Parallel transmission is a technology that was proposed in 2003 by Katsher et al (“transmit SENSE”). In pTx, the MRI signal is excited by transmitting the RF energy using several transmit antennas as opposed to a single one as in the ubiquitous birdcage coil approach. Although more complex than birdcage coil MRI excitation, pTx offers the advantages of 1) improved excitation quality and 2) reduced SAR at ultra-high fields. Indeed, at ultra-high fields (7T in the brain and 3T in the body, and higher), the wavelength of the RF energy in the body becomes comparable to the size of the body, which creates a strongly non-uniform transmit field profile. In addition, SAR (specific absorption rate), which is a measure of the RF energy absorbed by the body, increases with the field strength thus introducing safety concerns.
By solving these two problems simultaneously, pTx offers the promise of “unlocking” the benefits of ultra-high field MRI for human imaging. In our lab, we focus on:
- Optimization of pTx RF pulses with low global and local SAR
- Joint gradient trajectory/RF shapes for improved inner volume excitation using pTx
- Evaluation of pTx coils using electromagnetic simulation
- Development of electromagnetic solvers specifically designed for MRI (in collaboration with the Elfar Adalsteinsson and Luca Daniel groups at MIT)
- Direct optimization of pTx coil geometries.
Some of our code and methods are available here: http://ptx.martinos.org