Qun Zhao

Our research program is centered on one of the most advanced biomedical imaging modalities, MRI, and its applications such as functional MRI, diffusion tensor imaging, and spectroscopy.

MRI relies on the relaxation properties of excited nuclei, e.g. hydrogen nuclei in water and fat. When the object to be imaged is placed in a powerful, uniform magnetic field the spins of the atomic nuclei within the tissue all align in one of two opposite directions: parallel to the magnetic field or antiparallel. The magnetic dipole moment of the nuclei then precesses around the axial field. While the proportion is nearly equal, slightly more are oriented at the low energy angle. The frequency with which the dipole moments precess is called the Larmor frequency. The tissue is then briefly exposed to pulses of electromagnetic energy (RF pulses) in a plane perpendicular to the magnetic field, causing some of the magnetically aligned hydrogen nuclei to assume a temporary non-aligned high-energy state.

In order to selectively image different voxels (volume picture elements) of the subject, orthogonal magnetic gradients are applied. MRI allows encoding either in the principal axes of a patient (so that the patient is imaged in x, y, and z from head to toe), or completely flexible orientations for images. All spatial encoding is obtained by applying magnetic field gradients which encode position within the phase of the signal. In one dimension, a linear phase with respect to position can be obtained by collecting data in the presence of a magnetic field gradient. In three dimensions (3D), a plane can defined by "slice selection", in which an RF pulse of defined bandwidth is applied in the presence of a magnetic field gradient in order to reduce spatial encoding to two dimensions (2D). Spatial encoding can then be applied in 2D after slice selection, or in 3D without slice selection. In either case, a 2D or 3D matrix of spatially-encoded phases is acquired, and these data represent the spatial frequencies of the image object. Images can be created from the acquired data using the discrete Fourier transform (DFT).

In this project, students will have an opportunity to learn fundamentals on MR imaging, such as RF transmit / receive, gradients encoding, as well as learn MRI image processing methods. The new BioImaging Research Center (located in Coverdell Center) has a state-of-the-art General Electrical 3T magnet, which provides top quality MR imaging capabilities. We hope that students can build good hands-on experiences in MR systems.