Coaxial waveguide MRI

Abstract

As ultrahigh-field MR imaging systems suffer from the standing wave problems of conventional coil designs, the use of antenna systems that generate travelling waves was suggested. As a modification to the original approach, we propose the use of a coaxial waveguide configuration with interrupted inner conductor. This concept can focus the radiofrequency energy to the desired imaging region in the human body and can operate at different Larmor frequencies without hardware modifications, as it is not limited by a lower cut-off frequency. We assessed the potential of the method with a hardware prototype setup that was loaded with a tissue equivalent phantom and operated with imaging areas of different size. Signal and flip angle distributions within the phantom were analyzed, and imaging at different Larmor frequencies was performed. Results were compared to a finite difference time domain simulation of the setup that additionally provides information on the spatial distribution of the specific absorption rate load. Furthermore, simulation results with a human model (virtual family) are presented. It was found that the proposed method can be used for MRI at multiple frequencies, achieving transmission efficiencies similar to other travelling wave approaches but still suffers from several limitations due to the used mode of wave propagation. Magn Reson Med 67:1173-1182, 2012. V C 2011 Wiley Periodials, Inc.Key words: MRI; travelling wave MRI; radiofrequency electronics; waveguides Ultrahigh-field whole-body MRI systems operating at field strengths of B 0 ! 7 T offer higher signal-to-noise ratio (SNR), which enables enhanced image resolution and acquisition speed, while the higher susceptibility contrast is favorable for functional MRI (1). A technical challenge at high field strengths is the radiofrequency (RF) wavelength in tissue which is-other than at 1.5 T-in the order of the size of the anatomical structures. At 7 Tesla, the proton Larmor frequency is 297 MHz, which corresponds to a wavelength of 14 cm in tissue.When conventional transmit RF coils (2,3) are used, standing wave patterns with local signal maxima and minima are observed (4,5). The patterns cause an inhomogeneous RF excitation field and thus lead to a spatially variable image contrast that bears the risk to obscure anatomical or pathological details. For this reason, currently no large volume coil (i.e., body coil) concepts for ultrahigh-field MRI systems are available. A more homogeneous RF excitation over a large field-of-view can be achieved with RF coils with multiple transmit channels (parallel transmission) (6-9); however, this requires new RF systems with dedicated transmit coils in combination with multiple transmitters and power amplifiers.To avoid the standing wave problems, Brunner et al. (10) suggested the use of propagating or travelling waves for whole-body MRI at high magnetic fields. Using the bore of the magnet as a hollow circular waveguide, they transmitted the RF energy from a patch antenna at the far end of the bore. RF transmi...

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“…The lower B 1 + transmission efficiency of the TW excitation is due to a smaller B 1 + filling factor for the volume-of-interest. In comparison with volume resonators, the according B 1 + transmission efficiency decreases—a finding also reported by other groups [30],[31]—which constitutes the main disadvantage of the TW excitation approach. The 37% B 1 + transmission efficiency gap between both imaging approaches increases dramatically to approx.…”

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

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“…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 ]. Usually, birdcage volume coils confine the RF energy in the structure enclosed by the RF coil.…”

Direct imaging of radio-frequency modes via traveling wave magnetic resonance imaging

“…Traveling wave (TW) MRI is a far-field imaging technique that relies on rf mode propagation in a cylindrical magnet bore utilized as an rf waveguide. This approach was originally experimentally demonstrated at 7 T 17 and at clinically relevant field of 3 T 18,19 . However, the main challenge for the TW propagation in most conventional MRI scanners and spectrometers is the relatively small bore radius-to-wavelength ratio (a/λ) even at ultra-high field strengths.…”