Hyperpolarized Parallel MRI

Many diseases are characterized by changes in metabolism. This is true for cancer, cardiovascular diseases, diabetes, and for many other diseases. However, current methods in the clinic do not allow for assessment of metabolism without the use of radioactive tracers or biopsies. Hyperpolarized carbon-13 magnetic resonance imaging (MRI) is a new method to measure in vivo metabolism in a safe and non-radioactive manner. Hyperpolarization increases the magnetic resonance (MR) signal by many orders of magnitude. It involves injection of a hyperpolarized biological molecule that is taken up by the cells in the body and metabolized to other molecules through different metabolic pathways. The MR imaging of the substrate and metabolites, however, is constrained by the fact that the high signal of the hyperpolarized molecule decays away on a time-scale of only a few minutes. A popular method for fast data acquisition in conventional MRI is parallel imaging, which exploits the extra information given when MR signals are measured from multiple view angles simultaneously using multi-channel receive coils.

The objective of this PhD project was to investigate the use of parallel imaging for hyperpolarized carbon-13 MRI as a tool to improve imaging of in vivo metabolism through acquisition of larger volumes with better resolution compared to what is currently possible. The project involved characterization of multi-channel carbon-13 coils, which represent an integral part of parallel imaging acquisition, investigations of optimal sensitivity extraction from the coils, and finally in vivo demonstration of fast volumetric acquisition of metabolism using parallel imaging. In vivo studies were carried out in healthy pigs with imaging of kidneys and heart and in a healthy human volunteer, representing the first full volumetric acquisition of human abdominal metabolism. The results provide instructions for future carbon-13 coil design and show that parallel imaging can increase the resolution and coverage for imaging of in vivo metabolism if the right strategy is applied. This is especially useful for clinical applications of hyperpolarized carbon-13 MRI that require acquisition of large volumes, including imaging of cancer metastases.