Fast spin‐echo approach for accelerated B₁ gradient–based MRI

Abstract

Purpose To expand on the previously developed B1+$$ {\mathrm{B}}_1^{+} $$‐encoding technique, frequency‐modulated Rabi‐encoded echoes (FREE), to perform accelerated image acquisition by collecting multiple lines of k‐space in an echo train. Methods FREE uses adiabatic full‐passage pulses and a spatially varying RF field to encode unique spatial information without the use of traditional B0 gradients. The original implementation relied on acquiring single lines of k‐space, leading to long acquisitions. In this work, an acceleration scheme is presented in which multiple echoes are acquired in a single shot, analogous to conventional fast spin‐echo sequences. Theoretical analysis and computer simulations investigated the feasibility of this approach and presented a framework to analyze important imaging parameters of FREE‐based sequences. Experimentally, the multi‐echo approach was compared with conventional phase‐encoded images of the human visual cortex using a simple surface transceiver coil. Finally, different contrasts demonstrated the clinical versatility of the new accelerated sequence. Results Images were acquired with an acceleration factor of 3.9, compared with the previous implementation of FREE, without exceeding specific absorption rate limits. Different contrasts can easily be acquired without major modifications, including inversion recovery–type images. Conclusion FREE initially illustrated the feasibility of performing slice‐selective 2D imaging of the human brain without the need for a B0 gradient along the y‐direction. The multi‐echo version maintains the advantages that B1+$$ {\mathrm{B}}_1^{+} $$ encoding provides but represents an important step toward improving the clinical feasibility of such sequences. Additional acceleration and more advanced reconstruction techniques could further improve the clinical viability of FREE‐based techniques.

Correcting image distortions from a nonlinear B1+$$ {\boldsymbol{B}}_{\mathbf{1}}^{+} $$‐gradient field in frequency‐modulated Rabi‐encoded echoes

“…As detailed in the Theory section, this can be avoided by using the highest resolution grid. This issue potentially can also be solved by using a model‐based approach which reconstructs each pixel to its true resolution 26 . Another limitation of the used approach is that it requires the $$$ {B}_1^{+} $$$‐gradient field to be monotonic.…”

“…This issue potentially can also be solved by using a model-based approach which reconstructs each pixel to its true resolution. 26 Another limitation of the used approach is that it requires the B + 1 -gradient field to be monotonic. This work also suggests that, at 64 MHz ( 1 H frequency at 1.5T), the Biot-Savart law may suffice for estimating a B + 1 map, as needed for correction of image distortion and intensity variation.…”