Turbo Spin Echo
Similar in concept to EPI, is Turbo Spin Echo (TSE) in which multiple 180° RF pulses are used to continually refocus the decaying Mxy magnetisation. In this way, multiple MRI signals may be recorded from each excitation pulse. (TSE is also known as FSE and RARE.)
A Turbo Spin Echo pulse sequence, in which multiple 180° RF pulses are applied after each excitation pulse. The phase encoding is reset after signal measurement with an equal and opposite magnetic field gradient, and a new phase encoding gradient is applied, for each echo. The envelope of the echo peaks decays with a time constant T2.
As in the EPI readout, an effective echo time for in a sequence like this is defined as the time to the echo which has the least phase encoding.
The fourth echo in the figure above is the nearest to the centre of k-space (no phase encoding). The effective echo time is defined this echo. The TEeff will change for different phase encoding schemes.
Combinations of image acquisition acceleration strategies are possible, such as TGE or TSE with EPI. In each new acceleration method, a tradeoff will occur (e.g. poorer signal-to-noise ratio, increased image artefact prevalence) which must be weighed against the gain in speed.