reverse unsharp mask?

[Michael Doube]

Hi all

I'm the disappointed recipient of a bunch of CT scans of bones in air,
to which overzealous unsharp masking was applied (at least, that's how
they look) - dark black borders of air surrounding over-bright bone edges.

Is there any way to reverse this effect to get at least partially back
to the original contrast?

Cheers

Mike

[Kenneth Sloan-2]

On Apr 9, 2008, at 9:33 AM, Michael Doube wrote:

> Hi all
>
> I'm the disappointed recipient of a bunch of CT scans of bones in  
> air, to which overzealous unsharp masking was applied (at least,  
> that's how they look) - dark black borders of air surrounding over-
> bright bone edges.
>
> Is there any way to reverse this effect to get at least partially  
> back to the original contrast?
>

Well, let's see.  Unsharp masking works by subtracting a blurred  
image from the original, to get the USM image:


     Iusm  =   I - Ib

If  you can approximage Ib, then you can add it back:

     I' = Iusm + Ib'

I imagine that blurring Iusm might give an acceptable Ib'.

Of course, there are numerous parameters to this process that you'll  
also have to estimate.

[note: instead of subtracting and adding, I generally prefer  
blending. e.g.,

      Iusm = a*I + (1-a)*Ib

where (in the case of unsharp masking) a > 1.0.

the reverse process would use 0.0 < a < 1.0, and, of course, the  
imperfect approximation of Ib by Ib']

Interesting question - I'll toss it out to my class on Thursday and  
see if anyone has a spare hour to try it out.
If one of them picks up the ball and runs with it, can I get a sample  
image to test on?

--
Kenneth Sloan                                          
[hidden email]
Computer and Information Sciences                        [phone]
University of Alabama at Birmingham              FAX [phone]
Birmingham, AL 35294-1170                http://KennethRSloan.com/

[ctrueden]

Hi Kenneth,

In addition to any images from Mike, I would suggest applying some
"overzealous unsharp masking" to some images of your own and pass those out,
so that you have the originals for comparison with any results that your
students develop. Always nice to have a quantitative gauge of effectiveness,
right?

-Curtis

On Wed, Apr 9, 2008 at 2:48 PM, Kenneth Sloan <[hidden email]>
wrote:

> On Apr 9, 2008, at 9:33 AM, Michael Doube wrote:
>
> > Hi all
> >
> > I'm the disappointed recipient of a bunch of CT scans of bones in air,
> > to which overzealous unsharp masking was applied (at least, that's how they
> > look) - dark black borders of air surrounding over-bright bone edges.
> >
> > Is there any way to reverse this effect to get at least partially back
> > to the original contrast?
> >
> >
> Well, let's see.  Unsharp masking works by subtracting a blurred image
> from the original, to get the USM image:
>
>
>    Iusm  =   I - Ib
>
> If  you can approximage Ib, then you can add it back:
>
>    I' = Iusm + Ib'
>
> I imagine that blurring Iusm might give an acceptable Ib'.
>
> Of course, there are numerous parameters to this process that you'll also
> have to estimate.
>
> [note: instead of subtracting and adding, I generally prefer blending.
> e.g.,
>
>     Iusm = a*I + (1-a)*Ib
>
> where (in the case of unsharp masking) a > 1.0.
>
> the reverse process would use 0.0 < a < 1.0, and, of course, the imperfect
> approximation of Ib by Ib']
>
> Interesting question - I'll toss it out to my class on Thursday and see if
> anyone has a spare hour to try it out.
> If one of them picks up the ball and runs with it, can I get a sample
> image to test on?
>
> --
> Kenneth Sloan
> [hidden email]
> Computer and Information Sciences                        [phone]
> University of Alabama at Birmingham              FAX [phone]
> Birmingham, AL 35294-1170                http://KennethRSloan.com/
>