Novel techniques and innovative designs for the RF chain of magnetic resonance imaging scanners

Abstract

In this dissertation, novel techniques and innovative designs are proposed to serve one of the main goals of MRI, which is achieving the best image quality by improving signal-to-noise ratio (SNR) while confined to the patient’s safety restrictions determined by specific absorption rate (SAR) limits. This dissertation comprises five different contributions to the field. First, the co-simulation method is accelerated using the equivalent circuit model of the intended coil and introduced as a compact novel technique for the fast design of transmit array (TxArray) coils. It is shown that the existing co-simulation method can be accelerated by more than 20-fold for different types of TxArray coils. In another study, a simple strategy is introduced for optimizing the topography of transmit elements to reduce the local SAR at ultra-high field (UHF, B0 ≥ 7 T) MRI. It is shown that the peak local 10g-averaged SAR can be reduced more than 20% for different types of transmit elements at two different field strengths (i.e., 7 T and 10.5 T). As another contribution, confining to the safety limits, the first in vivo human head images at 10.5 T is acquired. To ensure the safety of subjects, the EM simulation model of the radiofrequency (RF) coil is validated using an expanded validation workflow, then, safe RF power levels were calculated. In another study, a novel short dipole-like element with improved SAR performance, non-unform dielectric substrate (NODES) antenna, is introduced which provides an opportunity to design highly-dense TxArray coils for UHF-MRI. Eventually, a nine-channel transmit/receive coil is built and human cadaver spine images are acquired at 10.5 T. In the final study, a new parameter, ultimate intrinsic SAR efficiency (UISARE), is introduced and calculated as the upper bound for SAR performance of the transmit coils at different field strengths. Overall, this dissertation proposes novel techniques and innovative structures for designing the TxArray coils, which mainly contribute to enhancing image quality and patient safety by improving SNR and reducing SAR.