PET scans are primarily used to gauge cerebral blood flow, which tends to correlate with minute changes in cerebral activity. Indeed, fMRI also primarily assesses cerebral blood flow-although these techniques can also monitor other facets of cerebral metabolism.
The measure of cerebral blood flow began in the 1960s, when Lassen and his colleagues injected xenon-133 gas in saline solution. The flow of this radioactive chemical through the carotid artery and into brain regions was then monitored by multiple detectors outside the head (see Lassen, Ingvar, & Skinhoj, 1978). Gradually, over time, rather than inject this solution, participants could instead breathe a mixture that includes xenon for a minute or so.
Although this technique could be used to measure only outer regions of the brain, the initial findings were promising. Blood flow to the cortical regions that are involved in vision, for example, increased when individuals observed moving pictures. Likewise, blood flow to the right hemisphere increased when participants completed a closure task-in which they received sparsely drawn pictures and needed to ascertain what these figures represent. In contrast, blood flow to the left hemisphere increased when participants completed a verbal analogies task (see Risberg, Hasley, Wills, & Wilson, 1975).
SPECT, or single photon emission tomography, yields a 3D representation of brain imagery, using a s series of cross sections to represent neural activity. In particular, radiopharmaceuticals, which are compounds that emit gamma rays, are injected into the bloodstream, and the flow of these compounds is monitored through detectors. A computer program then constructs the distribution of this compound to represent blood flow and, thus, neural activity. HMPAO is the most common tracer.
The image that is generated represents the cerebral blood flow at the approximate time the compound was injected. Hence, the compound is usually injected while participants undertake a specific task, to uncover the regions that underpin that activity. Likewise, the compound can be injected during a seizure.
The HMPAO tracer clears only after several hours. Therefore, this procedure is too slow to explore multiple conditions in the same participants. More recently, however, technology has been upgraded to enable researchers to use inhaled xenon mixtures-which can clear more rapidly, enabling multiple scans within a limited time.
Some substances emit positrons, which generate a pair of photons that travel in the opposite direction of one another. Detectors that are positioned 180 degrees apart from each other can monitor these photons to locate the substance somewhat precisely.
Many biological compounds can be traced-that is converted to a positron emitter. Hence, the distribution of glucose, oxygen, and neurotransmitters can all be ascertained. Nevertheless, most PET studies use positron labeled water to measure blood flow-similar in principle to xenon 133-but enabling 3D images. For example, both positron labeled water and xenon 133 clear from the brain rapidly, enabling researchers to study the same person many times across a limited duration.
Because cerebral blood flow can be measured rapidly, participants often perform a series of tasks, each one process more complex than is the previous activity. For example, in the first task, participants might merely observe words. In the second task, participants might verbalize the words. Changes in blood flow from one task to the next activity are assumed to represent the location of this additional process-in this instance, verbalization.
Some researchers have questioned whether blood flow, and thus neural activity, is the optimal means to characterize the regions that underpin behavior. One problem, for example, is that processes that originate in one hemisphere often activate the other hemisphere, via the corpus callosum. As a consequence, researchers often conclude that activation permeates both hemispheres, whereas activation actually originated in one hemisphere only.
Lassen, N. A., Ingvar, D. H., & Skinhoj, E. (1978). Brain function and blood flow. Scientific American, 239, 62-71.
Risberg, J., Hasley, J. H., Wills, E. L., & Wilson, E. M. (1975). Hemispheric specialization in normal man studied by bilateral measurements of the regional cerebral blood flow: A study with the 133Xe inhalation technique. Brain, 98, 511-524.
Springer, S. P., & Deutsch, G. (1998). Left brain, right brain (5th ed). New York: Freeman and Company
Last Update: 5/28/2016