PubMed Health⌕ Search

Biomedical subjects

H N Yeung

Publications and source records attributed to H N Yeung.

At least 19 recordsLinked to original sources
PubMed1999

Evaluation of liver diseases via MTC and contrast agent.

Our goal was to characterize pathological tissues of liver by magnetic resonance (MR)-related parameters such as T1 and magnetization transfer (MT) indices and to evaluate the clinical efficacy of MT contrast (C) in diagnosis of liver diseases via binomial pulsed saturation, with and without the administration of the paramagnetic agent gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA). Fifty-one cases of liver disorders were included in this study. Among the more important findings were the following: a) cirrhotic livers have significantly higher MT indices than normal liver, while hepatoma, metastatic tumor and fatty liver have sub-normal MT indices; b) in general, although with notable exceptions, images with MT give significantly better contrast indices than control images; and c) MT with Gd-DTPA rarely fares any better than the MT technique alone, although again with notable exceptions. MTC is a potentially powerful technique for diagnosing liver diseases, provided it can be optimally exploited for each individual disease type.

Carcinoma, Hepatocellular↗
PubMed1997

Temperature dependence and phase transition of proton relaxation of hydrated collagen in intact beef tendon specimens via cross-relaxation spectroscopy.

The role of water of hydration in proton relaxation in tissues as exemplified by hydrated collagen in beef tendon was studied as a function of temperature from -40 degrees to 37 degrees C by using cross-relaxation spectroscopy. Experimental data were fitted to a simple binary spin-bath model. The outcome of this procedure allows the construction of a semi-quantitative depiction of proton relaxation in a heterogeneous system and its change as one of the water fractions freezes at about -10 to -20 degrees C, a transition observed by NMR and confirmed independently by differential scanning calorimetry. Such physical depiction provides a crude but insightful interpretation of the role "bound" water plays in proton relaxation. This may be important in shedding light on the mechanism of tissue relaxation and its role in MRI diagnosis, particularly for those diseases such as liver cirrhosis where the water-macromolecular interaction plays a prominent role.

Animals↗
PubMed1996

Estimation of in vivo proton intrinsic and cross-relaxation rate in human brain.

By using on-resonance binomial pulse saturation of the immobile component in tissues and by monitoring the time development of the longitudinal decay of the free water magnetization, it has been shown that it is possible to estimate physical model parameters not directly measurable by conventional methods. Since pulsed saturation is easy and safe to implement in a clinical MRI machine, it should be possible to estimate these parameters in vivo by the same means. In this article, we report the results of such estimates of the parameters of the principal components (gray and white matter) in brains of two normal volunteers.

Adult↗
PubMed1996

A three-component model for magnetization transfer. Solution by projection-operator technique, and application to cartilage.

A projection-operator technique is applied to a general three-component model for magnetization transfer, extending our previous two-component model [R.S. Adler and H.N. Yeung, J. Magn. Reson. A 104, 321 (1993), and H.N. Yeung, R. S. Adler, and S.D. Swanson, J. Magn. Reson. A 106, 37 (1994)]. The PO technique provides an elegant means of deriving a simple, effective rate equation in which there is natural separation of relaxation and source terms and allows incorporation of Redfield-Provotorov theory without any additional assumptions or restrictive conditions. The PO technique is extended to incorporate more general, multicomponent models. The three-component model is used to fit experimental data from samples of human hyaline cartilage and fibrocartilage. The fits of the three-component model are compared to the fits of the two-component model.

Adult↗
PubMed1996

Azygos blood flow in cirrhosis: measurement with MR imaging and correlation with variceal hemorrhage.

PURPOSE: To evaluate magnetic resonance (MR) measurement of azygos blood flow (ABF) in assessment of risk of variceal hemorrhage in cirrhosis. MATERIALS AND METHODS: ABF in 50 patients with cirrhosis and 20 healthy control subjects was evaluated with phase-contrast cine MR imaging at the transaxial subcarinal plane. Group 1 included nine patients with cervical-drainage varices draining into brachiocephalic vein; group 2 included 41 patients with varices draining into azygos vein and was subdivided as group 2A, 15 patients with variceal hemorrhage, and group 2B, 26 without variceal hemorrhage. RESULTS: Azygos flow rate and velocity were increased in patients compared with that in control subjects. ABF in group 2A was faster than that in group 2B (cutoff, 15 cm/sec; sensitivity, 80%; specificity, 89%). Group 1 had ABF close to that of group 2B, but variceal hemorrhage occurred in seven of nine patients in group 1 versus 15 of 41 patients in group 2 (78% vs 37%; P = .029). CONCLUSION: Patients with cirrhosis with cervical-drainage varices or ABF greater than 15 cm/sec may have higher risk for variceal hemorrhage.

Azygos Vein↗
PubMed1994

Magnetization transfer contrast imaging of hepatic neoplasms.

A method of performing magnetization transfer contrast (MTC) using a pulse sequence based on a series of on-resonance binomial pulses preceding a conventional spin-echo sequence has been recently described. We investigated this technique in the evaluation of circumscribed hepatic neoplasms on a 0.5 T imager. Conventional spin-echo imaging was performed in 18 patients with hepatic neoplasms, 15 with primary or metastatic hepatic neoplasms, and 3 with cavernous hemangiomas. The T1-weighted sequence was repeated following MTC saturation in 7 patients and the proton density sequence was repeated following MTC saturation in 13 patients (both were used in two patients). The mean lesion-to-liver contrast for neoplasms worsened with T1-MTC (0.87 vs. 1.04), but improved slightly with proton density-MTC (1.10 vs. 1.22) when compared to the corresponding non-MTC spin-echo images. However, these differences were not statistically significant (p = .19 and p = .16, respectively). The limited data on cavernous hemangiomas does not suggest these have consistently different MTC imaging characteristics from other hepatic neoplasms. In conclusion, MTC imaging using a binomial saturation pulse did not offer significant improvement in contrast between hepatic neoplasms and normal liver. Refinements of the binomial pulse sequence will be necessary if a consistent improvement in lesion sensitivity is to be achieved. Evaluation of its role in increasing diagnostic specificity will require further investigation.

