Highly Accelerated Parallel Imaging Using Rotating Radiofrequency Coil Array at 7 T

Introduction: Previous studies have shown that the approach of sensitivity encoding using a mechanically rotating radiofrequency coil (RRFC) is capable of emulating a large number of decoupled coil elements [1, 2]. Recently, the RRFC concept has been successfully combined with phased-array coil (PACs) techniques to further improve the sensitivity encoding capability, using a naturally-decoupled RF coil array. A novel 4-element RRFC array (RRFCA) has shown to have better imaging acceleration capability than PACs with twice as many coil elements at 3T [3]. An enhanced acceleration capability of RRFCA is expected at higher fields as two advantages of the RRFCA can be strengthened: (1) the mutual coupling between RRFCA elements is dramatically reduced due to the shorter RF wavelength and (2) the B1 field (sensitivity profiles) become more inhomogeneous, which has the potential to improve encoding capability. In this novel study, the potential of RRFCA to accelerate imaging at 7T was investigated. A 4-element RRFCA in different operating modes was compared with an 8-element stationary PACs at various reduction factors. Method: The RRFCA consists of 4 identical coil elements evenly distributed around the cylindrical coil former of 280 mm in diameter. The length of each coil is 170 mm (longitudinal) with a subtended angle of 35 ̊. This arrangement naturally endows the excellent electromagnetic isolation between any two coil elements. After loading a 250 mm diameter homogeneous spherical phantom with εr=50.5 and σ=0.65 S/m (average of brain tissue at 300 MHz), all the mutual couplings are under 31dB without using any decoupling circuitry. This is much improved compared with -15dB mutual coupling of RRFCA elements at 3T. The RRFCA and phantom geometry are shown in Fig.1. The modelling and electromagnetic solutions were performed with the commercially available software package FEKO (EMSS, SA). The circular polarization of steady-state RF magnetic field (B1) was calculated according to [4]. In previous works [1-3], the RRFC was actuated pneumatically and rotated about the subject with constant velocity in fast-mode (typically over 1500rpm). Since the acquisition time (TACQ) is comparable to the rotational period, within the time of sampling each kspace line, acquisition experiences time-varying sensitivity profiles as shown in Fig. 2 from point A to B and C to D. However, fast rotating may bring multiple practical issues, which can be avoided by actuating RRFCA with a non-magnetic motor. In this way, the RRFCA sample at certain points (φ=0) in a stepped fashion with precise position-control, such as at point A and C. Compared with conventional PACs, the RRFCA provides the ability of encoding with much larger number of sensitivity profiles, in both fastand steppingmodes. The fast-mode provides us additional capability of optimising the sensitivity encoding performance, by adjusting the rotating speed (ω) in relation to the imaging parameters, such as repetition time (TR) and TACQ. The optimal speed of revolution can be obtained as following functions in Fast Low Angle Shot (FLASH) imaging sequence: