Scatter & Contrast
This interaction produces scattered photons that are a major cause of reduced radiographic contrast.
Compton Scatter
Reducing the x-ray field so that only the anatomy of interest is irradiated is known as this.
Collimation
The principal job of this device is to remove scatter before it reaches the image receptor.
A grid
A focused grid is used at an SID far outside the distance for which it was designed. This error occurs.
Off-focus grid cutoff
A grid has lead strips 2.0 mm high with interspaces 0.25 mm wide. Calculate the grid ratio.
8:1
How can scatter be managed?
Tight collimation
Proper grid use
Appropriate exposure techniques
When the x-ray field is made smaller, image contrast generally improves because...
Less scatter reaches the IR
A grid with taller lead strips and narrower interspaces has a higher value for this grid characteristic.
Grid ratio
A focused grid is accidentally placed upside down. The center may remain adequately exposed while marked cutoff appears in these portions of the image.
Both lateral edges
Each lead strip is 50 µm wide and each interspace is 200 µm wide. Calculate the approximate grid frequency in lines/cm.
40 lines/cm
What is the effect of increasing kVp?
X-ray beam energy increases
More transmission of photons through the patient
More compton scatter
Image contrast decreases
Less patient dose
This beam-limiting feature prevents the x-ray field from being larger than the selected image receptor.
PBL, positive beam limitation
Compared with a low-ratio grid, a high-ratio grid removes more scatter but normally requires an increase in this exposure factor.
mAs
During a mobile exam, the grid is angled while the central ray remains perpendicular to the receptor. This alignment error produces generalized grid cutoff.
Off-level grid cutoff
Image contrast measures 2.8% with a grid and 1.4% without a grid. Calculate the contrast improvement factor.
2
What is the effect of increasing the field size?
More tissue is irradiated
More scatter is produced
Image contrast decreases
Patient dose increases
The radiation leaving the patient contains both scattered photons and photons that passed through without interacting. What is this called?
Remnant radiation
During an exposure, a Potter-Bucky moves the grid so that these structures are less visible on the finished image.
Grid lines
A focused grid is correctly leveled and used at the proper SID, but the central ray does not enter at the grid’s center. This error is called this.
Off-center grid cutoff
A grid receives 1,500 remnant photons but allows 300 image-forming photons to pass through. Calculate the Bucky Factor.
5
What is the effect of patient thickness decreasing?
Less tissue is irradiated
Less x-rays are absorbed/scattered
Image contrast increases
Lower exposure factors can be used
Digital scatter-correction software may simulate the appearance of a grid, but it does not eliminate the radiographer’s responsibility to perform this basic radiation-protection practice.
Collimation
This grid design has lead strips running in two perpendicular directions and is very effective at scatter cleanup, but requires highly accurate alignment.
Crossed grid
Instead of using a grid, a radiographer increases the distance between the patient and receptor to allow scatter to miss the receptor. A major disadvantage is increased OID and....
Magnification
A mobile examination uses 5 mAs without a grid. The repeat will use an 8:1 grid. What mAs will maintain approximately the same receptor exposure?
20 mAs