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MRI Sequences

Ultrafast spin echo sequences

by Denis Hoa

Type of sequence Philips Siemens GE Hitachi Toshiba
Ultrafast SE SSH-TSE
UFSE
SSTSE
HASTE
SS-FSE FSE - ADA (Super)FASE
DIET

 

Generic diagram

 

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The echo train technique can be pushed to the limit to fill the entire Fourier plane with a single 90° pulse (TR is thus infinite) . These so-called « single-shot » sequences require the successive application of as many 180° pulses as there are k-space lines to fill.
The sequence can be further accelerated, avoiding the need to register the latest echoes (whose signal is much reduced by T2 relaxation) by partial k-space acquisition. Just over half the k-space lines are actually acquired and the missing lines are calculated using k-space symmetry properties. This reduces acquisition time by a factor close to 2, but to the detriment of the signal to noise ratio of the image.

 

 

Contrast and scan time

Given the length of the echo train, the images obtained are highly T2 weighted, since the majority of k-space lines are filled with long TE echoes.
With this type of sequences, a slice can be made in under a second.

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Duration of an ultrafast spin echo sequence

 

Duration = TE • number of phase encodings to acquire

 

Interest and limits

These sequences are well adapted to imaging non-circulating liquid structures appearing as a T2-weighted hypersignal (Cholangio-MRI and Uro-MRI) . Due to their rapidity, they have low sensitivity to movement and are compatible with apnea (mobile structures: liver, abdomen, heart).
The negative impact of very long echo trains is a decay in signal to noise ratio (weak signal from late echoes and very high effective TE) with low spatial resolution and blurring in the phase encoding direction.