[AUDIO LOGO] PROFESSOR: 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
thermionic 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 braking radiation. And that makes sense
because the radiation is created because the
electrons brake, 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 out 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 100 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 90 keV,
it'll slow down to almost 0 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 electron 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's helpful to look at
a graph of X-rays created during a single 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
represents the photon energies that appear most
often in the beam. The large bell curve are
the Bremsstrahlung X-rays. A single exposure
creates hundreds of millions of X-ray photons. And almost all of these photons
are Bremsstrahlung X-rays. The curve is short on
the left because most of the low-energy
Bremsstrahlung X-rays have been filtered
out of the beam. These low-energy photons
came from electrons that slowed down not much at all. They can't penetrate the tissues
of the patient, which is why they need to be filtered out. The curve is also
short on the right because very few
Bremsstrahlung X-rays are created with the high energy. This comes from electrons
that slow down to near 0 and release all of their
energy as a photon. The curve is tallest
in the center because most
Bremsstrahlung X-rays have a moderate
energy level, above 0 but less than the
maximum set by the kVp. In summary,
Bremsstrahlung radiation is the primary way that
X-ray photons are created during an X-ray exposure. Bremsstrahlung
radiation is created when high energy
electrons from the cathode interact with the
nucleus of tungsten atoms at the anode target. Bremsstrahlung X-rays
within the X-ray beam have a wide range of energies,
from nearly 0 up to the maximum set by the kVp. The exact energy is influenced
by the electron's proximity to the nucleus as it passes
through the tungsten atom.