How Xrays see through your skin Ge Wang
In 1895, a physicist named
Wilhelm Roentgen
was doing experiments
with a cathode tube,
a glass container in which a beam of
electrons lights up a fluorescent window.
He had wrapped cardboard around the tube
to keep the fluorescent
light from escaping,
when something peculiar happened.
Another screen outside the tube
was glowing.
In other words, invisible rays
had passed through the cardboard.
Wilhelm had no idea what those rays were,
so he called them X-rays,
and his discovery eventually won him
a Nobel Prize.
Here’s what we now know was happening.
When high energy electrons
in the cathode tube
hit a metal component,
they either got slowed down
and released extra energy,
or kicked off electrons
from the atoms they hit,
which triggered a reshuffling
that again released energy.
In both cases, the energy was emitted
in the form of X-rays,
which is a type
of electromagnetic radiation
with higher energy than visible light,
and lower energy than Gamma rays.
X-rays are powerful enough
to fly through many kinds of matter
as if they are semi-transparent,
and they’re particularly useful
for medical applications
because they can make images of organs,
like bones, without harming them,
although they do have a small chance
of causing mutations
in reproductive organs,
and tissues like the thyroid,
which is why lead aprons are often
used to block them.
When X-rays interact with matter,
they collide with electrons.
Sometimes, the X-ray transfers all of its
energy to the matter and gets absorbed.
Other times, it only transfers
some of its energy,
and the rest is scattered.
The frequency of these outcomes
depends on how many electrons
the X-rays are likely to hit.
Collisions are more likely
if a material is dense,
or if it’s made of elements
with higher atomic numbers,
which means more electrons.
Bones are dense and full of calcium,
which has a relatively high atomic number,
so they absorb X-rays pretty well.
Soft tissue, on the other hand,
isn’t as dense,
and contains mostly lower
atomic number elements,
like carbon, hydrogen, and oxygen.
So more of the X-rays penetrate tissues
like lungs and muscles,
darkening the film.
These 2-D pictures are only useful
up to a point, though.
When X-rays travel through the body,
they can interact with many atoms
along the path.
What is recorded on the film reflects
the sum of all those interactions.
It’s like trying to print 100 pages
of a novel on a single sheet of paper.
To see what’s really going on,
you would have to take X-ray views
from many angles around the body
and use them to construct
an internal image.
And that’s something
doctors do all the time
in a procedure called a CT,
Computed Tomography scan,
another Nobel Prize winning invention.
Think of CT like this.
With just one X-ray,
you might be able to see the density
change due to a solid tumor in a patient,
but you wouldn’t know how deep
it is beneath the surface.
However, if you take X-rays
from multiple angles,
you should be able to find
the tumor’s position and shape.
A CT scanner works by sending
a fan or cone of X-rays through a patient
to an array of detectors.
The X-ray beam is rotated
around the patient,
and often also moved down
the patient’s body,
with the X-ray source tracing
a spiral trajectory.
Spiral CT scans produce data
that can be processed into cross sections
detailed enough to spot
anatomical features, tumors,
blood clots, and infections.
CT scans can even detect
heart disease and cavities
in mummies buried thousands of years ago.
So what began as Roentgen’s happy accident
has become a medical marvel.
Hospitals and clinics now conduct over
100 millions scans each year worldwide
to treat diseases and save lives.