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library:echo_train_length [2025/06/16 20:53] – scottlibrary:echo_train_length [2025/06/18 03:03] (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 TR. This page will primarily focus on echo train length as used in the fast spin echo. See __HASTE__ or __3D FSE__ pages for more explanation of ETL in those sequences.+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. 
  
 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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 ====Echo Train Length and Effective TE==== ====Echo Train Length and Effective TE====
  
-When using a fast spin echo, the TE parameter behaves differently than in the traditional spin echo. The selected TE is now one of multiple echoes, and is placed at the center of k-space which allows it to dominate the image contrast; this is known as the 'Effective' TE. Unlike a traditional spin echo where there is only 'waiting time' before or after the TE, the fast spin echo will always start with the shortest TE and extend out to the end of the selected ETL. For example, in a fast spin echo where the shortest echo is 10ms and there is a 10ms space between each echo, an ETL of 12 will result in echoes at 10ms, 20ms, 30ms, 40ms, etc until 120ms. Any of those echoes can be selected as the effective TE and placed at the center of k-space. There a few important things to take away from this behavior:+When using a fast spin echo, the [[time_to_echo|TE]] parameter behaves differently than in the traditional spin echo. The selected TE is now one of multiple echoes, and is placed at the center of k-space which allows it to dominate the image contrast; this is known as the 'Effective' TE. Unlike a traditional spin echo where there is only 'waiting time' before or after the TE, the fast spin echo will always start with the shortest TE and extend out to the end of the selected ETL. For example, in a fast spin echo where the shortest echo is 10ms and there is a 10ms space between each echo, an ETL of 12 will result in echoes at 10ms, 20ms, 30ms, 40ms, etc until 120ms. Any of those echoes can be selected as the effective TE and placed at the center of k-space. There a few important things to take away from this behavior:
  
   - There are multiple echoes at varying TE values that will be acquired during a fast spin echo, always   - There are multiple echoes at varying TE values that will be acquired during a fast spin echo, always