My overall research goal is to develop Quantitative Susceptibility Mapping (QSM) techniques to improve Magnetic Resonance Imaging (MRI) of microbubbles. Although microbubbles are a well-established intravascular ultrasound contrast agent, a few studies have shown that they can enhance the image contrast in MRI. There is increasing interest in MRI-guided microbubble-mediated focused ultrasound treatments such as thermal surgery and targeted delivery of drugs, antibodies or genes, especially in regions like the brain which are difficult to treat using conventional surgery and treatments. State-of-the-art MRI techniques used to detect microbubbles exploit the local decrease in the MRI signal magnitude due to the magnetic susceptibility difference between microbubbles and the surrounding tissues. Magnetic susceptibility is a property telling us how much a tissue or material becomes magnetised in an external magnetic field and can be measured directly using QSM. Signal magnitude decreases occur not only around microbubbles but also close to veins or other strong susceptibility sources, making it difficult to separate these effects and confounding MRI of microbubbles. Furthermore, the signal magnitude has a non-linear and non-local dependence on microbubble size and volume fraction so it is difficult to relate signal decreases to the quantity of microbubbles at a specific location.
In this fellowship I have developed a fast MRI QSM technique to allow much more direct detection of microbubbles with the potential for dynamic tracking of microbubble concentration, destruction and clearance. MRI signal is complex, i.e. it has two constituents: magnitude and phase. QSM uses the phase, which has previously been discarded in MRI of microbubbles. Use of the phase information to visualise human brain anatomy has resulted in dramatic improvements in the image contrast-to-noise ratio, especially at high magnetic fields (3 Tesla and above). However, the phase contrast is non-local and orientation dependent, making it difficult to interpret. QSM overcomes these problems by calculating from the phase signal the underlying magnetic susceptibility, which more closely represents tissue composition, promising quantification of microbubble concentration or dose calculation in targeted delivery of drugs. The technique I have developed for fast imaging of microbubbles using QSM may facilitate broader application of MRI-guided microbubble-mediated focused ultrasound treatments in the future.
