Magnetic resonance imaging of immiscible fluid displacement in porous media.
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Journal Article↗
Biomedical subjects
S Patz
Publications and source records attributed to S Patz.
Rapid Fourier imaging using steady-state free precession.
Reversal of the read gradient in a SSFP imaging experiment allows a full spin echo to be collected in the interval tau between successive rf pulses. Orthogonal gradient pulses are used to dephase and subsequently rephase the transverse magnetization each tau enabling 2D or 3D Fourier techniques. The minimum data collection time per slice in the 3D technique is 3.1 s (128 X 256). For a 2D data collection, an oscillating bipolar sawtooth gradient is used to select the slice. Each phase-encode value must be averaged over an equivalent portion of the oscillating slice-selection gradient and this condition gives a minimum of 25 s for 2D data collection. Excellent slice selection is achieved with less than 5% of the signal lying outside the slice profile central lobe. Images at 0.14 T show tissue contrast may be manipulated by changing the rf pulse angle, an example of which is the presence or absence of gray/white matter contrast at rf pulse angles of 30 and 90 degrees, respectively. The pulse angle theta dependence of five samples with different values of T2/T1 was measured and numerically calculated with good agreement between theory and experiment for theta less than or equal to 90 degrees.
Brain↗
Fast imaging of CSF flow/motion patterns using steady-state free precession (SSFP).
Using a rapid Fourier SSFP imaging technique, which is sensitive to slow flow (approximately 1 mm/sec) in the plane of the image, we obtained 135 brain MRI examinations. The CSF flow/motion patterns were mapped by two images with orthogonal in plane flow sensitivity directions. Analysis showed significant deviations from the "normal" pattern in ventricular enlargements because of obstruction (no evidence of CSF flow/motion) or in normal pressure hydrocephalus (complex, intensive flow pattern in lateral ventricles) suggesting a diagnostic potential for this fast imaging technique.
Brain↗
The application of steady-state free precession to the study of very slow fluid flow.
An NMR imaging technique sensitive to slow flow (approximately 1 mm/s) using a conventional imaging gradient strength (0.025 mT/cm) is described. Two projections with different spatial magnetic periodicity (determined by the SSFP pulse interval), and thus with different flow sensitivities, are subtracted to give signal from flows in a velocity window.
Animals↗
Basic physics of nuclear magnetic resonance.
This review of basic physics of nuclear magnetic resonance (NMR) discusses precession of magnetic nuclei in a static external field, introduces the concept of the rotating frame, and describes excitation of nuclei by an RF field. Treats subject of T1 and T2 relaxation from the dual viewpoints of (1) phenomena of relaxation times for both the longitudinal and transverse magnetization and (2) relaxation resulting from local field fluctuations. It describes practical ways in which T1 and T2 are measured (i.e., inversion recovery and spin-echo) and gives the value of the nuclear magnetization in thermodynamic equilibrium with a static external field. It discusses the reduction of NMR signal resulting from saturation. These concepts are related to clinical use with a set of four spin-echo images of a human head.
Astrocytoma↗
Mapping of normal and abnormal cerebrospinal fluid flow/motion patterns using steady state free precession imaging.
In a comparative study more than 35 brain magnetic resonance imaging examinations were analyzed by mapping the CSF flow/motion pattern using the steady state free precession technique with two different flow sensitivity directions. Significant deviations from the normal pattern were seen in ventricular enlargements due to obstruction with no evidence of CSF flow/motion and in normal pressure hydrocephalus with complex flow pattern in lateral ventricles, suggesting a diagnostic potential for this fast imaging technique with sensitivity to very slow flow.
Cerebrospinal Fluid↗
The placing of many large superconducting magnets in a limited space.
