  Background: Parallel transmit (pTx) has introduced many benefits to MRI with
regard to decreased specific absorption rates and improved transmit field
homogeneity, of particular importance in applications at higher magnetic field
strengths. PTx has also been proposed as a solution to mitigating dangerous RF
induced heating of elongated conductive devices such as those used in cardiac
interventions. In this work we present a system that can augment a conventional
scanner with pTx, in particular for use in interventional MRI for guidewire
safety. Methods: The pTx system was designed to work in-line with a 1.5T MRI
while the RF synthesis and imaging control was maintained on the host MR
scanner. The add on pTx system relies on the RF transmit signal, unblanking
pulse, and a protocol driven trigger from the scanner. The RF transmit was
split into multiple fully modulated transmit signals to drive an array of
custom transceiver coils. The performance of the 8-channel implementation was
tested with regards to active and real-time control of RF induced currents on a
standard guidewire, heating mitigation tests, and anatomical imaging in a sheep
torso. Results: The pTx system was intended to update RF shims in real-time and
it was demonstrated that the safe RF shim could be determined while the
guidewire is moved. The anatomical imaging demonstrated that heart anatomy and
neighbouring structures can be fully imaged with the pTx system inline.
Conclusion: We have presented the design and performance of a real-time
feedback control pTx system capable of adding such capabilities to a
conventional MRI with the focus guidewire imaging in cardiac interventional MRI
applications.
