Electromagnetic exploration of a radiative antenna for 7T pelvic MR imaging

Abstract

Although an obvious (signal-to-noise ratio) SNR gain due to higher field is observed in 7T (magnetic resonance) MR images, the concomitant higher (radiofrequency) RF frequency of these systems results in strong RF attenuation, B1+ field inhomogeneity and increased (specific absorption rate) SAR levels. These challenges originate from increased electromagnetic interferences and dampening of the RF fields in the subject caused by shortened RF wavelengths. The design of the RF coil can be altered to mitigate these issues to a certain extent. Conventional designs for RF transmit coils used for body imaging in ultra-high field systems are based upon adapted "low field" designs. These coils are operated as near field antennas which are designed to deliver a high magnetic field in the near field by maximizing the currents on the coil by tuning it to resonance. However, for 7 T body MR imaging, the target is no longer in reactive near field. This results in a sub-optimal penetration of the fields in deeply situated target regions. This thesis considers an alternative antenna design to the classic, reactive coil. The radiative antenna achieves an efficient signal penetration outside the near field by directing the Poynting vector towards the target location. The aim of this thesis is to investigate various RF aspects regarding this new antenna design for 7 T body MR imaging. As the radiative antenna is compared with the microstrip coil in MR measurements and electromagnetic simulations, the radiative antenna yields a factor of two higher B1+ and SNR at the deep regions and shows low local SAR levels. The radiative antenna with a relative permittivity of 90, and with a size of 50 mm width and length is the ideal one to reach the optimal local SAR/B1+ ratio. For the 8 channel radiative antenna array, complex electric field contributions of neighboring elements requires full amplitude/phase monitoring of the full array for the subject-specific local SAR assessment. Four human body subjects with various muscle/fat ratio were simulated with the radiative antenna array to investigate the worst-case shimmed local SAR values. Subjects with a large BMI are experiencing lower local SAR levels, explained by the larger electrical spacing effect of the thicker subcutaneous fat layer. But, still local SAR/(B1+ )2 ratio is higher for more obese model compared to slim models. These findings indicate that for a worst-case local SAR, SAR setting derived from a slim patient model should be employed. We also investigated the various concepts known from optics and radio-communications, such as the critical angle, surface, evanescent and travelling waves with a 7 T MR imaging and with electromagnetic simulations. For this purpose, horizontal and vertical loop coil and horizontal electric dipole were placed on an air/ethylene glycol (EG), air/oil and air/water/EG layers. The critical angle and evanescent waves were observed in the MR images and simulations in the lower layer of the air/EG and air/oil. We observed that surface waves are excited in the water layer of air/water/EG. They may have role in enlarging the lateral FOV.