Multiphoton magnetic resonance in imaging: A classical description and implementation

Abstract

Funding information UC BerkeleyPurpose: To develop a classical geometric interpretation of multiphoton excitation and apply it to MRI. To investigate ways in which multiphoton excitation can enable novel imaging techniques. Theory and Methods: 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 singlephoton excitation. With a homebuilt low frequency coil, we execute a standard slice selective pulse sequence with all of its excitations replaced by their equivalent twophoton 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. Results: With Bloch-Siegert shift corrections, analytical multiphoton excitation expressions match with Bloch equation simulations. Two-photon gradient-echo images of a lemon and a pork rib match with their single-photon counterparts.Frequency-offset RF combined with oscillating gradients generate excitation where the RF alone does not. Conclusion: 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. K E Y W O R D Sadiabatic, Bloch-Siegert shift, multiband, multiphoton, selective excitation, two-photon ORCID Victor Han https://orcid.org/0000-0003-3093-2549 Chunlei Liu https://orcid.org/0000-0001-8816-4832

Selective excitation localized by the Bloch–Siegert shift and a B1+ gradient

“…Although their rapid dropoff in amplitude with N makes their use for imaging challenging, it may be possible to use BSSE pulses for multiband multiphoton excitation in implementations similar to those previously described. 26 However in general 𝜔 off should be increased to move multiphoton resonances outside the FOV.…”

“…However, excessive reduction of the frequency offset can produce out‐of band excitation (Figure 3B, at $$$ {\omega}_{\mathrm{off}} $$$ = 2.5 kHz) or move multiphoton resonances closer in $$$ {B}_1^{+} $$$ to the single‐photon resonance (Figure 6) and potentially into the FOV. Although their rapid dropoff in amplitude with $$$ N $$$ makes their use for imaging challenging, it may be possible to use BSSE pulses for multiband multiphoton excitation in implementations similar to those previously described 26 . However in general $$$ {\omega}_{\mathrm{off}} $$$ should be increased to move multiphoton resonances outside the FOV.…”

“…However, the new pulse class is reliant on the simultaneous application of two off-resonant pulses in the xy plane, which also produces additional multiphoton resonances. 24,25 Although imaging techniques that use multiphoton resonances for excitation have recently been demonstrated, 26 these additional resonances are here regarded as nuisances and their mitigation is discussed.…”

Nutation dynamics and multifrequency resonance in a many-body seesaw

“…An intensive review of this physics in the Rabi model can be found in reference [19]. This method has also been applied in medicine for magnetic resonance imaging [20,21]. Moreover, similar physics can also be found in other models without these energy levels, such as the nonlinear oscillator [22][23][24][25], nanomechanical resonators [26,27], and Josephson junctions [28,29].…”

Any‐nucleus distributed active programmable transmit coil

“…In our ongoing work, a specialized control board will be made to control one or more ADAPT Coils, and more complicated switching patterns could be designed to provide more optimal RF waveforms for higher RF amplitudes. Fields used for multiphoton MRI [47][48][49] and concurrently exciting multiple nuclei could also be implemented. Because the switching for each current direction does not have to be symmetrical, B 0 shimming and local B 0 gradients like in AC/DC coils [50][51][52][53] could also be produced.…”