How Brain Scans Work

How can we study human mental and psychology phenomena? For researchers, knowing the human brain activity is essential, since the brain works as the major control of mankind. Brain imaging or brain scanning, widely known as the “brain scan”, is a product of modern technology useful for studying more about the brain and its activity; and also detecting abnormalities that may exist in the brain.

There are several kinds of brain scans. People perhaps only know a few of them, for example EEG (Electroencephalography) and CAT (CT) scan, which are quite popular. Actually there are more, like PET (Positron Emission Tomography), MRI (Magnetic Resonance Imaging), and MEG (Magnetoencephalography). Here is a brief explanation about brain scans and how they work.

EEG (Electroencephalography)

EEG or Electroencephalography is a process of recording brain activity by measuring the electrical activity of the brain. The result can be used to know whether there are problems or not.

EEG uses several disks of electrodes, between 16 and 25 disks with different positions on the scalp. The electrodes will pick up the electrical signal produced by each brain cell. Sticky paste is used to prevent the air gap between the disks and the scalp, so the electrical signal can be transmitted easily from skin to the disks without air resistance.

The received signal will be transmitted by wires from each electrode to an amplifier. Since the electrical signal from the brain is pretty weak, the usage of an amplifier is quite important to amplify the signal so it can be read on the next destination, the recording machine. The recording machine converts amplified electrical signals into patterns that can show up on the computer monitor or be saved in a computer disk. Before the computer, a galvanometer hooked with a pen was  used to trace the signal onto graph paper. Since the galvanometer can detect small electrical signals, the process doesn’t need an amplifier. The result of EEG, whether it is patterns on screen or traces on paper, can give information about brain activity. Different states or conditions of people in tests, may lead to different pattern characteristics – therefore, it can be used to detect abnormalities by comparing the result with the patterns from a normal condition.

PET (Positron Emission Tomography)

By using radioactive glucose, PET can be used to observe blood flow and metabolism of tissues and organs inside the body, including the brain. The main principle of PET is this: The more glucose a part of an organ consumes, the more active it is. In other words, the amount of radioactive glucose left after absorption indicates how a part of an organ is functioning.

The test usually begins by injecting the radioactive glucose, called radiotracer, into the vein of arm. A more active brain part will absorb much of these, while a less active brain part won’t. In the PET scanner, the radioactive chemical will emanate an amount of energy, which is detected by the scanner and recorded, then converted into a 3D picture by using the computer. Different amounts of radioactive chemical left, emanate different amounts of energy, therefore resulting in different colors. For the brain, usually the color of “red” indicates a more active part, while the color of “blue” indicates a less active part. The cross sectional view also can be taken from any angle to detect functional problems in the deep part of the brain. With this capability, PET is one of the most popular brain scans among the neuroscience researchers.

CAT (Computed Axial Tomography)
CAT or CT (Computed Tomography) scan uses multi x-rays in 2D to generate a cross-sectional view and 3D images of internal organs, including the brain. The images later can be used for diagnosing problems in the organs.
In the process, the CT scan involves a machine that moves around the patient and takes many x-rays while it moves. By using a connected computer, the two dimensional x-ray pictures then can be combined into three dimensional images. The 2D pictures itself are the cross-sectional view of the 3D model, and can be monitored or be printed.

MRI (Magnetic Resonance Imaging)

The main principle of MRI (Magnetic Resonance Imaging) with the CT scan is actually the same, which is generating three dimensional images of internal body structure by using a multiple of cross-sectional views, and two dimensional images. The difference is on the media it uses. MRI uses magnetic field and radio waves to produce images, while CT scan uses x-rays.

Patients are usually asked to lie down on a table. An injection of contrast material is sometimes given to enhance the visibility of certain tissues or blood vessels. The table will then move into a tunnel that generates a magnetic field and radio waves. The magnetic field is produced by passing an electric current through wire coils in most MRI units. Other coils, located in the machine, send and receive radio waves, producing signals that can be detected by the coils

A computer then processes the signals and produces images which show thin slices of the body. It is just like the CT scan, but MRI can give a more detailed result; so it can be used to diagnose certain diseases that can’t be done by the CT scan.

MEG (Magnetoencephalography)

MEG or Magnetoencephalography is a new technology of brain scanning which provides the most accurate resolution of brain cells activity. MEG works by detecting an extremely weak magnetic field, about 10-12 Tesla, which is created by the brain. Because the magnetic signals are so weak, a super-sensitive detector is needed.

The detector is called SQUID (Superconducting Quantum Interference Device). SQUID needs to be bathed in liquid helium to gain its superconducting operating temperature which is -269 degrees C. In MEG, SQUID is placed over the subject’s head to detect the weak magnetic signals of the brain. The signals are then processed into images. One advantage of MEG is that the magnetic fields can be measured with a high temporal resolution, making it become the only brain scanning which can track the movement of activity around cortical networks at the speed at which the brain works.