TRASE 1D sequence performance in imperfect B₁ fields

Transmit Array Spatial Encoding (TRASE) is an MRI technique that uses radio-frequency (RF) magnetic field (B₁) phase gradients for spatial encoding. A TRASE pulse sequence consists of a long echo train in which each echo samples a different k-space point. Due to the need for accurate refocusing, TRASE imaging performance depends on |B₁| homogeneity. Although the CPMG echo train is often relied on to provide immunity against B₁ flip angle errors, this does not apply to TRASE echo trains. Due to the spatially dependent B₁ phases involved in TRASE imaging, the CPMG condition, where all spins flip about the y-axis in the rotating frame, can only be achieved at one single location within the sample. Moreover, CPMG only preserves one component of the transverse magnetization, the y-component, whereas TRASE requires both components to be retained. Here we investigate the performance of a set of variants of a 1-dimensional (1D) TRASE sequence under conditions of |B₁| errors. We varied the B₁ transmit pulse RF waveform phases in an effort to optimize the TRASE imaging point spread function (PSF). The performance of 256 sequence variants, including those previously reported in the literature was studied. Both Bloch equation simulations and experimental confirmations were completed. Off-resonance (B₀ inhomogeneity) effects were not considered so that the effects of B₁ inhomogeneity alone could be understood. Results show that, using optimum transmit pulse phases, high quality image encoding is achievable over ∼90% of the Nyquist field-of-view (FOV) for a practically realizable variation in B₁ amplitude (Δ|B₁|⩽±11%). This improves significantly upon the performance of a previously-reported sequence which generated ∼75% usable FOV within the Nyquist FOV.