The problem of minimizing the space needed to house two or three superconducting magnets was studied. Dipolar fields were used to approximate the stray fields. Field contours of a single dipole mu are reviewed. The three-dimensional contour surface is discussed in terms of its intersection with planes parallel to a base plane containing mu. The area within a field contour H in the base plane was numerically determined to be 142.55 m2[(mu/mu 0)(H0/H)]2/3 where mu 0 = 10(9) erg/G and H0 = 5 G. For the contours within planes parallel to the base plane, the contour area and its intercepts with the planar x and y axes are given as a function of the distance from the base plane. For two dipoles at one site and for a given mechanical stress, the torque-free orientations allow closer spacing. Field contours for the four torque-free orientations are given for mu 1 = mu 2. For one zero-torque configuration (mu 1 and mu 2 antiparallel, but perpendicular to the line joining them) and for a mutual force of 667 N (150 lb), the 5-G contour area is only 36% greater than that for a single dipole. General expressions for the forces, torques, and field gradients for two dipoles are given. Torque-free cases of three superconducting magnets (two of which are identical) at one site are described. Contour plots of the linear arrangements of the magnets as well as of two symmetrical triangular arrangements (corner angle is either 70.5 or 109.5 degrees) are given.
Equipment Safety↗
Hospital leads way in employee cost-sharing.
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Deductibles and Coinsurance↗
MRI of pulsatile CSF motion within arachnoid cysts.
Flow void due to pulsatile motion of cerebrospinal fluid (CSF) has recently been demonstrated by a variety of magnetic resonance techniques with sensitivity to slow flow. It has been suggested that within fluid collections not communicating with the physiologic CSF space, there is less signal loss than with the physiologic CSF spaces. Utilizing the SSFP MR technique, which is sensitive to flow as slow as 1 mm/sec, we evaluated three patients with isolated arachnoid cysts. Irregular signal loss consistent with fluid motion was noted within all of the cysts, as well as within the physiologic CSF spaces. Definitive anatomic evaluation of these lesions, though, required ventriculography, an invasive technique.
Arachnoid↗
Clinical experience with rapid 2DFT SSFP imaging at low field strength.
A retrospective analysis of clinical imaging using 2DFT SSFP at 0.14 T is presented. The technique's potential for tissue characterization and its utility for clinical diagnosis were tested by both in vitro measurements of various tissues and in vivo clinical images. Different pulse angles not only influenced image contrast, but also helped characterize lesions, particularly those containing fat. In addition, the pulse angle changed the signal from venous flow perpendicular to the imaged slice. The slow flow sensitivity of the 2DFT SSFP technique was demonstrated in the detection of CSF motion. Rapid SSFP offers flow sensitivity and adequate lesion detecting ability, along with high patient throughput.
Brain↗
Some factors that influence the steady state in steady-state free precession.
Steady-state free precession (SSFP) and particularly the spatially periodic magnetization response with wavelength lambda that results in the presence of an applied gradient is discussed. The maximum SSFP magnetization does not always occur at the Larmor frequency but rather depends on both the phase cycling of the rf pulses and on the rf tip angle. The slow flow sensitivity of SSFP also depends on lambda. Experimental data of the flow sensitivity is shown to be parameterized by a dimensionless dephasing parameter phi defined as the ratio of the distance traveled by a spin in one cycle of the SSFP sequence as compared to lambda. An example of the creation of a moving reference frame with the SSFP sequence is given. The modulation in image intensity that results when the gradients are nearly but not exactly compensated and lambda becomes large (i.e., tens of pixels long) is demonstrated. The pixel length must be an integral number of lambda's in order to have a uniform image intensity from a uniform phantom.
Fourier Analysis↗
Spin-lock techniques and CPMG imaging sequences: a critical appraisal of T1p contrast at 0.15 T.
