Shielding is accomplished by building rooms made of metals that
are resistant to penetration from magnetic fields, and by using elab-
orate electronic systems to measure and cancel sources of magnetic
noise. This is analogous to noise-canceling headphones—the physical
structure of the headphone over the ear blocks much of the ambient
sound, and the noise-cancellation electronic circuitry cancels out
much mote. To detect the very weak magnetic fields generated by
neural activity in the brain, MEG uses something called a SQUID, an
acronym for superconducting quantum interference device. SQUID
detectors are based on properties of superconducting currents in the
presence of magnetic fields. They were first constructed in the 1960s,
and soon after their construction they were applied to the detection of
magnetic fields generated by brain activity.
All this shielding and measurement technology ends up making
MEG a pretty expensive undertaking, far more expensive than high-
resolution EEG. In addition, the computational challenges in recon-
structing the location of sources of neural activity in the brain are also
formidable. These things serve to limit the widespread application of
MEG.
Another technology for imaging dynamic activity in the brain is
positron emission tomography, or PET. This technique uses the prop-
erties of particular radioactive chemicals to visualize cellular activity
in the brain. Let’s talk about radioactivity—radiation, radiate, radiant,
ray. To shine; to emit; to glow; to extend out from the center—a radius.
Every chemical element exists in several varieties, called isotopes,
some of which are unstable and undergo radioactive decay.
A chemical element is defined by the number of protons in its
atomic nucleus, its atomic number: one for hydrogen, two for he-
lium, three for lithium, six for carbon, seven for nitrogen, eight for