Friday, December 21, 2007

What color is your PET?

Contributed by Michael H. Brill, Datacolor
Appears in ISCC News, No. 430 (Nov-Dec 2007)

Last September I was an “accompanying person” at a conference on molecular imaging. It might seem close to my area of expertise, but it is not. Accordingly, my color-scientist thinking led to a strange take on concepts in that field. Ostranenie (from the Russian остранение) is the literary device of forcing an audience to see something in an unfamiliar way, to enhance perception of the familiar. An example is to refer to driving a car as sitting on top of repeated gasoline explosions, or to refer to the human brain as electrified meat. For lots of ostranenie, read any book by Kurt Vonnegut---or be an accompanying person.

At the conference, I heard about positron-emission tomography1 (PET). The PET scan is a diagnostic tool whose cuddly name hides the fact, salient at the conference, that the imaging events are positron-electron (matter-antimatter) annihilations that are made to happen inside your body. Now, an electron is a sizeable particle whose complete annihilation (and also that of an injected positron with the same mass) produces a lot of energy. “Is it safe?” I asked, with none of the menace of Szell in Marathon Man. “It’s been known to be safe for three decades,” was the reply, replete with the condescension I’ve come to associate fondly with the medical profession.

Seeking confirmation from my own meager knowledge, I tried to cast this problem in a framework familiar to color science: Find the wavelength associated with the energy of a positron-electron annhilation. Does it convey warmth by the mechanism of a heat lamp, sunburn you, incur the dose-calibrated ionization damage of a dental X-ray, or more generally rock your world? (In the title to this column, I loosely call “PET color” the PET-induced-photon spectrum.)

Everyone I asked at the conference knew that a positron-electron annihilation liberates two photons, each with 511 KeV of energy, in opposite directions (to conserve momentum). But nobody had turned that number into wavelength, so I did a back-of-the-envelope calculation:

PET photon energy 511 KeV converts to energy E = 8.186 x 10-14 Joules (J), via 1.602 x 10-19 J/eV,. Planck's constant is h = 6.626 x 10-34 J-sec, and c = 2.998 x 108 m/sec. Hence the PET photon wavelength is h c/E = 0.002427 nm. At this wavelength, a photon from PET has far more energy than one from a dental X-ray (~0.06 nm) or chest X-Ray (~ 0.03 nm), and in fact is near the shortest wavelength attributed to X-Rays (0.001 nm).

Given that PET positrons produce hard X-rays in our fragile bodies, how can this be safe? Presumably, the total dosage of radiation in a PET scan is low, even though the energy per photon is high. Let’s check the plausibility of this assumption. One application of PET is to see brain metabolism through uptake of a radioactive glucose analogue that emits (you guessed it) positrons. The pseudocolors that are used to encode the metabolism level in the PET image1 are, of course, correlated with radiation emitted from the affected brain cells when the positrons annihilate with local electrons. One would think the radiation dosage to those brilliantly metabolizing brain cells would be quite high, even for relatively low average dosages in the whole brain.

Ultimately, the dose depends on how many photons are needed for a PET scan. Although tomography implies a volume scan and hence a lot of photons needed to light up the right pixels, a PET scan has a spatial resolution of 5-6 mm (much coarser than most other diagnostic images). Relative to diagnostic X-rays, PET scans need fewer photons per pixel to combat shot noise. But a lot of photons are wasted, because the image is captured only in a short cylinder around the affected area. And surprisingly, the photons do less damage to the local tissue than ionization due to the original positron.

As I found out, the computation of radiation dose is serious business in radiochemistry. Empirical studies (mostly on mice) make model fits to vulnerability as a function of such variables as organ type, metabolic rate, uptake rate, and geometry. The complication quickly exceeded the envelope I was writing on.

Back to color science, then. Can we tweak those PET pseudocolors so they’re more informative to doctors? Don’t even try: doctors are used to the present colors. Okay, back to literary devices then, such as ostranenie. That seems safe enough.