Hey, everybody.
Welcome back.
This is the last video on
radiation interactions.
Here we'll be talking
about Compton scattering.
This is the most
common, but the least
desirable photon interaction.
So here's what happens.
The X-ray photon enters
an atom in the body
and the X-ray photon
energy is partially
absorbed by a loosely
bound outer shell electron.
This results in the
electron being knocked out
of its orbit, which
we call ionization.
The electron is commonly
called a Compton electron
because it was created
by Compton scattering
the rest of the photon
energy immediately exits
the atom as a scattered photon.
It has less energy than
the original photon
and it's going in a
different direction.
That's why we call it scatter.
So with Compton
scattering it's a photon
in with an electron
and a scattered photon
out. Make sure and understand
how Compton scattering is
different from the
other interactions.
With photoelectric effect
this interaction takes place
in an inner shell electron
and it results in only
an ionized electron.
There's no scattered
photon coherence scattering
is also different.
This occurs when the
incident photon interacts
with the entire atom
the energy, the photon
is temporarily
absorbed, and then
released as a scattered
photon. There's
no ionization no free electron
and no biological harm.
Once again Compton
scattering occurs
when the incoming photon
interacts with an outer shell
electron.
This energy is absorbed
by the electron, which
causes it to be knocked out
of its orbit at the same time,
a scattered photon
is also created.
So what are the effects
of Compton scattering?
Remember the three
areas that we should
be concerned with: patient dose,
the bystander dose,
and image quality.
Compton scattering
affects all three areas
and none of these
effects are good effects.
We'll start by talking
about the patient dose.
The effect of Compton scattering
on patient dose is not good.
Remember that Compton scattering
does result in ionization.
So this free electron
that gets created crashes
through the surrounding
tissues and cells and results
in biological
damage in the body.
The scattered photon can also
be absorbed in the patient's
tissues which causes even
more harm and more dose
to the patient.
All of this results in an
increase in the patient's dose.
Compton scattering also
has a negative effect
on the bystander dose
such as the radiographer.
The reason this happens
is because Compton
scattering, according to
its name, creates scatter.
This scatter sometimes
strikes the image receptor,
but it very easily can
strike the radiographer.
In fact Compton scattering
is the number one source
of technologist dose.
Last of all,
Compton scattering
also has a negative
effect on the image.
So why is that.?
Compton scattering
of course creates
scatter, which in turn
decreases the image contrast.
Remember, this principle:
the combination
of photoelectric
effect and transmission
creates what would otherwise be
a high contrast, high quality
image and what we
mean by this is
there's clearly visible
differences in the shades
of gray Compton scattering and
really any kind of scattering
adds meaningless
noise to the image.
When we add scatter such as
Compton scatter to the image.
This results in an overall
decrease in the image contrast.
So when does Compton
scattering occur?
Remember, this principle: more
matter equals more scatter.
For example, part thickness.
With more matter in the way more
photons interact by
Compton scattering.
This is the main reason
that larger patients as
in the example to the
right have more scatter
and therefore lower image
contrast. More Compton
scatter is created in
the thicker patient.
There's more matter
therefore more scatter.
The same idea applies to part
density or tissue density.
This refers to the
amount of matter packed
into the space of
the tissue. The chest
is mostly full of
air, which is lower
density, which means there's
less matter and less scatter.
The abdomen though,
is approximately
the same thickness
but much more dense.
This means there is
more matter more scatter
and the overall image
is going to be lower contrast
One more way we
apply this principle is
in regards to the field's size
or the amount of collimation.
If we decrease the collimation,
or use a larger field size,
there's more matter
being exposed therefore
more scatter and lower contrast.
One more factor that influences
the amount of Compton
scattering taking place
is the beam energy,
which of course, is
controlled by the kVp.
If we increase the kVp,
this proportionally increases
the amount of scatter
and therefore decreases
the total image contrast.
The ideal situation would
be to have as little scatter
radiation as possible.
So how can we reduce the
amount of content scatter.
Consider these factors.
We could decrease
the part thickness
we do this by compressing
the patient if possible.
We would also want to use the
smallest field size or the most
collimation possible.
And last of all, when we can
we'd want to use a lower kVp.
All of these factors
together decrease
the amount of content
scatter taking place.
Obviously, this is the
ideal, but sometimes this
is impossible.
Here's a summary of
Compton scattering
Compton scattering occurs when
the incoming photon interacts
with a loosely bound
outer shell electron.
The electron is going to
be immediately knocked out
of its orbit.
This process is
called ionization
and the free
electron is sometimes
called the Compton electron.
The remaining energy
leaves the atom
as a scattered photon, sometimes
called the Compton photon.
A few more principles
to remember.
Compton scattering occurs at
all energy levels.
But it becomes the predominant
or the main interaction
at high energies.
Compton scattering
increases the patient dose
because of ionization it
increases the bystander
dose because scatter
is being created.
And last of all
Compton scattering
decreases the image
quality because it
decreases image contrast.
