TWO DIFFERENT CLOCKS
TR repeats. TE listens.
Repetition time, or TR, is the time from one excitation pulse to the next corresponding excitation. It determines how much longitudinal magnetization can recover before the tissue is excited again.
Echo time, or TE, is the time from the excitation pulse to the center of the echo, where the signal is measured. It determines how much transverse decay occurs before the signal is sampled.
Time to repeat
Closely connected to T1 recovery and often to scan time.
RF TO NEXT RFTime to echo
Closely connected to T2 or T2* decay, depending on the sequence.
RF TO ECHOFOLLOW THE TIMELINE
Both begin with excitation, but they end at different events.
“Short” and “long” are relative descriptions. Their meaning depends on the tissue relaxation times, pulse sequence, field strength, and purpose of the examination. There is no single cutoff that applies to every MRI sequence.
CONVENTIONAL SPIN-ECHO WEIGHTING
Timing helps reveal different tissue properties.
Short TR preserves T1 differences. Short TE limits T2 influence.
Long TR reduces T1 influence. Long TE allows T2 differences to appear.
Long TR and short TE reduce both T1 and T2 influence.
These combinations are a foundation for conventional spin-echo thinking. Fast spin echo, gradient echo, inversion recovery, and other sequence families add controls that can change the final contrast.
THE REPEATED PHOTO ANALOGY
TR starts the next setup. TE takes the picture.
Imagine repeatedly preparing a group for a photograph. TR is the wait from one preparation command to the next. TE is the wait from the command until the camera captures that photograph.
Remember: TR is repeat time. TE is echo time.
CHECK YOUR UNDERSTANDING
Know which clock you are changing.
Which timing control determines how much T1 recovery can occur before the next excitation?
TR. A longer TR gives longitudinal magnetization more time to recover before the next corresponding excitation.
Which timing control determines how much transverse decay occurs before the echo is sampled?
TE. A longer TE allows more T2 or T2* decay before measurement, depending on the sequence.
In a conventional spin-echo sequence, which combination supports T2 weighting?
A long TR and a long TE. The long TR reduces T1 influence, while the long TE reveals differences in T2 decay.
LESSON 07 COMPLETE
You understand what TR and TE measure.Educational references
This lesson introduces TR and TE using conventional spin-echo principles. Exact timing choices depend on the sequence, anatomy, field strength, system, and clinical goal.