Hi Dave

Thank you for your reply. But I found some papers on recovering susceptibility-induced signal losses.

 

 some authors have reported methods for recovering susceptibility-induced magnetic field gradient (SFG)-induced signal losses. The simplest method uses thinner slices to reduce the field change across a slice (1). However, this method reduces both the signal-to-noise ratio (SNR) and the spatial coverage per unit of time. Another method (2每4) effectively compensates for field inhomogeneities
using multiple refocusing gradients. However, in practice, many repetitions are needed to sum up to a uniform image. This disadvantage limits its value in fMRI experiments. Some methods using high-order field compensation (5,6) and a two-shot technique (7,8) were more efficient but the repetition time (TR) was still doubled.
 

 

1. Young IR, Cox IJ, Bryant DJ, Bydder GM. The benefits of increasing spatial resolution as a means of reducing artifacts due to field inhomogeneities. Magn Reson Imaging 1988;6:585每590.

2. Frahm J, Merboldt KD, Hanicke W. Direct FLASH MR imaging of magnetic field inhomogeneities by gradient compensation. Magn Reson Med 1988;6:474每480.
3. Ordidge RJ, Deniau JC, Knight RA, Helpern JA. Assessment of relative brain iron concentrations using T2-weighted and T2*- weighted MRI at 3 Tesla. Magn Reson Med 1994;32:335每341.
4. Constable RT. Functional MR imaging using gradient-echo echoplanar imaging in the presence of large static field inhomogeneities. J Magn Reson Imaging 1995;5:746每752.

5. Cho ZM, Ro YM. Reduction of susceptibility artifact in gradientecho imaging. Magn Reson Med 1992;23:193每196.
6. Glover G, Lai S. Reduction of susceptibility effects in BOLD fMRI using tailored RF pulses. In: Proceedings of the 6th Annual Meeting of ISMRM, Sydney, Australia, 1998. p. 298.
7. Chen NK, Wyrwicz AM. Removal of intravoxel dephasing artifact in gradient-echo images using a field-map based RF refocusing technique. Magn Reson Med 1999;42:807每812.
8. Mao J, Song AW. Intravoxel rephasing of spins dephased by susceptibility effect for EPI sequences. In: Proceedings of the 7th Annual Meeting of ISMRM, Philadelphia, PA, USA, 1999. p. 1982.

 

So I think there are many people do this work. Am I right?

Shiliang

2008/4/14, Lythgoe, David <[log in to unmask]>:

Hi Shiliang,

The best software to use is the pulse sequence on your scanner. If you reduce the slice thickness, you'll reduce the through-plane de-phasing that causes most of the signal loss. If you increase the matrix size, you'll also reduce signal losses due to in-plane echo-shifting.
If that solution isn't available, sigloss in FSL can provide an estimate of the signal loss, which can then be used to "correct" the signal. You'll also need a field map.  One problem with this is it locally increases the noise. Also, in some regions the signal loss is so severe, any correction would be completely meaningless.  Finally, I haven't used sigloss directly for some time, but I found it underestimates susceptibility-induced signal losses. In fact, I would advise against attempting to correct for lost signal using post-processing. Others may disagree.


Dave





-----Original Message-----
From: FSL - FMRIB's Software Library on behalf of qiang wang
Sent: Mon 14/04/2008 15:03
To: [log in to unmask]
Subject: [FSL] Which software can solve the problem of susceptibility-induced signal losses in EPI images

Dear every one

I want to solve the susceptibility-induced signal losses in EPI images. Who
can tell me which software can solve this problem? Thank you.

Best wishes

Shiliang