Transcription
So, let's look at our final x-ray interaction with Mata mechanism: Rayleigh scatter. Now, you may hear this called elastic scatter or coherent scatter; they're all synonyms for the same event that's occurring.
Now, you may also hear of one more x-ray interaction known as pear production, and that happens at much higher energy ranges than our diagnostic energy ranges. Later on, when we're in our nuclear medicine module, I will touch on pair production then. But for now, I want to keep it at photoelectric effect, Compton scatter, and Rayleigh scatter.
Now, when we looked at the photoelectric effect and Compton scatter, both of those processes produced a photoelectron. And that photoelectron, traveling through tissue, the electrostatic force that it gives to other electrons within the tissue is the dose that is applied to our patient. We had one incident x-ray that ionized one atom. That single x-ray was responsible for the ionization of one atom. When that photoelectron travels through tissue, it ionizes thousands of atoms. That single electron is responsible for thousands of ionization events to occur. So, we can see it's not the actual x-ray itself; it's that photoelectron traveling through tissue that imparts dose on the patient.
Now, why am I mentioning that here in Rayleigh scatter? Well, Rayleigh scatter doesn't infer those to the patient. We do not get the release of a photoelectron into our tissues.
So again, let's just touch on the three interactions: transmitted, attenuated, or scattered. Rayleigh, elastic, coherent scattered happens; it's a scattering event here. It's occurring along this pathway here.
Now, what is Rayleigh scatter? Well, we have an incident x-ray again, coming from our x-ray machine, that interacts with an atom in our patient. Now, Rayleigh scatter generally occurs at larger wavelengths of our incident x-ray: larger wavelengths, lower frequencies, lower energies. And generally, when the atom that is interacting with that incident x-ray is smaller than the wavelength of the incident x-ray, we can get this phenomenon known as rayly scatter.
The incident x-ray interacts with all of the electrons within that atom, and those electrons oscillate; they vibrate in place at a set frequency. Now, when those electrons stop oscillating, come back to rest, they release electromagnetic radiation that is identical to the incident x-ray. But that x-ray release is at an angle to this incident x-ray. That's why it's called Rayleigh scatter; we get a scattered x-ray.
Now, this scatter angle is independent of the incident x-ray energy. Now, when we looked at Compton scatter, the scattering angle was dependent on the incident x-ray energy, as well as dependent on the amount of energy that we've given off to our photo electron. Here, this scatter angle is independent of this incident x-ray energy.
So, let's look at this finally on our complete graph that includes our photoelectric effect, our content scatter, and Rayleigh scatter. We see that Rayleigh scatter decreases with Photon energy. We see that as our Photon energy increases, our frequency increases, our wavelength shortens, and therefore, the likelihood of the Rayleigh scatter or coherence scatter to occur decreases exponentially here.
And when we look at diagnostic energy ranges, say between 30 and 80, for example, our contribution of Rayleigh scatter at these low energy ranges is very small compared to content and photoelectric effect. Again, this is not a linear scale on our y-axis. So, if we look at a photon energy of 30 here, we've got a mass attenuation coefficient of 0.03 here for Rayleigh scatter, and a mass attenuation coefficient of 0.3 for Compton and photoelectric effect, 10 times higher there.
And that's why Rayleigh scatter rarely comes up on its own within an exam. It's often part of a multiple choice question, or it's part of a list when you have to list the types of interaction that can happen in matter. And a really common question that comes up is, "Does really scatter contribute to Patient dose?" And we've seen before, there's no photoelectron being released; therefore, no dose released onto that patient.
So, I hope going through these three different types of interactions helps you understand how x-rays interact with matter. Next, we're going to look at how dose gets inferred to a patient and how we can measure those within the patient, as well as how we can measure the penetrability of our x-ray beam. After that, we will be looking at the x-rays that leave our patient—be transmitted and scattered X-rays—and see how they interact with our x-ray detectors: our screen foam detectors, our computed radiography detectors, as well as our direct and indirect digital radiography systems. So, I'll see you all in the next talk where we're going to look at the concept of linear energy transfer. I'll see you there. Goodbye.