So today we want to learn how a CT scanner works.

And a CT scanner is basically an X-ray device that captures multiple X-ray images and makes

use of these projection images, these X-ray images to reconstruct the 3D volume.

So the picture we have to keep in mind for the following discussion is the following.

We have here, let me just write the headline, computed tomography for dummies.

It's a very intuitive introduction.

So that's exactly what you remember.

Tomography what you have to remember basically for the oral exam.

So what we have to keep in mind is the following.

We have a point source that generates X-rays of a constant energy.

So we have a mono-energetic X-ray source.

So this is the mono-energetic X-ray source.

And then we have an object.

And this is a discrete grid where each cell, so each element of this grid carries a certain

material.

And we can characterize each cell by its X-ray attenuation properties.

So if I do put in something like bone, it takes way more energy from the X-ray particles

if they are propagated through than water.

So with each of these cells here, we associate a attenuation value.

And we call this function here in two dimensions, F, X, and Y.

And each value for X and Y coordinate, for the Y coordinate, gives us the attenuation

value.

You agree on that?

Yes.

Good.

And then we have here our detector.

Think about a flat panel detector.

And for all the upcoming discussions, we do the following.

We just consider the case we project a 2D function onto a one-dimensional detector.

We project a 2D function on a one-dimensional detector.

So this here is 1D, and the function is 2D that we want to reconstruct.

And now we switch on the X-ray source, and we generate a single beam that hits the detector.

So we generate a single beam.

Which color do you like for the X-ray beam?

What is the color of X-ray beams?

Red.

Red.

Okay, very creative.

I like you.

Okay.

And now you can think of this X-ray beam to be a set of particles that carry a certain

energy, and then they go through the function here, and they bump against the cells.

Independent on the attenuation properties of each cell, dependent on the material, they

lose some energy.

And at the end, we just measure here the energy that survived.

Okay?

So we bump, bump, bump, and at the end, we measure the energy that survived.

So let's think about a Porsche starting here, and this is a fence, and this is a little

wall and so on.

And the Porsche loses energy.