[Insight-users] Confused abour Optimizer Scales

[Luis Ibanez]

Hi Auron,


Your interpretation of the Optimizer Scales is correct.

In practice we mostly use them in order to normalize the
dynamic range of rotation units with respect to translation units.

Rotation tends to be expressed in Radians,
while, Translation tends to be expressed in Millimeters.

The ratio between the optimizer scales of Transform parameters
associated with rotation, and the Transform parameters associated
with translation should match your expected proportion between
rotation and translation.

A typical rule of thumb is to assume that rotating by 0.57 radians,
(45 degrees) is as dramatic as translating by half the image length.
Therefore, you want to compute the length of your image in millimeters
and compute its ratio to the 0.57 radians.

For the typical case of a CT scan, you may expect that a patient may
rotate by less than 10 degrees ( 0.17 radians), and the image length
is typically 512 pixels x 0.5mm/pixel = 256 millimeters.

Therefore the ratio that you want to use is :   0.57 : 256
which is about 1:500.

In the numeric example of your email, the ratio is 1:1000,
which is close enough...

Also,
In your email, you mention micrometers, which led us to suspect
that you may be working with Microscopy images.

In that case, the exercise is the same, except that you use
micrometers as the units of spacing.

Therefore, compute the length of one side of your image in
microns, and compute the ratio of that amount with respect
to 0.57 radians.


      Regards,


             Luis



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On Mon, Nov 16, 2009 at 3:28 AM, Auron Ford Huton <auron at hotmail.es> wrote:
> Hi ITK!!
>
> First of all, thak you all for answering me about my doubts regarding pixel
> spacing.
>
>
> Now I´m a bit confused about what exactly means the Optimizer Scales. I
> think, for what I coul read, that the optimizer scales are used to
> compensate the difference between the units of the rotation and the units of
> the translation, it´s that ok??
>
> I´m using a itkCenteredRigid2DTransform with a
> itkRegularStepGradientDescentOptimizer and my pixel spacing is about 2.83
> micrometres.
>
> Then, if I´m using the followin optimizer scale:
>
>   optimizerScales[0] = 1.0;
>   optimizerScales[1] = 1.0;
>   optimizerScales[2] = 1.0;
>   optimizerScales[3] = 1.0/1000.0;
>   optimizerScales[4] = 1.0/1000.0;
>
> what is the real relation between the angle and the translation??? I mean,
> for each rotation of 1.57 rad, how many micrometers my translation will
> move??
>
>
> Thank you in advance.
>
> Auron.
>
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