Dear Buyean,
I totally agree with what David Wack wrote
– misalignment between transmission and emission can be a major problem.
I also agree co-registration will be better suited than realignment.
You don’t say which tracer you work
with, but it must be one with slow kinetics seeing your acquisition timing J . If you use a “cortical”
tracer with lots of signal in the outer parts of the brain, your proposed
technique might already work “as is”. Particularly if you’re
using a “subcortical” tracer, however, where signal is more or less
confined to the basal ganglia (or the limbic system,…), or if you use a “cortical”
tracer with heterogenous kinetics, your late frames are likely to look so
different from early frames that even mutual information co-registration may
run into problems. Two strategies may help to correct post hoc (assuming you do not have
independent movement detection data e.g. from Polaris or other systems):
Hammers A, Asselin M-C, Turkheimer FE,
Hinz R, Osman S, Hotton G, Brooks DJ, Duncan JS, Koepp MJ. Balancing bias,
reliability, noise properties and the need for parametric maps in quantitative
ligand PET: [11C]diprenorphine test-retest data. Neuroimage 2007, 38(1):82-94.
(You correctly wondered about what happens
to actual values after co-registration and, therefore, necessarily reslicing
the dynamic images. Note a reviewer made us take out the detailed results of
the movement correction procedure, but importantly, the parametric values
obtained after applying the model did not
change in those subjects without substantial movements – this is now
briefly mentioned in Methods, p. 85 col.I).
- or a combination of the two.
Sorry about only quoting work from our
place – you’ll find references to others’ important contributions
in the papers cited.
Good luck and I hope this helps,
Alexander
From: SPM (Statistical Parametric Mapping)
[mailto:[log in to unmask]] On Behalf Of David
Wack
Sent: 01 April 2008 21:13
To: [log in to unmask]
Subject: Re: [SPM] Can one use
coregistration to correct head motion in PET?
Realignment routines typically expect the images to be more less the same
except they may be slightly out of alignment. What you likely have with
the
dynamic PET scans is that the individual frames don't look that much alike
(even if they were perfectly aligned). This is due to how the tracer is
being taken up.
For this reason using the Coregistration routine makes some sense.
Of course if you do have substantial motion you'll have to deal with
the fact that your transmission scan wasn't aligned with at least some
of your frames.
dave
Buyean Lee wrote:
Dear SPM users,
I would like to know if one can use Coregistration
(normalized mutual information) to register PET frames to each other (also
called, head motion correction).
My concern is that if Coregistration can introduce
artifact to the resliced image (or even after only estimation).
I know that realignment uses only translation and
rotation, which would not change the values in the image except the changes due
to interpolation.
Thank you,
Buyean
For someone who is interested in specifics, I will
describe the PET procedure and problem in detail below.
This is the PET acquisition protocol that we use.
1. 1st transmission scan for attenuation correction
2. Inject the isotope
3. 1st emission scan for 80 minutes
4. 20-minute break (the subject is removed from
the scanner bed).
5. Reposition the subject to the scanner bed
6. 2nd transmission scan for attenuation
correction for the 2nd emission scan
7. 2nd emission scan
Basically, we acquire two scans, even though we
are acquiring only one dynamic scan.
So far, I have been using 'Realignment' to correct
head motion (after making the two scans into one scan); the middle frame (17th)
was used as the first image for realignment.
By the way, the 17th frame (image) belongs to the
first emission scan.
Occasionally, I noticed that registration by
'realignment procedure' is not perfect.
Specifically, SPM5 fails to realign the frames
(images) from the 2nd emission to the first image (17th frame).
When I used coregistration (default, normalized
mutual information), I got better registration.
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