Zoltan:
I don't know much about segmentation, but I have done quite a bit of T1 mapping. I am not sure exactly what you are asking. Are you asking for the best 3D parameters to segment pia and dura? Or for the T1 times of pia and dura? Or how to measure the T1 times of pia and dura? I hope the following comments answer some of your questions.
To measure T1 in brain, I suggest a single-slice IR sequence with fast-SE acquisition, TE as short as possible, TR = 2500 (~2x your longest T1), and TI times of 50-100 (as short as possible), 500, 900, and ~2400-2900 (almost as long as possible for your TR). With FSE acquisition, echo train length = 4, and 128 phase encode steps with 3/4 FOV = 96 PE steps, this takes ~1-1.5 min per TI, or ~4-6 min per slice for 4 TIs. This method is insensitive to slice profiles and imperfect flip angles.
If you can estimate the T1 times of pia and dura, you can use the Ernst formula to maximize the signal for a given T1,
cos(theta) = exp(-TR/T1)
The Ernst angle should maximize the signal intensity (SI) for the selected T1 at a given TR, and I think it should also optimize contrast near the selected T1. You can confirm this by calculating the relative SI for two different T1 species,
SI = C*sin(theta)*[1-exp(-TR/T1)]/[1-cos(theta)*exp(-TR/T1)]
where C is a proportionality constant that includes proton density, T2 effects, hardware effects, etc. Since sin(theta) is the same for two different T1 species, you can incorporate it into C. You can apply the SI formula to two different T1 species, then vary TR and/or theta to maximize the SI difference, for example by using the Solver in Excel.
The following references may be helpful.
1. Kingsley PB, Ogg RJ, Reddick WE, Steen RG. Correction of errors caused by imperfect inversion pulses in MR imaging measurement of T1 relaxation times. Magn Reson Imaging 16:1049-1055, 1998.
2. Kingsley PB. Signal intensities and T1 calculations in multiple-echo sequences with imperfect pulses. Concepts Magn Reson 11:29-49, 1999.
3. Kingsley PB. Methods of measuring spin-lattice (T1) relaxation times: An annotated bibliography. Concepts Magn Reson 11:243-276, 1999.
4. Kingsley PB, Monahan WG. Effect of increased repetition time TR on precision of inversion-recovery T1 measurements. Magn Reson Imaging 19:279-282, 2001.
5. Ogg RJ, Kingsley PB. Optimized precision of inversion-recovery T1 measurements for constrained scan time. Magn Reson Med 51:625-630, 2004.
--
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 Zoltan Nagy
Sent: Saturday, March 19, 2005 3:56 PM
To: [log in to unmask]
Subject: [SPM] T1-relaxation times for Pia/Dura
hello,
suppose i wanted to collect some fantastic 3-D anatomical data for some VBM
and/or segmentation. what i would like to know is how to separate the gray
matter from the pia/dura matter on a T1-weighted image.
- is it possible?
- does anyone know the T1-relaxation times of pia/dura?
- can it be done with a normal spgr or should one use inversion recovery?
thanks in advance for any quotes, guesses or references,
zoltan
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