A Target Field Based Design of a Phase Gradient Transmit Array for TRASE MRI

Abstract

A target field method approach to the design of RF phase gradient fields, intended for TRASE MRI, produced a superposition of axial currents C_m*sin(m*phi) for m=1,2,3..., and a solenoidal current C_0*z (m=0), where C_m are constants. Omission of terms m>2 produced a phase gradient field with a linear phase and uniform magnitude within a target ROI of 2.5 cm diameter. A set of three RF coils (uniform birdcage, gradient mode birdcage, and 4-loop Maxwell) was found to be sufficient to generate both positive and negative x and y phase gradients. In addition, the phase gradient amplitude can be controlled by simply adjusting the power split to the three RF component coils. Bench measurements of an experimentally constructed 1.8 deg/mm transverse phase gradient showed excellent agreement with predicted results. A linear phase and magnitude within ± 4% of the median value was achieved within the ROI.