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T K Foo

Publications and source records attributed to T K Foo.

13 recordsLinked to original sources
PubMed1992

Reduction of RF penetration effects in high field imaging.

A method of correcting for the RF inhomogeneity in the body by dielectric loading of the coil-to-shield space in an RF resonator (coil and shield assembly) is described. The presence of the RF coil and RF shield have significant effects on RF homogeneity. Based on theoretical calculations, a method for adjusting the RF homogeneity by manipulating the axial propagation constant, kappa z, is proposed. This is accomplished by loading the coil-to-shield space with dielectric material of suitable relative permittivity so as to increase kappa z and decrease the radial propagation constant, kappa rho. In this manner, the radial wavelength (lambda rho = 2 pi/kappa rho) can be increased relative to the body dimensions, and the field amplitude variations in the axial plane minimized. Theoretical calculations indicate that a value of between 30 and 40 for the relative permittivity of the dielectric material in the coil-to-shield space would reduce the RF field inhomogeneity from +/- 15% to about +/- 3% over a central 30-cm-diameter region of a homogeneous 40-cm-diameter body at both 64 and 170 MHz. The theoretical model was verified in laboratory measurements of the [formula; see text] field generated in a test coil at 170 MHz which was scaled to correspond to a body at 64 MHz. However, the improved RF field homogeneity would be accompanied by increased RF power requirements and reduced coil sensitivity.

Humans↗
PubMed1992

Thick-section, single breath-hold magnetic resonance pulmonary angiography.

RATIONALE AND OBJECTIVES: Approaches to performing magnetic resonance angiography (MRA) of the pulmonary vasculature are described using very fast (repetition time [TR] less than 13 mseconds) radiofrequency (rf)-spoiled, gradient-recalled pulse sequences and the standard quadrature body imaging coil of a commercial 1.5-T MR imaging system. METHODS AND RESULTS: Signal-to-noise (SNR) is improved by signal averaging (Nex greater than or equal to 4) in a two-dimensional, single thick-section approach and by volume acquisition (Nex = 1) in a three-dimensional approach. Blood signal loss is minimized by using short, asymmetric echoes (echo time [TE] less than or equal to 2.7 mseconds). Respiratory motion is eliminated by keeping the scan time short enough (approximately 15 seconds) for image acquisition within a single breath-hold. Cardiac motion artifacts are reduced with section orientations that avoid intersecting the heart and/or use of small flip angle (alpha less than or equal to 25 degrees). CONCLUSIONS: Images of healthy volunteers showed that while single thick sections have superior SNR, the three-dimensional approach appears to produce better visualization of the peripheral vascular segments and offers improved ability to process the images to remove overlapping structures.

Adult↗
PubMed1992

High-resolution MR imaging of the wrist and eye with short TR, short TE, and partial-echo acquisition.

High-resolution spin-echo and two- and three-dimensional gradient-recalled acquisition in the steady state (GRASS) and spoiled GRASS images of the wrist and eye were obtained with a whole-body magnetic resonance imager by using trapezoidal phase-encoding gradient waveforms and by lowering receiver bandwidth to +/- 8 kHz, which results in a minimum field of view (FOV) of 4 cm and pixel resolution of 156 microns for a 256 x 256 matrix. Short echo times were achieved by using partial-echo acquisition. Poor signal-to-noise ratio resulting from smaller voxel sizes was overcome by using prototype receive-only eye and wrist coils. Images were obtained in reduced time and were superior to those obtained in studies performed with an 8-12-cm FOV and commercially available coils.

Adult↗
PubMed1992

Pulmonary vasculature: single breath-hold MR imaging with phased-array coils.

The authors obtained magnetic resonance images of the pulmonary vasculature with reduced artifacts caused by cardiac and respiratory motion by acquiring a series of moderately thin sections in a single breath-hold with an ultrafast gradient-echo pulse sequence. The series of two-dimensional images was postprocessed with a maximum-intensity projection algorithm. Time-of-flight inflow enhancement increased the signal intensity of arteries and veins while radiofrequency phase spoiling produced limited stationary spin suppression of the chest wall. Moderately thin (8-mm) section thicknesses were used to attain the resolution necessary to visualize smaller pulmonary vascular segments up to the chest wall while the number of acquired sections was minimized. Because the body coil did not provide an adequate signal-to-noise ratio (S/N) for a single excitation and thin-section acquisitions, phased-array coils covering either the right or left lung were used to single breath-holds prevented the misregistration and blurring that occurred in examinations performed with multiple breath-holds.

