Biomedical subjects

R S Hinks

Publications and source records attributed to R S Hinks.

28 records · Page 2Linked to original sources

PubMed1987

Kinetic studies of modifier effects on the carboxypeptidase A catalyzed hydrolyses of peptides.

A variety of modifiers of carboxypeptidase A (CPA) have been investigated in an effort to understand the structural requirements of inhibitors and activators of peptidase activity. It is proposed that an understanding of the mechanism of action of reversible activators of the enzyme may bear on the long standing question of whether the detailed mechanism of peptidase activity is different from that of esterase activity. An analog of the activator 2,2-dimethyl-2-silapentane-5-sulfonate, 5,5-dimethylhexanoate, was found to be a competitive inhibitor of the CPA-catalyzed hydrolysis of benzoylglycyl-L-phenylalanine. The modifier 4-phenyl-3-butenoate (styrylacetic acid) was determined to be an activator. The sulfonates benzene-sulfonate, p-toluenesulfonate, phenylmethanesulfonate, 2-phenylethanesulfonate, and 3-phenylpropanesulfonate were all found to be activators.

Carboxypeptidases↗

PubMedDate not supplied

Ghost phase cancellation with phase-encoding gradient modulation.

Motion artifacts are a dominant cause of magnetic resonance image quality degradation. Periodic or nearly periodic motion results in image replicates of the moving structures in spin-warp Fourier imaging. The replicates, or ghosts, propagate in the image in the phase-encoding, or y, direction. These ghosted images can be considered to consist of the time-averaged spin density I0 and a ghost mask g. A set of j ghosted images Ij may be acquired in which the ghost mask is intentionally phase shifted by varying amounts relative to I0 with interleaved acquisitions that have shifted phase-encoding orders or by acquiring multiple images during a single readout period in the presence of an oscillating phase-encoding gradient. The resulting complex images Ij have the same time-averaged spin density I0 but have ghost contributions gj that, on a pixel-by-pixel basis, trace part of a circle around I0. The source images Ij can then be used to estimate I0. Simulations and experiments with the phase-encoding gradient modulation method show good general ghost suppression for a variety of quasi-periodic motion sources including both respiratory-type artifacts and flow artifacts. The primary limitation of the method is the need for rapid gradient switching.

Artifacts↗

PubMedDate not supplied

Fast spin-echo MR imaging of the abdomen: contrast optimization and artifact reduction.

The effects of various fast spin-echo (FSE) magnetic resonance (MR) imaging parameters and artifact reduction techniques on FSE image contrast and quality were studied. The authors performed 139 abdominal MR examinations, comparing standard FSE images (echo train length [ETL] = 8, echo space [E-space] = 17 msec, bandwidth = +/- 16-kHz) with FSE images with an ETL of 16 (n = 22) or FSE images with a +/- 32-kHz bandwidth and an E-space of 11-14 msec (n = 22). FSE artifact reduction techniques were evaluated with spectral fat saturation (n = 40) or with a new flow compensation FSE sequence (n = 55). Images of liver lesions were reviewed qualitatively and with contrast-to-noise ratio (C/N) measurements. Decreasing the time of echo train sampling produced superior image quality, with increased anatomic sharpness, less image artifact, and improved liver-lesion C/N. Images obtained with an ETL of 16 showed more image blurring and a 23% decrease in relative contrast and 28% decrease in relative C/N for liver tumors. Increasing the bandwidth reduced E-space, producing a 12% decrease in background noise. Artifact reduction with fat saturation or flow compensation produced images with less ghosting artifact and superior overall image quality, with 39% and 20% increases in liver-tumor C/N, respectively. FSE image quality and contrast in the depiction of hepatic disease can be optimized with careful selection of imaging parameters and the use of artifact reduction techniques.

Abdomen↗

PubMedDate not supplied

Separation of fat and water in fast spin-echo MR imaging with the three-point Dixon technique.

A method for suppressing fat in fast spin-echo imaging with the three-point Dixon technique is described. The method differs from the three-point Dixon method used in conventional spin-echo imaging in that the readout gradient instead of a radio-frequency pulse is shifted. This method preserves the Carr-Purcell-Meiboom-Gill nature of the fast spin-echo sequence and hence is less sensitive to magnetic field inhomogeneities and resonance frequency mistuning. As in the original three-point Dixon technique used in conventional spin-echo imaging, three acquisitions are required to estimate the field inhomogeneity and completely separate fat and water. The extra time required is not excessive considering that the fast spin-echo method is frequently applied with multiple signal acquisition. Also, this technique achieves an expected signal-to-noise ratio comparable to 2.67 signal acquisitions, which is approximately 94% of the signal-to-noise ratio obtained with three signal acquisitions. The method is demonstrated with applications to phantoms and a human volunteer.

Adipose Tissue↗

PubMedDate not supplied

MR systems for image-guided therapy.

