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Biomedical subjects

B K Rutt

Publications and source records attributed to B K Rutt.

At least 55 records · Page 3Linked to original sources
PubMed1996

Geometric characterization of stenosed human carotid arteries.

RATIONALE AND OBJECTIVES: The geometry of stenosed carotid bifurcations was analyzed to determine average representations for several stenosis grades. METHODS: Film angiograms of 62 patients with internal carotid artery stenoses were digitized. Residual lumen boundaries were manually outlined. The outlines were processed with a computer to extract geometric measurements. The measurements were grouped according to stenosis grade and used to create average representations. RESULTS: Accuracy and precision of the outlining technique were +/- 0.020 common carotid diameters (CCD) and +/- 0.025 CCD, respectively. Maximum narrowing of the internal carotid artery occurred at 0.3 CCD +/- 1.5 (mean +/- standard deviation) distal to the flow divider. The region of significant narrowing extended axially 1.2 CCD +/- 1.0. Poststenotic dilatations were observed, with enlargement of 1.3 +/- 0.7 times the normal diameter of the distal internal carotid artery. A tendency toward smaller bifurcation angles with increasing stenosis severity was observed. CONCLUSION: Three-dimensional geometric models could be created for carotid bifurcations that were disease free (normal) and of arbitrary stenosis grade.

Aged↗
PubMed1995

MR gradient coil heat dissipation.

The temperature responses of five different gradient coil designs were modeled using simplified engineering equations and measured. The model predicts that the coil temperature approaches a maximum as an inverse exponential, where the maximum temperature is governed by two parameters: a local power density and a cooling term. The power density term is a function of position and is highest where the current paths have minimum widths and are closely packed. The cooling parameter consists of convective, conductive, and radiative components which can be controlled by (1) providing forced cooling, (2) having a coil former with high thermal conductivity and thin walls, and (3) varying the emissivity of the coil surfaces. For a given gradient strength, the average temperature rise is minimized by designing a coil with a small radius and thick copper. The model predicted the local temperature rise, which is also dependent on the current density, to within 5 degrees C of measured values.

Equipment Design↗
PubMed1995

Measurement of fluid-shear rate by Fourier-encoded velocity imaging.

A new technique for estimating the blood fluid shear rate at the vessel wall is presented. The technique uses Fourier-encoded velocity imaging to determine the velocity distribution within a spatial element (voxel) that straddles the blood-vessel wall interface. By appropriate processing, the velocity distribution (1) can determine the location of the wall-blood interface within the voxel and (2) estimate the velocity profile across the spatial extent of the voxel. From this information, accurate estimates of fluid shear rate may be obtained. Simulations are presented to illustrate this technique and to show the effects of various error sources, including differences in proton densities between blood and wall tissues and flow-related signal changes. Experimental evidence obtained for steady flow in straight tubes is also presented in support of the technique. The mean error in the experimental shear rate estimates found using the proposed technique was -15%. This represents a significant improvement over estimates obtained by extrapolation of the velocity profile over multiple voxels (mean error of -73%).

Blood Flow Velocity↗
PubMed1995

A novel transverse gradient coil design for high-resolution MR imaging.

The authors describe a new gradient coil design for high resolution human, animal, specimen, or phantom imaging with high gradient efficiency and a large region of excellent gradient uniformity. Important features of our new design are the simple analytical description of the wire patterns that comprise the design, and ease of construction. Wires are spaced in a sinusoidal distribution around the circumference of the cylinder, and curved in an arcsin shape along the length of the cylinder. This coil produces a magnetic field pointing in a direction transverse to the axis of the coil with a gradient in the direction parallel to the axis of the coil. The same arcsin coil can be used to create a magnetic field pointing in a direction parallel to its axis with a gradient in a direction perpendicular to the coil axis. A prototype coil was constructed; field and inductance calculations were verified. Geometric variations on this coil design were modeled and their performance characteristics compared. This coil design is ideal for rapid implementation of a transverse gradient coil, since no specialized design software is required.

Algorithms↗
PubMed1995

Novel low-dosage hormonal replacement therapy complements dietary treatment of moderately hypercholesterolemic postmenopausal women.

It is not known whether female hormonal replacement therapy (HRT) influences fasting plasma lipoprotein lipids in diet-treated hypercholesterolemic subjects. Thirteen moderately hypercholesterolemic postmenopausal women were studied during dietary treatment alone with a low fat, low cholesterol diet for the 3 months and during the subsequent 2 y of HRT with dl-norgestrel (0.075 mg daily) and estradiol-17 beta (1 mg, 25 of 28 days) with maintenance of the same diet. Hormonal replacement therapy decreased plasma total cholesterol by 11 +/- 3% (5.7 vs. 6.4 mmol/L, p < 0.005), due to a 17 +/- 3% mean reduction (p < 0.001) in the concentration of plasma low density lipoprotein cholesterol (3.9 vs. 4.7 mmol/L, p < 0.001). The ratio of plasma total cholesterol to high density lipoprotein cholesterol fell significantly by 17 +/- 4% (4.1 vs. 4.9, p < 0.005). Mean fasting plasma concentrations of total triglycerides (1.1 vs. 1.6 mmol/L, p < 0.005) fell by 31 +/- 6%, and very low density lipoprotein triglycerides (0.56 vs. 0.83 mmol/L, p < 0.02) by 33 +/- 9%. Hormonal replacement therapy was well-tolerated, improved mood levels, and increased the mineral content of the vertebral spine significantly, while effectively relieving vasomotor flushing. Hormonal replacement therapy complements the dietary treatment of hypercholesterolemia.

