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library:echo_train_length [2025/06/18 03:03] – scottlibrary:echo_train_length [2026/06/12 15:23] (current) – scott
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 ====What is Echo Train Length==== ====What is Echo Train Length====
  
-Echo train length (ETL), also known as turbo factor, determines the number of additional refocused echoes to be acquired in a spin echo based sequence and is used to decrease scan time. Depending on the sequence, this number can be selected by the user (as in FSE) or is controlled indirectly (as in HASTE) through other parameters such as phase encoding steps. When selecting an echo train length, it is important to remember that the refocused echo is created from an additional refocusing RF pulse. As will be discussed further down, this can have implications for controlling SAR and [[time_to_repeat|TR]]. This page will primarily focus on echo train length as used in the fast spin echo. +Echo train length (ETL), also known as turbo factor, determines the number of additional refocused echoes to be acquired in a spin echo based sequence and is used to decrease scan time. Depending on the sequence, this number can be selected by the user (as in FSE) or is controlled indirectly (as in HASTE) through other parameters such as phase encoding steps. When selecting an echo train length, it is important to remember that the refocused echo is created from an additional refocusing RF pulse. As will be discussed further down, this can have implications for controlling [[specific_absorption_rate|SAR]] and [[time_to_repeat|TR]]. This page will primarily focus on echo train length as used in the fast spin echo. 
  
 Consider 1 slice of a T2 weighted spin echo sequence with a phase encoding matrix of 256; to fully acquire the image 256 lines of k-space are needed, and will be acquired 1 line at a time. For an example, if the TR is 3000ms, this will need to be repeated 256 times: 3000msx256 = 768,000ms or ~12.8 minutes. This is quite inefficient, as only 1 echo is acquired with each TR, and the rest of the TR is just spent waiting. To reduce imaging time, let's acquire 16 echoes within that 3000ms TR, the math now looks like this: (3000msx256)/16 = 48,000ms or ~48 seconds. Much faster!  Consider 1 slice of a T2 weighted spin echo sequence with a phase encoding matrix of 256; to fully acquire the image 256 lines of k-space are needed, and will be acquired 1 line at a time. For an example, if the TR is 3000ms, this will need to be repeated 256 times: 3000msx256 = 768,000ms or ~12.8 minutes. This is quite inefficient, as only 1 echo is acquired with each TR, and the rest of the TR is just spent waiting. To reduce imaging time, let's acquire 16 echoes within that 3000ms TR, the math now looks like this: (3000msx256)/16 = 48,000ms or ~48 seconds. Much faster! 
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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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