Differences

This shows you the differences between two versions of the page.

--- library:echo_train_length [2025/03/25 00:11] – [Echo Train Length and Effective TE] scott
+++ library:echo_train_length [2025/03/31 15:06] (current) – [Selecting an appropriate ETL] scott
@@ Line 19: / Line 19: @@
 ====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.
+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.
 {{:library:etl_13-64_6_.gif|}}
@@ Line 27: / Line 27: @@
   * T1 Contrast: ETL 2-4, Maximum TE ~30ms
   * PD Contrast: ETL 7-11, Maximum TE ~100ms
-  * T2 Contrast: ETL 16-24, Maximum TE ~250ms
+  * T2 Contrast: ETL 16-24, Maximum TE ~200ms
 ====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**, as not all echoes are created equally. 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, have both stimulated and refocused echo signals, and tend to have sharper edge details. Very late TE's in the ETL may suffer from too much signal loss due to T2 decay. 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 is a broccoli, the edge detail changes are still demonstrative of this behavior.
+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, have both stimulated and refocused echo signals, and tend to have sharper edge details. Very late TE's in a long ETL may suffer from too much signal loss due to T2 decay. 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 is a broccoli, the edge detail changes are still demonstrative of this behavior.
 {{:library:t1_fse_echoes_1-4.gif}} {{:library:t2_te_changes.gif}}
@@ Line 41: / Line 41: @@
 {{:library:t1_etl_and_te.gif}}
+====ETL and Specific Absorption Rate (SAR)====
+The SAR model used by most scanners tends to be inaccurate in the setting of small animal scanning, in part due to the much smaller patient weights. Typically the ETL is comprised of 180 degree refocusing pulses that will contribute to patient heating and lead to high SAR calculations, which may force the scanner to pause in the middle of a sequence. To reduce actual and estimated patient heating with small animals, it is often necessary to reduce this refocusing flip angle. In practice, this parameter change will have relatively little effect on image contrast and can be as low as 110 degrees. In T2 weighted sequences the longer TR will tend to balance out the RF heating from the refocusing pulses, but this is not the case with the shorter TR of T1 weighted sequences, especially if multiple T1 weighted sequences are to be run in succession.
 ====Indirect Parameter Effects====
@@ Line 48: / Line 52: @@
 ===Bandwidth===
-The most influential secondary parameter will be receiver bandwidth. This parameter will affect the sampling rate, and therefore how long it takes to sample each echo. As bandwidth is increased, the length of time taken to fully sample an echo will decrease. This will have the effect of reducing the time between each echo, known as the __echo spacing__, which will also reduce the maximum and minimum TE's. This will also have the effect of reducing chemical shift artifact and improving edge details. For most older scanners, a good guideline is to try and keep the echo spacing around 8-12ms. For more information on bandwidth, see __here__.
+The most influential secondary parameter will be receiver bandwidth. This parameter will affect the sampling rate, and therefore how long it takes to fully sample each echo. As bandwidth is increased, the length of time taken to fully sample an echo will decrease. This will have the effect of reducing the time between each echo, known as the __echo spacing__, which will also reduce the maximum and minimum TE's. This will also have the effect of reducing chemical shift artifact and improving edge details. For most older scanners, a good guideline is to try and keep the echo spacing around 8-12ms. For more information on bandwidth, see __here__.
 ===Frequency Encoding Matrix===
-The frequency encoding matrix will determine how many samples are to be taken within the FOV and at a rate determined by the bandwidth. If the frequency matrix is increased, the time needed to fully sample the echo is increased, so the echo spacing and maximum TE will be increased.
+The frequency encoding matrix will determine how many samples are to be taken within the FOV at a rate determined by the bandwidth. If the frequency matrix is increased, the time needed to fully sample the echo is increased, so the echo spacing and maximum TE will be increased.
 ===Special cases: Single Shot Imaging with HASTE===