November 11 Nov 11
2020
10:00 AM - 11:00 AM
Wednesday Wed

Multiphoton Magnetic Resonance Imaging

RSL Invited Seminar Series

Today’s MRI assumes single-photon excitation. That is, for each nuclear spin, a single photon accompanies the transition between energy states. This photon must resonate near the Larmor frequency. In our work, we show that, instead of the usual single‐photon resonance, we can excite multiphoton resonances to generate signal for MRI by using multiple magnetic field frequencies, none of which is near the Larmor frequency. Only the total energy absorbed by a spin must correspond to the Larmor frequency. 

We present a fully geometric view of multiphoton excitation by taking a particular rotating frame transformation. In this rotating frame, we find that multiphoton excitations appear just like single‐photon excitations again, and therefore, we can readily generalize concepts already explored in standard single‐photon excitation. Here, we present several examples to illustrate this concept. With a homebuilt low frequency coil, we execute a standard slice selective pulse sequence with all of its excitations replaced by their equivalent two‐photon versions. In the case of no extra hardware, we use oscillating gradients as a source of extra photons for excitation. Finally, with the multiphoton interpretation of oscillating gradients, we present a novel way to transform a standard slice selective adiabatic inversion pulse into a multiband version without modifying the RF pulse itself. The addition of oscillating gradients creates multiphoton resonances at multiple spatial locations and allows for adiabatic inversions at each location. 

The multiphoton interpretation presents new flexibilities for imaging. Excitation needs not be bound to the Larmor frequency, which opens doors to RF pulse design beyond the usual filter design and the potential for further imaging innovations. 

Location

RSL Group Meeting ZOOM link

Lucas Center for Imaging
1201 Welch Road
Palo Alto, CA 94305
United States

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Lucas Center for Imaging

1201 Welch Road
Palo Alto CA, 94305
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Speaker

Guest Speaker: Victor Han 

Victor Han is currently a fourth-year PhD student in the Department of Electrical Engineering and Computer Sciences (EECS) at the University of California, Berkeley. He is advised by Professor Chunlei Liu. Mainly interested in developing new imaging techniques with custom-built hardware, he would like to develop methods that deviate from standard practice that hopefully push the boundaries of what is possible.