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Abbreviations Sequences

SPLICE

Split acquisition of fast spin echo signals for diffusion imaging.

In Brief

Acquisition of both a spin echo and a stimulated echo, to allow faster and higher resolution diffusion-weighted imaging (DWI) than single-shot TSE diffusion, as well as lower distortion than EPI-based diffusion imaging.

More Detail

Diffusion imaging with TSE readout violates classical spin-echo (CPMG) conditions, because the large diffusion-weighting gradients cause phase shifts of the transverse magnetization when there is linear motion which, in vivo, is unavoidable. In addition, the 180 degree refocusing pulses are not absolutely perfect across the anatomy. As a result, a complex combination of primary spin echoes, subsequent spin echoes and stimulated echoes are produced. They interfere destructively, reducing SNR, and can produce ghosting artefacts and even signal voids. Therefore, conventional TSE-DWI sequences use modifications to remove sensitivity to phase changes due to motion. In particular, a 90 degree pulse is applied at half the echo time before the first refocusing pulse with the same phase as the refocusing pulses; this rotates the unwanted component of magnetisation to the longitudinal axis where it is invisible in the subsequent readout, leaving a clean signal for measurement.

But this is throwing away a lot of signal.

SPLICE doesn’t do this. First, it replaces the first spin echo with a diffusion-sensitive stimulated echo. (I.e. 90x-90x-90x instead of 90x-180y.) Second, SPLICE uses an imbalanced readout gradient. This separates the spin echo and stimulated echo and by using a longer readout gradient they can be recorded separately: so-called split echo acquisition. Separate images from the two echoes are reconstructed, which means that it doesn’t matter that the CPMG conditions are not met. The two images are finally merged to produce higher SNR than conventional TSE-DWI.

“The split echo acquisition mode does not require any phase relation of the signal phase and the phase of the refocusing pulses. Thus, the modified method works.”

Fritz Schick

Read more about SPLICE: https://doi.org/10.1002/mrm.1910380418

Vendor implementations

Philips: Diffusion TSE XD