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I J Lowe

Publications and source records attributed to I J Lowe.

17 recordsLinked to original sources
PubMed2000

Measuring flow reattachment lengths downstream of a stenosis using MRI.

Flow reattachment lengths (l(r)) are measured downstream of an abrupt axisymmetric 75% stenosis, located inside a cylindrical channel, for steady flow using ultra-fast magnetic resonance imaging (MRI). The MRI results are compared with those from other similar (non-MRI) studies. The MRI data confirm the existence of three flow reattachment regimes (laminar, fully turbulent, and transition) related to the flow Reynolds number (Re) measured inside the stenosis. Based on the MRI experiments, the laminar regime occurs at a stenotic Reynolds number below 250 with a slope (l(r)/Re) of 0.086. The fully developed turbulence occurs at a stenotic Reynolds number above 3600 with a minimum observed reattachment length of 5 step heights. The transition regime (occurring between the laminar and fully turbulent regimes) is characterized by a reattachment length plateau and then a drop with Re(-1.1). J. Magn. Reson. Imaging 2000;12:939-948.

Arterial Occlusive Diseases↗
PubMed2000

Imaging obstructed ventilation with NMR using inert fluorinated gases.

We partially obstructed the left bronchi of rats and imaged an inert insoluble gas, SF(6), in the lungs with NMR using a technique that clearly differentiates obstructed and normal ventilation. When the inhaled fraction of O(2) is high, SF(6) concentrates dramatically in regions of the lung with low ventilation-to-perfusion ratios (VA/Q); therefore, these regions are brighter in an image than where VA/Q values are normal or high. A second image, made when the inhaled fraction of O(2) is low, serves as a reference because the SF(6) fraction is nearly uniform, regardless of VA/Q. The quotient of the first and second images displays the low-VA/Q regions and is corrected for other causes of brightness variation. The technique may provide sufficient quantification of VA/Q to be a useful research tool. The noise in the quotient image is described by the probability density function for the quotient of two normal random variables. When the signal-to-noise ratio of the denominator image is >10, the signal-to-noise ratio of the quotient image is similar to that of the parent images and decreases with pixel value.

Animals↗
PubMed1999

Observing curved flow using RUFIS.

The rotating ultra-fast imaging sequence (RUFIS) is used to image spins flowing through the curved portion of a 180 degrees U-tube with a circular cross section and a ratio of inner diameter to curvature diameter of 0.41. A velocity-encoding preparation sequence is used with RUFIS to measure quantitatively the axial velocity distribution for spins under steady-flow conditions. The resulting velocity contours confirm that higher velocity spins migrate toward the outer wall of the curve as the Dean number is increased.

Blood Flow Velocity↗
PubMed1999

Transforming NMR data despite missing points.

Some NMR experiments produce data with several of the initial points missing. The inverse discrete Fourier transform (IDFT) assumes these points are present so the data cannot be so transformed without artifact-ridden results. This problem is often particularly severe when projection imaging with free-induction decays (FIDs). This paper compares recent methods for obtaining a projection from incomplete data and elaborates on their strengths and limitations. One method is to write the transform that would take the desired projection to the truncated data set, and then solve the matrix equation by singular value decomposition. A second replaces the missing data with zeros, so that an IDFT produces a projection with unwanted artifacts. Then one solves the matrix equation that takes the desired projection to the artifact-ridden projection. A third uses the same artifact-ridden projection, but fits the region outside the bandwidth of the sample with as many sinusoidal functions as there are missing data. The coefficients of these functions are estimates of the missing data, and the projection is obtained by transforming the completed FID or subtracting the extrapolation of the fitted curve from the region containing the object. We show that when all three methods are applicable, they theoretically produce the same result. They differ by ease of implementation and possibly by computational errors. They give a result similar to that of the previous method that iteratively corrects the FID and projection after repeated IDFTs and DFTs. We find that one can obtain a projection despite missing a substantial number of data.

Artifacts↗
PubMed1998

Ultra-fast velocity imaging in stenotically produced turbulent jets using RUFIS.