Humans↗
PubMed1992

Kinematic MR imaging of the patellofemoral joint: comparison of passive positioning and active movement techniques.

Thirteen patients underwent magnetic resonance (MR) imaging of the patellofemoral joint in an evaluation of passive positioning and active movement kinematic MR imaging techniques. Sixteen joints were symptomatic, and 10 were not. Delineation of normal and abnormal patellar alignment and tracking was similar with the two techniques, but kinematic MR imaging performed with active movement allowed a substantial reduction in examination time while permitting evaluation of the contribution of associated activated muscles and soft-tissue structures to patellofemoral joint function.

Adolescent↗
PubMed1992

Occlusion and narrowing of the pharyngeal airway in obstructive sleep apnea: evaluation by ultrafast spoiled GRASS MR imaging.

Morphologic abnormalities of the pharyngeal airway are frequently found in patients with obstructive sleep apnea. These structural alterations in the pharyngeal airway can be detected in awake patients by using rapid imaging techniques. Ten patients with clinically proved obstructive sleep apnea had ultrafast spoiled gradient-recalled acquisition in the steady state (GRASS) MR imaging of the pharyngeal airway to determine the presence of occlusions and/or narrowings. Twelve sequential images were obtained at one midsagittal plane and at eight transverse planes through the pharyngeal airway. The scans were obtained at the rate of one image per 1.04 sec while the patient was breathing quietly. Occlusions or narrowings of the pharyngeal airways were detected on MR images in all patients. The site(s) of the occlusions and the site(s) and extent of the narrowings varied. Six patients had occlusions and four had narrowings of one or more sites. This study shows that ultrafast spoiled GRASS MR imaging can be used to evaluate patients with obstructive sleep apnea during tidal breathing and is useful for determining the presence of occlusions and narrowings of the pharyngeal airway.

Adult↗
PubMed1991

Lung calcium-dependent phospholipid-binding proteins: structure and function.

Distinct peptide maps of two rabbit lung Ca2(+)-dependent phospholipid-binding proteins (PLBPs), 36,000 and 33,000, were generated by cyanogen bromide (CNBr) cleavage, trypsin or Staphylococcus aureus V8 proteinase digestion. The amino acid sequence of a CNBr-cleaved peptide of the 36,000 PLBP was aligned to the amino terminus of human lipocortin I with more than 77% identity, but had no identity with the known amino terminal sequence of other known annexins. Partial amino acid sequence of a 33,000 PLBP peptide demonstrated a close (56%) relationship to endonexin II, human placental anticoagulant protein, and porcine intestine protein II, but shared only 32% identity with lipocortin I, 30% with lipocortin II. Antiserum generated against purified 36,000 PLBP reacted strongly with the 33,000 PLBP, but did not react with any other rabbit lung cytosolic proteins. Both PLBPs inhibited the phospholipase A2 reaction when dioleoyl phosphatidylcholine and phosphatidylglycerol vesicles or monolayers were used as substrates. In the vesicle assay, the phospholipase A2 reaction was inhibited at lower substrate phospholipid concentrations but not at nearly saturating substrate concentrations. In the monolayer assay, the phospholipid-binding proteins did not inhibit phospholipase A2 at a low phospholipid surface concentration of 3.8.10(-3) molecules/A2, but they did at higher surface concentrations between 1.1 x 10(-2) and 3.8 x 10(-2) molecules/A2. The inhibition of phospholipase A2 by rabbit lung phospholipid-binding proteins is most likely due to the prevention of penetration by phospholipase A2 into the interface, a requirement for the enzyme to act on the substrate.

Amino Acid Sequence↗
PubMed1991

An analytical model for the design of RF resonators for MR body imaging.

