production of X rays pt 2 Flashcards
mA (milliamperage):
controls
heating of the
filament
Exposure time: controls
the
time in which x-rays are
produced
kV (kilovoltage): controls
the acceleration of
electrons from cathode to
anode
Electricity:
Flow of electrons through an electrical conductor.
Current:
Amount of electrons flowing through a conductor per second. Measured in amperes (A).
Circuit:
Path of electrical current
Voltage (Potential difference) :
Difference in electrical potential energy between two points in an
electric circuit. Measured in volts (V).
During each half-cycle (1/120 of a second),
anode is positive and attracts the electrons from
the cathode (x-radiation is produced).
During each alternate half-cycle (1/120 of a second),
anode is negative, therefore, no
attraction for electrons exists and no x-radiation is produced (inverse voltage).
USA: – cycles per second
60
Full wave rectification (4)
- Changing alternating current into
direct current - Full-waive rectification, high frequency
power supply - Essentially constant potential between
cathode and anode. - Higher mean energy of beam
compared to AC.
Constant potential and direct current
(4)
- Shorter exposure times
- More consistent beam intensity
- Higher mean energy of beam
- Decreased radiation dose
How are x-ray produced?
X-rays are produced whenever
high-speed electrons are
suddenly decelerated or
brought to a stop when they
pass close to the nuclei of a high
Z # absorbing material (in this
case tungsten 74W)
1.Bremsstrahlung radiation
(3)
◦ AKA Breaking radiation
◦ Electron to nucleus interaction
◦ The fast-moving electrons either slow down
or stop when they come close to the nucleus
of the atoms and part of their energy is
transferred as X-rays.
- Characteristic radiation
(3)
◦ Electron to electron interaction
◦ A few electrons interact with tungsten target orbital
electrons, imparting enough energy to ionize the
tungsten target.
◦ When electrons displace inner shell electrons,
characteristic radiation is produced.
Bremsstrahlung radiation
—- electrons directed
toward the target material.
High-velocity
Bremsstrahlung radiation
All electrons do not attain the same
velocity. Some move at
higher
velocity(ies) than others (depending
on kV)
Bremsstrahlung radiation
equation
KE = ½ mV^2. The higher the velocity,
the greater the KE of electrons.
Bremsstrahlung radiation increases
with (2)
increasing the voltage (kV) and
the atomic number of the target ( Z#)
Bremsstrahlung radiation
steps (2)
- Impinging electron is deflected
and decelerated. Kinetic energy
lost is emitted as an x-ray photon. - Head-on collision with nucleus:
Electron brought to rest producing
a maximum energy photon.
Bremsstrahlung radiation
This type of radiation has a wide distribution
of
wavelengths (heterogeneous).
Bremsstrahlung radiation
Electrons lose their energies in
random
fashion when they interact with tungsten
atoms.
Bremsstrahlung radiation
Multiple interactions between (2)
electrons and
tungsten atoms.
Summary of Bremsstrahlung (3)
Electrons (with different
Velocities) are
accelerated towards the
anode under the
influence of high
kilovoltage.
At the anode they either
interact with the nucleus
or rapidly decelerate and
get deflected (change of
direction).
The process results in
the release of
energy in the form of
x-ray photons (eV)
Characteristic Radiation
steps (4)
. Arises when cathode electron collides with inner
orbital electron of tungsten atom (target) and
removes it from orbit.
2. Atom is now ionized and unstable.
3. Immediately, the hole left by the electron is filled
by an electron from an outer shell, and energy is
emitted from this electron in the form of x-
radiation characteristic of tungsten and the
involved shell.
4. The energy emitted by the electron is equivalent
to the difference in the binding energies of the
two shells/orbitals
Characteristic Radiation
K=69,500
L=12,100
M=2,800
L to K transition →
M to K transition →
69,500 – 12,100 ev = 57,400 ev
69,500 – 2,800 ev = 66,700 ev
Summary of Characteristic Radiation
◦ —% of diagnostic x-ray beam is characteristic radiation
30
Summary of Characteristic Radiation
Electron strikes and ejects an inner shell electron of a tungsten
atom.
◦ This creates a
hole or vacancy in the atom that makes it
unstable.
Summary of Characteristic Radiation
The atom responds by
shifting electrons inward to fill the
vacancy. Whenever an outer shell electron is shifted inward it
must give up some of its energy in the form of radiation called a
characteristic photon.
Summary of Characteristic Radiation
In this manner discrete energy is imparted to the — which is “characteristic” of the atom type and orbital
from which it came, thus the name characteristic radiation.
newly formed
radiation
Bremsstrahlung radiation makes up the majority of the
x-ray beam and produces a heterogeneous
beam of varying photon energies.
Characteristic radiation is only a minor source of radiation from the
x-ray tube and produces photon
energies specific for the target material.
