For those of you who don’t know, I’m a Master’s Student in Medical Physics. Let me beat you to the punch: Medical Physics is a wide discipline, but deals largely with two distinct fields. The first is Medical Imaging or diagnostics, which uses different techniques to image the body and diagnose pathology, namely cancer. The second is Medical Treatment, which I happen to be interested in and, therefore, what the subject of my thesis. About 90% of what we as MedPhysicists deal with is cancer, and my side of things deals almost exclusively with how to use radiation to treat cancers, or how to improve the quality of life for a patient.
When I was asked, or rather coerced, into posting something scientific, I thought about what someone with a modest scientific background would find interesting. I decided to eighty-six my research about treatment and go with the other side of the coin, the diagnostic imaging part . Although I am by no means an expert on this, (hell, there are kids in my program who know a lot more than I do about this) I figured it might be more interesting and take less scientific explanation to go this route. (If I was way off base, I apologize. Next time I can lecture about why not to stare at the sun) So here we go. Strap yourself in.
Diagnostic Imaging uses many different techniques to image the body. The one we’re probably most familiar with is the radiograph, or x-ray. Wilhelm Roentgen discovered these wonderful beams of death (just ask Marie and Pierre Curie) in 1895 and received the first Nobel Prize in physics in 1901. Things have been moving by leaps and bounds since then, but the really interesting stuff has happened in the last 30 or so years. The radiograph is about the same as it was back then; you accelerate electrons toward a source and just the bending of the electrons paths near a positively-charged nucleus (usually Molybdenum) causes an x-ray to shoot off. You can steer this ray through a patient and onto a fluoroscopic film on the other side. Do this a few million times, and the ones that hit air will go straight through, most of the ones that hit tissue will go through, and very few of the ones that hit bone will make it out on the other side. Hence the spooky skeleton business. What a radiograph measures, then is absorption. Dense stuff absorbs more, etc. This was all well and good, except radiographs kinda sucked, and guess what, they still do. But they’re cheap, and techs can’t screw them up as easily. So what came after Roentgen? Without going into all the advancements in medical imaging, let me hit some major steeping stones along the way.
In the 70’s some wacky dudes in the Midwest figured out that if you took little slices (~1 mm thick) from the front, back, sides and some angles in between you could use this new, kick-ass thing called a computer to assemble all the projections to give an image! Do this every millimeter down the length of the body and damned if you don’t get a 3-D image. This is a CT scan, or “cat” as you might hear it called. It stands for Computed Tomography, and although it gives you a seriously substantial dose of radiation, it is an excellent and widely-used imaging modality in modern times. Then you have ultrasound, which kinda blows but is cheap and doesn’t cook you, and MRI which still blows my mind but is way too complicated to get into here. So, if you’re still reading this, you’re probably asking when does the cool shit start? Right now, baby.
What do all of these things have in common? They image anatomy, but not physiology. They tell you what’s in there but not what’s going on. This brings me to the PET scan (Positron Emission Tomography). If you’re science-savvy, or, rather if you haven’t been living under the proverbial rock for the last 60 years, you probably noticed the word ‘positron’. No, a positron isn’t something we come across day to day. In fact they were theorized by some wicked smart quantum physicists last century and took a hell of a long time to find physically. They are anti-matter. I could bore you with WHAT anti-matter is all day, but this is a long post anyhow. Lets just say that for every little bit of matter, there is an identical piece of anti-matter, and whenever anti-matter touches matter, a lot of energy comes out (how much, we’ll see later). So a positron is the anti-particle of our electron, and only comes out through nuclear decays, the occasional cosmic ray, and hardcore experiments like the Fermilab or CERN in Switzerland
So what the hell does anti-matter have to do with medical imaging? Some seriously brilliant minds figured out that when certain radionuclides decayed, they gave off positrons, but that wasn’t too interesting because they wouldn’t travel any appreciable distance before they hit some matter and annihilated. But they figured out this might be of interest, because, after all, the annihilation produces our old friend Mr. x-ray (I’m kinda lying here, if actually produces gammy rays, but they’re very similar). So, first, a little graphical help: this is what happens when a positron hits an electron. See Figure 1 So here we are. Notice that the two photons (x-rays) are moving in completely opposite directions and have the same energy. This arises from the laws of conservation of energy and momentum, which I refuse to explain (pick up a high-school textbook, you lazy bum). But why 511 keV you might ask? 1) What the hell is a keV and 2) why such an arbitrary number? First, a keV is a unit of evergy, called a kilo-electron-volt. No matter what the hell it really means, but lets say it’s VERY small. Next question. A little more schooling in physics (trust me on this) shows that Einstein was actually right, the crazy bastard, and energy and mass ARE interchangeable. Which means you can express how massive something is in terms of energy and vice versa. So particle physicists often express masses, or weights of particles in units of energy (strange, I know, but useful mathematically). Long story short, the rest-mass (if it were moving it would be heavier, also something we can thank Albert for) of the electron is 1.022 MeV (Mega-electron volt….a WHOLE LOT of energy), and when it disappears in the annihilation, that much energy has to come out; since there are 2 of the same thing, they split the energy equally (Don’t worry, they don’t have mass). (Side-note…what I just told you explains fission, fusion, nuclear weapons and nuclear power…think about it…maybe the topic for the next lecture?) Anyhow, this means that wherever the radionuclide, 2 photons would shoot off in exactly opposite directions. If you could somehow detect these 2 photons, you could draw a line between them and know that the annihilation, and therefore the radionuclide existed on that line. That’s exactly what the scientific community did. See figure 2. The patient sits in a big gantry, surrounded by a ring of detectors, and when two photons are detected at the same time the computer software uses complex algorithms to figure out where it came from. So the last question is how does this really apply to cancer? Different chemicals in the body are used by different organs, cells and processes. Your brain uses a lot of sugar, for instance. It’s like the brain’s food. On the other hand, Iodine is sucked up by the Thyroid gland in the neck. Normally these chemicals are normal and come from food and the world around us, but when this whole PET thing came about, people started to figure out you can change a few atoms on these chemicals and all the sudden they’re radioactive. Now relax, they’re not THAT radioactive, but they still decay by emitting positrons, which we like. Something called FDG is a sugar used by the brain. It’s actually called fluorodeoxyglucose, but that’s the point. You see the word fluoro in there? That’s usually a normal Fluorine atom on that big chemical chain, but we can throw on F-18, which is a positron emitter. The brain doesn’t know the difference chemically, but as soon as it goes there, the decay starts and bam, a picture of the brain comes out. What’s more, is that if there happens to be a tumor there, its using a lot more food to grow and spread, so more FDG is sucked up, and therefore, more shows up on the scan. Similarly, we use a chemical called Technetium 99m because it gets sucked up by the bones, and shows bone tumors. This is the ONLY modality that shows the body’s function and anatomy. Although these aren’t as high quality as conventional radiographs, they give info the x-ray simply can’t.
So people willingly inject anti-matter into their bodies. It can save lives, and does every day. The same principles that allow nuclear power and the wonderful ability to destroy mankind also allows us to see the body and its processes in ways Wilhelm Roentgen could never have imagined. Please feel free to send questions my way regarding the content here, and there are many more resources online, but I must warn you, none of them are this in your face, hardcore, and downright sexy. Rock and roll, Don Herbert. Rock and roll.— Matt Goss