Thanks.
This greatly helps me. thanks for your advice.
adil
--- On Thu, 3/11/10, Burkhard Mädler <[log in to unmask]> wrote:
> From: Burkhard Mädler <[log in to unmask]>
> Subject: Re: [FSL] DTI imaging
> To: [log in to unmask]
> Date: Thursday, March 11, 2010, 6:08 AM
> Hi Adil,
> From the scan parameters you mentioned it is not possible
> to get an reconstructed
> isotropic dataset with 2x2x2 mm^3.
> I just set it up in the Philips MR-simulator. With a
> FOV=240 and an acquisition
> matrix of 120x118 you cannot reconstruct the data in the
> same matrix size because
> of FFT-issues. The recon size of your data need to be
> multipliers of 16, therefore the
> reconstructed matrix can only be:
> 96x96, 112x112, 128x128 and so on.
> If you want to achieve an iso-resolution of 2x2x2mm^3 in
> your recon-data you have to
> change your FOV or accept a larger voxel size,e.g.:
> voxel-size: 2x2x2mm^3
> matrix: 112X112 or 128x128
> FOV: 224x224cm or 256x256cm
>
> as you see, you only have two practical choices to achieve
> an iso 2x2x2, either FOV=224
> or FOV=256 unless you slightly increase your voxel size.
> With single shot EPI acquisition this decision is difficult
> to make. EPI-images tend to
> get more distorted as longer the read-out train gets.
> Aiming for a 128 matrix is pushing
> the envelope for single shot EPI-DTI. On the other hand you
> also lose a lot of signal
> because with a longer EPI-readout train your effective TE
> needs to be longer, hence
> less signal due to T2*-decay.
>
> Without knowing more about your study purpose it is very
> difficult to give advice
> remotely.
>
> To your second question:
> you never lose information by reconstructing images to
> higher (!) resolution but there
> are other issues to consider:
> - for fiber tracking a non-isotropic voxel size is not
> recommended.
> - the amount of memory you need for postprocessing grows
> very fast with interpolated
> data,e.g.
> 60 slices DTI-high (32 directions) with 128x128
> matrix = ca. 65MByte
> 60 slices DTI-high (32 directions) with 256x256
> matrix = ca. 260 MByte
>
> for some fiber tracking programs the later one can become
> already a challange!
>
> Cheers,
> Burkhard.
>
> Burkhard Mädler, PhD
> Tel.: (+49) 228 287 16186
>
>
> -----Original Message-----
> From: FSL - FMRIB's Software Library [mailto:[log in to unmask]]
> On Behalf Of Adil Javed
> Sent: Thursday, March 11, 2010 1:20 AM
> To: [log in to unmask]
> Subject: [FSL] DTI imaging
>
> Hi,
> for those who are experts in DTI:
> I am acquiring DTI images on 3.0T Phillips scanner using 32
> directions, FOV 240, acquired matrix is 120 x 118 with
> acquired voxel dimensions of 2x2x2.
> In order to overlay DTI FA/MD/ or tracts on a FLAIR or T1
> image, I need to reconstruct the matrix to 256 with voxel
> RECON to 0.94 x 0.94 x 2 so they align with my FLAIR
> or T1 images.
>
> Question: is this ok or do you see any problems with
> this approach? By reconstructing voxels down to 0.94 x
> 0.94 x 2
> after they are Acquired in 2x2x2 size, am I losing any
> information?
>
> many thanks for your help.
>
> adil
>
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