A pathway towards a two‐dimensional, bore‐mounted, volume body coil concept for ultra high‐field magnetic resonance imaging

Abstract

Lack of a body‐sized, bore‐mounted, radiofrequency (RF) body coil for ultrahigh field (UHF) magnetic resonance imaging (MRI) is one of the major drawbacks of UHF, hampering the clinical potential of the technology. Transmit field (B1) nonuniformity and low specific absorption rate (SAR) efficiencies in UHF MRI are two challenges to be overcome. To address these problems, and ultimately provide a pathway for the full clinical potential of the modality, we have designed and simulated two‐dimensional cylindrical high‐pass ladder (2D c‐HPL) architectures for clinical bore‐size dimensions, and demonstrated a simplified proof of concept with a head‐sized prototype at 7 T. A new dispersion relation has been derived and electromagnetic simulations were used to verify coil modes. The coefficient of variation (CV) for brain, cerebellum, heart, and prostate tissues after B1+ shimming in silico is reported and compared with previous works. Three prototypes were designed in simulation: a head‐sized, body‐sized, and long body‐sized coil. The head‐sized coil showed a CV of 12.3%, a B1+ efficiency of 1.33 μT/√W, and a SAR efficiency of 2.14 μT/√(W/kg) for brain simulations. The body‐sized 2D c‐HPL coil was compared with same‐sized transverse electromagnetic (TEM) and birdcage coils in silico with a four‐port circularly polarized mode excitation. Improved B1+ uniformity (26.9%) and SAR efficiency (16% and 50% better than birdcage and TEM coils, respectively) in spherical phantoms was observed. We achieved a CV of 12.3%, 4.9%, 16.7%, and 2.8% for the brain, cerebellum, heart, and prostate, respectively. Preliminary imaging results for the head‐sized coil show good agreement between simulation and experiment. Extending the 1D birdcage coil concept to 2D c‐HPLs provides improved B1+ uniformity and SAR efficiency.

Investigation of alternative RF power limit control methods for 0.5T, 1.5T, and 3T parallel transmission cardiac imaging: A simulation study

“…UHF MR was not considered in this work since our focus was on the –normally vendor‐built– body coil and very few such coils exist, so far for $$$ {B}_0\ge 7\kern0.3em \mathrm{T} $$$ 3,84–90 . The key elements of the described approach can also be applied at UHF, of course, where pTx is indispensable to cope with $$$ {B}_1^{+} $$$ inhomogeneities 7–9 …”

“…UHF MR was not considered in this work since our focus was on the -normally vendor-built-body coil and very few such coils exist, so far for B 0 ≥ 7 T. 3,[84][85][86][87][88][89][90] The key elements of the described approach can also be applied at UHF, of course, where pTx is indispensable to cope with B + 1 inhomogeneities. [7][8][9] Only static RF shimming is considered in this work, since it can be immediately applied to all existing pulse sequences and is thus the natural first step for pTx in the clinical market.…”