Multiphoton simultaneous multislice imaging.

Area of Science:

• Magnetic Resonance Imaging
• Pulse Sequence Design
• Biomedical Engineering

Background:

• Simultaneous multislice (SMS) imaging accelerates MRI acquisition but faces challenges with specific absorption rate (SAR).
• Multiphoton excitation offers a potential solution for reducing SAR in SMS imaging.
• Controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) is a technique to improve parallel imaging acceleration.

Purpose of the Study:

• To develop and evaluate multiphoton excitation techniques for SMS imaging.
• To assess the specific absorption rate (SAR) benefits of multiphoton SMS.
• To enhance multiphoton SMS image reconstruction quality using a novel CAIPIRINHA design.

Main Methods:

• Developed multiphoton excitation by combining conventional single-slice RF pulses with oscillating gradient fields.
• Implemented gradient-echo and spin-echo sequences with multiphoton CAIPIRINHA-SMS excitation for in vivo human imaging at 3T.
• Designed a custom CAIPIRINHA phase-encoding strategy for multiphoton SMS.

Main Results:

• Multiphoton SMS achieved a 51% SAR reduction for 3 slices and 66% for 5 slices compared to conventional superposition SMS.
• Multiphoton SMS demonstrated superior SAR reduction over PINS and MultiPINS, particularly with short pulse durations, thin slices, or large slice spacing.
• The custom CAIPIRINHA design significantly improved multiphoton SMS reconstruction quality.

Conclusions:

• Multiphoton SMS excitation, using combined RF and oscillating gradients, provides substantial SAR benefits over conventional SMS.
• A novel CAIPIRINHA design enables higher acceleration factors in multiphoton SMS imaging.
• This technique holds promise for safer and more efficient accelerated MRI.