Magnetic Resonance Technology Development
Four major technical capabilities support the CFMM research programs; (1) the simulation, design and construction of multi-channel RF coils for parallel transmission (pTx) and parallel reception (pRx), (2) the quantification of the safety of the electromagnetic (EM) fields produced by the pTx coils (to calculate Specific Absorption Rate or SAR), (3) determination of the MRI safety and compatibility of the growing array of devices and implants used to facilitate research (recording electrodes, motion trackers) and to perform therapy (e.g. deep brain stimulators (DBS), implanted infusion pumps) and (4) the generation of new pulse-sequences and acquisition methodologies on high and ultra-high field MRI scanners.
pTx/pRx. Advances in RF technology are a valuable and cost-effective way to further improve homogeneity, sensitivity and encoding capabilities in MR. At 7T, pTx capabilities are essential to compensate for the RF transmit inhomogeneity and for controlling SAR in human studies. At all field strengths, pRx systems are essential to speed up individual image acquisition times (particularly for echo planar imaging (EPI) which is ubiquitous for fMRI and is sensitive to field distortions), while achieving the highest SNR possible. The Menon lab was amongst the first to build pTx/pRx arrays at the challenging field strengths of 4T and above over a decade ago. Arrays for pTx and/or pRx can only function efficiently if the coil elements are mutually decoupled, but eliminating this coupling has been a longstanding challenge. We recently introduced a decoupling method that uses structures of artificially engineered materials with negative magnetic permeability (µ) inserted between array elements to suppress the electromagnetic field responsible for the mutual coupling phenomena (US Provisional No. 61/631,883) and this is a major area of focus for the RF group.
Electromagnetic (EM) interactions. Many of the benefits of 3T and 7T MRI come at the expense of an increase in SAR, which causes potentially dangerous heating if not well controlled. Vulnerable populations, especially neonatal and pediatric populations, have no capability to report discomfort, so extensive characterization is necessary for safe applications. Higher magnetic fields also come with greater challenges in optimizing magnetic field homogeneity. SAR calculations are also particularly important for tight fitting conformal pTx coil designs. Beyond homogenizing the RF field, RF shimming has the additional potential to reduce global and local SAR at 3T and 7T. For example, high degrees of pTx freedom allow us to “steer” the SAR away from implanted objects such as neurostimulators, pacemakers or recording electrodes. Prof. Chronik is a world expert in the area of EM interactions with tissue,having collaborated with international groups, vendors and regulatory agencies. The Chronik lab is also designing novel magnetic shim coils to enhance magnetic field homogeneity and additional gradient coil inserts for enhanced DTI at 7T.
MRI safety and compatibility of implants & devices. Techniques such as Deep Brain Stimulation and transcranial direct current stimulation have considerable therapeutic potential in neurological disorders, but there is a very poor understanding of how they work and how to optimize them. While fMRI and NHP studies have the potential to shed light on this, the first objective has to be assuring their safe use in the MR environment using our NHP model. Similarly, many devices that we might want to use in the MRI scanner require modification, either to work at 3T, or for those that do work at 3T, to make them work at 7T. Examples of this are the M-R Eye motion capture system that the Chronik lab adapted to 3T and the future implementation of the EGI MR300 electrode cap at 7T. The Bartha and Menon labs also have experience in this area, having demonstrated the first implanted electrodes at 9.4T (in a rat model of epilepsy) and electroencephalography (EEG) on humans at 4T.
Pulse-sequences. All five of the MR team members have one or more decades of pulse-sequence programming experience on the Agilent and/or Siemens platforms. Prof. Bartha is extending the use of the LASER sequence for spectroscopy that was developed with Prof. Garwood for many of his MRS studies. Prof. Bartha is also exploring the use of pTx and novel pulses for reducing transmit power and improving fat-saturation and has already incorporated pRx for numerous spectroscopic measurements. He is developing new analysis tools for volumetric imaging at 3T and 7T, leveraging the ADNI trial dataset. Prof. Théberge is developing both single voxel and spectroscopic imaging (CSI) sequences on the 7T for neuropsychiatric patient studies of glutamate and GABA. On the 3T, Prof. Cusack is exploring the use of “silent” MR sequences for auditory studies, and neonate applications (for which he just received a NSERC/CIHR CHRP award). He is also evaluating the slice-multiplexed sequences such as the multi-band fMRI sequence (available from the Minnesota group). The Cusack lab is a major developer of multi-voxel pattern analysis (MVPA) and other fMRI analysis tools to exploit functional information at a finer spatial scale. Prof. Menon is developing new pulse sequences for fMRI at all fields, which utilize the orthogonality between phase and magnitude information to improve artifact rejection and provide independent measures of confounding variables such as breathing. His group has extended the use of RASTAMAP to make accurate isotropic 3D measurements of tissue frequency and apparent transverse relation rate (R2*). These sequences are being used at 3T and 7T in studies of multiple sclerosis and are being extended to other neurodegenerative diseases, in particular Alzheimer’s, MCI and mTBI. Utilizing pTx capabilities, the Menon group is also developing whole-brain low SAR versions of FSE, MP2RAGE and other essential anatomic imaging sequences, which are often limited in slice-count or coverage at 7T.