 295 on the regulatory properties of Escherichia coli ADP-glucose pyrophosphorylase were studied. In previous studies, this residue, altered from proline to serine (P295S) in the gene of a mutant strain of E. coli, resulted in a high-activity form of enzyme [higher activity in absence of activator fructose 1,6-bisphosphate (FBP), higher apparent affinity for FBP and substrates, and lower apparent affinity for the inhibitor, AMP]. The effects of size and charge on this site were explored by replacing Pro with Gly, Asp, Asn, Gln, or Glu. All mutant enzymes were expressed and purified for kinetic analysis. All mutant enzymes, to varying extents, were in more active form than the wild-type enzyme. Enzymes with a substituted negative charge (P295D, P295E) had the highest activity in the absence of FBP, while the P295G enzyme was most similar to the wild type. The P295D and P295E enzymes had the lowest apparent affinities for AMP; this effect was partially abolished by the neutral substitutions P295N and P295Q. Another mutation, G336D, had previously been found to produce an even higher activity enzyme form. In order to examine interactions between substitutions at the 295 and 336 positions, the double mutant P295D-G336D was constructed and characterized. The double mutant enzyme was more active in the absence of FBP, with a higher affinity for FBP and a lower apparent affinity for AMP than either single mutated enzyme. The significance of residue 295 in regulation is discussed.

Adenosine Monophosphate↗
PubMed1998

Batten down the hatches.

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Centers for Medicare and Medicaid Services, U.S.↗
PubMed1998

Infertility.

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Ethics, Medical↗
PubMed1997

Retrospective correction of surface coil MR images using an automatic segmentation and modeling approach.

The use of surface coils in magnetic resonance imaging offers significant improvements in the signal-to-noise ratio over volume coils for many applications. However, the inhomogeneous reception profile of surface coils hampers their usefulness by introducing significant nonuniformities or intensity variations which can vary by greater than six-fold across the sample. In this study, we evaluated an automatic technique for retrospective correction of intensity variations observed in a high-resolution surface coil MR image of the rat brain obtained using an adiabatic magnetic resonance imaging sequence. The images are shown to have a coefficient of variation less than 12% following application of this correction algorithm. This image intensity correction technique can be applied retrospectively to all data sets and corrects both sample/patient dependent effects (e.g. attenuation of overlying tissue) or sample independent effects (e.g. coil geometry or position). This approach should also prove valuable in improving regions of interest analysis, volume histograms and thresholding techniques.

Animals↗
PubMed1997

Mutual information for automated unwarping of rat brain autoradiographs.

An automated multimodal warping based on mutual information metric (MI) as a mapping cost function is demonstrated. Mutual information (I) is calculated from a two-dimensional (2D) gray scale histogram of an image pair, and MI (= -I) provides a matching cost function which can be effective in registration of two- or three-dimensional data sets independent of modality. Most histological image data, though information rich and high resolution, present nonlinear deformations due to the specimen sectioning and need reconstitution into deformation-corrected volumes prior to geometric mapping to an anatomical volume for spatial analyses. Section alignment via automatic 2D registrations employing MI as a global cost function and thin-plate-spline (TPS) warping is applied to deoxy-D-[14C]glucose autoradiographic image slices of a rat brain with video reference images of the uncut block face to reconstitute a cerebral glucose metabolic volume data. Unlike the traditional feature-based TPS warping algorithms, initial control points are defined independently from feature landmarks. Registration quality using automated multimodal image warping is validated by comparing MIs of the image pair registered by automated affine registration and manual warping method. The MI proves to be a robust objective matching cost function effective for automatic multimodality warping for 2D data sets and can be readily applied to volume registrations.

Algorithms↗
PubMed1997

Ultrasonic characterization of in vitro osteoarthritic articular cartilage with validation by confocal microscopy.

The majority of adults over the age of 65 y develop osteoarthritis (OA), a joint disease characterized by degeneration of articular cartilage and subchondral sclerosis. Early in the disease, the articular cartilage surface begins to change histologically from a smooth to a rough or fibrillated appearance. A prerequisite for any chondroprotective pharmacological intervention is detection of OA in its preclinical phase. Current diagnostic imaging modalities, such as radiographs or (nuclear) magnetic resonance imaging, either cannot directly image the cartilage surface or lack sufficient resolution to detect surface fibrillations. We have developed an ultrasonic technique that can be used to characterize these surface fibrillations directly. We present our in vitro results with validation by laser-based confocal microscopic imaging.

Adult↗
PubMed1997

Demonstration of accuracy and clinical versatility of mutual information for automatic multimodality image fusion using affine and thin-plate spline warped geometric deformations.

This paper applies and evaluates an automatic mutual information-based registration algorithm across a broad spectrum of multimodal volume data sets. The algorithm requires little or no pre-processing, minimal user input and easily implements either affine, i.e. linear or thin-plate spline (TPS) warped registrations. We have evaluated the algorithm in phantom studies as well as in selected cases where few other algorithms could perform as well, if at all, to demonstrate the value of this new method. Pairs of multimodal gray-scale volume data sets were registered by iteratively changing registration parameters to maximize mutual information. Quantitative registration errors were assessed in registrations of a thorax phantom using PET/CT and in the National Library of Medicine's Visible Male using MRI T2-/T1-weighted acquisitions. Registrations of diverse clinical data sets were demonstrated including rotate-translate mapping of PET/MRI brain scans with significant missing data, full affine mapping of thoracic PET/CT and rotate-translate mapping of abdominal SPECT/CT. A five-point thin-plate spline (TPS) warped registration of thoracic PET/CT is also demonstrated. The registration algorithm converged in times ranging between 3.5 and 31 min for affine clinical registrations and 57 min for TPS warping. Mean error vector lengths for rotate-translate registrations were measured to be subvoxel in phantoms. More importantly the rotate-translate algorithm performs well even with missing data. The demonstrated clinical fusions are qualitatively excellent at all levels. We conclude that such automatic, rapid, robust algorithms significantly increase the likelihood that multimodality registrations will be routinely used to aid clinical diagnoses and post-therapeutic assessment in the near future.

Abdomen↗
PubMed1997

A call to action.

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Delivery of Health Care↗