Legacy · Direct Application
Medical Imaging
In a PET scanner, matter and antimatter annihilate inside the patient and E = mc² turns their mass into the light that makes the picture.
How a PET Scan Works
PET stands for positron emission tomography. The patient is given a small dose of a sugar or other molecule tagged with a radioactive atom. The body carries it to wherever that molecule is being used most actively, such as a tumor or a busy region of the brain.
As the radioactive atoms decay, each emits a positron: the antimatter counterpart of the electron. Within a millimeter or so, the positron meets an electron, and the two annihilate.
The Equation at Work
Annihilation is mass–energy equivalence in its purest form. The entire mass of both particles disappears and is replaced by two photons of gamma radiation, flying off in opposite directions.
Each photon carries precisely 511 kiloelectronvolts, the energy equivalent of the mass of one electron. Scanners are built to look for exactly that energy.
A ring of detectors around the patient registers the two photons arriving at the same instant on opposite sides. The annihilation must have happened somewhere on the line between them. Millions of such lines, combined by a computer, build a three-dimensional map of where the tagged molecule has gone.
Detecting the Photons
The detectors that catch these gamma rays convert them to flashes of visible light and then to electrical signals, using the photoelectric effect. A PET scanner uses two of Einstein's 1905 papers at once.
Radiotherapy
The machines that treat cancer with beams of radiation accelerate electrons to nearly the speed of light. Their design has to use special relativity, because at those speeds Newton's mechanics gives the wrong answers.
How Direct Is the Link?
For PET, direct: the number the scanner is tuned to is for an electron. For radiotherapy, supporting: relativity is a necessary part of the engineering, not the idea behind the treatment.