[Insight-users] Image to Spatial Object Metric

Wed Jan 28 04:43:32 EST 2009

Hello again, it seems i could overcome some of my difficulties re-defining a
metric. This is what I did:

   - First I tried a SpatialObjectToImageStatistis calculator, so that I
   would be able to sum the values of pixels of the image included inside the
   Spatial Object. I couln't make it, and struggled for several days on that.
   So I decided I would look for another solution that might not be as
   efficient, but could be used meanwhile.
   - So I did the following, I used an SpatialObjectToImageFilter and
   counted the pixels of that Spatial Object. This has given me a metric that
   uses the points inside the image ratio, and the pixels that includes the
   spatial object. With this I am able to register, but I face the following
   problem, which is why I ask you for some guidance:

Given that my registration algorithm using this unefficient metric is quite
slow, i would certainly need to have the right scales for dynamic range of
paratemers. So what's the rule to choose them? I can see examples in the ITK
Software Guide, but I'd rather have some more information about why those
values were chosen.

Thank you in advance,

Antonio Foncubierta

2009/1/16 Antonio Foncubierta <antonio.foncubierta at gmail.com>

> Thank you very much for your quick answer, Luis. As requested, I will give
> as many details as possible:
>
>
>    1. I create an itkImage using a SpatialObjectToImageFilter. This image
>    contains an ellipsoid with radii 10 15 20, for instance.
>    2. I want to make a registration of a certain Spatial Object with that
>    Image. The starting Spatial Object is another ellipsoid with radii, for
>    example, 5, 5, 5. I set the Spatial Object offset in a way so that the
>    ellipsoid centre fits the Image Gravity Center (I use a MomentCalculator to
>    get that value).
>    3. I start the registration method using an Affine Transform, because
>    the starting ellipsoid needs a scale change.
>    4. The result is that the optimizer doesn't have anything to optimize,
>    because the SimpleModelToImageMetric GetValue method returns the number of
>    Spatial Object points in the Image (every point in the image has value=1).
>    Therefore, the metric is already maximized. The result is a small ellipsoid
>    fully contained inside the image.
>    5. In some other cases, I located the starting spatial object outside
>    the gravity center of the image and then started the registration method.
>    The result then was a randomly shaped ellipsoid (it depends on where I put
>    the center and the original size of it) which has all of its points inside
>    the image. The Optimizer returns the same parameters once it achieves a
>    maximum in the metric, no matter how many iterations I allow the
>    registration method to run.
>
> Those results have made me think that I must change the metric so that it
> has a maximum when all the points are inside the image and the spatial
> object volume is as high as possible.
>
> Any ideas?
>
> I also would like to apologize if you understood that I was reporting a
> problem in the registration methods or examples (which obviously work
> perfectly). My intention was to find what I am doing wrong or what I could
> probably use to solve _my_ problem.
>
> Thank you in advance,
>
> Antonio Foncubierta
>
>
> 2009/1/16 Luis Ibanez <luis.ibanez at kitware.com>
>
>
>> Hi Antonio,
>>
>> the ImageToSpatialObjectRegistrationMethod<> that is illustrated in the
>> examples
>>
>>    Insight/Examples/Registration
>>
>>               ModelToImageRegistration1.cxx
>>               ModelToImageRegistration2.cxx
>>
>> should, in principle, work with any of the ITK Transforms.
>>
>> It is likely that the problem that you faced when using an Affine
>> transform are more related to:
>>
>>
>>        A) Poor choice of parameter scaling values (the array
>>           that is passed to the Optimizer in order to compensate
>>           for the difference in dynamic range between Translation
>>           and Rotation/Scale components.
>>
>>        B) Poor choice of Optmizer parameters.
>>
>>
>> Of course, the option of writing your own Metric is always worth
>> considering, but that is independent of the need for solving (A)
>> and (B) properly.
>>
>>
>> In general, when you report that something "doesn't work", it is
>> very helpful to describe *exactly* what the result was, and how
>> do you arrive to the conclusion that "it doesn't work".
>>
>> Please give us more detail on what you observed....
>>
>>
>>    Thanks
>>
>>
>>       Luis
>>
>>
>>
>> ---------------------------
>> Antonio Foncubierta wrote:
>>
>>> Hello,
>>>
>>> I am trying to develop a Model Based Registration Method, and I started
>>> using the example mentioned by the Itk Software Guide and included in the
>>> 3.6 version of Insight: ***ModelToImageRegistration1.cxx***. I have been
>>> trying to adapt the example to work with my data: 3D Images. Achieving that
>>> was not a big deal, but I started to struggle when I wanted the Transform to
>>> be an itkAffineTransform, which includes a scale change. The example
>>> explicitly says that the SimpleImageToSpatialObjectMetric is designed
>>> considering no scale changes, and it says it right, because the metric
>>> doesn't work as I required. (It does work as expected, though).
>>>
>>> So my main problem now is finding a way to define a metric that maximizes
>>> both the ****number of Spatial Object Points inside the Image**** and the
>>> ****Spatial Object Volume****. The first one is easy, as it is exactly what
>>> the above mentioned example does, but I haven't found a way to compute the
>>> Transformed SpatialObject Volume.
>>>
>>> And that's why I ask you if any of you have found a similar problem and
>>> how did you solve it. Also, if any of you know a not-as-simple metric which
>>> I could use.
>>>
>>> Thank you in advance,
>>>
>>> PS: The explanation migh be very blurry, so if you don't undersrtand
>>> anything of it, please, just ask. I'd rather repeat things in a different
>>> way, or attach images to not having help at all ;)
>>>
>>> Antonio Foncubierta
>>>
>>>
>>> ------------------------------------------------------------------------
>>>
>>> _______________________________________________
>>> Insight-users mailing list
>>> Insight-users at itk.org
>>> http://www.itk.org/mailman/listinfo/insight-users
>>>
>>
>
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