PET Scans, CAT Scans, No Dog Scans

PET Scans are amazing. Not only do they provide information to physicians about the extent of malignancy, in preparation for various treatments, they also provide insight into biology and physics!

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Anterior view and lateral view. White areas are places of high glucose absorption. This is normal in the brain, with subtle outlining of blood vessels in the neck and arms. Any muscle that moves will take up glucose, which is why technicians ask patents to lie still. But everybody swallows their saliva, so muscles of the face and throat tend to light up slightly. There is glucose in the blood, so you can vaguely see blood vessels, and background haze of the liver, and the chronic muscular action of the bowel is vaguely outlined as well. The radioactive material exits via the urine, so you see hot spots in the abdomen where the kidneys are. And the heart keeps pumping, so it is dull grey. The extremely white area next to the heart in the anterior view (look at the lateral view: it’s near the back while the heart is near the front) is the tumor, plastered up against the chest wall on the inside.

 

The above image may not look like much: it is simply two slices, anterior and lateral, from an MIP construction of the PET scan.If you look carefully, you can sort of see that the image extends from the level about half way through the skull down to the waist level. See the vague outline of the arms going up over the head? The two slices are actually part of a 3-D image that a we can rotate in space.

To understand what it is showing, you need to know a little biology, particularly of cancer, a little physics, particularly Einstein’s Theory of Relativity (believe it or not), and a little medicine.

My book, Trojan: Hollow Moon of Jupiter, highlights the use of PET scans in the investigation of deaths of street people occurring in the cities of Trojan. In this scene, Donald Garth is a physician, son of the famous James Garth from the book Trojan: Nefra Contact. Garth describes to the two TPD Detectives, Rosie Williams and Simeran, just how a PET Scan works, and what it shows in Dicky’s (the victim’s) brain:

Garth turned to his laptop to control the view of the monitor. The lights of their room dimmed, and the monitor on the wall, built into the wall really, brightened in comparison, showing the image of a white brain with dark grey surrounding structures of bone and soft tissue.

“PET scans highlight areas that take up glucose,” Garth said. “Not like CT scans, CAT scans—we have PET scans, CAT scans, but no dog scans, not yet. CT scans work from penetration values of non-ionizing radiation, like multiple x-rays rapidly taken around the patient. A computer calculates a value of each point in 2-D space at each slice, depending on how the radiation penetrates along that particular line.

“Computerized Axial Tomography or CAT scan, shortened to CT scans in recent years. I guess because it is not always axial, anymore. CT scans on Dicky showed us nothing. No hemorrhage, no infarct, though those often don’t show up so soon after the event. Well, not completely nothing, but the findings on CT are really subtle—I’ll show you that later.

“PET Scans are Positron Emission Tomography. The stuff of Flash Gordon.” Garth saw the confusion on the faces of his audience of two. “An old Earthside TV program of pretty archaic science fiction, some of it just laughable, demonstrating the lack of knowledge of the 1950’s audience. Nobody knew any physics back then. Really, it was quite appallingly amazing. You should see the old films. They’ve got representation of some space ship traveling along with smoke coming out of the tail and traveling straight up as it rose in the atmosphere, even though it was supposed to be out in space. I don’t even know if the film editors had any idea what smoke in weightless airless space would look like.

“Flash Gordon was the hero, and ‘stuff of Flash Gordon’ just suggests unbelievable science.”

“Sounds like it was unbelievable science,” Rosie said.

“Yeah, it was non-science—nonsense,” Garth answered. “Only a handful of scientists of that era understood Einstein’s theory. Barely nobody the General Theory, which really is difficult, I guess, but few enough believed the Special Theory. Remember, the General Theory was only proved in 1919, only a hundred and fifty years ago, when Eddington demonstrated the bending of light by massive bodies.”

Garth paused and looked at the two TPD. “I know,” he said quietly. “Your history is likely different. But for us, well, E = mc^2 was just wild, and it comes out of Special Relativity, after just two assumptions: that there is no preferred frame of reference, and that the speed of light is the same in empty space, no matter what your frame of reference is. Simple math and you work that all out. It’s quite amazing, really.”

“Hey, you’re not just a pretty face,” Rosie said.

