The echo time (TE) represents the time from the center of the RF-pulse to the center of the echo. For pulse sequences with multiple echoes between each RF pulse, several echo times may be defined and are commonly noted TE1, TE2, TE3, etc.
The repetition time (TR) is the length of time between corresponding consecutive points on a repeating series of pulses and echoes.
Advanced Discussion (show/hide)»
The simple definition for TE found in the ACR Glossary and other sources is not always so easy to apply.
Asymmetric RF pulses. The "middle of the RF pulse" is an older definition for symmetrical pulses, but what about the bizarrely shaped adiabatic and SLR-generated pulses used in many of today's sequences?
Asymmetric echoes. Here it is better to say the echo time ends at the center of k-space rather than the center of the echo. Even still there is a problem if the area covered by the echo is out in some corner of k-space and does not include any axis or point of symmetry.
Sequences where the peaks of the spin echo and gradient echo components do not coincide. Here the default is usually to use the center of the GRE rather than the SE to define echo time.
Hendrick RE (ed). Glossary of MR Terms (5th ed). Reston, VA: American College of Radiology, 2005.
How do you produce multiple GRE's from a single pulse?