[MINC-users] Conversion of volume

[Jason Lerch]

Ed Gronenschild wrote:

>Hello,
>
>Since I didn't receive any response I tried to figure it out myself.
>I think that I'm wrong about the proposed conversion. I guess
>that it is much simpler:
>
>vol_native = vol_tal / scale_factor
>		= vol_tal / 1.4388
>		= vol_tal * 0.6950
>
>(see eg. Pruessner et al, J. Neurosci, 2001, 21(1), 194-200)
>
>I hope that I can have some feedback now.
>  
>
This looks correct. A good sanity check, if you are unsure of 
conversions like this, is to invert the transform that takes the volume 
into stereotaxic space (using xfminvert), then resample the labels 
(using mincresample -nearest_neighbour) using the inverted transform, 
and then measuring the volume in native space. If it's close to your 
computed volume then you were right!

Jason

>Ed
>
>On 7 Sep 2006, at 14:44, Ed Gronenschild wrote:
>
>  
>
>>Hello
>>
>>I'm doing volumetric measurements of brain structures in
>>stacks which have been transformed linearly in stereotaxic
>>Talairach space (mritotal) en resampled in 1.00 mm cubic
>>voxels (mincresample). The native voxel size is 0.94 * 0.94
>>* 1.50 = 1.3254 mm^3). To derive the volume in native
>>space I do the following:
>>
>>Let's define the native voxel size "voxel_size_native",
>>and the Talairach voxel size "voxel_size_tal". Furthermore,
>>the native volume is defined as "vol_native" and the volume
>>in Talairach as "vol_tal".
>>
>>With xfm2param I derive the following transformation
>>parameters:
>>
>>-center         0.00000    0.00000    0.00000
>>-translation -129.92584  111.43100   97.71683
>>-rotation      -7.68466   -1.81075   -0.82142
>>-scale          1.06879    1.11325    1.20931
>>-shear          0.00000   -0.00000   -0.00000
>>
>>The scale factor ("scale_factor") is the product of the scale
>>factors in X,Y, and Z, so
>>scale_factor = 1.06879 * 1.11325 * 1.20931 = 1.43882.
>>By the way, this is equal to the determinant of the transformation
>>matrix.
>>
>>The conversion from Talairach to native is then:
>>
>>vol_native = vol_tal * voxel_size_native / voxel_size_tal /  
>>scale_factor,
>>
>>or
>>
>>vol_native = vol_tal * 1.3254 / 1.0 / 1.4388
>>
>>		= vol_tal * 0.9211
>>
>>Can you agree with me?
>>
>>Regards,
>>
>>Ed
>>    
>>
>
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