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Multiphoton magnetic resonance in imaging: A classical description and implementation.

Victor Han1, Chunlei Liu1,2

  • 1Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, California, USA.

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|February 6, 2020
PubMed
Summary
This summary is machine-generated.

Researchers developed a geometric view of multiphoton excitation for MRI, enabling novel imaging techniques. This approach simplifies complex excitations, allowing for advanced pulse sequences and multiband imaging without RF pulse modification.

Keywords:
Bloch-Siegert shiftadiabaticmultibandmultiphotonselective excitationtwo-photon

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Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Quantum Optics
  • Geometric Phase

Background:

  • Multiphoton excitation is crucial for advanced imaging but lacks a simple geometric interpretation.
  • Existing MRI techniques are limited by the Larmor frequency constraint.
  • Novel methods are needed to enhance imaging capabilities and pulse sequence design.

Purpose of the Study:

  • To establish a classical geometric interpretation of multiphoton excitation.
  • To apply this interpretation to Magnetic Resonance Imaging (MRI).
  • To explore novel imaging techniques enabled by multiphoton excitation.

Main Methods:

  • A rotating frame transformation was employed to simplify multiphoton excitations to resemble single-photon excitations.
  • A homebuilt low-frequency coil was used to implement two-photon versions of standard slice-selective pulse sequences.
  • Oscillating gradients were utilized as an additional photon source for excitation, enabling multiband imaging.

Main Results:

  • Analytical multiphoton excitation expressions were validated against Bloch equation simulations with Bloch-Siegert shift corrections.
  • Two-photon gradient-echo images of phantoms (lemon, pork rib) demonstrated comparability to single-photon counterparts.
  • The combination of frequency-offset radiofrequency (RF) pulses and oscillating gradients successfully generated excitation where RF alone was insufficient.

Conclusions:

  • The geometric interpretation of multiphoton excitation offers significant flexibility for MRI.
  • This approach liberates excitation from the Larmor frequency constraint, enabling innovative RF pulse designs.
  • The findings pave the way for further advancements and innovations in MRI technology.