Affect↗
PubMed1994

Quadrupole gradient coil design and optimization: a printed circuit board approach.

Three different dual-axis quadrupole gradient coils for quantitative high resolution MR imaging of small animals, phantoms and specimens were designed and built using printed circuit board technology. Numerical optimization of the conductor positions was used to increase the volume of 0.4% gradient uniformity by up to a factor of four. In one coil, the volume of 5% gradient uniformity occupied 88% and 83% of the overall diameter and length of the coil, respectively. A systematic error of 0.5% in the wire placement was shown to cause a reduction in the volume of 0.4% gradient uniformity by a factor of two, though the region of 5% gradient uniformity was not significantly affected. Heat transfer calculations were used to determine maximum peak and root-mean-squared currents that could safely be applied to the coils.

Copper↗
PubMed1994

Turbine flow sensor for volume-flow rate verification in MR.

A turbine flow sensor for MR flow experiments has been evaluated using reference volume-flow rate measurements obtained using an electromagnetic (EM) flow meter measurements and simultaneous phase contrast (PC) MR acquisitions. After calibration, the device was found to have accuracy (compared with the EM flow meter), linearity, and precision of better than +/- 1%, +/- 3.5%, 3.5%, respectively, in constant flow mode (0 to 30 ml s-1). The frequency response of the flow sensor was flat (within +/- 10%) up to 13.9 Hz. Volume-flow rate measurements on constant and simulated physiologic flow waveforms were in close agreement with both the electromagnetic (EM) flow meter and the gated MR PC estimates.

Calibration↗
PubMed1994

Poster production by color laser imager.

The scientific poster is an important educational tool at most radiology meetings. The visual impact of the exhibit is important in attracting attention and conveying content to the reader. A new method of designing and producing full-color transilluminated posters in the radiology department using a personal computer and color laser imager is described. The advantages of this approach are author control over the production process and production of a visually interesting exhibit.

Audiovisual Aids↗
PubMed1993

Generalized K-space analysis and correction of motion effects in MR imaging.

A new approach to understanding and reducing motion artifacts in magnetic resonance imaging (MRI) is introduced. This paper presents a novel technique for correcting generalized motion artifacts arising from translation, rotation, dilation, and compression, or any combination thereof. We also describe a new pulse sequence and a specialized postprocessing technique required to suppress these motion artifacts. The correction algorithm corrects for generalized motion. The theoretical basis of the correction scheme is founded upon the (k,t)-space formalism and the concept of pulse sequence contrast mapping functions. The proposed (k,t) formalism is based on the Fourier projection slice theorem and allows us to determine how motion artifacts arise. The correction technique currently suffers from some spatial resolution and signal-to-noise ratio limitations, and works better for small objects than large objects. These problems will be investigated in subsequent studies.

Artifacts↗
PubMed1991

Fast Fourier projection for MR angiography.

We have developed a technique called fast Fourier projection which rapidly produces projections through images and is particularly useful for generating MR angiograms. Based on the projection-slice theorem of Fourier transform theory, this method extracts planes from three-dimensional spatial frequency space and computes projections at arbitrary viewing angles by two-dimensional inverse Fourier transformation. Typical computation times are on the order of 1 s per projection. This performance makes possible interactive selection of optimal projection directions for visualizing the desired vasculature in single or stereo-pair angiographic images and drastically reduces the time required to generate sequences of projections for display in movie loops compared to the conventional ray-casting approach. The method is easily implemented on off-line workstations or directly on MRI computer systems.

Angiography↗
PubMed1991

Magnetic resonance techniques for blood-flow measurement and vascular imaging.

The authors review the history and physical principles behind vascular magnetic resonance imaging (MRI) techniques, developed to measure blood flow noninvasively and to display images of the vasculature. All these techniques have been used to create magnetic resonance angiograms, in which the vasculature is shown in a projection format similar to x-ray angiography. Signal loss limits the effectiveness of "white-blood" magnetic resonance angiography techniques, since slow flow and complex flow often cause a drop in signal and consequently a loss of accuracy in depicting vessel anatomy. "Black-blood" magnetic resonance angiography is described as a method that avoids these problems of signal loss. Selective black-blood magnetic resonance angiography is introduced as a technique for improving the visualization of the vasculature when other signal-void structures are present in the volume of interest.

