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library:echo_train_length [2026/04/27 16:57] – scottlibrary:echo_train_length [2026/06/12 15:23] (current) – scott
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 ====Selecting the Effective TE==== ====Selecting the Effective TE====
  
-So, which echo is the **best** echo in the echo train? Generally speaking, it is good to select an effective TE near-ish the midpoint of the echo train, at a time appropriate for the desired contrast. Very early echoes do tend to demonstrate some degree of blurring, and greater flow artifacts. TE's around the mid point of have achieved some degree of steady state signal, influences from both stimulated and refocused echo signals, and tend to have sharper edge details. This behavior can even be seen in relatively short ETL's! The gif below is a T1 FSE with 4 echoes, 8ms apart, therefore TE's at 8ms, 16ms, 24ms, and 32ms. All parameters are constant other than the effective TE. Note how the flow artifact from the vessels decreases as the effective TE is longer, and how the edge details become more crisp. The second gif on the right is a T2 FSE with min/max TE's at 10ms and 180ms. Although it's a broccoli, the edge detail changes are still demonstrative of this behavior. Very late TE's may suffer from too much signal loss due to T2 decay. +So, which echo is the **best** echo in the echo train? Generally speaking, it is good to select an effective TE near-ish the midpoint of the echo train, at a time appropriate for the desired contrast. Very early echoes do tend to demonstrate some degree of blurring, and greater flow artifacts. TE's around the mid point of have achieved more signal stability , influences from both stimulated and refocused echo signals, and tend to have sharper edge details. This behavior can even be seen in relatively short ETL's! The gif below is a T1 FSE with 4 echoes, 8ms apart, therefore TE's at 8ms, 16ms, 24ms, and 32ms. All parameters are constant other than the effective TE. Note how the flow artifact from the vessels decreases as the effective TE is longer, and how the edge details become more crisp. The second gif on the right is a T2 FSE with min/max TE's at 10ms and 180ms. Although it's a broccoli, the edge detail changes are still demonstrative of this behavior. Very late TE's may suffer from too much signal loss due to T2 decay. 
  
 {{:library:t1_fse_echoes_1-4.gif}} {{:library:t2_te_changes.gif}} {{:library:t1_fse_echoes_1-4.gif}} {{:library:t2_te_changes.gif}}
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 Single Shot Fast Spin Echo sequences take the ETL to the extreme; instead of the required phase encoded steps being 'chunked' into a neat echo train and then repeated over multiple TR's, HASTE and similar sequences acquire every phase encoding step in a single TR, resulting in an echo train length that is equivalent to the total number of phase encoding steps. In these sequences, if a phase matrix of 256 is chosen, the echo train length may be as long as 256, although this is frequently shorted by a number of different techniques. Parameters that will affect the phase encoded steps , and therefore the echo train length, are as follows: Phase matrix, Phase FOV, Parallel Imaging, and Partial Fourier. Single Shot Fast Spin Echo sequences take the ETL to the extreme; instead of the required phase encoded steps being 'chunked' into a neat echo train and then repeated over multiple TR's, HASTE and similar sequences acquire every phase encoding step in a single TR, resulting in an echo train length that is equivalent to the total number of phase encoding steps. In these sequences, if a phase matrix of 256 is chosen, the echo train length may be as long as 256, although this is frequently shorted by a number of different techniques. Parameters that will affect the phase encoded steps , and therefore the echo train length, are as follows: Phase matrix, Phase FOV, Parallel Imaging, and Partial Fourier.
-For a more detailed review of the HASTE sequence, see __here__.  
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