In this week, we are going to learn about contrast in imaging and the art of segmentation. The first part, the imaging contrast has to do with what effects it is that creates the contrast in the images. This is determined by how the X-rays interact with the materials as it passes through the material. This is what determines how dark an object will look next to another object in the image, and this contrast is what we later on we'll use in our segmentation step in order to differentiate between the various elements in the material. But first, let us hear more about this X-ray matter interaction, and how it determines contrast. This is what Manuel will be speaking about now. Any electromagnetic waves, X-rays, optical, they all experienced a change of amplitude and phase when they traverse a material. So, here you can see in the little sketch that you have a wave that is propagating in free space in the upper part, and the wave propagating through a material in the lower part. So, here you can see that as the wave traverses the material, the amplitude gets reduced, so this is basically amplitude contrast or absorption all materials have some absorption contrast. But also because the speed of light inside the material is different than free space propagation, you also get a phase effect, you get retardation of the wave effect. So, basically if you follow one of these peaks, and compare it to the peak of the propagating wave on the air, you can see that there is a delay between these two crests over here, and this is basically what we call phase contrast. So, what happens as the X-rays passes through the material is that the amplitude is attenuated, the amplitude of the waves representing the X-rays dampened as they go through the material, but also the wave fronts of the wave are shifted with a certain amount. This is what is called the phase contrast and the formula is called the amplitude contrast. The formula, the damping of the amplitude is what creates the shadow images that we know for example from classical radiography where a heavy material will cause a dark shadow in the image as the X-rays are passing are being absorbed by the heavy material in the matter. The phase shift on the other hand is more enlightened to refraction of rays, so the bending of rays as they pass through the sample and the wave-fronts are shifted relative to one another as they go through the material. We have a small example here, we can show you that perhaps exemplifies this in a more intuitive way. If we look at a pair of glasses where a point source far away a light source in the form of a concentrated focused light hits the glasses as they are next to our detector in this case our screen, we see that when we are in what we call the contact region really close to the detector, receive mainly the shadowing of the glasses the mainly the rim and the bars of the glasses, and very little contrast from the glasses themselves, but as we move out the glasses from the detector plane, we start to see the refraction effect the bending of the rays which is called caused by the phase shift of the light rays as they go through the material. As you could see it's relatively easy to construct a simple experiment that will show you the effect of phase contrast for visible light, it is however much more difficult to do with X-rays because of the refractive index for X-rays in matter is a very small amount smaller than one which means that the refraction angles are much much smaller than they are for visible light. So, what you will normally see will be the amplitude contrast images that you know as the classical X-ray radiographs, and this Manuel has more about. So, normal detectors cannot measure the phase, they don't have access to the phase, they just can measure the attenuated wave that went through the object or this shadow object, and so basically many techniques are limited to measuring with absorption contrast, so here you see the equation for absorption or intensity contrast. Although as Manuel says the typical X-ray detectors do not detect phase shifts directly and thus not phase contrast, it can however be interesting to get access to the phase contrast. There are other ways of doing that, and that is because the phase contrast is sensitive to the properties of the material in a different way than the absorption contrast is. Manuel has some more explanation about this. So, now talking a little bit about phase contrast versus absorption contrast, when you move to higher and higher photon energies, the difference between the real and the imaginary part of the index of refraction becomes larger and larger as you can see in the plot that I put there on the slide. So, when you're using this for example this 10 keV energies, these high energies for imaging, the contrast that you can get by using the phase can be up to two orders of magnitude better than what you get using the absorption, and this plot for example is particular for calcite, and you can see really that almost two orders of magnitude better signal can be obtained if you can get access to the phase, but as I mentioned before this information is normally not directly available to intensity detector. Again you heard this mentioning of keV, which is a term that Manuel has used to before which refers to the energy of the X-rays. This is directly related to the wavelength, so that's another way of talking about wavelength of X-rays, and the energy is measured typically in kilo electron volt, and that is what the keV refers to. So, higher keV means higher energy of the X-rays, which also means that they are able to penetrate through more material, but without losing too much on the other hand at the same time of the phase contrast as compared to the amplitude contrast. So, what Manuel is saying here is that as you go through the harder higher energy X-rays, you can still observe using appropriate methods phase contrast while having only a weak attenuation of the X-ray beam. We will return to the concept of amplitude contrast and phase contrast in a little bit more detail in the coming videos you notice that we skipped rather quickly across some of the formulas, but this we will dive into in more detail now.
