6 - Radiation Physics and X-ray I Flashcards
What are x-rays?
X-rays are ionizing radiation
- It removes an electron from an atom
- It is also non-particulate and uncharged radiation
What are the ways we measure x-rays?
- Roentgren (R) or Air Kerma
- Rad or Gray (Gy)
- Rem (r)
- Curie or Becquerel
Roentgren (R) or Air Kerma
QUANTITY of exposure
Rad or Gray (Gy)
QUANTITY of absorbed dose
10-20 Gy is lethal to humans
Rem (r) or Sievert (Sv)
Rem = “roentgen-equivalent-man”
QUANTITY of effective dose equivalent received by radiation workers
4 Sv is lethal dose to humans
Curie or Becquerel
QUANTITY of radioactivity
Exposure
Amount of ionization that is produced when radiation passes through matter
Absorbed dose
Amount of energy absorbed by matter when radiation passes through it
Dose equivalent
- Measure of biological damage caused by radiation
- Product of absorbed dose multiplied by a quality factor that accounts for the observation that for the same amount of absorbed radiation, different types of radiation cause different amounts of biological damage
Effective dose equivalent
- Measure of biological damage caused by radiation to a specific organ
- Product of dose equivalent multiplied by the weighting factor that is a measure of the risk from exposure to that organ compared to the risk from whole body exposure to the same dose
What are the numbers of doses you need to know?
** KNOW THIS **
- 1 Gy = 100 rad
- 1 Sv = 100 rem
FYI - 1 rem = .01 Sv = 1cSv = 10 mSv
Describe what an x-ray is
X-rays, along with gamma rays, are also a form of electromagnetic radiation (another form of ionizing radiation) which have no mass or charge
Describe photons
- Photon-smallest quantity of any type of electromagnetic radiation-has high energy and short wavelength
- Photon energy is measured in electron volts
- Diagnostic imaging uses 30 to 150 kVp (discussed later)
Attenuation
- X-rays are partially absorbed by matter which is known as attenuation
- Results in different shades of gray on film
- Matter that absorbs x-rays appear white and are radiopaque ***
- Matter that allows total penetration of x-rays appear black and are radiolucent***
Radiopaque
- ABSORBS x-rays
- Shows up WHITE (bone)
Radiolucent
- PENETRATION of x-rays
- Shows up BLACK (soft tissue)
Inverse square law
Inverse square law: light intensity from a source is inversely proportional to the square of the distance between the object and the source
Equation: [I₁/I₂ = (d₂/d₁)²]
Example: X-ray intensity is 1 rad at 2 feet and at 4 feet is 0.25 rad
Components of an x-ray unit
- Control console
- Transformer
- High-voltage generator
- Tubehead
- Beam limitation device
What are the three factors under control of the x-ray operator?
** IMPORTANT **
- kVp (kilovoltage peak)
- mA (milliamperage)
- Timer
Describe kVp (kilovoltage peak)
- Energy of the x-ray beam
- 50-70 kVp’s
- Manipulates primarily the energy or QUALITY of x-rays and to a lesser extent the quantity of x-rays ***
Describe mA (milliamperage)
- Tube current and controls the number of photons produced
- 10-30 mA’s
- Manipulates quantity of x-rays ***
Describe the timer
Listen to this for what he said will be on the test
- Controls length of exposure
- 1/60th to 1/100th of a second
- Manipulates quantity of x-rays ** KNOW FOR EXAM **
What are the components of the tubehead?
- Supporting arm: holds tubehead firmly in place and keeps the source-to-image distance (SID) constant and is at minimal 12 inches from the patient
- X-ray tube
- Beam limitation device
Describe the protective housing of the tubehead
Filled with oil, surrounds and insulates the x-ray tube
Describe the x-ray tube
- Enclosed in glass or metal and maintains a vacuum
- Has area which allows for passage of the x-rays called the useful beam
2 basic components
- Cathode
- Anode
Describe the cathode
Negative electrode of the x-ray tube
What does the cathode contain?
- Filament: composed of tungsten and produces electrons when heated-the number of electrons produced is determined by the temperature of the filament and is measured in milliamperes
- Focusing cup: directs the filament electrons to the anode
Describe the anode
Positive electrode of the x-ray tube
What does the anode contain?
- Target: area to which electrons produced by cathode are directed and made of tungsten
- Focal spot: area on target from which x-rays are emitted
Describe filtration
- Inherent: tube housing absorbs off-focus radiation that does not contribute to the useful x-ray beam
- Added: filters the useful x-ray beam and absorbs low energy x-rays that cannot reach the image receptor-usually made of 1.5 to 2.5 mm thick aluminum
Notes: Not every x-ray beam will be focused or used for diagnostics, so you want to filter out the x-ray beams that you don’t need. The lead will absorb some of the scatter radiation (INHERENT) and then just before it leaves, there is another filter that absorbs low-energy radiation and this will then never reach the patient (ADDED)
Beam limitation device
Shape the dimensions of the useful x-ray beam so that only the area of interest is x-rayed, a process called collimation
Describe the process of using a beam limitation device
- First step is to turn on a light source which shines on the dimensions of the part to be examined and will correspond to the area exposed by the useful beam
- The center of the light source has crosshairs or a circle to accurately position the central beam
Notes: Nothing more than a flashlight that allows you to control the length and width of the field you will expose the patient to - this field of exposure is called collimation
Choose the correct units that primarily determine the quantity of x-rays that reach the object during conventional x-ray of a foot.
