Pilar-
SHORT ANSWER:
Sounds like the first acquisitions are showing typical "T1" effects. Due to
the physics involved, the first few EPI volumes typically appear darker
than later volumes. With EPI acquisition you normally discard the first few
volumes of the sequence.
MRIcro always precisely converts DICOM image data to Analyze image data.
The conversion is LOSSLESS.
DETAILED ANSWER:
When you open an image in MRIcro (and SPM), it automatically scales the
image brightness for the maximum and minimum values in that volume. This
usually gives a fairly balanced representation of the image brightness. If
the early slices include T1 effects, they will appear darker than the later
slices. If you open the later slices on their own (without the earlier
slices), MRIcro will readjust the contrast and the resulting image will
appear darker than before (as the image intensity range is on average
higher). I think that is why your images appear differently on the screen
depending on how you segment them.
It is important to realize that MRI intensity levels are relative (unlike
CT and PET scans where the voxel values are precisely calibrated to a
certain number of counts). To precisely set the contrast/brightness in
MRIcro, select 'Precise Contrast' from the 'View' menu. A window appears
(http://www.psychology.nottingham.ac.uk/staff/cr1/contrast.gif) that allows
you to set the 'Center' [radiology speak for brightness] and 'Width'
[contrast] of your image. When you do this to set a specific
contrast/brightness, you will find a specific slice will appear identical,
irregardless of how you segmented the image.
-chris
At 12:15 09/05/2001 +0100, Pilar Salgado Pineda wrote:
>Dear SPMers,
>
>
>I have realized that when reconstructing a multivolume
>file from the DICOM raw data by using the mricro, the
>resulting one is such as that always the first volume
>is darker than the others. Even if ,i.e, I have 30
>slices and I create a unique file with two volumes of
>15 slices each the first one is darker than the second
>which is lighter. however if I create one volume for
>each group including 15 slices, then both volumes are
>darker. The same happens with 3, 4...
>I would like to know why this happens and what should
>I do to avoid it
>
>
>Tanks in advance
__________________________________________
Dr. Chris Rorden
Department of Psychology
University of Nottingham
Nottingham NG7 2RD, UK
Tel: +44 [0]115-951-5294
Fax: +44 [0]115-951-5324
http://www.psychology.nottingham.ac.uk/staff/cr1/
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