“Rosie’s a physics major. Mother works for ArfenCorp. Physicist,” Simeran said.

The ponytail was going up and down.

“OK, so you understand. But positrons are the weird thing. They are anti-matter. The positron is the anti-electron. Exactly the same, except opposite charge. Same mass to the decimal point, same spin, same charge magnitude, just different sign. Positrons occur naturally as radiated particles from Fluorine-18, an isotope of Fluorine which is used to label glucose. So the glucose we inject into the patient produces positrons. Now, as anti-matter, the positron travels about a millimeter before it runs into an electron. There are not many positrons in our universe, but there are a shitload of electrons. When they hit, being anti-particles to one another, they annihilate, and produce two photons. Since momentum is conserved—”

“Very good, doctor,” Rosie said.

Garth grinned. “—the two photons are equal and opposite, and you can calculate the energy of the photons by the equation—” Garth waited for the answer, as Rosie, smiling conspiratorially, looked at Simeran.

“E = mc^2,” Rosie finally said.

Simeran frowned slightly. “With ‘m’ being the mass of the electron?” Simeran said slowly.

“Or the positron, they’re the same,” Garth said. “Comes out to 511 KEV. We can detect those photons with the PET scanner, and calculate the same kind of values as the CT scan, using axial tomography as the basis of the mathematics. So anywhere glucose goes is hot, white, intensity telling us how much. Anti-matter-matter annihilation is the stuff of Flash Gordon. Well, Einstein really. This stuff is real.

“So,” Garth pointed back to the monitor, “there is a lot of glucose up in the brain, because that is the energy source for the brain, exclusively, more or less, and anywhere that glucose doesn’t light up, is an area of brain that isn’t functionally working. Dead areas will be relatively black in this representation.”

So, I won’t tell you what they found in Dicky’s brain; that would be a spoiler. The point is that PET Scans are fascinating in their own right. They light up wherever glucose goes, and the biology of most cancers demands that the cancer cell absorbs a lot more glucose than their neighbors. About ten times as much, for each cell. That is because for most cancers, the breakdown of glucose does not go through the Kreb’s cycle, the tricarboxylic cycle, but stops at the lactate level. Since the Kreb’s cycle is a source of most of the ATP (or equivalent, some GTP, some NADH), the molecule(s) that carries usable chemical energy in the body, the cell has to absorb ten times the glucose to create enough ATP, just to keep up.

With increased rate of growth on top of that, some cancers can absorb up to 200 times the amount of glucose of their surrounding tissues. This was first suggested in the Warburg Hypothesis (Otto Heinrich Warburg, Nobel Laureate). This is part of the explanation why a patient with half a kilogram of cancer total in their body can become so emaciated … it’s taking a lot of the glucose!

So if glucose is labelled with a radioactive material that can be detected, one can ultimately obtain pictures like the one above (and many more, of course). Because this actually works, it is supportive evidence for the Warburg Hypothesis, and Einstein’s Special Relativity.

Two particles of matter and anti-matter really can, and do, come together to annihilate one another, producing pure energy (no mass any more) in the form of light, and the energy of that light (and therefore the color) can be calculated form Einstein’s famous equation (btw, for those truly picayune among you, I know I ignored the neutrino).

Unfortunately, perhaps, for us, the light produced by positron electron interaction is not in the visible spectrum (and so has no color), it’s actually in the X-ray region, but we can detect it, and then project something we can see in these images.

Take another long look at the PET Scan above. See the little tiny hot spot over near the right armpit (we call them axillae!). Unfortunately, that is a deposit of cancer, actually in the right lung near the edge, that could not be seen on routine plain x-rays, and just looked like an old scar on CT scans.

PET scans can be used in the brain too, though with a different intensity level than shown here, because there is a lot of uptake in the brain. Dicky, the victim in this mystery who is residing in the Ortho Burn unit under the care of Dr. Donald Garth, has some peculiar findings that help to clinch the investigation into a number of deaths. He is in the Trojan Ortho Burn unit because of extensive burns to his body, and the only hospital in the solar system which uses ‘no gravity’ suspension as part of its therapeutic plan.

But, sorry, to learn more about all that, you’ll just have to read the books.

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