Principals of X-ray Production Flashcards
How are x-rays produced?
X-rays are produced when highly accelerated electrons are brought to a sudden stop by hitting a stationary object, thus causing some of the kinetic energy of the electron to be converted to x-rays
What are the 2 main mechanisms for x-ray production?
- Bramsstrahlung (electron-to-nucleus)
2. Characteristic radiation (electron-to-electron)
Which is more common, complete or partial conversion of kinetic energy of an electron to x-ray?
Partial conversion is more common
Define Bremsstrahlung radiation
High velocity electrons are suddenly decelerated (stopped or slowed) at the target causing a complete or partial conversion of electron kinetic energy to x-ray energy (photon) measured in keV
Do all electrons move at the same velocity?
NO! not all electrons attain the same velocity – some move at higher velocities than others
What are the 3 requirements for Bremsstrahlung?
- Electrons
- High velocity (electrons)
- High Z number absorbing material/target (eg tungsten)
What is the relationship between the velocity and KE of an electron?
DIRECTLY related. The higher the velocity of the electron, the greater the kinetic energy (and vice versa)
What is the clinical implication of electrons moving at different velocities?
Differences in velocities affect the final quality of the x-ray beam and therefor the radiograph
How is a continuous spectrum (varying energy level) of x-ray photons achieved with Bremsstrahlung?
Some electrons are directly hit by the nucleus and give up ALL of their KE with a single high energy x-ray photon being produced….however, MOST electrons only pass NEAR the nucleus, decelerate, and give up only some of their KE thus producing x-ray photons with various energies.
What 3 “variations” result in Bremsstrahlung (polychromatic x-ray)?
- continuously varying voltage difference
- Varying distances b/t electrons
- Multiple (various) interactions b/t many participating electrons
What percent of diagnostic x-ray beams are Bremsstrahlung?
70% (most common)
What mechanism for x-ray production results in a polychromatic (heterogeneous) x-ray beam?
Bremsstrahlung
What happens when a k-shell electron is ejected?
It is replaced by an L, M, or N shell electron, or by a free electron
What is the mechanism, Characteristic Radiation, for x-ray production?
Incident electrons interact with K (or L-shell) electrons of target (tungsten) atoms overcome the binding energies and cause ionization
How does a shortage of 1 electron in K shell used for x-ray photon production?
Shortage of one electron in K-shell leads to a higher energy electron from L or M shell to move into the K shell and in the process, the excess energy is given off in the form of an x-ray photon
In characteristic radiation, which electron transition produces a photon of the highest binding energy difference?
A photon representing a binding energy difference of a free- electron moving into k-shell produces the most energy (next high is M to K, and then L to K)
What percent of diagnostic x-ray beams are characteristic radiation?
30%
How does the x-ray beam of characteristic radiation compare to that of Bremsstrahlung?
Characteristic radiation also produces a heterogeneous x-ray, but it has a much narrower energy spectrum than Bremsstrahlung
Where are the x-rays produced in an x-ray tube?
At the center of the anode = “Focal Spot”
What is the primary energy source for an x-ray tube?
Electricity
What is responsible for the speed of electrons, and therefore the energy of the x-ray beam in an x-ray tube?
The kilocoltage peak (kVp)
What is the positive electrode?
Anode
What is the function of the cathode?
To supply electrons through a heated filiment
What is the purpose of the anode?
To stop the electron and change its KE into an x-ray photon
What is the negative electrode?
Cathode
Is the cathode a low or a high voltage circuit?
Cathode is low-voltage, goes thru a step-down transformer
What material is used to make the focusing cup?
Molybdenum
What is the filament at the cathode made of?
Tungsten wire
What is another name for the “boiled off” electrons at the cathode?
Electron cloud
Is the anode a high or low voltage circuit?
Anode is high-voltage, goes thru a step-up transformer
Why is there a radiator at the anode?
Anode requires heat absorber (radiator) b/c over 99% of energy is given off as heat, not x-rays
Which is larger, the effective focal spot or the focal spot/target?
The focal spot/target is larger than the effective focal spot
Describe the anode focal spot/target
Made of tungsten embedded in a copper anode, situated at an angle
What are the dimension (and the significance) of the actual focal spot vs. the effective focal spot?
Actual focal spot dimension are 1x3 mm, but anode is angulated to produce a beam from a smaller area (1x1mm) called the “effective focal spot”
What are the 6 major reasons Tungsten is used as the target?
- High melting point/Heat stable
- Highly ductile
- Produces predictable & optimal x-rays
- High atomic number
- High thermal conductivity
- Low vapor pressure
Why does high atomic number help make Tungsten a good target material?
High atomic number elements produce predictable and useful energy level photons
Why is it important for Tungsten to have a low vapor pressure?
X-ray tube needs to have NO air or moisture in it, otherwise it will not last for very long
In what 3 ways does a collimator influence the x-ray beam?
- Diaphragm limits the size of divergent beam
- External collimator (BID) filter reduces the beam size providing a means of aiming the beam
- Collimator device also aids in increasing the “mean” central beam
What are the 2 kinds of collimators?
- Round
2. Rectangular
How is a rectangular collimator advantageous?
