UC Berkeley

Magnetic resonance imaging (MRI) consists of three main components: a main static magnetic field for producing magnetization in a sample or subject, spatially varying magnetic fields for spatial encoding of the magnetization, and radiofrequency (RF) magnetic fields for manipulating the magnetization. This dissertation is mainly concerned with the RF fields. The frequency of the RF fields is set to the resonance determined by the strength of the main magnetic field and the properties of the object being imaged. For the usual case of imaging Hydrogen nuclei, the frequency is on the order of hundreds of MHz on superconducting clinical MRI scanners. While standard MRI only uses RF near the resonance frequency, we propose the use of multiphoton excitation in MRI, where multiple RF fields whose frequency sums or differences equal the resonance frequency are used instead. No RF at the resonance frequency is required.

We begin by verifying multiphoton resonances in MRI via simulation and experiment. Practically, this is done using a combination of RF in the kHz and RF in the MHz, for example 30 kHz and 127.71 MHz for a 127.74 MHz resonance frequency. While multiphoton excitation with RF at a single subharmonic frequency, such as with half-resonance frequency RF at 63.87 MHz for a 127.74 MHz resonance, is possible, it is not practical for human scale MRI due to excessive power requirements. Although the RF fields in MRI are often praised for being safe in that they are non-ionizing, they still pose the danger of potentially excessively heating a subject.

By using RF in the kHz and the MHz, the power efficiency of multiphoton excitation is greatly increased, and the kHz RF provides negligible additional power deposition in the human body. By taking advantage of this fact, we can actually reduce overall power deposition in the body in more spatially complex excitation patterns such as multiband excitation by simultaneously making use of multiple multiphoton resonance conditions at different spatial locations. With the addition of power-deposition-negligible kHz RF, the power-deposition intensive MHz RF can be reused for multiple excitations at different spatial locations instead of just one spatial location.

Next we move onto providing a more rigorous analysis of spatially selective excitation with multiphoton resonances. With this analysis we see that amplitude and frequency modulation can be traded-off between the different RF fields in a multiphoton resonance, granting additional flexibility. After demonstrating this additional flexibility with a homemade extra kHz-range RF coil, we also show its practicality on an in vivo human head. As future MRI systems increasingly gain complexity, we believe this additional flexibility will enable novel applications.