 |
 |
Hi - > > 1) After completing BEDPOSTX, there are only 50 samples at each > voxel. I > don't know which parameters are set to generate the number of > samples. In > GUI display, I cannot find the configuration. I want to extend the > number to > using them well. > It is available on bedpostx command line, but not the GUI. If you type: bedpostx <subject_directory> -j <number_of_jumps> -s <sample_every> Then the number of samples per voxel will be <number_of_jumps>/ <sample_every> > 2) During BEDPOSTX, which sampling technique is employed, still > Markov Chain > MonteCarlo as PROBTRACKX? which model, diffusion tensor model or > partial > volume model? Bedpostx is running Metropolis Hastings (a variation on MCMC) on the partial volume model. Probtrackx is performing numerical integration using the posterior pdf calculated by bedpostx. (the integral given by eq 21 of the 2003 paper). > > > 3) In the paper "Characterization and Propagation of Uncertainty in > Diffusion-Weighted MR Imaging" written by T.E.J Behrens and > published on the > journal MRM, does local parameter estimation denote BEDPOSTX > process, and > global connectivity estimation corresponds PROBTRACKX process? if > so, the > former is 1000 samples and the latter is only 50 samlples in the > paper. > Quite the contrary to the description above. The 1000 samples are created in order to show an example posterior distribution from a single voxel or slice, but for storage reasons, bedpostx only calculates 50 samples in practice. This is stated at the bottom of page 1085 of the paper. I am not sure where exactly it is said that probtrackx only calculates 50 samples?? > > > I am appreciated for any suggestions. Cheers, Saad. > > > Liang Wang > --------------------------------------------------------------------------- Saad Jbabdi, Postdoctoral Research Assistant, Oxford University FMRIB Centre FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK +44 (0) 1865 222545 (fax 222717) [log in to unmask] http://www.fmrib.ox.ac.uk/~saad ---------------------------------------------------------------------------