This is to follow-up my earlier post.

In computed tomography of 2D parallel projection, its scanning process can be regarded as a real world example of 2D Radon transform.  Let us call density function (or beam attenuation coefficient or beam absorption coefficient) of body to be scanned $f(x, y)$.  Then scanned data for each projection angle $\theta$, after correcting exponential decay of beam due to attenuation, can be seen as Radon transform of $f(x, y)$: $\displaystyle Rf(\theta, X) = \int_{-\infty}^\infty f(X \cos \theta -Y \sin \theta, X \sin \theta + Y \cos \theta) dY.$

Where $(X, Y)$ are rotated coordinates: $\displaystyle x = X \cos \theta -Y \sin \theta,$ $\displaystyle y = X \sin \theta + Y \cos \theta.$

Taking Fourier transform of $f(x, y)$ we get: $\displaystyle F(\xi, \eta) = \int_{-\infty}^\infty \int_{-\infty}^\infty f(x, y) e^{-i(\xi x + \eta y)} dx dy.$

Then $F(\xi, \eta)$ in polar coordinates $\xi = w \cos \theta, \eta = w \sin \theta$ would look like: $\displaystyle F(w \cos \theta, w \sin \theta) = \int_{-\infty}^\infty \int_{-\infty}^\infty f(x, y) e^{-i(\xi x + \eta y)} dx dy.$

I want to change variables from $(x, y)$ to $(X, Y)$ in order to relate $F$ to the Radon transform $Rf$.  Because of the change to polar coordinates, the exponent $\xi x + \eta y$ becomes: $\displaystyle \xi x + \eta y = xw\cos \theta + yw \sin \theta = w(x \cos \theta + y \sin \theta) = wX$.

And Jacobian is: $\displaystyle \dfrac{\partial x \partial y}{\partial X \partial Y} = \left | \begin{array}{ccc} \cos \theta & -\sin \theta \\ \sin \theta & \cos \theta \end{array} \right | = 1$.

So $F(w \cos \theta, w \sin \theta)$ is rewritten as: $\displaystyle F(w \cos \theta, w \sin \theta) = \int_{-\infty}^\infty [ \int_{-\infty}^\infty f(X \cos \theta - Y \sin \theta, X \sin \theta + Y \cos \theta) dY ] e^{-iwX} dX$.

Gazing the inner integral with respect to Y for a while… carefully… This is just the projection of $f$ in the direction of $\theta$, which is… Radon transform: $\displaystyle F(w \cos \theta, w \sin \theta) = \int_{-\infty}^\infty Rf(\theta, X) e^{-iwX} dX$.

Okay, finally gazing this last integral for another while… patiently… It turns out that this is rightly Fourier transform of $Rf$ with respect to $X$, doesn’t it?  My boss used to call this “Fourier slice theorem” or “central slice theorem“.  In CT, “filtered back projection” is a most important method for reconstructing $f(x, y)$ from projection data $Rf(\theta, X)$, and the method is based on the slice theorem.

But this theorem does not force us to go only backward (reconstruction) .  If we have an image $f(x, y)$ at hand, then we can go forward to get lots of projection data using the same theorem.  Getting lots of projection data is same thing as plotting on $(r, \theta)$ plane in Hough transform.  Steps to get projections using this idea are:

1. Fourier transform the $f(x, y)$ –> $F$,
2. Resample $F$ radially to convert to polar coordinates –> $P$,
3. Inverse Fourier transform $P$ –> projection data $p$.

Proper resampling should be difficult in practice.  It is like filtering in filtered back projection is difficult.  But I think it is still nice to be aware of the theorem, maybe just for fun.

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### 3 Responses to Radon and Hough Transform

1. voodoo child says:

Please do not try this at home unless you are experienced, as you may not be able to withstand the immediacy of the shock waves.

Also Note: Should your body start moving in accordance with the ultra heavy frequencies do not panic. This is entirely normal. What you are experiencing is known as the “Groove Transform”.

Step 1) Put on one pair of devilishly cool shades.

Step 2 )Turn on your speakers full blast

Step 3) Assume the air guitar position (mirror optional).

Step 4) Radon transform this. Sucker!!!!!

http://www.buddyguy.net/site.html

• azumih says:

Best damn cool.
Thanks a lot for the pointer.

2. voodoo child says: