Hey, everybody. Welcome back. This video is an overview of
Bremsstrahlung radiation-- one of the target interactions
that creates X-ray photons in the X-ray tube. First, let's remember there are
three essential requirements for creating X-ray photons. First, a source
of free electrons, secondly, a means of
accelerating those electrons, and thirdly, a process
for deceleration of those electrons. Each of these
steps is controlled by different components
and processes inside of the X-ray tube. Free electrons are created
at the cathode filament through the process
of thermonic emission. These electrons are accelerated
through the X-ray tube by a voltage. That's the kVp. These electrons are
decelerated when they interact with the tungsten
atoms in the anode target. And through that process,
they create x-rays. Bremsstrahlung radiation
is one of the processes through which these X-ray
photons are created. So here's how it works. A tungsten atom, like all
atoms, has a positively charged nucleus because of
the protons, and it's surrounded by negatively
charged electrons. All of these charged particles
have an electric field that radiates around them. The nucleus has a
positive electric field, and the electrons have a
negative electric field. When electrons from the
cathode enter the anode, some electrons are attracted
to the positive charge of the nucleus within
these tungsten atoms. And that makes sense-- the
negatively charged electron is attracted to that positive
charge in the nucleus. But when this happens, the
electrons slow down or break, and they also change
direction, called deflection. This lost energy is released
in the form of an X-ray photon. This process has a name. It's actually called
Bremsstrahlung radiation. Bremsstrahlung is
the German word that means breaking radiation,
and that makes sense because the radiation is created
because the electrons break or slow down. So you can kind of
think of it like this. If you throw a ball
at a glass window, the ball will slow
down and usually get deflected another
direction, just like when the electrons pass
near the tungsten nucleus. But what happened
to all of the energy that the ball was carrying? It had to go somewhere. The law of conservation of
energy says that energy cannot be created or destroyed. It can only change in form. So some of that original
energy is left in the ball, but some of it leaves the
ball and creates cracks in the glass. And this is just like how
some of the electron energy leaves the electron and
creates an X-ray photon. The energy of a Bremsstrahlung
radiation photon depends on a few
factors, one of them being the energy of
the incoming electron. If the incoming electron
has an energy of 100 keV, the maximum energy photon that
can be created is 100 keV. They won't always have the
maximum energy, but they can. Here's another example. If the incoming electron
has an energy of 75 keV, the absolute highest
energy photon that we can get outs of that
electron would be 75 KeV. The maximum energy of a
bremsstrahlung X-ray photon is controlled by the kVp,
so the tube potential-- if it's set to 100 kVp-- will create Bremsstrahlung
photons that have a maximum energy of
100k keV, and of course if we change the
kVp, this changes. If the tube potential
is set to a kVp of 75, the maximum Bremsstrahlung
X-ray photons will have an energy of 75 keV. Not many Bremsstrahlung photons are going to have
this maximum energy. And why is that? The energy of
Bremsstrahlung radiation also depends on how close the
electron passes to the nucleus. If the electron passes very
close, as in this example, it will slow down significantly,
change directions completely, and give up most of its
energy as an X-ray photon. For example, if the
electron approaches with an energy of 90k keV,
it'll slow down to almost zero and release a photon with
an energy of about 90 keV. But these electrons don't always
pass very close to the nucleus. Here's another example. Let's say, in this
scenario, the electron passes at a much greater
distance from the nucleus, and things will change. The attraction between the
electron and the nucleus is weaker. The electrons slows down
some but not completely. It changes direction
some but not completely, and it only gives up some
of its original energy. For example, if the
electron approached the atom with an energy of 90
keV but slowed down to an energy of 60 keV,
the resulting photon would have an energy of 90 keV
minus 60 keV, which is 30 keV. All of the energy
here is accounted for. It might be helpful to look
at a graph of x-rays created during a single X-ray exposure. The x-axis is the
different photon energies in the X-ray beam. The y-axis is the number of
photons with that energy, so the tallest part of the curve
represent the photon energies that appear most often
in the X-ray beam. And it's right in the middle. This large bell curve represents
the Bremsstrahlung x-rays. A single exposure
creates hundreds of millions of X-ray photons,
and almost all of these are Bremsstrahlung x-rays. The curve is short on the left
because very few Bremsstrahlung x-rays have such a low energy. This would come
from electrons that slowed down not much at all. The curve is also very
short on the right, and that's because
very few Bremsstrahlung x-rays are created with the
highest possible energy. These photons would come
from those electrons that slow down to nearly zero
and release of their energy as a photon. The curve is tallest
in the center because most Bremsstrahlung
x-rays have a moderate energy level above zero but less
than the maximum energy sent by the kVp. So what's that large
spike on the right? That is characteristic
radiation, which is the topic
of the next lesson.
