Advanced Imaging

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Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

The Inside Image

Truth or lie? Brain imaging may become the ultimate test
functional magnetic resonance imaging (fMRI)
© University of Wisconsin (Madison) Laboratory for Affective Neuroscience
The discovery of functional magnetic resonance imaging (fMRI) opened the door to picturing the dynamics of cognition.
fMRI
© University of Wisconsin (Madison) Laboratory for Affective Neuroscience
fMRI shows great promise—and is receiving the most journalistic exposure—for brain mapping; understanding the physical events which beget human sensation, attention and awareness.
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By Lee J. Nelson
Contributing Editor

The medial anterior prefrontal cortex and inferior and superior lateral gyri are the portions of the brain that appear to be involved in deception. Activation related to truth-telling tends to locate posteriorly to those structures, supporting the paradigm that dishonesty is a more memoryintensive and complex task versus truthfulness. So far, fMRI shows great promise—and is receiving the most journalistic exposure—for brain mapping; understanding the physical events which beget human sensation, attention and awareness.

One fMRI approach, aimed at maximizing the effect produced by oxygenated hemoglobin and minimizing the time needed to obtain an image, is known as Blood Oxygenation Level Dependent fMRI (BOLD fMRI). As with the polygraph, the magnitude of any BOLD fMRI indication is meaningful only in relation to a baseline. Selection of a valid control condition for fMRI is as critical as is the background signal behind a "probable truth" or "probable lie" polygraph examination.

Although positron emission tomography (PET) dates to the early 1950s, it was the mid-1980s when the technology came into widespread use for medical diagnoses and dynamic studies of human brain activity. PET imaging affords a million-fold sensitivity advantage over other techniques in studying regional metabolism and neuroreceptor functionality. After injecting the subject with a biological tracer, a molecule that transports a positron-emitting isotope, it begins to accumulate in an area of the body for which the molecule has affinity. Labeled glucose, for instance, concentrates in the brain where it serves as the primary energy source. A sensor array collects positrons discharged by decaying radioactive nuclei and a microprocessor assembles those signals into images, portraying site-specific metabolism.

Similar to electroencephalography (EEG), magnetoencephalography (MEG) measures tiny electrical currents arising inside the brain. From them, MEG generates a representation of magnetic fields produced by the neurons. But unlike EEG, the skull and surrounding tissues impact less upon MEG, diminishing signal distortion and yielding greater precision. That permits more usable and reliable localization of brain function as well as highly accurate temporal resolution of nerve cell activity...down to the millisecond.

Functional transcranial Doppler sonography (fTCD) directs ultrasound waves (2 MHz) from a piezoelectric probe along the anterior, middle and posterior cerebral arteries. Moving blood cells reflect the transmissions that are detected by the same probe. The frequency of those echoes is directly proportional to blood-flow velocity. And, as a perfusion-sensitive neuroimaging technique, fTCD relies on the same positive correlation between regional cerebral blood-flow and degree of neural activation.



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