12.
Modern imaging
X-rays
gave scientists the first method for examining the arrangement
of atoms inside matter. X-rays, with wavelengths a thousand
times smaller than the wavelengths of visible light,
allowed Bragg and others to look through otherwise solid
objects at atoms so small they can never be seen with
visible light.
Their
techniques were developed around the world. In America,
powder diffraction was developed by Debye. Rather than
aligning different faces of his crystals at right angles
to the X-rays, Debye ground his crystals down. A single
X-ray would hit all the possible planes of atoms in
this crystal powder, and the reflections could then
be measured on a film wrapped completely around the
experiment. In 1953 X-ray crystallography was used to
find the Double Helix structure of DNA.
Today
we have many other ways of seeing inside objects. Computed
Tomography (CT) uses many X-ray slices at different
angles to build up three dimensional images inside the
body. Ultrasound uses echoes from sound waves to locate
different organs inside the body, and can be used to
monitor the growth of unborn babies. Magnetic Resonance
Imaging (MRI, or NMR) uses powerful magnets to measure
the water content of different types of body tissue.
It is a slow process, but generates incredibly detailed
images of the body. Positron Emission Tomography (PET)
detects radioactive tracers that are injected into the
body, and Electroencephalography (EEG) detects electrical
activity in the brain. Physics has allowed us to develop
some of the most powerful diagnostic tools used in medicine.
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