Horse bone results
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anita scampton
May 9, 2012, 12:06:45 AM5/9/12
to bonej-users-a...@googlegroups.com
Hi again Michael,
Sorry (again!) for the long delay in replying.
I got a little more savvy about using ImageJ when I took some time not to think about it, and have very easily managed to upload entire CT images for analysis. Not sure where my brain block was before, but I'll blame it on first trimester :-/
Anyway, I have managed to very easily perform a MOI analysis on an entire image.
Would you mind having a quick look at the data output, and tell me if you think I'm on the right track?
Also, if I'm reporting MOI results, which I-value should I be reporting? I'm not 100% sure what the different I-values relate to (eg Icxz cf Icyz). Or could you direct me to somewhere that would explain the different values.
Thanks again for your patience and help.
Kindest regards,
Anita
Michael Doube
May 10, 2012, 6:31:21 AM5/10/12
to bonej-users-a...@googlegroups.com, anita scampton
Hi Anita,
> Anyway, I have managed to very easily perform a MOI analysis on an
> entire image.
Great!
> Would you mind having a quick look at the data output, and tell me if
> you think I'm on the right track?
Yes, looks good. Please make sure that the threshold values used by the
Moments plugin are sane, by checking them with the
Image>Adjust>Threshold command. The only thing to bear in mind is that
the calculation by default assumes a bone density of 1.8 g/cm^3 for all
pixels within your threshold value. That may or may not be correct for
your specimens but it's a reasonable guess in the absence of a
calibration curve. Better calibration is one of the things on BoneJ's
long and growing to-do list.
> Also, if I'm reporting MOI results, which I-value should I be reporting?
> I'm not 100% sure what the different I-values relate to (eg Icxz cf
> Icyz). Or could you direct me to somewhere that would explain the
> different values.
Wikipedia is gold:
http://en.wikipedia.org/wiki/Moment_of_inertia#Definition
http://en.wikipedia.org/wiki/Moment_of_inertia#Principal_axes_of_inertia
And you can get a hint from bonej.org
http://bonej.org/moments
Probably the most relevant values for you are the I1, I2 and I3 values
(the principal moments), because these tell you about the disposition of
mass around the centroid of your bone, irrespective of the coordinate
frame of your image. The Icxx etc. values tell you about the moments
with respect to the (x, y, z) axes of your scan.
"Here I_{xx} denotes the moment of inertia around the x-axis when the
objects are rotated around the x-axis, I_{xy} denotes the moment of
inertia around the y-axis when the objects are rotated around the
x-axis, and so on." (Wikipedia)
The principal moments are derived from the Icxx moments by Eigenvector
decomposition. As a fellow vet, I feel your pain if this is the first
time you heard of this stuff.
Michael
> Anyway, I have managed to very easily perform a MOI analysis on an
> entire image.
> Would you mind having a quick look at the data output, and tell me if
> you think I'm on the right track?
Moments plugin are sane, by checking them with the
Image>Adjust>Threshold command. The only thing to bear in mind is that
the calculation by default assumes a bone density of 1.8 g/cm^3 for all
pixels within your threshold value. That may or may not be correct for
your specimens but it's a reasonable guess in the absence of a
calibration curve. Better calibration is one of the things on BoneJ's
long and growing to-do list.
> Also, if I'm reporting MOI results, which I-value should I be reporting?
> I'm not 100% sure what the different I-values relate to (eg Icxz cf
> Icyz). Or could you direct me to somewhere that would explain the
> different values.
http://en.wikipedia.org/wiki/Moment_of_inertia#Definition
http://en.wikipedia.org/wiki/Moment_of_inertia#Principal_axes_of_inertia
And you can get a hint from bonej.org
http://bonej.org/moments
Probably the most relevant values for you are the I1, I2 and I3 values
(the principal moments), because these tell you about the disposition of
mass around the centroid of your bone, irrespective of the coordinate
frame of your image. The Icxx etc. values tell you about the moments
with respect to the (x, y, z) axes of your scan.
"Here I_{xx} denotes the moment of inertia around the x-axis when the
objects are rotated around the x-axis, I_{xy} denotes the moment of
inertia around the y-axis when the objects are rotated around the
x-axis, and so on." (Wikipedia)
The principal moments are derived from the Icxx moments by Eigenvector
decomposition. As a fellow vet, I feel your pain if this is the first
time you heard of this stuff.
Michael
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