The Travelling-Wave Primate System: A New Solution for Magnetic Resonance Imaging of Macaque Monkeys at 7 Tesla Ultra-High Field

Abstract

IntroductionNeuroimaging of macaques at ultra-high field (UHF) is usually conducted by combining a volume coil for transmit (Tx) and a phased array coil for receive (Rx) tightly enclosing the monkey’s head. Good results have been achieved using vertical or horizontal magnets with implanted or near-surface coils. An alternative and less costly approach, the travelling-wave (TW) excitation concept, may offer more flexible experimental setups on human whole-body UHF magnetic resonance imaging (MRI) systems, which are now more widely available. Goal of the study was developing and validating the TW concept for in vivo primate MRI.MethodsThe TW Primate System (TWPS) uses the radio frequency shield of the gradient system of a human whole-body 7 T MRI system as a waveguide to propagate a circularly polarized B1 field represented by the TE11 mode. This mode is excited by a specifically designed 2-port patch antenna. For receive, a customized neuroimaging monkey head receive-only coil was designed. Field simulation was used for development and evaluation. Signal-to-noise ratio (SNR) was compared with data acquired with a conventional monkey volume head coil consisting of a homogeneous transmit coil and a 12-element receive coil.ResultsThe TWPS offered good image homogeneity in the volume-of-interest Turbo spin echo images exhibited a high contrast, allowing a clear depiction of the cerebral anatomy. As a prerequisite for functional MRI, whole brain ultrafast echo planar images were successfully acquired.ConclusionThe TWPS presents a promising new approach to fMRI of macaques for research groups with access to a horizontal UHF MRI system.

Improved traveling-wave efficiency in 7 T human MRI using passive local loop and dipole arrays

“…The low efficiency of traveling-wave MR signal reception can be reduced using separate receive-only arrays [8,9], but the low transmit efficiency remains a central disadvantage of the technique. One way to improve the transmit efficiency is to deliver traveling-wave power directly to the target imaging region with a coaxial waveguide [10].…”

Metamaterial-based transmit and receive system for whole-body magnetic resonance imaging at ultra-high magnetic fields

“…The choice to select a macaque, which is about 45 cm tall, as an in vivo subject for imaging is motivated by the goal to acquire data from complex biological structures under the above-mentioned spatial restrictions of the MRAS-equipped bore of the MRI system. A second rationale was to provide the neuroscience research group of our site working with macaques monkeys [ 21 , 24 , 32 ] with a new and potentially better data acquisition concept. For the experiments with the water-NaCl-isopropanol phantom and crab-eating macaque, the MRAS was configured as a two ring antenna systems both for transmit and receive which were placed at the end positions of the VoI ( Fig 5A ).…”

“…A step in this direction is the so-called travelling-wave (TW) excitation approach that makes use of the RF shield as part of the gradient system in combination with a simple patch antenna [ 20 ]. This approach yields a sufficiently uniform B 1 + field distribution for the imaging of a human extremity (leg) [ 20 ] or for neuroimaging of smaller non-human primates [ 21 ], thus enabling the imaging at VoIs of intermediate size. Unfortunately, the B 1 + transmit efficiency, i.e., the required RF energy to generate the B 1 + field is lower than that of both the body coils conventionally used in clinical 3 T MRI systems [ 22 ] and of the UHF body-part volume resonators of birdcage architecture [ 23 , 24 ].…”

A Review on the RF Coil Designs and Trends for Ultra High Field Magnetic Resonance Imaging

“…The use of traveling wave antennas as RF coils for MRI has been proposed along with patch antennas (90). One of the first trials of to a travel wave system (TWS) used a CP patch antenna that comprised a of polymethylmethacrylate (PMMA) substrate separating the copper plates (91); the substrate measured 20 mm in thickness and 44.0 cm in diameter. The magnet bore was use as a circular waveguide; the bore measured 64 cm in inner diameter and 158 cm in length.…”