The twisted solenoid RF phase gradient transmit coil for TRASE imaging

[Display omitted] •High spatial resolution MRI can be performed using RF phase gradients only.•A solenoid can be twisted to generate a novel RF phase gradient coil.•The twisted solenoid retains the geometric and efficiency advantages of solenoids.•Power balancing between geometrically decoupled array elements is not necessary. TRASE is an MRI k-space encoding method that uses radio-frequency (RF or B1) transmit phase gradient fields to achieve millimeter-level spatial resolution. Image quality is critically dependent upon the efficient generation of B1 fields with uniform magnitude and strong phase gradients. We present the design of a new family of phase gradient transmit coil based upon a solenoid twisted about a transverse axis. This design has many attractive geometric, electrical and magnetic characteristics, including the capability to spatially encode in the direction of the main static B0 field without obstructing access to the bore. Analytical, numerical simulation and experimental results are presented, including demonstration of 1-dimensional TRASE encoding without the use of PIN diode switches. Twisted solenoid coils significantly expand the capabilities of TRASE MRI.