The use of MRI to guide and monitor interventional procedures requires the merging of surgical and MRI environments. The ideal magnet shape for homogeneity and efficiency is spherical, but this design provides no access. Opening the sphere to provide both patient and surgeon access suggests cylindrical or biplanar magnets. Cylindrical magnets have poor surgical access but provide good imaging capabilities, which can be used in conjunction with a neighboring but distinct surgical environment. Biplanar magnets provide more and better approaches to the patient, but generally with lower field strength. Vertical biplanar systems allows surgical approaches from above but reduce the access of support staff to the patient. A hybrid magnet design, which combines the benefits of both cylindrical and biplanar magnets, can provide increased access with simultaneous approach from two sides of the patient. Application-specific magnets can target a smaller region, leading to compact magnet designs that greatly expand access for both surgical intervention as well as patient support. As the field of interventional MRI matures, the suitability of each design to specific applications will be better understood, leading to more integrated system designs tailored to the needs of image-guided therapy.

Equipment Design↗

PubMedDate not supplied

MRI monitoring of interstitial microwave-induced heating and thermal lesions in rabbit brain in vivo.

The purpose of this experiment was to use MRI to monitor microwave heating and thermal damage of brain tissue in vivo. Interstitial microwave antennas were implanted into the cerebral hemispheres of seven anesthetized rabbits. Variable power of 30 to 100 W was applied for periods of 5 to 15 minutes and tissue temperature was monitored continuously. MR images were obtained throughout the procedure at 20-second intervals, using a spoiled gradient-echo sequence, without significant artifact. Magnitude, phase, and complex difference images all demonstrated temperature-related signal changes during heating. The findings were better visualized on the phase and complex difference images. Phase difference image analysis revealed an approximately linear relationship between phase change and temperature. Post-treatment thermal lesions measured up to 2.0 cm in size on pathologic specimens and exhibited a zonal pattern on spin-echo MR images.

Animals↗

PubMedDate not supplied

Dynamic scan-plane tracking using MR position monitoring.

An MR-based method for tracking subject motion is presented. The technique identifies subject motion from the three-dimensional positions of three small samples attached to the subject in a fixed, triangular configuration. The updated positions of these samples relative to their initial positions determine a rigid body transformation. Applied to the MRI scan prescription via adaptive feedback controls, this transformation yields an updated MRI scan plane that tracks the prescribed imaging section as the subject moves. The scan-plane tracking procedure is demonstrated experimentally for two-dimensional imaging of a standard imaging phantom and the head of a human subject. Sets of images were acquired sequentially, with motion (translations and/or rotations) introduced between image frames. The scan-plane tracking system provides well registered image slices of the same section, adaptively compensating for the subject motion.

Artifacts↗

PubMedDate not supplied

Gradient coil design considerations for iron core interventional magnets.

The requirements for access and imaging performance compete in the design of open-concept MR magnets and gradient coils. We conducted a theoretical and experimental investigation of gradient coil design using both solid and laminated pole piece construction to determine whether adequate eddy current control can be obtained without shielded gradient coils while maintaining good patient access and high gradient performance. Eddy currents, gradient characteristics, gradient efficiency, and magnet openness are compared and contrasted for various construction options based on a compact, .27 T iron yoke magnet. The resulting pole pieces and gradient coils have high efficiency for an interventional open-configuration magnet while taking up minimal space between the poles for improved patient access.

Equipment Design↗

PubMedDate not supplied

Problems with organic materials for magnetic resonance imaging phantoms.

Organic liquids are frequently used as magnetic resonance (MR) phantom materials. It is often not appreciated that indirect spin-spin coupling can dramatically affect signal amplitudes in spin-echo experiments. These effects are evidenced in three ways: (1) modulation of spin-echo amplitudes; (2) decrease in measured T2 relaxation times; and (3) dependence of spin-echo modulation and measured T2 values on the echo time (TE) in multiecho Carr-Purcell-Meibroom-Gill experiments. This paper illustrates these effects and warns against the inappropriate use of organic liquids as MR phantoms.

Copper↗

PubMedDate not supplied

Comparison of gradient-recalled-echo and T2-weighted spin-echo pulse sequences in intramedullary spinal lesions.

Nineteen consecutive patients with spinal intramedullary lesions were studied on a 1.5-T system to compare the quality of T2-weighted spin-echo and gradient-recalled-echo (GRE) pulse sequences. Direct comparisons were made in the sagittal and/or axial planes. Twenty-four studies were performed in the 19 patients. The gradient echoes were usually performed at 300/14 (TR/TE) with a flip angle of 10 degrees. Although no significant diagnostic differences were noted in the sagittal plane, there was a distinct anatomic advantage for GRE imaging over spin-echo imaging in the axial plane. This is believed to be the result of CSF time-of-flight effects in the slice-select direction, which are not compensated for by flow-compensating gradients on the spin-echo images, but which are insignificant in the GRE sequence used in this study. Pathology was seen equally well or better on GRE in 79% (19/24) of the sequences. In the other five cases, the spin-echo image showed a brighter intramedullary signal than that seen on GRE, although GRE showed the lesion in all cases. Our results indicate that properly optimized GRE imaging on a high-field-strength system can replace spin-echo imaging in the spine when intramedullary disease is suspected and that the benefits of GRE are most striking in the axial plane.

Adolescent↗