One problem is that the bias field is dependant on the subject, as it
depends also on the receiver coils position. Therefore any a priori
information would be hard to come up with. This might be less of a problem
for brain imaging where bird cage coils are used giving a relatively flat
B1.
When dealing with surface array coils with much stronger sensitivity
fall-off, I use the following method: 1) threshold the voxels with signal,
2) fit a polynomial function directly to those voxels (3rd or 4th order), 3)
initialize the bias with this function.
I guess any fast method not based on tissue modeling would do, like
homomorphic filtering for example.
Olivier
__________________________________________________
Olivier Salvado, PhD
Case Western Reserve University | Case Center for Imaging Research
University Hospitals of Cleveland | Department of Radiology | Wearn B49
11100 Euclid Av. | Cleveland, OH 44106
Ph. (216) 983 3426 | Fax: (216) 844 4987
-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On
Behalf Of Marko Wilke
Sent: Monday, February 26, 2007 03:07
To: [log in to unmask]
Subject: [SPM] Bias field
Dear All,
I stumbled upon the following passage in the segmentation help text:
'A more accurate estimate of a bias field can be obtained by including prior
knowledge about the distribution of the fields likely to be encountered by
the correction algorithm.'
I wonder if this only refers to the amount of inhomogeneity (likely a
ballpark number) or it is it possible to, for example, use another
algorithms output as a more informed starting estimate. Alternatively, one
could take the bias field from a first iteration and feed that into a second
pass segmentation run. I realize this would take more time but for very
inhomogeneous data (surface coils, high-field etc.) it may result in an
improvement of tissue labeling.
So bottomline, my question is: Has anyone experimented with providing 3D
inhomogeneity estimates as starting parameters for the spm5 segmentation
algorithm?
Best,
Marko
--
=====================================================================
Marko Wilke (Dr.med./M.D.)
[log in to unmask]
Universitäts-Kinderklinik University Children's Hospital
Abt. III (Neuropädiatrie) Dept. III (Pediatric neurology)
Hoppe-Seyler-Str. 1, D - 72076 Tübingen
Tel.: (+49) 07071 29-83416 Fax: (+49) 07071 29-5473
=====================================================================
