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library:time_to_echo [2026/06/12 15:00] – [What is TE?] scottlibrary:time_to_echo [2026/06/12 15:11] (current) – scott
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 ====TE in Spin Echo==== ====TE in Spin Echo====
  
- 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.+ 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 most tissues except for pure fluids to decay, creating myelographic contrast. 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 40-70 ms range.
  
 These behaviors are relatively similar when considering the Fast Spin Echo, though there are important caveats discussed in more detail  in the [[echo_train_length|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.  These behaviors are relatively similar when considering the Fast Spin Echo, though there are important caveats discussed in more detail  in the [[echo_train_length|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. 
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 ==In and Out of Phase TE== ==In and Out of Phase TE==
  
-When utilizing fast 3D gradient echo sequences, a TE time may no longer be selectable, and instead the options are limited to an 'in-phase' or 'out-of-phase' option. In-phase and out-of-phase describe the relationship between fat and water vectors as they rapidly precess; at certain time points, fat and water vectors may be pointing in the same direction (in-phase) or opposite directions (out-of-phase) and the resulting images will demonstrate characteristic differences. In out-of-phase images, voxels containing both fat and water will will have signals that cancel out, resulting in Chemical Shift artifact. The specific times where fat and water are in and out of phase depend on the field strength, but at 1.5T are approximately 2.2ms, 4.4ms, 6.6ms etc. with the out of phase TE occurring earliest. Unless imaging the abdomen, typically select in-phase whenever possible.+When utilizing fast 3D gradient echo sequences, a TE time may no longer be selectable, and instead the options may be limited to an 'in-phase' or 'out-of-phase' option. In-phase and out-of-phase describe the relationship between fat and water vectors as they rapidly precess; at certain time points, fat and water vectors may be pointing in the same direction (in-phase) or opposite directions (out-of-phase) and the resulting images will demonstrate characteristic differences. In out-of-phase images, voxels containing both fat and water will will have signals that cancel out, resulting in the 'india ink' artifact that looks like a black line outlining most organs. In-phase images will not have the black outlines. The specific times where fat and water are in and out of phase depend on the field strength, but at 1.5T are approximately 2.2ms, 4.4ms, 6.6ms etc with the out of phase TE occurring earliest. Unless imaging the abdomen, typically select in-phase whenever possible for the highest readability.
  
-{{:library:ax_op_fspgr.gif|}}  {{:library:ax_ip_fspgr_2_.gif|}}+===Out of Phase=== 
 + 
 +{{:library:ax_op_fspgr.gif|}}   
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 +===In Phase=== 
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 +{{:library:ax_ip_fspgr_2_.gif|}}