representation of the brain’s internal structure. CT scans may be used
to visualize not only the brain but just about any internal body struc-
ture and are widely used as an aid to diagnosis in clinical medicine.
X-rays are not benign—their high energy is damaging to any
molecules they encounter, by breaking covalent chemical bonds and
disrupting the structure of the molecule. If the molecule is a protein
or lipid, its structure is likely to be permanently damaged and its
function destroyed. If the molecule is DNA, damage to its structure
may sometimes be repairable by enzymes that specialize in fixing
damaged DNA. However, sometimes this kind of repair process gener-
ates changes in the nucleotide sequence of the DNA, especially if the
damage is extensive. Thus, x-rays give rise to mutations in genes and
other kinds of anomalous activity in the transcription of genes. X-
radiation is toxic, and exposure to it needs to be limited if health is to
be maintained.
Another method of structural brain imaging that uses the power of
computers to manipulate large quantities of information is mag-
netic resonance imaging, or MRI. Like CT, MRI can produce a three-
dimensional reconstruction of the internal structure of a living brain
or other parts of the body and was introduced into medicine in the
1980s. Today, many medical centers use both CT and MRI technolo-
gies to image the body. For some kinds of visualization, CT may be the
desired method, and for others, MRI may be preferred.
MRI uses not x-rays to penetrate the skull and other tissue but
an entirely different process. It is based on a mysterious physical
phenomenon called quantum spin. This is a property of subatomic
particles, in particular, of the protons and neutrons in atomic nuclei.
Although no one really knows what quantum spin is, it is possible to
describe with great precision how quantum spin behaves. It is called