Brain Imaging

Learning Objectives

  • Describe the types of techniques available to clinicians and researchers to image or scan the brain
You have learned how brain injury can provide information about the functions of different parts of the brain. Increasingly, however, we are able to obtain that information using brain imaging techniques on individuals who have not suffered brain injury. In this section, we take a more in-depth look at some of the techniques that are available for imaging the brain, including techniques that rely on radiation, magnetic fields, or electrical activity within the brain.

Techniques Involving Radiation

Image (a) shows a brain scan where the brain matter’s appearance is fairly uniform. Image (b) shows a section of the brain that looks different from the surrounding tissue and is labeled “tumor.”

Figure 1. A CT scan can be used to show brain tumors. (a) The image on the left shows a healthy brain, whereas (b) the image on the right indicates a brain tumor in the left frontal lobe. (credit a: modification of work by “Aceofhearts1968″/Wikimedia Commons; credit b: modification of work by Roland Schmitt et al)

A computerized tomography (CT) scan involves taking a number of x-rays of a particular section of a person’s body or brain (Figure 1). The x-rays pass through tissues of different densities at different rates, allowing a computer to construct an overall image of the area of the body being scanned. A CT scan is often used to determine whether someone has a tumor, or significant brain atrophy.

A brain scan shows different parts of the brain in different colors.

Figure 2. A PET scan is helpful for showing activity in different parts of the brain. (credit: Health and Human Services Department, National Institutes of Health)

Positron emission tomography (PET) scans create pictures of the living, active brain (Figure 2). An individual receiving a PET scan drinks or is injected with a mildly radioactive substance, called a tracer. Once in the bloodstream, the amount of tracer in any given region of the brain can be monitored. As brain areas become more active, more blood flows to that area. A computer monitors the movement of the tracer and creates a rough map of active and inactive areas of the brain during a given behavior. PET scans show little detail, are unable to pinpoint events precisely in time, and require that the brain be exposed to radiation; therefore, this technique has been replaced by the fMRI as an alternative diagnostic tool. However, combined with CT, PET technology is still being used in certain contexts. For example, CT/PET scans allow better imaging of the activity of neurotransmitter receptors and open new avenues in schizophrenia research. In this hybrid CT/PET technology, CT contributes clear images of brain structures, while PET shows the brain’s activity.

A brain scan shows brain tissue in gray with some small areas highlighted red.

Figure 3. An fMRI shows activity in the brain over time. This image represents a single frame from an fMRI. (credit: modification of work by Kim J, Matthews NL, Park S.)

Techniques Involving Magnetic Fields

In magnetic resonance imaging (MRI), a person is placed inside a machine that generates a strong magnetic field. The magnetic field causes the hydrogen atoms in the body’s cells to move. When the magnetic field is turned off, the hydrogen atoms emit electromagnetic signals as they return to their original positions. Tissues of different densities give off different signals, which a computer interprets and displays on a monitor.

Functional magnetic resonance imaging (fMRI) operates on the same principles, but it shows changes in brain activity over time by tracking blood flow and oxygen levels. The fMRI provides more detailed images of the brain’s structure, as well as better accuracy in time, than is possible in PET scans (Figure 3). With their high level of detail, MRI and fMRI are often used to compare the brains of healthy individuals to the brains of individuals diagnosed with psychological disorders. This comparison helps determine what structural and functional differences exist between these populations.

Techniques Involving Electrical Activity

In some situations, it is helpful to gain an understanding of the overall activity of a person’s brain, without needing information on the actual location of the activity. Electroencephalography (EEG) serves this purpose by providing a measure of a brain’s electrical activity. An array of electrodes is placed around a person’s head (Figure 4). The signals received by the electrodes result in a printout of the electrical activity of his or her brain, or brainwaves, showing both the frequency (number of waves per second) and amplitude (height) of the recorded brainwaves, with an accuracy within milliseconds. Such information is especially helpful to researchers studying sleep patterns among individuals with sleep disorders.

A photograph depicts a person looking at a computer screen and using the keyboard and mouse. The person wears a white cap covered in electrodes and wires.

Figure 4. Using caps with electrodes, modern EEG research can study the precise timing of overall brain activities. (credit: SMI Eye Tracking)

Try It

Brain Review

Review the content from the module thus far in the following activity.

Glossary

computerized tomography (CT) scan: imaging technique in which a computer coordinates and integrates multiple x-rays of a given area
electroencephalography (EEG): recording the electrical activity of the brain via electrodes on the scalp
functional magnetic resonance imaging (fMRI): MRI that shows changes in metabolic activity over time
magnetic resonance imaging (MRI): magnetic fields used to produce a picture of the tissue being imaged

positron emission tomography (PET) scan: involves injecting individuals with a mildly radioactive substance and monitoring changes in blood flow to different regions of the brain

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