MORE THAN ONE ECHO PER TR
Echo train length tells us how many echoes help fill k-space after one excitation.
In fast or turbo spin echo, one excitation is followed by multiple refocusing pulses. Each resulting echo can fill a different k-space line. The number of echoes collected in the train is commonly called echo train length, ETL, or turbo factor.
Echo spacing is the time between neighboring echoes. Effective TE is associated with the echo that contributes to the center of k-space and strongly influences image contrast.
THE ECHO TRAIN
Later echoes do not carry the same signal as earlier echoes.
EARLY ECHOEST2 decay across the trainLATE ECHOES
As the train continues, transverse signal changes because of T2 decay and the refocusing-pulse pathway. Different k-space lines therefore receive different signal weighting. Long trains, especially with long echo spacing, can create T2-related blurring and alter edge appearance.
SPEED NEEDS CONTROL
ETL and bandwidth shape the same acquisition from different directions.
Basic FSE scan time falls as more phase-encoding lines are collected each TR. The exact gain depends on the full acquisition.
T2 decay and stimulated-echo pathways can increase blurring and change contrast or edge definition.
Wider receiver bandwidth can shorten readout and echo spacing, reduce chemical shift and off-resonance distortion, and permit a shorter minimum TE.
Higher bandwidth reduces SNR when other conditions are held constant. Lower bandwidth gains SNR but lengthens sampling and may increase displacement or blurring.
Echo ordering determines which echo fills central k-space. Effective TE is therefore not simply “the last echo” or “the middle echo” in every implementation. Sequence design and vendor ordering matter.
THE DELIVERY TRAIN
Each echo delivers one package of k-space data.
A longer train delivers more packages after one departure, so the job finishes sooner. But the packages change as the trip continues because signal decays. Shorter spacing gets each delivery to its stop sooner.
How many echoes are in the train?
How much time separates the echoes?
Which echo fills central k-space?
Remember: More echoes save time; shorter spacing protects sharpness.
CHECK YOUR UNDERSTANDING
Follow the echo train from speed to image quality.
Why does a higher ETL usually shorten a basic FSE acquisition?
More phase-encoding lines are collected after each excitation, so fewer TR cycles are needed to fill k-space.
Why can a very long echo train blur the image?
Signal changes across the train because of T2 decay and refocusing pathways. This weights k-space lines differently and can smooth detail in the phase direction.
What may happen when receiver bandwidth increases?
SNR generally decreases, while chemical-shift displacement and off-resonance distortion may improve. Readout and echo spacing can shorten, which may reduce blurring and permit a shorter TE.
LESSON 26 COMPLETE
You understand how ETL, echo spacing, effective TE, and bandwidth work together.Educational references
ETL, turbo factor, echo spacing, bandwidth, and echo ordering vary by sequence and vendor. Follow approved protocols and manufacturer guidance.