[MINC-users] Re: hires vs. lowres registration

[minc-users@bic.mni.mcgill.ca]

Hi Andrew,

Thanks very much for your comments.  We're continuing to look at this 
issue.  

>> First, this difference has (well should have!) nothing to do with
>> mincresample so let's concentrate on minctracc/mritoself.

I'm not sure if mincresample does not have an effect.  For example, the
process of reducing the resolution and changing coordinates to match the
T1 image to the T2 space takes place when this tool is run.  So perhaps
mritoself calculates the correct transformation (.xfm) but could error or
noise be then introduced afterward during the resampling?

>>   * Given that you are using mutual information and that the input T1
>> may be somewhat noisy, the mutual information "image" that is
>> calculated internally as part of the fit may be somewhat spurious. 
>> Down sampling the data first is effectively blurring it.  Thus your
>> input T1 is more smooth in the low-res version.
>>
>>    * Lots of other things....
>>
>> You can easily test the second by blurring the input high-res data
>> before fitting at the same level as the downsampling would achieve.

I plan to go ahead and try this - to manually reduce the hi-res data, and 
to see if this also improves the registration/alignment quality.

Recently I tested the registration (i.e. mritoself and mincresample) on a
series of scans with varying quality from different scanners.  In all
cases registering using the lo-res T1 image worked extremely well for some
reason.  So if we can be sure that this is happening for a valid reason,
it would represent a good technique or strategy for us (and perhaps in
general), during the image registration stage of analysis.

All comments or thoughts are greatly appreciated!!


Ernest Lo
NeuroRX Research
Montreal, Canada


On Wed, 4 May 2005 elo@neurorx.com wrote:

> Hello,
> 
> I've encountered a curious situation with image registration and would 
> like to know if anyone has any insight or comments:
> 
> I'm aligning two images that represent MRI scans of the same object (i.e. 
> the human brain) but in two different modalities (i.e. T2 and T1).  The 
> registration process is essentially mritoself which calculates the 
> transform function needed to register one image to the other, followed by 
> mincresample, which applies the transform and places one image in the same 
> space as the other.
> 
> Initially we decided to use the hi-res T1 images and to align them to the 
> lower resolution T2 images, with the rationale that providing more 
> information about the image should lead to a better fit.  But we have 
> since found that using the lo-res T1 images actually produces better 
> registration (i.e. alignment) results.  Does this make sense and what 
> could be happening here within the mritoself and mincresample algorithms 
> to cause this?
> 
> The lo-res T1 image resolution matches the T2 image resolution, but 
> theoretically the hi-res T1 image should be less prone to partial volume 
> etc. errors.
> 
> Any comments or ideas would be appreciated!
> 
> 
> Ernest Lo
> NeuroRX Research
> Montreal, Canada
> 
> 
>