scan time equation

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scan time equation

Postby jschedel » Thu Dec 02, 2010 3:23 am

Taking my MR registry. Can anyone give me an EASY mathematical equation to calculate scan time? Thanks!
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Re: scan time equation

Postby Dave Higgins » Thu Dec 02, 2010 3:39 pm

Ask yourself:
  • How may phase encoding steps are there?
  • How long is each TR?
  • How many phase encoding steps will I acquire per TR?
...and you should be able to work it out.

Simple 2D spin echo or gradient echo:

Scan time = TR·Ny·NSA

(TR: repetition time; Ny: number of phase encoding steps; NSA: number of averages)

For 3D, just multiply by number of phase encoding steps in z:

Scan time = TR·Ny·Nz·NSA

For Turbo Spin Echo, or Turbo Gradient Echo, or EPI, you'll acquire more than one line per TR, so just divide by the echo train length (aka turbofactor in TSE or TGE):

Scan time = TR·Ny·NSA/ETL

After that things can get more tricky (e.g. combining TGE and EPI) (TR·Ny·NSA/(ETLTGE·ETLEPI)). Plus there are other parameters which affect the Ny and Nz which you need to acquire (phase oversampling, parallel imaging, partial Fourier). Maybe that's more advanced than the questions you'll get?

Here "scan time" means data acquisition time, excluding the few seconds the scanner takes to prepare to acquire data after you press Start Scan.

Dave
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Re: scan time equation

Postby jschedel » Thu Dec 02, 2010 4:54 pm

Perfect! Thank you!
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