Transcription
We've spent the last three talks now looking at how x-rays that we've created at the X-ray tube interact with matter, interact with the tissues of the patient that we are imaging, and specifically, we've looked at diagnostic energy x-rays. Now, we know that some of those interactions result in ionization of atoms within our patient, and it's ionization, the process of losing an electron from an atom, that results in the detrimental biological effects within our patient. So, what are those two main interactions?
Well, the first is the photoelectric effect, where we get an inertial electron being released from an atom within our patient. Here, we've created an ion and we created a photoelectron. The second is Compton scatter, where an incident x-ray deflects off an outer shell electron, releasing a photoelectron into the tissue, and that x-ray has been scattered off at an angle. In both cases here, we've ionized an atom. A single x-ray has ionized a single atom.
Now, you may think that this x-ray interaction with a single atom is what's responsible for most of the biologic effects within our patients, and actually, that's not the case. What's responsible for most of the damage is this photoelectron that will travel through tissue. One x-ray has resulted in one ion. This photoelectron, this charged particle moving through tissue, will result in thousands of ionization events occurring. This is where the dose is imparted on our patient, and that process of transferring energy over a set path is what's known as linear energy transfer, the primary mechanism for the biological effects of ionizing radiation.
So, linear energy transfer can be divided into two specific categories. One is called high LET, high linear energy transfer, and the second is called low LET. Now, we get the transfer of energy into tissues when a charged particle travels through matter. We can either have a high linear energy transfer, where a large charged particle, such as an alpha particle here, travels in a direct line and ionizes surrounding atoms, or we can get low LET, where we get a small charged particle, such as an electron, traveling through tissue and interacting with the electrostatic force, repelling adjacent electrons, causing ionization events in these surrounding tissues.
Now, the distance from where that particle starts in a tissue to where it rests, that direct line, as the crow flies, here, is what's known as the range. That distance traveled, the straight line from start to finish. Now, the actual path in a low LET event is longer. Here, it goes all the way across. So, our range is the short distance. Our path, the actual path the electron travels, is a longer distance here. Now, when we have heavy particles like this, like an alpha particle or a proton, think of that as a bowling ball going through bowling pins. The bowling ball knocks over all those pins, makes a destructive path. And think of this low linear energy transfer as a marble going through bowling pins. That marble is going to bounce around within the bowling pins, take a much longer path until it gets to a resting point.
Now, the reason that x-rays themselves aren't causing all of this ionization is because x-rays are uncharged. Linear energy transfer happens when a charged particle moves through a tissue. That charge is what's imparting the energy onto those surrounding atoms. So, today, we're going to be looking at low linear energy transfer, an electron, a photoelectron that's been released either through the photoelectric effect or through Compton scatter into our tissue. We see what that electron does as it travels through tissues.
Now, we can see that that electron travels through a random path as it interacts with surrounding electrostatic forces of other electrons within our tissues, and as that electron moves, it loses kinetic energy and imparts more and more dose on our patients. Now, the linear energy transfer, the transfer of energy from a charged particle traveling through tissue, is proportional to the square of the charge of that particle. The higher the charge of the particle, the more energy that is imparted to surrounding tissue, and it's inversely proportional to the energy of that particle, which may be counter-intuitive at first. The faster an electron is traveling through a tissue, the less chance it has to impart that energy on surrounding tissues. That electrostatic force of repulsion has less chance the faster it is for traveling.
Now, the way I like to think of this is if I'm skimming a stone off a lake. The faster I throw that skimming stone, the more that stone spends time in the air, not interacting with the surface of the water. And initially, when it bounces off that water, it's imparting very little kinetic energy to the water, and the distances between each bounce is far. Now, as that stone gets slower and slower, more and more energy is being imparted to the surrounding tissues, and the distance between each skip is getting more and more. As the kinetic energy decreases of the stone, the more energy is being released into the surrounding water until it comes to a stop, where all that kinetic energy has now been released into the water. The same thing is happening as an electron is traveling through tissue. Initially having a fast or a high kinetic energy, we can see that as electron energy is really high, we get very low levels of linear energy transfer, kiloelectron volts per micrometer, the amount of energy transferred per unit distance. As that electron slows down, as it loses kinetic energy, more and more energy is transferred to surrounding tissues. So, we can see that as the electron gets further along in its path, it releases more and more energy within the tissue.
Now, this actual distance here within our patient is very small, but it is responsible for thousands of ionization events. And as we move on later into our nuclear medicine block, we're going to look at high LET interactions, high energy transfer interactions, and we'll see that those alpha particles or protons impart much more dose over a much shorter distance and have larger biologic effects within our patients. So, linear energy transfer is the process that's responsible for most of the ionization events when we are dealing with x-rays in the diagnostic energy range. That initial x-ray is what sets off that photoelectron and then sets off the sequence of events that results in energy being deposited within our patients, creating ions within our patients, hydroxy ions, free radicals that are responsible for the biological damage that we see in x-ray radiation.
So, when asked about whether an x-ray ionizing a specific atom is responsible for the patient dose, it's indirectly responsible. It's the release of that photoelectron, and that photoelectron is responsible for most of the dose imparted on the patient. Now, again, as a reminder, I've linked a question bank that I've created below. If you are interested in that and you're studying for your part one Radiology exams or your registry radiography exams, go and check that out. You might find that useful. Otherwise, in our next talk, we're going to look at the linear attenuation coefficient, how we can predict how x-rays interacting with matter are attenuated as they travel through tissue. So, I'll see you all in that talk. Goodbye everybody.