1 - mA of 100 2 - kVp of 50 3 - mA of 50 4 - Timer of 1/70 of a second 5 - Timer of 1/30 of a second
What we are asking here is what PRIMARILY controls quality
Answer: Timer of 1/70 of a second
This is because it is a conventional foot x-ray (correct range)
X-ray production summary
- Electrons generated at the cathode end at the filament
- Focusing cup directs electrons toward the anode target
- Electrons interact with the target where x-rays are produced
What are x-rays (photons) produced by?
- Characteristic radiation
- Bremsstrahlung radiation
Describe characteristic radiation
- Electron interacts with atom’s inner shell electron and removes the electron which causes ionization
- X-ray photons produced as outer-shell electrons fill the inner shells
Describe Bremsstrahlung radiation
- Electron slows down or “brakes” as it passes by + charged nucleus
- Changes course of electron which causes it to lose energy in the form of x-ray photons
Describe x-ray QUANTITY
X-ray quantity (output intensity, x-ray intensity, x-ray exposure) is the number of x-rays
Describe x-ray QUALITY
X-ray quality is the penetrability of the x-ray or how much energy it has
What do you need to know about x-ray contrast?
- The more x-rays that strike the film, AND the more energy x-ray strikes the film, the blacker it makes the film and the less contrast there is (can’t tell white from black as well)
- You WANT to have as much contrast as possible without exposing the patient to excess x-ray
- More contrast means you can clearly tell white from balck
Describe the x-ray emission spectrum
- X-ray emission spectrum is the range of x-rays energies present in a quantity of x-rays-influenced by mAs, kVp, and added filtration
- Increasing mAs increases quantity but not quality
- Increasing kVp increases both quality and quantity
- Filtration decreases quantity, increases quality
What does increasing MAs do?
- Increases quantity
- Does NOT increase quality
What does increasing kVp do?
- Increases BOTH quality and quantity
What does decreasing filtration do?
- Decreases quantity
- Increases quality
Heel effect
- Useful x-ray beam has greater intensity on the cathode side than the anode side which is a result of the line-focus principle
- Position the thicker portion of the part to be examined closer to the cathode
The thicker part of the body part that you want to look at, put it closer to the cathode because it will be exposed to more x-rays
Matching REVIEW:
* TEST QUESTION*
1 - X-rays are produced by this process…
2 - Place the thickest section of the object closest to the cathode…
3 - X-ray emission spectrum…
4 - Affect quality of x-ray…
5 - Affect quantity of x-ray…
Listen for the rest of the answers
1 - X-rays are produced by this process… Bremsstralung radiation
2 - Place the thickest section of the object closest to the cathode… Heel effect
3 - X-ray emission spectrum… Range of energies in x-ray beam
4 - Affect quality of x-ray… kVp, filtration
5 - Affect quantity of x-ray… kVp, mA, filtration, time of exposure, BUT mA PRIMARILY controls quantity
X-ray interaction with matter occurs in three ways…
Occurs in 3 ways which have application to podiatric radiography:
- Coherent scattering
- Compton scattering
- Photoelectric effect
Describe coherent scattering
As x-ray photons excite the target atom, a secondary photon of equal energy is released in different direction which is absorbed by the patient but does not contribute to diagnostic image
Describe Compton scattering
- Occurs when x-ray photon interacting with matter ejects the outer-shell electron causing ionization
- Original x-ray continues in different direction with decreased energy
- This creates “fog” that impairs the image quality by reducing its contrast
Describe the photoelectric effect
- X-ray photon is totally absorbed and an inner-shell electron is ejected during ionization
- This process causes beam attenuation and image formation
- Also contributes to patient-absorbed dose
What is the diagnostic formed by?
