Biomedical subjects

H M Gach

Publications and source records attributed to H M Gach.

7 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↗