Time to Echo (TE)

TE, meaning Time to Echo, is a user selectable parameter which determines when the MR signal should be sampled. Across different pulse sequences, TE selections can vary greatly, but generally determine how much dephasing is allowed to occur with the transverse magnetization vectors.

The Spin Echo is the most straightforward example: TE will control how much T2 decay processes can influence image contrast. Long TE's (80-120) will allow a moderate amount of T2 decay to occur, allowing tissues with short T2 times to lose signal and become dark, creating contrast with long T2 tissues. Short TE's (<30) will restrict the amount of decay allowed, and reduce the influence of T2 decay processes to influence the image. Very long TE's (500+) will allow all tissues except for pure fluids to decay, creating myelographic contrast. As more T2 decay is allowed to occur, SNR will decrease; longer TE sequences will have less SNR than short TE sequences, all other parameters held constant. For this reason, sequences with inherently low SNR like STIR or T2 Fat sat may use reduced TE's in the 40-60 range.

These behaviors are relatively similar when considering the Fast Spin Echo, though there are important caveats discussed in more detail here in the ETL section. Note in the image series below as TE is increased from 15-500 how different tissues decay at different rates, moving the contrast from Proton Density through to clinical T2 ranges, to very heavily T2 weighted myelographic contrast.

The most commonly used gradient echo sequences are rapid steady-state 3D sequences, or T2* weighted sequences for detecting hemorrhage. The selectable TE's will vary, depending on the sequence chosen, and will be discussed in more detail below:

Without the strong refocusing pulses of a fast spin echo, transverse magnetization will decay rapidly due to the compounding effect of both T2 and T2* processes. This much shorter decay time means that TE's need to be much shorter, generally less than 30ms, or all the signal will have been lost.