Looking inside the human body using positrons

PET or Positron Emission Tomography scans have become an important aspect of medical imaging and diagnoses, allowing doctors to look into the human body as never before. PET scans are different from other medical imaging techniques because they do not actually look at the body itself. Instead, PET scans look at bodily process by detecting the decay products from radioactive tracers injected into the body. Radioactive tracers are designed to mimic naturally occurring substances and tend to deliver less radiation than an X-Ray.

When the radioactive tracers decay inside the body, they release a positron, the antimatter equivalent of electrons. When the positron encounters one of the billions and billions of electrons inside your body, it annihilates in a flash of light called gamma rays-it is these gamma rays that are the positron emissions which PET scanners detect. The donut ring of the PET scanner is lined with gamma ray detectors.

Alternative Medical Imaging Techniques

There are many types of medical imaging techniques available to doctors and patients. Each techniques use slightly different technologies, allowing the doctor to focus on different aspects of the human body.

The simplest and most common technique used to look into the human body is the X-Ray. X-Rays are high-powered beams of light that can pass through some parts of the human body, like skin, but are stopped by other parts, like bone. Using X-Rays to expose film (much the same way that visible light exposes film in a camera) allows doctors to examine the skeletal structure with ease.

CT or CAT, Computerized Axial Tomography, scans also use X-rays but in a slightly more complicated way. A CAT scanner consists of a donut shaped ring containing numerous X-Ray tubes that shoot out beams of X-Rays. Patients slide through the donut hole while X-Ray detectors that are also around the donut ring record the X-Rays after they have traveled through the body. The computerized component of CAT scans involves interpreting all the X-Ray signals and combing them into a coherent image. Because CAT scanners only take X-Ray pictures of your body in thin slices, computers are needed to combine each thin slice into a comprehensive three-dimensional image. This technique of looking at the body in thin slices is called Tomography. CAT scans image the body in slices perpendicular to the axis stretching from the feet to the head. These slices fall in the axial plane of the body.

		Figure 1. How the PET scan works: Radioactive tracers are injected into the patient before the PET scan. When these decay, the emit a positron which annihilates with electrons present in the body producing two gamma rays in opposite directions. The PET scanner detects all rays resulting from these sorts of interactions and a computer produces a final image.

Ultrasound imaging uses a completely different technology than X-Rays. An Ultrasound probe emits ultrasonic sound waves, sounds above the range of human hearing, into the body. While most of these ultrasonic sound waves travel straight through the body, a small amount reflect off the transition layers between different types of tissue. The echoes from these transmission layers are picked-up by the Ultrasound probe, fed into a computer and then rendered into a two-dimensional image. Ultrasound technology uses sound echoes to look into the human body much the same way bats and submarines use sound echoes to see.

MRI, or Magnetic Resonance Imaging, uses magnetic fields to peer into the human body. The MRI machine contains a very large magnet that surrounds the patient with an intense magnetic field. When patients are immersed inside this magnetic field all of the billions and billions of hydrogen atoms inside their body align with it. Inside the MRI machine, radio frequency pulses are applied to specific parts of the body to excite some of these hydrogen atoms. Images are created based on how the exited hydrogen atoms lose their energy. These images are fed into a computer to generate an incredibly detailed look inside the human body.

How is PET different?

PET scans are different from all the other types of medical scans because they never actually look at the human body itself. Instead, PET scans use radioactive chemicals to look at different processes inside the human body. The actual PET scan machines are similar to CAT scan machines as they are both donut shaped and use axial tomography. But unlike CAT scanners, the donut ring of PET scanners is lined only with detectors. These detectors are used to pick up positron emissions.

But what on earth is a positron? The simplest answer is that a positron is an anti-electron. So what is an anti-electron? There are billions and billions of electrons in your body. They are the negatively charged parts of atoms which orbit the positively charged nucleus. Every one of your atoms has electrons and electrons are considered to be what physicists call matter. Positrons, or anti-electrons, are antimatter-having many of the same properties as matter but with opposite electric charges. An electron is negatively charged while the positron is positively charged (electrons are sometimes called negatrons to differentiate them from positrons).

The important thing about positrons is that when they encounter electrons, they annihilate in bursts of light called gamma rays. These gamma rays are the positron emissions that the PET scanners detect.