Adenoma↗
PubMed1994

Evaluation of quantitative magnetic resonance imaging as a noninvasive technique for measuring renal scarring in a rabbit model of antiglomerular basement membrane disease.

Renal function tests are an insensitive means of detecting progressive scarring of the kidney such as occurs in chronic allograft rejection and lupus nephritis. Alternative approaches for the measurement of small progressive changes in renal structure on a repetitive basis are needed. Quantitative magnetic resonance imaging (proton T1 relaxation time) was assessed for this purpose. A rabbit model of antiglomerular basement membrane disease that develops glomerular and interstitial inflammation followed by scarring of the renal cortex was used. Longitudinal studies of cortical T1 showed a marked prolongation of T1 during the initial inflammation and edema associated with glomerular crescent formation (Day 7). The T1 remained prolonged up to Day 120 in animals with significant fibrosis and crescent formation when the wet/dry weight ratio (a measure of edema) had returned to baseline. T1 was a more sensitive index of renal injury than was serum creatinine for all of the end points measured (cortical hydroxyproline per dry weight, percent crescent formation, or histologic fibrosis score). It was concluded from these studies that measurement of renal cortical T1 is quite a sensitive index of renal injury, probably more sensitive than the measurement of serum creatinine, but that this method does not discriminate between edema and scarring.

Animals↗
PubMed1992

Experimental analysis of T1 imaging with a single-scan, multiple-point, inversion-recovery technique.

Look and Locker's single-scan, multiple-point, T1 determination technique modified for imaging applications was evaluated experimentally for its accuracy and reliability with respect to the pulse sequence parameters, in particular, to the interpulse delay, tD, and the tip angle, alpha. T1 imaging experiments were performed with a 0.5-T imaging system on a phantom which consisted of an array of vials containing 2% agarose gels doped with various amount of Mn2+ using different combinations of the parameter set (tD, alpha). T1 results obtained with this technique were compared with those measured by a conventional inversion-recovery procedure using the same spectrometer. Strategic choice of pulse sequence parameters to minimize experimental errors and the criteria for these choices will be discussed.

Humans↗
PubMed1992

Magnetization transfer contrast with periodic pulsed saturation.

Magnetic resonance imaging with magnetization transfer contrast (MTC) was implemented on a 0.5-T clinical system by using a stream of binomial radio-frequency pulses for the selective saturation of the bound protons in tissues. Images were obtained of an egg phantom and of the calf, head, and abdomen in healthy human volunteers. Nonspecific saturation resulting in diminished signal intensity was manifested in the plots of the ratio of MTC signal to control signal for egg yolk, raw egg white, cerebrospinal fluid, and bone marrow fat. This signal degradation resulted partly from the cumulative effects of T2 on the observed transverse magnetization during presaturation. MTC imaging is dependent on repetition time; this dependence arises from the dual but conflicting effect of the selective saturation on tissues rich in macromolecular constituents. Substantial changes in tissue contrast were observed with MTC presaturation. The effect was most marked in muscle, as expected from the high protein content of this tissue.

Abdomen↗
PubMed1990

Proton-enhanced 13C imaging/spectroscopy by polarization transfer.

Carbon-13 magnetic resonance imaging/spectroscopy (CMRI/S) was performed using polarization transfer techniques where sensitivity of the carbon signal was enhanced by transferring the proton spin order to the carbon nuclei. The experimental feasibility of using polarization transfer techniques at 2.0 T was demonstrated with a phantom and an intact chicken egg. The potential clinical applications of CMRI/S with polarization transfer include the assessment of prostate cancer. Preliminary results using human prostate specimens are presented.

Animals↗
PubMed1988

Single-acquisition chemical-shift imaging of a binary system with use of stimulated echoes.

A method for separating binary chemical-shift components with a single image data acquisition by means of stimulated echoes is demonstrated. With a strategy analogous to the modified Dixon method, three stimulated echoes were acquired to form three complex images. In each of the images, the complex pixel intensities were imparted, by design of the pulse sequence, with a phase factor carrying chemical-shift or field inhomogeneity information. With these three images, true fat/water separation can be obtained in biologic tissues. Studies at high field strength (4.7 T) on a toluene phantom, a pseudo-binary chemical-shift system, were used to evaluate the applicability of the method. Its clinical feasibility was demonstrated on a healthy human subject in a 0.6-T whole-body imaging system.

Humans↗
PubMed1987

NMR imaging of 13C in animal tissues.

Natural-abundance 13C NMR imaging of animal tissues is demonstrated using high-field (4.7-T) and surface coil techniques. Undecoupled images of an unfertilized egg yolk and an oxtail are presented. Analysis indicates the images map mainly methylene carbon or lipid signal intensity.

Animals↗
PubMed1987

Proton transverse relaxation rate of 17O-enriched water.

Proton transverse relaxation rates of 17O-enriched water at pH in the neutral range were determined with isotopic compositions of 17O varying from natural abundance to 5%. Meiboom's account of the proton T-1(2) caused by the scalar interaction between the proton and 17O is generalized to accommodate new data. The generalized formulation covers 17O enrichment above the limit to which Meiboom and his co-worker constrained all their work.

Contrast Media↗