• Object ID: 00000018WIA3015A870GYZ
  • Topic ID: id_40025400 Version: 1.3
  • Date: Apr 4, 2022 2:33:43 PM

Echo Train Length (ETL)

Echo Train Length (ETL) is the number of echoes per Time to Repeat or Repetition Time (TR) that you prescribe. The ETL determines the amount of image data per TR that fills k-space, thereby decreasing acquisition time.

  • In FSE, the initial 90° pulse is followed by the acquisition of 2 to 128 echoes within a single TR (depending on the system configuration).
  • Long TR acquisitions can be performed in reasonable scan times because the scan time is divided by the ETL.
  • The time between each echo in the echo train is called the echo space.
  • Each echo is acquired with a different phase-encode gradient and each echo fills a unique line of k-space.
Figure 1. FSE PSD with an 8 echo train length
Table 1. FSE PSD image legend
NumberDescription
190° RF pulse
2180° RF pulse
3Echo Space
Figure 2. Each echo of the 8 ETL acquisition fills 8 lines of k-space within a single TR period.

Select an ETL to prescribe the number of echoes acquired per TR.

As ETL increases:

  • Proton Density (PD) and T1 contrast decreases
  • T2 contrast increases
  • number of slices decreases
  • scan time decreases
  • edge blurring increases