Flashcards in properties and production of xray Deck (24):
what form of radiation are x-rays?
EM (electromagnetic radiation)
Because they are forms of electromagnetic radiation, x rays... what does that mean for them?
- are invisible (except for visible light, all forms of e-m radiation are invisible)
• travel in straight lines
• travel at the speed of light
• travel in a wave-like motion
• behave as discrete bundles or “particles” of energy called “photons” (A single "packet" of x radiation is called a photon. As the photon moves, it creates electric and magnetic fields perpendicular to one another.)
• have a range of energies
what is the velocity equation for all EM radiation?
VELOCITY = WAVELENGTH x FREQUENCY
(velocity is the speed of light, constant for all forms of electromagnetic radiation)
x-rays are higher energy than visible light and UV rays. True or false?
if it is higher energy what does it mean about the wavelength and the frequency?
• longer wavelength, lower frequency, LOWER ENERGY..
• shorter wavelength, higher frequency, HIGHER ENERGY..
what are the 7 properties of x-rays?
1. X rays are a form of electromagnetic radiation and thus are similar in many ways to light rays, radio waves, microwaves, etc.
2. X rays are able to penetrate various materials.
3. X rays are differentially absorbed by different forms of matter.
4. X rays affect photographic emulsions (useful in production of film-based radiographs).
5. They are able to cause certain substances to fluoresce or phosphoresce (give off visible light) when they interact with them. The ability to cause fluorescence is of importance in producing radiographs using “film-screen” systems (see "Extraoral Radiology" lecture). However, x ray film is not commonly used now.
6. Are capable of changing the energy state of orbital electrons in some materials (important in production of digital images).
7. X rays are capable of ionizing atoms or molecules, and thus potentially may produce biologic effects. X rays, gamma rays, and cosmic rays are forms of ionizing radiation.
are xrays and gamma rays similar?
X rays and gamma rays are similar except for their origin; gamma rays are emitted from the nucleus of a radioactive element as it decays, while x rays are produced outside the nucleus, mostly from interactions occurring in the electron orbital system.
SHOULD I LOOK AT THIS LECTURE TO SEE THE DIAGRAMS?
what happens when the x-ray machine is turned on?
Once the x-ray machine is turned on, the filament circuit is activated. The step-down transformer supplies approximately 3 to 5 volts to the current in this circuit. The current causes the tungsten filament to heat up and "boil off" electrons into the air; this process is called thermionic emission. This results in the formation of an electron cloud around the filament. The temperature of the filament, the number of electrons flowing through the filament circuit, the number of electrons in the cloud, and ultimately the number of electrons which will be propelled across the gap between the cathode and anode in the x ray tube to participate in x ray production, are all controlled by the mA (milliamperage) selector, or the filament current control. mA is a measure of the tube current. In most dental x ray machines, the mA is fixed.
what do the autotransformer, step-up transformer, and high tension circuit do?
The autotransformer is used to select the kilovotage ultimately used for the exposure. The kVp selector determines the number of coils used on the autotransformer. Depending on the number of coils used, a certain voltage will be "tapped off" the incoming electrical supply and fed to the step-up transformer in the high-tension circuit when the exposure is made. For example, the incoming electrical supply is 110 volts and if 7/11 of the autotransformer coils is used, the resultant voltage fed to the high-tension circuit during the exposure will be 70. In many dental x ray machines, this “tapped off” voltage is constant (most often 70).
what is the line focus principle?
“Line focus principle” - angulation of face of target on anode to create smaller “effective” focal spot size…improves sharpness of final image (see “Rules for Accurate Shadow Casting” – lecture 5)
what happens when the exposure switch is activated? what does the step-up transformer do and what is the potential difference used for? where does x-ray production actually occur?
When the exposure switch is activated, the step-up transformer increases the voltage selected at the autotransformer by a factor of 1000; this factor remains constant. For example, if 70 volts were selected, the step-up transformer increases it to 70,000 volts or 70 kilovolts during the x ray exposure. The kilovoltage is applied across the x-ray tube between the cathode and anode; it supplies the force necessary to propel the electrons across the cathode-anode gap and allow x ray production to occur. A high potential difference between the cathode and anode is necessary in order for this to take place.
At the moment the exposure switch is activated, the potential difference across the tube accelerates the electrons from the cloud around the filament across the gap at blinding speed. The electrons strike the tungsten target of the anode at the focal spot, where x ray production actually occurs. Braking radiation (Bremsstrahlung) accounts for the production of most of the x rays; characteristic radiation accounts for the remainder, but its contribution is minimal compared to Bremsstrahlung.
what controls the total number of electrons moving across the gap during an exposure?
The total number of electrons moving across the gap during an exposure is controlled by the mA; therefore, mA controls the QUANTITY of x rays produced. The quantity of x rays produced is, of course, also controlled by the time of the exposure. There is a direct relationship between mA or exposure time, and the number of x rays produced, during the exposure.
what is BREMSSTRAHLUNG (BRAKING) RADIATION?
