Abstract
The technology of magnetic resonance imaging is developing towards higher magnetic fields to improve resolution and contrast. However, whole-body imaging at 7 T or even higher flux densities remains challenging due to wave interference, tissue inhomogeneities, and high RF power deposition. Nowadays, proper RF excitation of a human body in prostate and cardiac MRI is only possible to achieve by using phased arrays of antennas attached to the body (so-called surface coils). Due to safety concerns, the design of such coils aims at minimization of the local specific absorption rate (SAR), keeping the highest possible RF signal in the region of interest. Most previously demonstrated approaches were based on resonant structures such as e.g. dipoles, capacitively-loaded loops, TEM-line sections. In this study, we show that there is a better compromise between the transmit signal {{bf{B}}}_{{bf{1}}}^{{boldsymbol{+}}} and the local SAR using non-resonant surface coils generating a low electric field in the proximity of their conductors. With this aim, we propose and experimentally demonstrate a leaky-wave antenna implemented as a periodically-slotted microstrip transmission line. Due to its non-resonant radiation, it induces only slightly over half the peak local SAR compared to a state-of-the-art dipole antenna but has the same transmit efficiency in prostate imaging at 7 T. Unlike other antennas for MRI, the leaky-wave antenna does not require to be tuned and matched when placed on a body, which makes it easy-to-use in prostate imaging at 7 T MRI.
Highlights
The technology of magnetic resonance imaging is developing towards higher magnetic fields to improve resolution and contrast
Their operation is based on the excitation of standing waves. This approach necessarily results in the excitation of strong reactive electric and magnetic fields in a coil’s vicinity. It is well-known in antenna engineering that the higher the Q-factor, the stronger the near reactive fields, i.e., the magnetic fields directly created by currents and electric fields directly created by charges
In ultrahigh field body imaging, the magnetic components in the reactive near field are concentrated near the surface coil and, in contrast to radiative fields, do not contribute to the signal in deeply located region of interest (ROI)
Summary
The technology of magnetic resonance imaging is developing towards higher magnetic fields to improve resolution and contrast. Unlike birdcage coils used for whole-body RFexcitation at the low frequencies of clinical scanners[10], pTx allows for the manipulation of the transmit field distribution This is achieved by using multiple surface coils placed directly onto a body and driven with customized phases and amplitudes, which allows steering the signal voids away from the region of interest. This approach is still not allowed in clinical MRI as it typically requires careful preliminary determination of individual transmit phases and constancy of coil tuning and matching for each subject Another limitation is the potentially high peak local specific absorption rate (SAR) created by each antenna element typically due to its proximity to the body surface. Solving the SAR issue, which is necessary for the potential clinical application of pTx, is strongly related to the design of surface coil elements
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