The addition of a spin-lock preparatory sequence to a Carr-Purcell-Meiboom-Gill (CPMG) imaging sequence provides a method which allows an accurate and simple comparison of T1p and T2 contrast. Sagittal and axial brain images, produced with the application of a three pulse preparatory spin-lock sequence prior to a sixteen-echo CPMG imaging sequence, are compared with images acquired without the spin-lock sequence. The CPMG sequence uses non-selective refocusing pulses. Therefore, observed echo signals accurately reflect T2 relaxation. This allows a convenient method for assessing the degree to which T1p and T2 contrast differ. The spin-lock CPMG (SL-CPMG) images were acquired with a spin-locking field amplitude of 0.4 G and resemble heavily T2-weighted images at 0.15 T. Quantitative analyses of signal intensities from edema and normal brain tissue confirm the qualitative observations. This in vivo method should prove useful for determining when the additional RF power deposition associated with spin-locking techniques will provide an alternate form of tissue contrast than that available from additional echo collection.
Adult↗
Tortuosity measurement and the effects of finite pulse widths on xenon gas diffusion NMR studies of porous media.
We have extended the utility of NMR as a technique to probe porous media structure over length scales of approximately 100-2000 microm by using the spin 1/2 noble gas 129Xe imbibed into the system's pore space. Such length scales are much greater than can be probed with NMR diffusion studies of water-saturated porous media. We utilized Pulsed Gradient Spin Echo NMR measurements of the time-dependent diffusion coefficient, D(t), of the xenon gas filling the pore space to study further the measurements of both the pore surface-area-to-volume ratio, S/V(p), and the tortuosity (pore connectivity) of the medium. In uniform-size glass bead packs, we observed D(t) decreasing with increasing t, reaching an observed asymptote of approximately 0.62-0.65D(0), that could be measured over diffusion distances extending over multiple bead diameters. Measurements of D(t)/D(0) at differing gas pressures showed this tortuosity limit was not affected by changing the characteristic diffusion length of the spins during the diffusion encoding gradient pulse. This was not the case at the short time limit, where D(t)/D(0) was noticeably affected by the gas pressure in the sample. Increasing the gas pressure, and hence reducing D(0) and the diffusion during the gradient pulse served to reduce the previously observed deviation of D(t)/D(0) from the S/V(p) relation. The Pade approximation is used to interpolate between the long and short time limits in D(t). While the short time D(t) points lay above the interpolation line in the case of small beads, due to diffusion during the gradient pulse on the order of the pore size, it was also noted that the experimental D(t) data fell below the Pade line in the case of large beads, most likely due to finite size effects.
Diffusion↗
Prototype miniature endoluminal MR imaging catheter.
PURPOSE: The feasibility of a miniature endoluminal magnetic resonance (MR) detection coil was investigated for imaging mural and perimural anatomy of small, tubular structures. MATERIALS AND METHODS: To this end, remotely tunable, single-loop, multiturn, receive-only radio-frequency coils, housed in 6-9-F arterial sheaths, were built. A 1.9-T imager was used. Phantom excitation was accomplished with a 62-mm-diameter bird-cage quadrature coil, and ex vivo specimen excitation was accomplished with a single-turn, untuned wire loop. Phantom images obtained with use of a 9-F catheter coil showed a signal-to-noise improvement on the border of 20 dB compared with images obtained with the quadrature coil. An 8-F catheter coil was used to obtain high-resolution (100 microns in-plane pixel size, 500 microns section thickness) spin-echo images (repetition time = 2,400 msec, echo time = 53 msec) of the wall of a fresh ex vivo human popliteal artery. RESULTS: Prospectively, these images were suggestive of the presence of diffuse intimal hyperplasia, medial calcification, and focal atherosclerotic plaque. These findings were confirmed histologically. Three-dimensional restacking of the axial images simplified examination of the normal layers and pathologic changes within the wall. The improved signal-to-noise characteristics of these miniature coils permit fast high-resolution imaging, allowing visualization of microscopic anatomic details. CONCLUSIONS: With further development, this technology may be useful for studying atherosclerosis and for providing imaging guidance during endoluminal MR interventions.
Arteries↗