A method for rapidly producing velocity images is presented. This sequence combines a modified bipolar gradient pulse to magnitude encode the velocity with the rotating ultra-fast imaging sequence (RUFIS) to image the encoded spins. Velocity encoding is done in 3 msec, and RUFIS acquires 32 projections in 8 msec. The method is applied to turbulent jets associated with a 75% stenosis in a 15-mm inner diameter glass pipe. Data is acquired upstream and downstream from the stenosis for Reynolds numbers from 560 to 3750. In addition, a robust method of reconstructing the unobserved short time region of a free induction decay is presented and incorporated into the image processing.

Image Processing, Computer-Assisted↗
PubMed1998

A programmable pre-emphasis system.

MRI systems often use magnetic field gradient and shim pulse-shaping networks (pre-emphasis) to correct for magnetic field distortions caused by eddy currents. A pre-emphasis system that uses up to 16 fixed resistor-capacitor (RC) time constants per channel with programmable amplitude coefficients is described. The magnetic fields induced by the pre-emphasis RC time constants serve as a set of basis functions for compensating eddy-current fields induced by the gradient set. The resultant time-varying magnetic field gradient accurately reflects the gradient specified by the pulse programmer. Reductions in eddy-current fields are demonstrated for actively shielded and unshielded gradient sets.

Artifacts↗
PubMed1998

Characterization of flow emerging from a stenosis using MRI.

MRI ultra-fast imaging techniques are used to characterize flow emerging from streamlined and abrupt stenoses inside cylindrical channels. Reattachment lengths of the shear boundary to the channel wall are measured using rotating ultra-fast imaging sequence (RUFIS) in-flow imaging. Velocity profiles of flow are created using velocity (sine and cosine)-encoded RUFIS sequences. The sine-encoded images permit one to identify reverse flow (i.e., eddies) that arise within the region of flow reattachment. The ratios of peak velocities (downstream/upstream of the stenosis) derived from the cosine-encoded images are used to identify the transition from the laminar to the turbulent regimen. Based on these experiments, the transition from the laminar to turbulent regimen occurs at a stenotic Reynolds Number of 350, whereas fully developed turbulence occurs at a stenotic Reynolds Number of 2600. These results are compared with the results from invasive studies.

Blood Flow Velocity↗
PubMed1998

Radial echo-planar imaging.

A new ultrafast magnetic resonance imaging pulse sequence named radial echo-planar imaging (rEPI) is introduced. The sequence is based on a modification of the echo-planar imaging (EPI) sequence to scan k-space radially, in an attempt to combine the speed of EPI with the benefits of radial sampling. Like in EPI, all the desired lines in k-space are scanned consecutively in opposite directions. The unique feature of this new sequence, however, is that the orientation of the readout gradient is incrementally rotated, so that all the echoes are refocused through the center of k-space. Therefore, rEPI data are acquired in a polar grid, and image reconstruction can be done either by means of filtered back-projection or by regridding the data to a Cartesian matrix followed by 2D Fourier transform. First results show that rEPI images can be acquired with the same speed and signal-to-noise ratio of EPI images. rEPI images are also shown to be less sensitive to off-resonance effects than EPI images. Further studies are underway to investigate the usefulness of rEPI for spectroscopic imaging and applications affected by motion.

Echo-Planar Imaging↗
PubMed1995

Optimized ultra-fast imaging sequence (OUFIS).

The DUFIS sequence can make ultra-fast images (approximately 10 ms) without the use of rapidly switched gradients. The RF excitation sequence is spatially selective so that only a small fraction of the magnetization in each imaging pixel is used which produces a poor imaging signal to noise ratio (SNR). We have developed several alternative RF sequences that use RF pulses with multiple phases, and also with just 0 degrees and 180 degrees phases to excite almost all the magnetization in a pixel and greatly improve the SNR. The optimization of these pulse sequences (now called OUFIS) have been conducted both analytically and by numerical searches, with various linear and nonlinear models. Both theoretical and computational methods used in the optimizations are described in detail. Preliminary experimental results are briefly presented with several possible applications of the OUFIS excitation sequences suggested.

Algorithms↗
PubMed1995

Ultra-fast imaging using low flip angles and FIDs.