A closed form, analytical solution describing the RF fields generated by an RF body coil resonator for MR imaging at 1.5 and 4.0 T is presented. This solution extends the results of earlier studies of RF penetration in the body by explicitly including the RF coil, the RF shield, and the field variation along the z axis for high-pass bird cage coils. A salient feature of this treatment is the calculation of the axial propagation constant, kz, which determines the z dependence of the RF field. We have determined the relative power deposition in the body, the B1 field homogeneity, and coil losses, which are functions of the coil-to-shield separation and body size. The relative power deposition in the body has been calculated to vary as the 1.58 power of the body radius. The calculations have also predicted that the field homogeneity in the z direction exhibits greater degradation at higher frequencies in a high-pass coil than in a low-pass coil. The model predicts an increase in coil losses by a factor of 2.8 as the coil-to-shield separation is reduced from 5 to 2 cm in a standard body resonator. Although the results for only a homogeneous cylindrical object or body are presented, the theory can be extended to a multilayered heterogeneous object of varying permittivity and conductivity.

Body Constitution↗
PubMed1991

Patellofemoral joint: evaluation during active flexion with ultrafast spoiled GRASS MR imaging.

An ultrafast spoiled gradient-recalled acquisition in the steady state pulse sequence was developed that permits multiple images to be obtained at a temporal resolution suitable for examining the patellofemoral joint during active flexion. This pulse sequence was used to perform kinematic magnetic resonance imaging of patellar alignment and tracking in five healthy subjects and seven patients with a provisional clinical diagnosis of abnormal patellofemoral joints.

Female↗
PubMed1990

The use of a multichannel scaling method to detect radioactive species in spread monolayers.

This study describes a simple and inexpensive method for monitoring radioactive species spread as monolayers at the air/water interface. The combination of a discriminator and multichannel scalar counter with a personal computer functions to unify all measurements, to simplify the operational process and data acquisition, and to provide a real-time display of the data. Its use is demonstrated by following the hydrolysis of L-alpha-[1-14C]dioleoyl phosphatidylcholine by the enzyme, phospholipase A2, isolated from the porcine pancreas.

Animals↗
PubMed1989

Projection flow imaging by bolus tracking using stimulated echoes.

Previous investigators have employed the concept of bolus tracking using either spin echoes or gradient echoes. In this paper we introduce two methods of bolus tracking using planar- and volume-selective stimulated echoes. The planar method employs a selective 90 degrees rf pulse which tags all spins in a particular plane. At a time tau 1 later, a nonselective 90 degrees rf pulse is employed, followed after a time tau 2, by another nonselective rf pulse. Only spins which experience all three rf pulses form a stimulated echo at time tau 1 after the third rf pulse. A balanced pair of flow-compensated dephasing (crusher) gradients further ensures that the stimulated echo is due only to the effect of all three rf pulses while minimizing flow dephasing. The first part of this gradient pair is applied after the initial rf pulse in the first tau 1 period to dephase the tagged spins. The second part of this gradient pair is applied after the third rf pulse to rephase the spins. Since the plane of the excited slice is orthogonal to the readout direction, flowing spins are imaged in an angiographic manner as they move away from the excited slice. A modification to this basic sequence excites only a small volume. In this manner, the suppression of stationary spins is effected by volume-selective excitation. In both the planar- and the volume-selective techniques, the excited spins undergo T1 and T2 relaxation during the tau 1 period but only T1 relaxation in the tau 2 period. In blood, where T1 is much greater than T2, keeping tau 1 as short as possible minimizes signal loss due to T2 dephasing. These methods demonstrate increased sensitivity compared to similar bolus tracking methods using either spin echoes or gradient echoes.

Blood Flow Velocity↗
PubMedDate not supplied

Dynamic study of the upper airway with ultrafast spoiled GRASS MR imaging.

Dynamic magnetic resonance (MR) imaging of the upper airway was not possible previously because of poor temporal resolution. Recently, a rapid technique has been developed that provides the means to obtain multiple images at different section locations with sufficient image quality and temporal resolution to allow a comprehensive, dynamic study of the upper airway. The authors describe an ultrafast spoiled GRASS (gradient-recalled acquisition in the steady state) pulse sequence for dynamic studies of the upper airway. The authors believe that this procedure has potential for identifying and characterizing upper airway abnormalities such as nonfixed occlusions and/or narrowings that may exist in patients with obstructive sleep apnea.

Humans↗