Applications of parallel radiofrequency transmission to ultra-high-field magnetic resonance imaging

Thesis

Increasing the static magnetic field (B0) strength of magnetic resonance imaging (MRI) scanners improves the signal-to-noise ratio (SNR). Higher SNR may be used for more sensitive, faster, and higher resolution imaging.

Despite its potential, the biggest challenges for ultra-high field MRI is inhomogeneity in the radio-frequency field (B1+), which causes spatially varying flip angles when using fixed-frequency pulses, leading to undesirable perturbations in signal and contrast. The most flexible solution to B1+ inhomogeneity at UHF is parallel transmission (pTx). A pTx enabled system consisting of more than one RF transmit channels which may be independently driven.

This thesis seeks to extend the methodology and applications of pTx magnetic resonance imaging (MRI) in the brain at 7 tesla. Specifically, this thesis focuses on two spin labelling techniques: arterial spin labelling (ASL) that measures cerebral perfusion and T2-Relaxation-Under-Spin-Tagging (TRUST) that measures venous oxygenation.

The ASL study demonstrates that labelling efficiency can be improved by the incorporation of pTx static shimming in the labelling pulses, but indicates that the temporal SNR does not yet exceed optimised 3 tesla protocols in the absence of further developments. The TRUST study investigates the use of spatially-selective pTx pulses in a modified TRUST sequence to measure regional oxygen extraction fraction (OEF) in the brain. Results show that localised saturation was achieved by repeatedly exciting (using pTx pulses) and saturating spins outside the region-of-interest.

Authors: Tong, Y

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: TRUST; PCASL; perfusion; parallel transmission; ultra-high field; arterial spin labelling
• Subjects: Brain--Magnetic resonance imaging; Magnetic resonance imaging