Development of Frequency-modulated Techniques for MRI of Fast Relaxing Spins

Abstract

In conventional MRI sequences, the signal from fast relaxing spin like in highly ordered tissues (e.g., tendons and knee meniscus) and highly mineralized tissue (e.g., bone, teeth) are hard to detect since the time elapsing between spin excitation and signal acquisition is typically too long. Short T2 sensitive sequences like ultra-short echo time (UTE), zero echo time (ZTE) and SWeep Imaging with Fourier Transformation (SWIFT) have been developed to detect signals from fast relaxing spins.

The work described in this thesis centered on the SWIFT sequence, including the MRI sequence development, reconstruction strategies and various applications.

Firstly, the applications of SWIFT sequence in the musculoskeletal imaging, including imaging of cortical bone with off-resonance saturation in the ex-vivo mandibular tumor invasion study, T1 quantification of fetal skeleton with variable flip angle method and the development of a 3D adiabatic T1ho mapping method for steady state sequence were demonstrated and discussed. The potential application for SWIFT in the detection of tumor invasion, abnormalities or early degeneration in the musculoskeletal system has been shown.

Secondly, a 3D Look-Locker T1 mapping method for radial sampling sequence was developed and implemented in the SWIFT sequence. The ability of the developed SWIFT Look-Locker method to provide quantitative distribution of the IONPs at high concentration is verified in both phantom and in-vivo studies. The potential application of this method on providing heating prediction in magnetic nanoparticles hyperthermia was also demonstrated. SWIFT Look-Locker T1 mapping should prove to be a valuable tool in magnetic nanoparticles hyperthermia research and practice.

Finally, an extension of the SWIFT sequence with one more degree of freedom, called GM-SWIFT, which includes gradient modulation, was developed. The correlation steps needed to reconstruct the images from GM-SWIFT data were established. The advantages of GM-SWIFT, including SAR reduction, RF power saving, and controllable T2 sensitivity, were demonstrated. The tradeoff has been discussed. GM-SWIFT provides a bridge between the UTE-type sequences and SWIFT. By manipulating the gradient, GM-SWIFT can provide a way to optimize the settings that balance RF amplitude, SAR, scan time, and image quality considerations.