Biomedical subjects

Jianhui Zhong

Publications and source records attributed to Jianhui Zhong.

9 recordsLinked to original sources

PubMed2005

A simulation algorithm based on Bloch equations and product operator matrix: application to dipolar and scalar couplings.

A product operator matrix is proposed to describe scalar couplings in liquid NMR. Combination of the product operator matrix and non-linear Bloch equations is employed to describe effects of chemical shift, translational diffusion, dipolar field, radiation damping, and relaxation in multiple spin systems with both scalar and dipolar couplings. A new simulation algorithm based on this approach is used to simulate NMR signals from dipolar field effects in the presence of scalar couplings. Several typical coupled spin systems with both intra-molecular scalar couplings and inter-molecular dipolar couplings are simulated. Monte Carlo methods are incorporated into simulations as well to analyze diffusion process in these complicated spin systems. The simulated results of diffusion and relaxation parameters and 2D NMR spectra are coincident with the experimental measurements, and agree with theoretical predictions as well. The simulation algorithm presented herein therefore provides a convenient means for designing pulse sequences and quantifying experimental results in complex coupled spin systems.

Algorithms↗

PubMed2004

High-resolution NMR spectra in inhomogeneous fields via IDEAL (intermolecular dipolar-interaction enhanced all lines) method.

Intermolecular double-quantum technique is used to yield high-resolution NMR spectra in inhomogeneous magnetic fields. The method exploits the distant dipolar interactions between the solvent and solute nuclear spins. Chemical shifts, J couplings, multiplicity patterns, and relative areas are retained with the method. Except for a 1.5-fold change in the scale factor of J couplings, other parameters are consistent with those extracted from one-dimensional spectra obtained in a homogeneous field.

Magnetic Resonance Spectroscopy↗

PubMed2004

Diffusion measurements free of motion artifacts using intermolecular dipole-dipole interactions.

Diffusion encoding, or diffusion weighting, is commonly achieved by applying a pair of balanced pulsed-field gradients during spin evolution. An alternative way to obtain diffusion measurements is to select dipolar correlation distances using the distant dipolar field (DDF) in systems with abundant spin density, such as water in tissues. Diffusion weighting using this effect is unique in that the refocusing "gradient" is carried within the sample, and thus the macroscopic motion of the sample is not expected to interfere with signal formation. The experiments presented here demonstrate that in moving phantoms, the phase shift of the signal due to linear motion is minimal in diffusion-weighted (DW) DDF measurements, and that motion artifacts in images of moving phantoms and the abdomen of live mice are small compared to standard pulsed-field-gradient methods. The technique may facilitate the use of DWI in typically motion-prone regions such as the abdomen, lungs, and heart.

Abdomen↗

PubMed2004

Separation and characterization of different signals from intermolecular three-spin orders in solution NMR.

In this paper, signals originating from a pure specific coherence of intermolecular three-spin orders were separated and characterized experimentally in highly polarized two-component spin systems. A modified CRAZED sequence with selective radio-frequency excitation was designed to separate the small signals from the strong conventional single-spin single-quantum signals. General theoretical expressions of the pulse sequence with arbitrary flip angle pulses were derived using dipolar field treatment. The expressions were used to predict the relaxation and diffusion properties and optimal experimental parameters such as flip angles. For the first time, relaxation and diffusion properties of pure intermolecular single-quantum, double-quantum, and triple-quantum coherences of three-spin orders were characterized and analyzed in one-dimensional experiments. All experimental observations are in excellent agreement with the theoretical predictions. The theoretical results show that the quantum-mechanical treatment leads to exactly the same predictions as the dipolar field treatment. The quantitative study of intermolecular multiple-quantum coherences of three-spin orders presented herein provides a better understanding of their mechanisms.

Journal Article↗

PubMed2004

Modeling liver motion and deformation during the respiratory cycle using intensity-based nonrigid registration of gated MR images.

We present a technique for modeling liver motion during the respiratory cycle using intensity-based nonrigid registration of gated magnetic resonance (MR) images. Three-dimensional MR images of the abdomens of four volunteers were acquired at end-inspiration, end-expiration, and eight time points in between using respiratory gating. The deformation fields between the images were computed using intensity-based rigid and nonrigid registration algorithms. Global motion is modeled by a rigid transformation while local motion is modeled by a free-form deformation based on B-splines. Much of the liver motion was cranial-caudal translation, which was captured by the rigid transformation. However, there was still substantial residual deformation (approximately 10 mm averaged over the entire liver in four volunteers, and 34 mm at one place in the liver of one volunteer). The computed organ motion model can potentially be used to determine an appropriate respiratory-gated radiotherapy window during which the position of the target is known within a specified excursion.