Rectangular collimator reduces patient exposure to primary radiation by shaping the beam to size and shape of the image receptor
What is the role of the positioning instrument when using a rectangular collimator?
Positioning instrument facilitate both beam and receptor alignment
What percent of total energy is given off as heat when taking a radiograph?
Over 99% is heat (Highly Ineffective)
How is heat conducted away from the beam and the x-ray tube?
By having a copper stem at the anode, and also by oil emersion of the x-ray tube to reduce overheating of the target and tube
What is a portal?
A channel for passage of x-rays that is liked by lead to reduce scatter
What is the role of the filter?
Filter absorbs the less penetrating (long wavelength/less energetic) x-rays which are harmful to the patient
What happens to longer x-rays if there is no filter?
X-rays with longer wavelengths are mostly absorbed by skin and do not reach the target tissue
What material is the filter composed of?
Aluminum
What kinds of beams produce harmful secondary radiation?
Low energy/long wavelength beams
What does “Half-value layer” mean?
Half Value Layer (HVL) is the thickness of aluminum in the filter which will reduce the number of x-ray photons passing thru it by half
What is the definition of collimation?
Shaping of the beam
Why do we use collimators?
Collimators cause x-ray beams to be greatly reduced in their amount of unnecessary exposure to patients
Most x-ray tubes use what shape and size collimator?
Round, 7cm
What kind of collimator reduces patient exposure significantly?
Rectangular
How does a rectangular collimator beam compare in size to the image receptor?
Rectangular collimation beam is slightly larger than the size of the image receptor
What are the 4 conditions necessary for x-ray production?
- Separation of electrons
- Concentration of electrons
- Imparting high speed to electrons
- Sudden stopping of electron stream to produce x-rays
What is “Thermionic Emission”?
A necessary process for x-ray production resulting in the separation of outer electrons of the tungsten atoms by having a filament current (low voltage) heat up the tungsten filament (at the cathode) to incandescence producing an “electron cloud” which has highly active electrons bumping into each other thus causing free electrons as a result.
What is the behavior of electrons in an electron cloud?
At the filament, electrons w/in the cloud are more active and by random collisions they produce free electrons
How is concentration of electrons (a necessary step in x-ray production) achieved?
A small molybdenum collar (focusing cup) at the cathode serves to concentrate the electrons
What becomes of the concentrated electrons at the cathode during x-ray production?
When the high-voltage circuit at the anode is activated, these concentrated electrons form a beam that hits a small area (focal spot) located on the face of the copper anode
How is the necessary step of imparting high velocity on the electrons achieved during x-ray production?
A high potential difference is created between the filament cathode and the target anode by activating a high voltage circuit at the anode…this causes electrons that have “boiled off” at the cathode filament to rush toward the anode target at high speed
How is the necessary step of sudden stopping of electrons onto a target during x-ray production achieved?
Kinetic energy of the electron stream interacts with the tungsten target and undergoes conversion. Although over 99% of the energy is given of as heat, a fraction is converted to x-ray photons by the sudden “breaking radiation” or “characteristic radiation” effects.
Into what 2 forms of energy does the kinetic energy of an electron transform into?
- X-rays (99%)
What 3 factors are controlled at the control panel?
- Milliamperage (current) = # electrons produced
- Exposure time = (length of time for voltage difference)
- Kilovoltage (power) = energy of the beam
What 5 factors regarding the receptor itself influence a radiographic image?
- Density, contrast, and resolution
- Distortion due to placement/technique
- Filtration and Collimation
- Source to receptor distance
- Object to receptor distance
What does an objects attenuation (absorption) depend upon?
- Density of object
- Atomic number of object
- Thickness of object
* Differential Absorption*
What 3 factors influence the penetrability of x-rays?
- Photon energy
- Thickness of object
- Density of object
What are the 4 possible interactions that an x-ray can have with matter?
- Complete beam absorption
- Partial beam absorption
- Complete penetration to receptor
- Scatter
What is the “Photoelectric Effect”?
Describes the type of interaction an x-ray has with matter in which the beam is completely absorbed by the object and no beam passes thru to the receptor (ex amalgam)
What type of interaction of beam to matter results in a radiopaque (white) area on the image?
Radiopacity occurs when SOME of the beam is completely absorbed by the patient
What type of interaction of beam to matter results in radiolucent (dark/black) area on the image?
Some of the beam basses thru the patient without loss of energy and reaches the film
What is fog?
Fog is degradation of the image and loss of resolution sharpness due to some of the beam scattering in a new direction away from the primary beam
What is (an example) of complete penetration to receptor?
Describes the type of interaction an x-ray beam has with matter in which the beam is not absorbed at all by the object and therefore all of the beam will reach the image receptor (ex. maxillary sinus filled with air)
What is “Coherent/Thompson” scattering?
Describes the type of interaction an x-ray has with matter in which the beam is almost completely absorbed by the object but not entirely. Thus some of the beam will be able to reach the image receptor (ex cortical bone)
What is Compton Scattering?
Describes the interaction an x-ray has with matter in which the is only partial or tangential exposure to the receptor, thus resulting in “fog” which is an undesirable but always occurring phenomenon.