Deceleration of electron due to electrostatic attraction…
It is like when an electron travels beside a nucleus and therefore beside other electrons and the interaction causes a bending and deceleration of that propelled incoming electron and that will give off a photon of energy.
A direct hit with the nucleus is a rare event. (this will give off a photon of maximum energy!!)
Far more common is the brensstrahlung radiation (when the electron direction is altered and decelerated produces a photon of lower energy (THIS IS BREMSSTRHLUNG RADIATION)
characteristic radiation is minimal compared to brensstrahlung radiation but what is characteristic radiation?
1 - Ejection of inner shell electron of tungsten by incoming electron
2 - Creation of “vacancy” in electron shell
3 - Electron from next shell drops in to fill “vacancy”, photon of “characteristic” energy produced…..
Areas under the curves represent radiation created by Bremsstrahlung at different kV settings; characteristic radiation produced is shown by “spikes” (arrows) (these are pretty much straight lines going up created at high energy but smaller amounts of photons produced by them)
BREMSSTRAHLUNG (BRAKING)RADIATION produces SIGNIFICANTLY MORE x rays than characteristic radiation in the diagnostic energy range…
the seed at which electrons cross the gap is controlled by what?
The speed at which the electrons travel across the gap is controlled by the potential difference (kilovoltage) applied across the x-ray tube. The greater the speed of the electrons striking the target, the greater will be their kinetic energy, and the greater will be the average energy of the x rays produced. Since not all of the electrons striking the target have the same kinetic energy, and not all of them are decelerated to the same degree, the resultant x ray beam will be composed of photons of varying wavelengths and energies (a heterochromatic or polychromatic beam). The kVp controls the average energy of the x rays produced. An x ray beam with photons of higher average energy has greater penetrating power. kVp also affects quantity of x rays produced (see below).
what does heat have to do with anything and how is it controlled?
Most of the total energy produced at the focal spot is in the form of heat. To prevent thermal damage to the tungsten target, it is imbedded in copper which dissipates the heat (copper is a good thermal conductor). In addition, oil within the tube head between the glass envelope of the tube and the tube housing helps to keep the anode cool.
What x-rays form the primary x-ray beam? where do the rest of the x-rays go?
The x rays that are produced at the focal spot are given off in all directions. Those that exit the tube through the window or port form the PRIMARY X-RAY BEAM. The majority of the remainder of the x rays that are given off in directions other than through the window is absorbed by the lead lining of the tube head housing. The small number of x ray photons that may escape the lead housing are termed leakage radiation.
At or close to the window are two devices, the filter and the collimator (see lecture on filtration and collimation).
what are the two types of tube rectification?
2. Direct Current (DC)
what are self-rectified machines? what does kVp have to do with these? (read in textbook about these)
Self-rectified (half-wave rectified) machines
Some older dental x ray machines are self-rectified (half-wave rectified) from the manner in which x ray production occurs during the alternating current (AC) electrical cycle. For these machines, the term “kVp” is used to describe the peak kilovoltage used to produce x rays. For a complete explanation of this, please refer to your textbook and the PowerPoint slides.
what are DC xray machines?
Direct current (DC) x ray machines
• Constant potential is maintained between cathode and anode during the exposure
• Higher mean energy of x ray beam
- fewer lower energy x rays are produced
- more efficient production of diagnostically useful x rays
• For a given kilovoltage setting, patient dose is lower compared to self-rectified machines
what does increase in kVp/kV do ? (assuming no other factors change)
• Kinetic energy of electrons striking anode increases…
- increase in average energy of photons in x ray beam
- greater number of x ray photons produced overall
Will also be increase in scatter radiation
*Voltage rating on x ray machines usually indicated by the term kVp (kilovolt peak); on DC machines, the term “kV” is usually used
a. Increases the kinetic energy of the electrons striking the target of the anode. As a result, the average energy of the photons of the x-ray beam will increase, and the beam will be more penetrating. Also, the number of x rays produced per unit of time will also increase. An increase in the potential difference across the x ray tube results in the electrons moving faster and thus having greater kinetic energy. As they interact with the tungsten nuclei in the target, more of them will yield x ray photons; this causes production of more x rays per unit of time.
b. Increases the amount of scatter radiation. This will be discussed further in the lecture on x ray interactions with matter. The increase in scatter radiation causes reduction in the contrast of the final radiograph.
c. Increases the darkness of the final image (more x rays can penetrate tissue and reach the image receptor, and the beam is more intense).
what does increasing mA do? what does mA mean?
• Increase in number of electrons striking the anode / second; therefore, number of x rays produced / second increases
• Will cause image to be darker
*mA means “milliamperes” - an indication of current flow across x ray tube
a. Increases the number of x rays produced per unit of time during the exposure. This is because the number of electrons striking the target of the anode per unit of time increases.
b. Increases the darkness of the image (more x rays striking the image receptor). Contrast will not be markedly affected unless the film is badly under- or overexposed.
The milliamperage (mA) controls the current flowing in the filament circuit of the x-ray machine. The higher the mA, the more electrons in the circuit, and thus the more electrons in the electron cloud around the tungsten filament of the cathode. At the time of exposure, there will be more electrons available to participate in x ray production.