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library:echo_train_length [2026/06/12 15:20] – 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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 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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