Blood↗
PubMedDate not supplied

Visualizing three-dimensional flow with simulated streamlines and three-dimensional phase-contrast MR imaging.

Three-dimensional (3D) velocity maps acquired with 3D phase-contrast magnetic resonance (MR) imaging contain information regarding complex motions that occur during imaging. A technique called simulated streamlines, which facilitates the display and comprehension of these velocity data, is presented. Single or multiple seed points may be identified within blood vessels of interest and tracked through the velocity field. The resulting trajectories are combined with a 3D MR angiogram and displayed with 3D volume visualization software. Mathematical analysis highlights potential applications and pitfalls of the technique, which was implemented both in phantoms and in vivo with excellent results. For example, single streamlines reveal helical flow patterns in aneurysms, and multiple streamlines seeded in the common carotid artery reveal branch filling-time relationships and slow filling of the carotid bulb. The technique is helpful in understanding these complex flow patterns.

Blood Flow Velocity↗
PubMedDate not supplied

Computer-controlled flow simulator for MR flow studies.

A novel computer-controlled flow simulator for use in magnetic resonance (MR) flow experiments was evaluated. The accuracy in constant-flow mode was better than 1%. The accuracy in pulsatile-flow mode was found to be dependent on the interconnecting tubing. The short-term and long-term reproducibilities of pulsatile waveforms were less than or equal to 0.4 mL/sec (1 standard deviation). Increased response times due to the lengths of tubing required in MR flow experiments were surmounted by using a modified tubing configuration and precompensated waveforms. Piston reversal was found not to cause major difficulties in MR flow experiments.

Blood Circulation↗
PubMedDate not supplied

Frequency response to retrospectively gated phase-contrast MR imaging: effect of interpolation.

Retrospectively gated phase-contrast (PC) magnetic resonance velocity and volume flow measurements were evaluated in both in vitro and in vivo experiments. The accuracy of these measurements was found to be affected by the interpolation window width required in the reconstruction of retrospectively gated data. Interpolation modified the frequency content of the series of temporal measurements by decreasing the response at higher frequencies. With a series of sinusoidal flow waveforms, the frequency response of one specific implementation of retrospectively gated PC velocity measurements was experimentally determined. The experimental response agreed with the theoretical response predicted from an analysis of the interpolating function (2.2% root-mean-square difference). In vitro experiments with a simulated carotid flow waveform demonstrated errors in the systolic measurements that were a direct result of the modified frequency response. A volunteer study was also undertaken and confirmed the in vitro findings.

Adult↗
PubMedDate not supplied

Effects of physiologic waveform variability in triggered MR imaging: theoretical analysis.

One of the assumptions inherent in most forms of triggered magnetic resonance (MR) imaging is that the pulsatile waveform (be it cardiac, respiratory, or some other) is purely periodic. In reality, the periodicity condition is rarely met. Physiologic waveform variability may lead to image artifacts and errors in velocity or volume flow rate estimates. The authors analyze the effects of physiologic waveform variability in triggered MR imaging. They propose that this variability be treated as a modulation of the underlying motion waveform. This report concentrates on amplitude modulation of the velocity waveform, which results in amplitude and phase modulation of the transverse magnetization. Established Fourier and modulation theory and the recently described principles of (k,t)-space were used to derive the appearance of physiologic waveform variability artifacts in triggered MR images and to predict errors in time-averaged and instantaneous velocity estimates that may result from such motion effects, including effects such as ghost overlap. Simulations and experimental results are provided to confirm the theory.

Algorithms↗
PubMedDate not supplied

Frequency response of prospectively gated phase-contrast MR velocity measurements.

The authors developed and experimentally verified expressions that describe the frequency response of prospectively gated phase-contrast magnetic resonance velocity measurements. Both interleaved and noninterleaved phase-contrast techniques were evaluated. The primary determinants of the frequency response were (a) the number of interleaved acquisitions (N), (b) the time between acquisitions (delta T), and (c) the degree of balance between the first moments of the velocity-encoding gradients. To quantify the last factor, an imbalance parameter (U) was defined. Depending on the chosen implementation and U, deviations from the ideal frequency responses were predicted and observed. The expressions also revealed an advantage of interleaved acquisitions that use a one-sided gradient configuration: no changes in the frequency response. The effects of concurrently encoding orthogonal velocity components with a Hadamard four-point scheme were examined.

Magnetic Resonance Imaging↗
PubMedDate not supplied

Understanding acceleration-induced displacement artifacts in phase-contrast MR velocity measurements.

A theoretical framework for understanding acceleration-induced errors in phase-contrast magnetic resonance velocity measurements has been developed. An important result of this framework is the interpretation of acceleration-induced velocity errors as displacement artifacts due to the delay between velocity and spatial encoding. A rotating-disk phantom was used to confirm the theoretically predicted displacement times (the difference between theory and experiment was 8.2%). Errors were also observed in velocity profiles measured in regions of fluid acceleration downstream from a step stenosis. The magnitude of these errors could be predicted and corrected by using the analytic framework.

Artifacts↗