Adult↗

PubMed2003

fMRI for monitoring dynamic changes in tissue oxygenation/blood flow: potential applications for tumor response to carbogen treatment.

The ability to differentiate between well-oxygenated and poorly-oxygenated tumors may play an important role in selecting an optimal therapeutic regime for tumor treatment of the individual patient. We present preliminary results in the development of a dynamic functional MRI method for mapping tissue oxygenation and blood flow distribution in humans simultaneously. We applied interleaved Blood Oxygenation Level Dependent (BOLD) and Flow-sensitive Alternating Inversion Recovery (FAIR) sequences to detect signals as a subject is inspiring gases of varying oxygen concentration. The method allows quantitation of the spatial distribution and time course of the important physiological functions that are easily registered with high resolution anatomic MR images. It may be used to critically evaluate the efficacy of varying durations of carbogen breathing in tumor patients, and allow a quantitative evaluation of the roles of carbogen and other radiosensitizers as potential adjuncts to radiotherapy and drug therapies.

Carbon Dioxide↗

PubMed2003

A four-element phased array coil for high resolution and parallel MR imaging of the knee.

A four-element phased array coil for MR imaging of the knee was designed, built and tested for clinical use at 1.5 Tesla. In routine imaging, it provides over twofold increase in signal-to-noise (SNR) compared to two commercially available knee coils, and supports higher spatial image resolution. The phased array knee coil was also tested for its compatibility with parallel MR imaging that reduces imaging time by several folds over conventional MR technique. Results obtained using SiMultaneous Acquisition of Spatial Harmonics (SMASH) technique shows that our phased array knee coil can be used with parallel MR imaging. These improvements may enhance knee diagnosis with higher image quality and reduced scan time.

Adult↗

PubMed2003

Frontal cortex, laterality, and memory: encoding versus retrieval.

The cerebral hemispheres differ in their capabilities and response to verbal versus nonverbal visual material. A priori, it might thus be expected that the right hemisphere would be best activated during a mnemonic task with fMRI when using nonverbalizable images, and the left hemisphere with verbal material. However, previous psychological tests had shown a high degree of similarity in measures of memory for these disparate items. It was thus hypothesized that extensive commonality in the areas activated would prevail when this previously tested material was employed with fMRI. Six subjects underwent fMRI with four types of trials in blocks: fixating; passively viewing 12 words and 12 nonverbalizable images; endeavoring to remember (encoding) another set of 12 words and images; endeavoring to recognize (retrieve) previously viewed words or images. Passive viewing produced small islands of activation in left versus right frontal cortex for words and images, respectively. Endeavoring to remember enlarged the areas of activation and produced some bilaterality. Retrieval greatly augmented activation as well as bilaterality, and some 20% of the activated frontal volume was shared by words and images. Thus, on the one hand, the distribution of activation upon retrieval differed substantially for words versus images, but on the other, as predicted, there was considerable commonality. Predominant laterality of activation in some areas shifted between encoding and retrieval (HERA), importantly involving different regions for words versus images. Of course, processes other than memory per se are undoubtedly involved in these distributions of fMRI activation in frontal cortex, yet the nature of the to-be-remembered items is clearly a major factor, in accord with the asymmetric lateralization in their basic representation.

Adult↗

PubMed2002

Optimization of blood oxygenation level-dependent sensitivity in magnetic resonance imaging using intermolecular double-quantum coherence.

PURPOSE: To optimize timing parameters in an intermolecular double-quantum coherence (iDQC) imaging pulse sequence for overall image signal-to-noise ratio (SNR) and blood oxygenation level-dependent (BOLD) sensitivity for brain functional imaging. MATERIAL AND METHODS: Fresh human blood was measured under different oxygenation conditions, and human brain functional magnetic resonance (fMR) images in three normal volunteers were obtained, using iDQC techniques at 1.5 T. The dependence of SNR and BOLD sensitivity was measured as a function of time delays after the iDQC evolution period. RESULTS: A time delay after the iDQC evolution period tau can be adjusted either to refocus the dephasing accumulated during tau, thus increasing SNR, with full rephasing occurring at delay = +/-2tau (for iDQC order n = +/-2), or to enhance BOLD effects with consequent reduced image SNR at delay = 0. CONCLUSION: Image SNR and BOLD sensitivity often impose different requirements for iDQC image sequence design and timing parameter selections. It is therefore important to select properly relevant parameters for different applications.

Acoustic Stimulation↗