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library:echo_train_length [2026/04/27 16:57] – scottlibrary:echo_train_length [2026/07/20 13:42] (current) – scott
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 ====Selecting an appropriate ETL==== ====Selecting an appropriate ETL====
  
-When selecting the ETL, it is important to consider the contributions of all the echoes within the ETL to image quality, as there are some special behaviors that occur with trains of RF pulses and echoes. Very long ETL's will result in echoes that may extend out far beyond the desired effective TE. The further out the echo, the more T2 decay will occur and the 'quality' of the echo will degrade and contribute to image blurring. Additionally, a long ETL means that many more refocusing pulses will be required; this will increase SAR and also lead to some degree of tissue saturation and magnetization transfer effects. Long ETL's also take up significantly more space within the TR, so there will be a tipping point where increasing ETL will no long decrease imaging time, as the TR will have to be increased to make room for all the additional echoes. Below is an example of a T2 fast spin echo with different ETLs: 13, 24, 32, 64. Notice how edge details rapidly become blurred as the ETL is increased, and the blurring is preferentially in the phase encoding direction. Keep an eye on the maximum TE when selecting the ETL, it will increase rapidly as ETL is increased. In general, select the shortest ETL possible to achieve a reasonably short scan time and appropriately selectable TE's.+When selecting the ETL, it is important to consider the contributions of all the echoes within the ETL to image quality, as there are some special behaviors that occur with trains of RF pulses and echoes. Very long ETL's will result in echoes that may extend out far beyond the desired effective TE. The further out the echo, the more T2 decay will occur and the 'quality' of the echo will degrade and contribute to image blurring. Additionally, a long ETL means that many more refocusing pulses will be required; this will increase SAR and also lead to some degree of tissue saturation and magnetization transfer effects. Long ETL's also take up significantly more space within the TR, so there will be a tipping point where increasing ETL will no long decrease imaging time, as the TR will have to be increased to make room for all the additional echoes. Below is an example of a T2 fast spin echo with different ETLs: 13, 24, 32, 64. Notice how edge details rapidly become blurred as the ETL is increased, and the blurring is prmarily in the phase encoding direction. Keep an eye on the maximum TE when selecting the ETL, it will increase rapidly as ETL is increased. In general, select the shortest ETL possible to achieve a reasonably short scan time and appropriately selectable TE's.
  
 {{:library:etl_13-64_6_.gif|}}  {{:library:etl_13-64_6_.gif|}} 
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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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