Hi,
The BW is in Hz, not Hz/pixel. So the value you need to use is 2240 * 64 = 143 360 Hz,
which is in the range that the warning message suggests.
For the MR parameters, these only depend on field strength, not on scanner.
They are intrinsic properties of the tissues at a specific field strength.
So you can just use the values provided if you are at the right field strength.
For whole brain, long time series there is a lot of calculation and it is best
suited to a cluster. If you had 50 cores then it would around 3 days.
However, if you do not have a cluster then you can save time by not doing
whole brain, but just a limited number of slices. Also, the estimated time is
only approximate - it is based on our server about 4 years ago. So it is worth
doing a small test and seeing how accurate the estimate is. If you are lucky
and your machine is substantially faster then it might be quite a lot less time.
All the best,
Mark
On 15 Dec 2011, at 18:59, YAN Chao-Gan wrote:
Dear all!
Currently
I am trying to using POSSUM to simulate BOLD signals, however, I experienced some difficulties. Our sequence
parameters can not pass POSSUM's consistency check.
The problem is the BW (read-out bandwidth, Hz) parameter. For our
sequence, BW is 2240 hz/px. When I set BW=2240, TE=0.03s, then POSSUM's
consistency check said "Try changing the BW to a value between 82336 and
782120 OR Try changing the TE to greater than 0.91736s." Do you have
any idea to set the correct BW?
Other parameters: TR=2.5s; TRslice=0.064; Number of voxels X=64, Y=64,
Z=39; Voxel size X=3, Y=3, Z=3; FOV=192*192; k-space coverage=100%; flip angle=80; Number of Volumes=2;
Gap=0.3mm.
Some additional questions:
The MR parameters
(relaxation times T1, T2*, spin density &rho, and chemical shift
value &delta): Could I just use the table provided by possum as
MRpar_3T, or I need
to specify the values which is specific to our own scanner. Are these parameters varied a lot between scanners? If the
latter, how can we calculate them?
Run time: POSSUM need 1.4 days for just two volumes!! If we
want to simulate 200 volumes with head motion, it need 140 days!!! I
don't know why it is so slow. It seems it is very difficult to simulate
head motion affected BOLD images for all the subjects.
Thanks!
Chao-Gan