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From: Arun Kook on 28 May 2010 06:33
Hi all,

I have imaged a circular object in an MRI scanner. I am trying to measure the 2d distortion of the image of this object. I have loaded this image as an matrix in matlab. I want to find out the largest possible circle that could be fit into the image of the circular object. Then I want to work out what area of the imaged object is not covered by this circle that i have fitted. I am going to use this as a measure of the 2d distortion of the image.
Many Thanks,
Arun
From: ImageAnalyst on 28 May 2010 06:47
Arun:
Can you threshold your image to get a binary image of the circular
area? That would be the first step. Then find the centroid and the
three points closest to the centroid, and you're practically done.
ImageAnalyst

From: Arun Kook on 28 May 2010 07:21
ImageAnalyst <imageanalyst(a)mailinator.com> wrote in message <f8f2544d-6e99-4d3f-a1e0-ec0f29707186(a)k31g2000vbu.googlegroups.com>...
> Arun:
> Can you threshold your image to get a binary image of the circular
> area? That would be the first step. Then find the centroid and the
> three points closest to the centroid, and you're practically done.
> ImageAnalyst

Image Analyst:

I have have done the thresholding. I am in the process of finding out how to find the centroid of the ROI.
From: Bruno Luong on 28 May 2010 07:49
ImageAnalyst <imageanalyst(a)mailinator.com> wrote in message <f8f2544d-6e99-4d3f-a1e0-ec0f29707186(a)k31g2000vbu.googlegroups.com>...
> Arun:
> Can you threshold your image to get a binary image of the circular
> area? That would be the first step. Then find the centroid and the
> three points closest to the centroid, and you're practically done.

If the circular happens to be a perfect disc, any point in the circumference is the closest to the centroid, no? And the set of the centroids any three points located on the circle is the disc itself, no?

It seems this method is not very robust, at least for the circle.

What about find the biggest (has largest radius) circumscribed circle of all Delaunay triangles of the set of the points on the *boundary* of the object?

Bruno
From: Rik on 28 May 2010 08:57
Why don't you use a "shape factor" for this purpose? cfi:
http://en.wikipedia.org/wiki/Shape_factor_%28image_analysis_and_microscopy%29

you may want to check out the regionprops function of the image processing toolbox as well ...

Rik

"Arun Kook" <arun_kook(a)yahoo.co.uk> wrote in message <hto65i$iu$1(a)fred.mathworks.com>...
> Hi all,
>
> I have imaged a circular object in an MRI scanner. I am trying to measure the 2d distortion of the image of this object. I have loaded this image as an matrix in matlab. I want to find out the largest possible circle that could be fit into the image of the circular object. Then I want to work out what area of the imaged object is not covered by this circle that i have fitted. I am going to use this as a measure of the 2d distortion of the image.
> Many Thanks,
> Arun
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