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NEUROSCIENCE OF PSYCHOACTIVE SUBSTANCE USE AND DEPENDENCE

advantages of having the highest spatial resolution among imaging techniques,
and does not require the use of ionizing radiation, thus it provides increased
experimental safety and the ability to retest subjects multiple times. Magnetic
resonance spectroscopy is used to gather information on the chemical composition
of a discrete brain region.
Positron emission tomography
Positron emission tomography (PET) is a technique for viewing the activity in different
regions in the brain. PET scans provide information about the metabolic activity in
a certain brain region. Most commonly, the person is injected with a radioactive
compound that can be followed through the bloodstream in the brain. This is usually
labelled 2-deoxyglucose (2-DG), which is taken up by active neurons due to its
similarity in structure to glucose. Thus, areas that are more metabolically active
will take up more glucose and 2-DG. Unlike glucose, 2-DG is not metabolized, and
therefore accumulates in the neurons. This can be visualized as two- or three-
dimensional images, with different colours on a PET scan indicating different levels
of radioactivity (blues and greens indicating areas of lower activity, and yellows
and reds indicating areas of higher activity). Using different compounds, PET scans
can be used to show blood flow, oxygen and glucose metabolism, and drug
concentrations in the tissues of the living brain. Regional cerebral blood flow can
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be measured using PET imaging using a “flow tracer” such as [ O] water to look
at blood flow in a given area. Selective labelling of radiotracers allows highly selective
biochemical specificities at low concentrations of tracers.
Single photon emission computed tomography
Similar to PET, single photon emission computed tomography (SPECT) uses
radioactive tracers and a scanner to record data that a computer uses to construct
two- or three-dimensional images of active brain regions. However, SPECT tracers
are more limited than PET tracers in the kinds of brain activity they can monitor,
and the SPECT tracers also deteriorate more slowly than many PET tracers, which
means that SPECT studies require longer test and retest periods than PET studies.
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 considerably less expensive than PET and can
address many of the same drug dependence research questions that PET can.
Electroencephalography
Electroencephalography (EEG) uses electrodes placed on the scalp to detect
and measure patterns of electrical activity emanating from the brain due to the
communication between neurons. EEG can determine the relative strengths and
positions of electrical activity in different brain regions within fractions of a second
after a stimulus has been administered. However, the spatial resolution of EEG is
not as good as with other imaging techniques. As a result, EEG images of brain
electrical activity are often used in combination with other techniques such as
MRI scans to better pinpoint the location of the activity within the brain.
Sources: Aine CJ, 1995; National Institute on Drug Abuse , 1996; Volkow et al., 1997;
Gatley & Volkow, 1998.

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