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Leina:
Robert has given some reasons to acquire several slices and use at least a moderate TR of ~1 s. You want to make sure that TR > 3T2, otherwise you may get unwanted echoes interfering with your data. This sets a minimum TR ~ 300 ms. The following arguments consider only SNR, not unwanted echoes or the number of slices. This SNR is for averaging multiple acquisitions, but the results should be applicable to multiple acquisitions that are processed separately instead of being averaged. For more information, see [Kingsley PB. Signal intensities and T1 calculations in multiple-echo sequences with imperfect pulses. Concepts Magn Reson 1999; 11: 29-49.]
No matter how many slices you choose to acquire, once you have chosen a TR, the optimum SNR is given by the Ernst angle, cos(theta) = exp(-TR/T1). [Ernst RR, Anderson WA. Application of Fourier transform spectroscopy to magnetic resonance. Rev Sci Instr 1966; 37: 93-102.] Some approximate results are
TR/T1 :4 2 1 0.5 0.3
angle (degrees) :89 82 68 53 42
Alternatively, you can set a flip angle, then choose the TR/T1 that gives the best SNR. Some approximate results are
angle (degrees) :90 60 45 30
TR/T1 :1.25 0.86 0.77 0.72
Notice that setting TR/T1 and calculating the optimum theta does not give the same result as setting theta and calculating the optimum TR/T1.
If you are considering a range of T1, such as WM ~ 700 and GM ~ 1200, I suggest using the longer T1 in the Ernst formula.
You can compare the "average SNR per unit time" for any given pair of theta and TR/T1 from the formula
SNR ~ {sin(theta)[1-exp(-TR/T1)]/[1-cos(theta)*exp(-TR/T1)]}/sqrt(TR)
These formulas apply to a single-pulse (gradient echo) acquisition. For spin-echo, see the Concepts Magn Reson paper for more details.
-----
Peter B. Kingsley, MRI Physicist [log in to unmask]
Department of Radiology / MRI
North Shore University Hospital
300 Community Drive, Manhasset, NY 11030, USA
Tel (516) 562-2842 Fax (516) 562-3561
-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Robert Welsh
Sent: Thursday, November 03, 2005 9:46 PM
To: [log in to unmask]
Subject: Re: [SPM] about TR
Leina,
You should look at your experimental design and let that drive your
choice of TR in part. If you have an event related design where the
duration of the stimulus is short then you may opt for a shorter TR.
Since you are only acquiring a single slice you can go quite short,
250ms or 500ms. But I would actually advise taking a few more slices,
very few studies now days get just a single slice, and in fact usually
the reason to get a single slice is driven by a desire to acquire them
very rapidly. Typically an echo planar (and spiral) acquisition per
slice is on order 20-45 ms. If you don't need to acquire quickly then
get move coverage. The coverage is especially good for any realignment
and coregistrations etc.
In much of cognitive neuroscience literature experiments typically have
a TR or 2 or 3 seconds (state of the art now days) which 30-40 slices.
You should match you flip angle according to the TR. Short TR, less of a
flip, as Peter has indicated.
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Robert C. Welsh, PhD
Research Investigator
Department of Radiology
University of Michigan
(734) - 764 - 2412 (fax)
[log in to unmask]
>>> LeiNa Hua <[log in to unmask]> 11/03/05 8:35 PM >>>
Hi SPMer,hi Peter:
When we want to aquire single slice,How should we choose the TR? It
also
need TR ~3-4T1?
Thanks
Leina
On 11/3/05, Kingsley, Peter <[log in to unmask]> wrote:
>
> It depends how long. With 90-degree pulses, "average" SNR per unit
time is
> best when TR/T1 ~ 1.25, so you certainly don't want to use TR <
1.25*T1unless you use a lower flip angle. At
> 1.5 T, T1 is ~700 ms for white matter and ~1200 ms for gray matter,
and
> these numbers increase by 10-20% at 3 T. Therefore, for gray matter,
TR
> should be at least 1.5 s at 1.5 T and 2 s at 3 T. Beyond 4-6 seconds,
you
> will not gain much SNR because you are already ~3-4x T1 for gray
matter, and
> therefore almost fully relaxed. A longer TR allows more slices, but I
see no
> advantage in setting TR much longer than needed, so TR usually is
chosen to
> be between 1.5 s and 5 s.
>
> -----
> Peter B. Kingsley, MRI Physicist [log in to unmask]
> Tel (516) 562-2842 Fax (516) 562-3561
>
>
> ------------------------------
> *From:* SPM (Statistical Parametric Mapping)
[mailto:[log in to unmask]] *On
> Behalf Of *chin wei
> *Sent:* Thursday, November 03, 2005 6:01 AM
> *To:* [log in to unmask]
> *Subject:* [SPM] about TR
>
> hi all;
> Who can tell me what is the advantages and disadvantages of a long TR
> time in EPI scan?
> thanks in advance
>
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