Chapter 9 - Scatter Radiation and Its Control Flashcards
Coherent scattering
Coherent scattering is also known as Thompson scatter. This type of interaction takes place at relatively low energy levels (less than 10keV).
Compton effect
The Compton effect occurs at energy levels throughout the diagnostic x-ray range of 40 to 125 kVp. The incoming x-ray photon interacts with an outer orbital electron of an atom, removing it from the atom (ionization), and then proceeds in a different direction.
As the kVp is increased
As the kVp is increased, Compton interactions are increased.
photoelectric effect
The photoelectric effect is similar to that which forms characteristic radiation in the x-ray tube (see Chapter 5). In this case, however, the incoming energy is an x-ray photon interacting with an atom in the body rather than an electron interacting with the tungsten anode. In a photoelectric interaction, the incoming photon from the primary beam collides with an inner orbital electron of an atom. The photon is totally absorbed in the process and creates an absorbed dose in the patient. The electron’s departure leaves a “hole” in the orbit, which is filled by an electron from an outer shell. The difference in binding energy between the two shells is emitted as a new x-ray photon
As kVp is increased
As kVp is increased, photoelectric effect is decreased.
scatter radiation
scatter radiation creates fog that reduces both contrast and the visibility of the spatial resolution
Factors Affecting Scatter Radiation Fog
↑ Volume of tissue =↑ scatter =↑ fog
↑ Kilovoltage =↑ scatter =↑ fog
↑ Field size =↑ scatter =↑ fog
↑ Density of matter =↓ scatter =↓ fog
The thicker or larger the body part is
The thicker or larger the body part is, the greater are the scatter and the fog.
Higher kVp
Higher kVp results in more scatter radiation fog.
The denser the body part,
The denser the body part, the less the scatter. This is because there is more photoelectric effect (absorption). A very dense body part (higher atomic number), such as a bone, will absorb a large quantity of primary radiation.
As collimation is increased, or made larger,
As collimation is increased, or made larger, scatter radiation fog increases.
grid
A grid is used when the body part becomes greater than 10 to 12cm in thickness or kVp settings are greater than 60.
grid frequency
The number of lead strips per inch is called the grid frequency. Grid frequencies range from 60 to 196 lines/inch.
grid radius
The lead strips of a focused grid are precisely aligned with the x-ray beam at a specific source–image receptor distance (SID), which is called the grid radius.
focused grids
Grids for general-purpose use are called focused grids because the lead strips are aligned in the direction of the diverging primary x-ray beam
focal range of the grid
Because the alignment does not need to be exact for the useful photons to pass through the grid, there is a range of distances within which the grid will not absorb an undue amount of useful radiation. The SID used with a grid should always be within the grid’s focal range. The most commonly used SIDs are 40, 48, and 72 inches.
Grid Focal Range
The range of source–image receptor distances at which the grid will not absorb significant amounts of primary radiation.
Example: A grid with a 40-inch radius may have a focal range of 36–48 inches.
Moving Grid
A moving grid is called a Bucky.
Moving the grid during the exposure blurs the image of the grid lines so that the grid image is not visible on the film.
Bucky grids typically have a ratio of 12:1 to 16:1 and a frequency of 85 to 103 lines/inch.
Two methods are used to prevent objectionable grid lines:
the grid may be moved during the exposure or the grid may have a very high frequency (i.e., many fine lines very close together).
Stationary Grids
*Do not move during the exposure
*Should have many very fine lines (high frequency) to avoid objectionable grid lines on images
*Commonly used today in upright cassette holders
Stationary grids for permanent installations typically have a ratio of 8:1 or 12:1 and a frequency of at least 103 lines/inch.
grid cassette
A grid cassette is a special cassette with a grid built into the front side.
Grid cassettes typically have lower ratios than the grids used in permanent installations. Ratios of 5:1, 6:1, and 8:1 are common.
grid cutoff
Excessive absorption of useful radiation by the grid is called grid cutoff. Grid cutoff appears as decreased radiographic density on the side of the image
grid cutoff
Grid cutoff occurs when the x-ray tube is centered to one side of the grid rather than to the focal center line (Fig. 9.14). Cutoff also occurs when the x-ray tube is angled toward one side of the grid, rather than perpendicular to its center.
No grid cutoff occurs when the x-ray beam is correctly aligned with the grid
The higher the grid ratio,
The higher the grid ratio, the more precise the alignment must be.
parallel grid
A grid with strips that are parallel to each other, rather than focused, is called a parallel grid
crosshatch grid
sometimes called a crossed grid. This is actually a composite of two grids with the lead strips at right angles to each other (Fig. 9.19). A crosshatch grid is desirable because it has an effective ratio that is greater than the ratios of the two grids combined.
Methods of Scatter Radiation Fog Control
*Use grid device: Grid placed between patient and film to absorb scatter
*Use air gap: Increased object–image receptor distance decreases scatter intensity at film
*Minimize field size: Decreases volume of scattering tissue
*Decrease kilovoltage: Decreases energy of scatter and increases contrast
coned-down image
This term refers to a radiograph of a very small area of the subject. Most coned-down images are taken using an 8- × 10-inch IR and a 5- or 6-inch square field. They are centered precisely to the area of interest. Coned-down images demonstrate increased contrast compared with images of the same anatomy seen within a larger field. When a specific area of interest is centered to the field, as in a coned-down image, the decreased distortion also improves image quality.
quality control (QC) check
This check is easily done with a collimator template and a beam alignment cylinder (Fig. 9.22). With the collimator test tool, an exposure is made with the template placed on the IR, and the light field is collimated to the indicated square. The standard control limit for the collimator is that the x-ray light field and the radiation field must be within ±2% of the SID.
Grid Cutoff
Caused by misalignment between grid and x-ray beam.
*Lateral decentering
*Source–image receptor distance outside focal range
*Lateral angulation or grid off level
*Grid reversed
lower kVp levels
this narrows the scale of contrast, decrease the energy of the scatter radiation and decrease the fog
