Special Report: Brain Imaging Research
Volume 11, Number 5
The Basics of Brain Imaging
By Robert Mathias, NIDA NOTES Staff Writer
The major neuroimaging techniques used in drug abuse research are positron
emission tomography (PET), single photon emission computed tomography (SPECT),
and magnetic resonance imaging (MRI), along with electro-encephalography
(EEG), an earlier technique for monitoring brain activity. Advances in all
these techniques are enabling scientists to produce remarkably detailed
computer-screen images of brain structures and to observe neurochemical
changes that occur in the brain as it processes information or responds
to various stimuli such as drugs of abuse or drug abuse treatment medications.
PET, SPECT, MRI, and EEG are noninvasive procedures that can measure biological
activity through the skull and reveal the living human brain at work. Each
has its own advantages and each provides different information about brain
structure and function. For this reason, scientists increasingly are conducting
studies that integrate two or more techniques. For example, merging a PET
scan image that shows activity at brain molecular sites, or receptors, with
a highly detailed MRI image of brain structure can produce a composite image
that makes it possible to identify more precisely where in the brain the
activity is occurring.
PET-Positron Emission Tomography
PET measures emissions from radioactively labeled chemicals that have been
injected into the bloodstream and uses the data to produce two- or three-dimensional
images of the distribution of the chemicals throughout the brain and body.
PET studies involve use of a machine called a cyclotron to "label"
specific drugs or analogues of natural body compounds, such as glucose,
with small amounts of radioactivity. The labeled compound, which is called
a radiotracer, is then injected into the bloodstream, which carries it to
the brain. Sensors in the PET scanner detect the radioactivity as the compound
accumulates in different regions of the brain. A computer uses the data
gathered by the sensors to construct multicolored two- or three-dimensional
images that show where the compound acts in the brain.
Using different compounds, PET can show blood flow, oxygen and glucose metabolism,
and drug concentrations in the tissues of the working brain. Blood flow
and oxygen and glucose metabolism reflect the amount of brain activity in
different regions and enable scientists to learn more about the physiology
and neurochemistry of the working brain.
In drug abuse research, PET scans are being used to identify the brain sites
where drugs and naturally occurring neurotransmitters act, to show how quickly
drugs reach and activate a neural receptor, and to determine how long drugs
occupy these receptors and how long they take to leave the brain. PET is
also being used to show brain changes following chronic drug abuse, during
withdrawal from drugs, and while the research volunteer is experiencing
drug craving. In addition, PET can be used to assess the brain effects of
pharmacological and behavioral therapies for drug abuse.
SPECT-Single Photon Emission Computed Tomography
Similar to PET, this imaging procedure also uses radioactive tracers and
a scanner to record data that a computer uses to construct two- or three-dimensional
images of active brain regions.
Generally, SPECT tracers are more limited than PET tracers in the kinds
of brain activity they can monitor. SPECT tracers also deteriorate more
slowly than many PET tracers, which means that SPECT studies require longer
test and retest periods than PET studies do. However, because SPECT tracers
are longer lasting, they do not require an onsite cyclotron to produce them.
SPECT studies also require less technical and medical staff support than
PET studies do. While PET is more versatile than SPECT and produces more
detailed images with a higher degree of resolution, particularly of deeper
brain structures, SPECT is much less expensive than PET and can address
many of the same drug abuse research questions that PET can.
MRI-Magnetic Resonance Imaging
MRI uses magnetic fields and radio waves to produce high-quality two- or
three dimensional images of brain structures without injecting radioactive
In the procedure, a large cylindrical magnet creates a magnetic field around
the research volunteer's head, and radio waves are sent through the magnetic
field. Sensors read the signals and a computer uses the information to construct
an image. Using MRI, scientists can image both surface and deep brain structures
with a high degree of anatomical detail, and they can detect minute changes in these structures that occur over time.
Within the last few years, scientists have developed techniques that enable
them to use MRI to image the brain as it functions. Functional MRI (fMRI)
relies on the magnetic properties of blood to enable scientists to see images
of blood flow in the brain as it is occurring. Thus researchers can make
"movies" of changes in brain activity as patients perform various
tasks or are exposed to various stimuli.
An fMRI scan can produce images of brain activity as fast as every second,
whereas PET usually takes 40 seconds or much longer to image brain activity.
Thus, with fMRI, scientists can determine with greater precision when brain
regions become active and how long they remain active. As a result, they
can see whether brain activity occurs simultaneously or sequentially in
different brain regions as a patient thinks, feels, or reacts to experimental
An fMRI scan can also produce high-quality images that can pinpoint exactly
which areas of the brain are being activated. For example, fMRI can produce
an image that distinguishes structures less than a millimeter apart, whereas
the latest commercial PET scanners can resolve images of structures within
4 millimeters of each other.
In summary, fMRI provides superior image clarity along with the ability
to assess blood flow and brain function in seconds. To date, however, PET
retains the significant advantage of being able to identify which brain
receptors are being activated by neurotransmitters, abused drugs, and potential
Electroencephalography uses electrodes placed on the scalp to detect and
measure patterns of electrical activity emanating from the brain.
In recent years, EEG has undergone technological advances that have increased
its ability to read brain activity data from the entire head simultaneously.
EEG can determine the relative strengths and positions of electrical activity
in different brain regions. By tracking changes in this activity during
such drug abuse-related phenomena as euphoria and craving, scientists can
determine brain areas and patterns of activity that mark these phenomena.
The greatest advantage of EEG is speed-it can record complex patterns of
neural activity occurring within fractions of a second after a stimulus
has been administered. The biggest drawback to EEG is that it provides less
spatial resolution than fMRI and PET do. As a result, researchers often
combine EEG images of brain electrical activity with MRI scans to better
pinpoint the location of the activity within the brain.
Aine, C.J. A conceptual overview and critique of functional neuro-imaging
techniques in humans: I. MRI/fMRI and PET. Critical Reviews in Neurobiology
From NIDA NOTES, November/December, 1996
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