No 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...

A new method to localize and track medical devices in airfilled body cavities is proposed that uses active microcoils with a semisolid filling. In air spaces, e.g., the lung, microcoils require an independent signal source, which should be made of a biocompatible, solid and sterilizable material with a long shelf time. In a measurement of the T 1 and T* 2 and the relative spin density of several semisolid materials, latex was identified as a suitable material from which a prototype catheter was constructed with a microcoil at its tip. In a dual-echo tracking pulse sequence, the very short T* 2 of the rubber material allowed suppressing the background signal from surrounding tissue with a subtraction technique and additional dephasing gradients. With a roadmapping reconstruction, the microcoil's trajectory could be visualized on a previously acquired reference image set with a tracking rate of up to 60 Hz at a spatial resolution of better than 2mm. In a real-time tracking implementation, an image update rate of 4 Hz was achieved by combining the tracking with a fast real-time imaging sequence. Both methods were successfully applied in vivo to track the catheter in the lung of a pig. Magn Reson Med 64:271-279,