Modern brain imaging techniques like PET and MRI (Magnetic Resonance Imaging) are becoming indispensible to researchers studying addiction and its effects on the brain. That is because addiction research requires looking inside the brain at areas where both drugs and natural chemicals act. Researchers can now determine how quickly drugs reach
receptors in the brain and how long they stay there. Or, view changes in brain activity after long-term drug use, during craving
or withdrawal, or following various treatments
for drug abuse and addiction.
Before a PET scan begins a patient is given a safe dose of a radioactive compound. If the purpose
of the PET scan is to study brain activity, doctors and scientists choose FDG (fluorodeoxyglucose), which is a modified glucose molecule.
Glucose is a type of sugar, and it is the main energy source for the brain. The injected or inhaled FDG will enter the person's bloodstream, where it can travel to the brain. If a particular area of the brain is more active, more glucose or energy will be needed there. The more glucose is used, the more radioactive material is absorbed.
FDG is a normal glucose molecule that has been attached artificially to a radioactive isotope of flourine. FDG can be absorbed by cells just like normal glucose.
The PET scanner measures energy that is emitted when positrons (positively charged particles) from
the radioactive material collide with electrons (negatively charged particles) in the person's brain.
The scan can take between 30 minutes to two hours to complete.
A computer then turns these measurements into multicolored two- or three-dimensional images. The result is a colorful picture showing which parts of the brain were most active, based on the amount of glucose being used there.
To measure the amount of radioactive material absorbed by the brain, a person lies on a moveable bed that slides into the tunnel-like opening of a device called a PET scanner.
To create the colorful PET image, a computer displays each measurement as a series of tiny dots. The color of each dot indicates the intensity of the energy that is recorded. Red indicates the highest intensity-in other words, the area of greatest brain activity.
Very High Activity
A new technique called functional MRI can also be used to measure brain activity. MRI detects
changes in blood flow rather than the quantity of a radioactive tracer.
When a particular site in the brain experiences increased activity, there is a sudden rush of blood flow to that area. This blood replenishes the oxygen used by the hard-working brain cells. By tracking variations in blood flow, functional MRI can detect active sites in the brain in real time.
An MRI machine looks a lot like a PET scanner but it has the added feature of an invisible magnetic field.
This is useful because certain atoms (like hydrogen, a major component of water) give off a wave
of energy when surrounded by giant magnets.
Since blood contains lots of water molecules and therefore lots of hydrogen atoms, the hydrogen atoms will produce pulses of energy when a person is immersed in a magnetic field. The energy emitted reflects increases in blood flow (and therefore brain activity) and can be detected by a computer.
MRI also differs from PET in that the energy pulse detected by the computer is in the form of radio waves rather
than gamma rays. (A PET scanner detects gamma rays that are produced when positrons from the
radioactive tracer collide with electrons in the brain.)
A form of energy called radiation travels through the atmosphere in waves that can be detected by a computer. The shorter the wavelength, the greater the energy. For example, gamma rays (detected by PET) contain much more energy than radio waves (detected by MRI). Frequent and prolonged exposure to high-level radiation such as ultra-violet waves and gamma rays is known to cause DNA damage. However, small, short doses of radiation (such as in PET scans) are generally considered relatively safe.
MRI machines detect and record radio waves, while PET scanners measure gamma rays.