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Abstract

Introduction: MRI is increasingly moving towards higher magnetic field because
of the inherent high sensitivity and greater spectroscopic resolution.
To realize these advantages, the front end, the RF coil and the receive chain,
needs to be optimized. A Quadrature Microstrip Transmission Line (MTL)
resonator was built and SNR is compared with a commercially available
shielded linear birdcage coil. This work also includes development of a TR
switch module (consisting of T/R switch and preamplifier) and a Microstrip
transmission line based Quadrature hybrid.
Subjects and Methods: Experiments were performed on a 16.4T/26cm horizontal
bore Magnex Magnet attached to a Bruker biospec Spectrometer. The
imaging gradient system has an inner diameter of 12cms.
The resonator (Figure 1) is built on Teflon cylinder with wall thickness 5mm.
It consists of 8 equally spaced λ/2 MTL resonators. Electromagnetic coupling
among the 8 resonators makes the entire MTL volume coil resonate at the
desired frequency. In this case, each resonator is tuned to 725MHz to resonate
the volume coil to 705MHz in the unloaded condition.
The TR switch module consists of a diode network and a 2 stage low noise
amplifier (LNA). The first stage of the LNA has PHEMT ATF35143 and the
second stage has BJT BFR193. The TR switch module has a gain of 30dB and
noise figure of 1.1dB at 698MHz.
Results: The losses in the receive chain is reduced by connecting the TR
switch module closer to the birdcage coil. An SNR gain of 1.4X is observed as
shown in Table 1. The MTL resonator has double the volume of the birdcage
resonator and has comparable SNR as that of the birdcage resonator (Table
1). The linear and combined phantom images are shown in Figure 2. Figure
3 shows in-vivo Axial and Sagittal rat brain RARE images with in-plane spatial
resolution of 117μm.
Conclusion: A Quadrature MTL resonator with good SNR and signal uniformity
has been built. This paves the way for developing actively decoupled
resonators and to use smaller receive only surface coils for improved
Sensitivity at 16.4T.