[AUDIO LOGO] PRESENTER: Some
people less astute than the average
radiography student have wondered, where
do X-rays come from? Are they created by magic? Or are they harvested
from unicorns? Maybe they're not even real. This lesson is about the main
principles of X-ray production. X-rays are real,
and they have to be created in a very specific way. There's three requirements
of X-ray production-- a source of electrons, a
means of rapidly accelerating or energizing those
electrons, and a means of rapidly decelerating or
de-energizing those electrons. If any of these
requirements are missing, X-rays won't be produced. In radiography, all three
steps of X-ray production take place inside
of the X-ray tube. Electrons are produced
in the X-ray tube at the cathode filament. The specific process is
called thermionic emission, which is the release of
electrons in response to heat. A current running
through the filament causes it to become
extremely hot, so hot that electrons are actually
dissociated from the metal and form an electron
cloud around the filament. This is like the filament in
an incandescent light bulb. A current running through
the light filament creates heat, which
creates light. In an X-ray tube,
the filament gets so hot it creates free electrons. So a cloud of electrons
doesn't do as much good. So what happens next? That's where the unicorns
come in, not really. The second step of
X-ray production is accelerating these
electrons, which gives them extremely high kinetic energy. This is accomplished by applying
an electrical voltage called the kilovoltage peak, or kVp. The kVp creates a
strong negative charge in the filament that forces the
electrons across the X-ray tube to the positively charged anode. And this makes sense
since opposites attract. The electrons are repelled by
the negatively charged cathode and attracted to the
positively charged anode. The last step in
X-ray production is decelerating the electrons. These highly energetic
electrons slam into the anode of the X-ray tube. In the process of decelerating,
they release their energy as heat and, more
importantly, as X-rays. The number of X-rays
and the energy of X-rays are actually controlled by the
operator using the mA, kVp, and the exposure time. Increasing the mA
or the tube current increases the
number of electrons produced in the X-ray tube. Increasing the
number of electrons increases the number of photons. This is the idea of
intensity or quantity. The effect of changing the
kVp is slightly different. Increasing the kVp--
that's the tube potential-- increases the energy of
electrons in the X-ray tube. The higher voltage
makes them move faster. Increased electron
energy results in increased X-ray energy. And this is the idea
of beam quality. Increased electron energy also
increases the total number of X-rays produced. And that's mainly because
high-energy electrons are more likely to be
converted into X-rays. So increasing kVp increases both
the number and energy of X-rays created. Increasing the exposure time
also increases the total number of X-ray photons created. The exposure time is
just the amount of time that electrons are
flowing through the tube. And X-rays are being created. It is exactly like unicorns. The longer they eat Skittles,
the more rainbows they make. In summary, X-rays are not
produced by magic or unicorns. X-ray production
requires three things-- a source of electrons--
that's thermionic emission; a means of accelerating the
electrons-- that's the kVp; and a means of decelerating
the electrons-- that happens at the anode. MA and exposure time
influence the total number of X-rays created. kVp influences both the number
and the energy of X-rays created.