A new ultra-fast imaging technique that does not place extreme demands on the speed of the gradient system is described. When used with comparable MRI systems, the rotating ultra-fast imaging sequence (RUFIS) can acquire images 4 to 5 times faster than gradient-moment nulled EPI and more than twice as fast as DUFIS, OUFIS, or BURST techniques. Because the technique uses free induction decays instead of echoes, it can be made particularly insensitive to effects of motion, flow, and diffusion. Preliminary images of turbulent flow are presented to demonstrate this insensitivity. However, with appropriate encoding, flow effects may be imaged.

Algorithms↗
PubMed1994

A novel eddy current compensation scheme for pulsed gradient systems.

We describe a modified pre-emphasis network that uses a number of fixed time constants, only whose amplitudes are adjustable. In comparison with variable time constant pre-emphasis networks we have used, the fixed time constant network is more effective and easier to set on the fly. We have further developed a method to set the fixed time constant network mathematically using a linear least squares technique.

Algorithms↗
PubMed1993

A simple method of measuring gradient induced eddy currents to set compensation networks.

We describe a technique for measuring the time dependence and field distortions of magnetic fields due to eddy currents (EC) produced by time-dependent magnetic field gradients. The EC measuring technique uses a sample with short T1, T2 and many rf excitation pulses and free induction decays (FIDs) to measure the out-of-phase component of the FIDs, which are proportional to gamma delta B, the amount by which the signal is off resonance. The measuring technique is sensitive, easy to implement and interpret, and useful for setting preemphasis compensation networks.

Magnetic Resonance Imaging↗
PubMed1988

Rotating-frame relaxation studies of slow motions in fluorinated phospholipid model membranes.

Rotating-frame relaxation experiments have been carried out on 19F-labeled dimyristoylphosphatidylcholine model membranes. The lipids are labeled with a single CF2 group in the 4-, 8-, or 12-position of the 2-acyl chain. Both oriented lipid bilayers and multilamellar liposomes have been investigated. The relaxation rate has been measured as a function of the locking-field strength, the sample orientation, the label position, and the temperature. Our results have confirmed that extensive slow motions exist in the bilayer and dominate the low-frequency relaxation. The relaxation rate is quite sensitive to the label position. However, many other features of the relaxation are very similar for all three lipid isomers. The temperature dependence of the relaxation rate for the multilamellar liposomes differs from the oriented bilayers, which may imply that the motions are also different. To fit our data, a working model consisting of a superposition of an anisotropic reorientation term and a director fluctuation term has been proposed. We have also verified that almost all of the relaxation process is caused by modulations of the intramolecular interactions. Based on this, a view of the slow motions at a molecular level is discussed in this paper.

Dimyristoylphosphatidylcholine↗
PubMed1984

Fluorine-19 nuclear magnetic resonance investigation of fluorine-19-labeled phospholipids. 1. A multiple-pulse study.

A multiple-pulse nuclear magnetic resonance technique has been used to measure the order parameter, SFF, at 40 MHz for dimyristoylphosphatidylcholine labeled with a difluoromethylene group at the 4-, 8-, or 12-position of the sn-2-acyl chain dispersed in water in the liquid-crystalline phase. The Carr-Purcell-Meiboom-Gill multiple-pulse sequence can resolve the homonuclear dipolar coupling between the two fluorine nuclei, thus making a direct determination of the order parameter, SFF, for the F-F internuclear vector possible. Other interactions, such as the 19F chemical shift anisotropy, heteronuclear dipolar couplings, and field inhomogeneity, which normally obscure the dipolar splitting, are effectively canceled. The order parameters obtained in this work compare well with those obtained by 19F nuclear magnetic resonance line-shape analysis of the 19F-labeled phospholipids reported in the following paper [Dowd, S. R., Simplaceanu, V., & Ho, C. (1984) Biochemistry (following paper in this issue)] as well as comparable SCD order parameters, determined for the deuterium-carbon internuclear vector of deuterium-labeled phospholipids [Oldfield, E., Meadows, M., Rice, D., & Jacobs, R. (1978) Biochemistry 17, 2727-2740]. The present results clearly show the usefulness of using nuclear magnetic resonance spectroscopy to investigate lipid-lipid and protein-lipid interactions, especially for those systems containing a difluoromethylene group in the acyl chain of a phospholipid molecule.

Chemical Phenomena↗