The diagnostic image is formed by the photoelectric effect and remnant radiation (x-rays that pass through patient and strike the image receptor)
Describe what causes image fog
Most x-rays interact with matter by Compton scattering which causes image fog
Low kVp
- Favors photoelectric effect, but increases dose of radiation (more radiation is absorbed by the body)
High kVp
- Favors Compton scattering (more radiation makes its way to the x-ray film)
Describe differential absorption
- Differential absorption is manifested by attenuation which is the reduction in x-ray beam intensity as it is absorbed and scattered by matter
- Bone absorbs x-rays to a greater degree (appears white) than fat and soft tissue (appear black)
Matching REVIEW
* TEST QUESTION*
1 - Forms the diagnostic image…
2 - Forms image fog…
3 - Favors photoelectric effect…
4 - Favors Compton scattering …
1 - Forms the diagnostic image… Photoelectric effect
2 - Forms image fog… Compton scattering
3 - Favors photoelectric effect… Low kVp
4 - Favors Compton scattering… High kVp
Describe what a high-quality x-ray should exhibit
- Sufficient radiographic density
- Acceptable contrast
- Optimal detail
- Minimal distortion
Resolution
Ability to separate and distinguish between two separate objects
Spatial resolution (image sharpness)
- Ability to distinguish between two objects which are side by side and have different contrasts (bone vs. soft tissue)
- High spatial resolution is able to distinguish between two objects that are aligned closely together
Contrast resolution
Ability to distinguish between differences in density or intensity
Radiographic density
The about of “darkness” or “blackness” on the film
AKA “optical density”
Describe radiographic density
- Amount of darkening in a radiograph
- Along with contrast (adequate differences in density), are required to give visibility to the structural detail of a subject
What is the primary controlling factor for radiographic density?
- mAs is the primary controlling factor and is the product of milliampere multiplied by exposure time in seconds (mA x seconds)
- Can have the same mAs but different milliamperes and seconds
What happens as mAs increase?
Film becomes DARKER
Known as the law of reciprocity
How do you make an adjustment?
- If you need to make an adjustment in density either increase the mAs by a factor of 2 (to increase darkness) or decrease by a factor of ½ (to increase lightness)
- Change either mA or s, but not both
Note that if you need to make an adjustment, you will need to increase or decrease mAs by at least 30% or the human eye cannot tell a difference
What other things can effect the radiographic density other than mAs?
- Kilovoltage (kVp)
- Distance (SID)
- Film/screen combinations
- Compensation filter
- Film processing
Describe the characteristics of kVp
- Governs penetrating power of electrons
- Alters both quality (energy) of x-ray beam and quantity of photons produced
- Abides by a 15% rule
How does kVp govern the penetrating power of electrons
- Low kVp produces high contrast (tissues appear black or white without much gray
- High kVp produces low contrast (tissue appears uniformly gray)
How much does kVp alter the quality (energy) of x-ray beams and quantity photos produced?
Need a 4% to 8% change in kVp (3 to 7 kVp) for the human eye to see a visible difference in the x-ray
Give the 15% rule of kVp
- A 15% increase in kVp will cause the same change in radiographic density (make image look darker) as doubling the mAs
- A 15% decrease in kVP will cause image to look lighter to the same extent as will halving the mAs
What does changing the distance between source to image do to the x-ray image?
Source-to-image distance (SID) will alter number of photons striking the film
Inverse square law:
- If tube is too far from film, too few photons strike film producing a lighter image
- If too close to film, more photons produce a darker image
Film screen combinations
- Film/screen speed-sensitivity of intensifying screen in cassette to the x-ray photons
- The higher the speed of the film/screen, the less radiation is necessary to produce an image, but the less image detail is produced
Compensation filter
- Forefoot is thinner than rearfoot; as a result, radiographic density varies between toes and tarsus (toes are thinner than tarsus)
- If exposure technique selected for toes, tarsus will be underexposed (look lighter)
- If exposure technique selected for tarsus, toes will be overexposed (look darker)
*** Need compensation filter to balance unequal radiographic density between forefoot and rearfoot
Film processing
As temperature and immersion time of film in developer increase, radiographic density increases (image darker), and vice versa
Don’t need to worry about this because you will never have to do this
Contrast
Allows visibility of detail
Two types
- Radiographic contrast
- Subject contrast
Radiographic contrast
- Result of difference in densities that allows viewer to discern between two adjacent densities within the same image
- Affected by type of intensifying screen, film’s density, characteristic curve, and processing
Subject contrast
- Result of attenuation differences as x-rays pass through body
- Affected by tissue thickness, type, atomic number and density
What are the two scales of contrast?
- Short
- Long
Short scale of contrast
- Used in foot and ankle imaging
- Distinguishes bone margins from adjacent soft tissues
Long scale of contrast
- Used in abdomen
- Visualize different soft tissue structures that have more similar densities
What is the PRIMARY factor in controlling contrast?
kVp
Describe how kVp controls contrast
** KNOW THIS FOR EXAM **
- Higher kVp’s produce low contrast and vice versa
- To produce shortened contrast, increase mAs by 2 and decrease kVp by 15%
- To produce lengthened contrast, decrease mAs by ½ and increase kVp by 15%
** NOTE: need to adjust mAs to maintain the same radiographic density (degree of “blackness”), so if your contrast is off, you need to change TWO things, you can’t just change the kVp **
If you don’t manipulate the mA you will either get a too-dark or too-light image
** KNOW THIS **