[Elastix] (no subject)

Kasper Marstal kaspermarstal at gmail.com
Sat May 3 14:08:33 CEST 2014


Dear Tom,

I had the same problem when initializing registration of MRIs of heads
using a rigid transformation (I can see you have specified "EulerTransform"
in your parameter file). In my case, some heads would align nicely and
other would seem to get stuck in random places yet close to the solution.
This typically happens when you apply rigid transformations (scaling,
rotation, translation) to registrations where they cannot account for
anatomical differences between scans that violates this rigid body
assumption (fx in inter-patient registrations).

I solved my problem using an affine transform which also allows for
stretching and shearing of the deformation field. This typically works well
for bones. Use this transform by changing the line in your parameter file
from (Transform "EulerTransform") to (Transform "AffineTransform").

If you are feeling adventurous you might even consider subsequent
application of a non-rigid transform, such as the  (Transform
"BSplineTransform"), to capture the remaining differences near bone edges
that the affine transformation is unable to recover. This really depends on
the assumptions about your problem that you are willing to make and/or
sacrifice, however.

Let us know how it goes!

Cheers,
Kasper


On 30 April 2014 12:10, Tom Knoop <t.h.knoop at student.utwente.nl> wrote:

> Dear All,
>
> I am trying to register the long bones in the leg (femur) in multiple CT
> scans of the same patients. There are a number of reasons to do this, but
> one of them is that I would like to get a segmentation of the consecutive
> femurs by transforming the femur segmentation I made for the first image to
> the other images in the series. Because the rotation of the legs are
> different in every image, I do this per femur
>
> To do this, I use the segmentation of the first image as a fixed mask, and
> mask out the controlateral side of the body in my moving mask. I have build
> a parameter file that uses AdvancedMeanSquares metric and five smoothing
> steps with a high number of iterations to make the process more robust. See
> also parameterfile rigid_bone_forward.txt in the zip-file which you can
> find at the download-link here:
> https://filesender.surfnet.nl/?vid=1dbe6839-ef04-b228-03e8-00001a643b33
>
> This works fine for most of my images, with final SSD of around 5000.
> However, on some images, the registration just doesn't seem to work. It
> does get in the right direction (the femurs definitely have more overlap
> after the registration than before) but the voxel-accuracy I normally get
> is never reached. I have tried fooling around with the sp_A parameters and
> the resolution steps and maximum stepsizes, but to no avail. Also in the
> zip file are the logfiles of a successfull and failed registration.
>
> Unfortunately, I cannot share the scans themselves. However, it are CT
> scans of the lower body (containing complete femurs, and part of the knee
> joint and pelvis) made on an philips brilliance big bore scanner, with a
> resolution of 512x512x±210 voxels with a voxel size of .9375x.9375x3mm. The
> images are saved as mhd double datafiles, with their anatomical position
> taken from the original dicom data. masks are saved as binary mhd files.
>
> Has anyone have any idea about where this goes wrong? Visually, I do not
> see a difference between the failed and succesfull scans. It is possible
> that a single scan fails, while another scan for the same patient does
> register succesfully. What can I do to make this more robust?
>
> Many thanks for your help!
>
> Best,
>
> Tom
>
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