RAD 350 Chapter 8 X-ray Production
Primary function of the 3 important components of an x-ray unit
(console, tube and high voltage generator) is to accelerate electrons
(sometimes called “projectile electrons”) across the x-ray tube from
cathode to anode (about 1 cm) and then suddenly stop them,
creating x-ray photons of various energies.
A couple of things to remember - kinetic energy is energy of
MOTION; 99% of the electrical energy is converted to heat.
MA is the source for the abundance of electrons to be
accelerated across the tube
kVp is the controlling item for the amount of EMF (positive
potential energy) positive charge on the anode side of the tube
as kVp increases, the efficiency of the x-ray tube to produce
x-ray photons INCREASES slightly
Only two processes convert the rapidly accelerated electrons into
x-ray photons:
“CHARACTERISTIC RADIATION” occurs when an
accelerated electron “ejects” an orbital electron from the tungsten
target. The K-shell binding energy of tungsten is 69 KEV.
Therefore, if a K-shell electron was ejected (binding energy of 69
KEV) and an outer MORE LOOSELY bound binding (say from the
“P-shell where it’s binding energy is only slightly more than .01
KEV) energy electron “falls” into the void in the K-Shell. The outer
electron must “give up” some of it’s own “on board energy” in order
to have the greater binding energy overcome it’s “on board energy”
and pull it into the K-shell void. THE DIFFERENCE IN BINDING
ENERGY IS THE ENERGY “GIVEN OFF” IN THE FORM OF AN
X-RAY PHOTON. i.e. - in this case k-shell of tungsten binding
energy is 69 KEV and the “0" shell less than .01 = an x-ray photon
with an energy of 69 KEV - and THUS “CHARACTERISTIC” of the
energy difference of the two shells involved (and remember all atoms
have different binding energies).
Characteristic can come from any orbital shells, BUT only
those with the greatest differing amount of BINDING energy shells
will produce photons with high energy that is useful at all. Since
tungsten has only 15 potential differences in binding energy levels of
“combinations” caused by the “void” in the shell where the electron
was ejected and the source of the electron to “fall into” the void.
The energy of the CHARACTERISTIC photons can only be one of
the 15 possible combinations.(see figure 8.3 on page 141)
Bremstrahlung (BREMS) radiation is produced when an fast
moving electron (one negative charge) is affected by the SUM of
positive charges contained in the nucleus of a target (tungsten) atom.
Since the nucleus has much more charge and the charge is positive,
the single negatively charged electron changes it’s course (vector)
due to the opposite charge and higher number of positive charges.
This causes the “projectile electron” to slow down and change
direction. This slowing down and changing direction causes the
electron to “emit” an x-ray photon as it loses it’s kinetic energy by
“braking” or slowing down.
Brem’s energy and be any energy from the highest kVP down to
zero. (see figure 8-5 on page 143)
As the kVp increases, the x-ray emission spectrum is “skewed” to the
right of the emission curve. As mA is increased, there is NO
CHANGE IN SHAPE, but the curve is bigger (if you double the mA,
the curve is TWICE as high!) See figure 8-12 on page 147.
Remember, as kVp increases the wavelengths shorten ( min.)
MOST PHOTONS IN THE DIAGNOSTIC IMAGING RANGE OF
ENERGY ARE BREMS!
The total number and energy of all the x-ray photons emitted is
called the x-ray emission spectrum. Since it has MANY energies, it
is called a POLYCHROMATIC SPECTRUM beam.
Since the polychromatic beam has all energies ranging from the kVp
set (if you set 80 kVp, there will be at least one x-ray photon with an
energy of 80 KEV and many more photons - based upon mAs - at
lower levels), ADDED filtration is added to ABSORB/ATTENUATE
the lower, non-useful photons thus saving the patient some useless
radiation. This action INCREASES the ‘average” energy of the beam
(by eliminating the photons with low energy, the remaining beam is a
higher average energy). This is also referred to as ‘hardening the
beam” due to the increases “average” energy. The added filtration is
made of aluminum since the “characteristic radiation” produced
within the aluminum atoms has very low energy and will be absorbed
in the air prior to reaching the patient.