Application of Phase Imaging at High Field - MR Thermometry at 7 Tesla

The main purpose of this research was to develop improved methods for RF coil characterisation, and for non-invasive spatio-temporal mapping of temperature in the living body, in order to utilise the full potential of magnetic resonance imaging (MRI) at high magnetic fields by ensuring radiofrequency (RF) safety. Current RF power limits are often overly conservative, unnecessarily limiting the full potential of MRI, especially at high field. Thus it is useful to monitor tissue temperature while running MR imaging sequences which may deposit high RF power. Proton resonance frequency (PRF) MR thermometry can employ the phase of the complex MR signal to estimate temperature change over time. However, the shift of the water PRF with temperature is relatively small, making phase-based MR thermometry inherently sensitive to any extraneously caused changes of local frequency or MR phase. A potential source of error to PRF MR thermometry is a change in surround air susceptibility. The considerable impact of air susceptibility changes on PRF MR thermometry was demonstrated and quantified in experiments and magnetic field simulations. One way of correcting MR thermometry is to use a chemically shifted reference substance, in combination with a phase-sensitive chemical shift-selective MR thermometry sequence. The requirement of having a reliable separation of substances based on their resonance frequency was met by a novel frequency-selective phase-sensitive spin-echo (SE) MR thermometry sequence. This sequence was thoroughly tested in phantom and in-vivo experiments as well as in extensive Bloch simulations. The sequence limitations and advantages are discussed in detail. This technique acquires unsaturated water and fat images in rapid succession at the same position in space. The acquisition of a water and fat slice in less than 100 ms allows the correction of rapid field fluctuations in the brain caused by breathing and heartbeat, while still ensuring the correction of long term drift. With no assumptions required regarding temperature distribution in the tissue, this novel MR thermometry technique can measure brain temperature within a single (1.5 mm)3 voxel with a very low standard deviation (SD) of 0.3 K. Using an MRI phantom with a dimethyl sulfoxide reference, heating experiments achieved a MR temperature measurement with an SD of approximately 0.1 K in a single (1.5 mm)3 voxel. In conclusion, the work presented in this thesis assists the development of a real-time in-vivo temperature monitoring system that guarantees patient RF safety at high field