scatter control & grids; imaging II ch 12
Describe the appearance of the moiré pattern artifact on a PSP digital image and how would a technologist obtain this type of artifact on their final image?
An wavy artifact caused by grid lines running parallel to the laser scanning motion; zebra pattern artifact occurs when grid is put in bucky tray (double grid) or if grid frequency similar to scanning frequency in CR imaging)
Focused grid:
lead lines that are angled to match the angle of divergence of the primary beam therefore they allow more transmitted photons to reach the image receptor
Parallel (non-focused) grid:
lead lines that run parallel to one another. Completely straight lines therefore these grids are commonly used during mobile examinations.
List the characteristic of a typical grid used in radiography. (Box 12-2)
is long dimension linear instead of crossed, is focused instead of parallel, is of mid ratio (8:1-12:1), has a focal range that includes an SID of 40 or 72"
List 2 major factors that affect the amount of scatter radiation produced and exiting the patient.
kVp & volume of tissue irradiated (part thickness & field size)
Parallel grids are commonly used during what imaging studies?
Fluoro & mobile imaging
What is the radiographic primary effect of grid cut-off on a film-screen & digital image?
Further reduction in the number of photons reaching IR; Decreases image quality & not useful information; further reduction in number of photons reaching IR in digital, noise increases from decreased photons reaching IR due to the further reduction in number of photons reaching IR.
What is the mathematical formula a technologist must determine when they utilize or use a grid for their radiographic procedure? (Know Grid Conversion
factors & how this affects mAs selection) In class Quiz: 10/29/20 Worksheet when grid added, mAs increased by factor indicated to maintain same number of photons reaching IR, this requires mult by the GCF for the grid ratio
Grid cap:
type of grid that is permanently mounted grid that allows you to slide the cassette or IR behind it
Grid frequency:
# of lead lines per unit length in inches or cm
Describe the location of the grid in relationship with the radiography table & patient.
After patient but before IR. (b/w pt & IR) under tabletop/ patient and just before IR
What is the purpose of an automatic collimator or positive beam-limiting device?
Automatically limits the size/shape of primary beam to the size/shape of the IR to limit amount of exposure to patient & increase quality
Why does the use of a grid have no effect on brightness, spatial resolution, or distortion?
Brightness comes from the computer, spatial resolution comes from pixels (pitch and density), distortion is geometric factors; Only cleans up scatter and contrast.
What is the purpose of placing a lead shield on the table behind the area of exposure when performing a lateral lumbar spine exam?
Cleans up scatter; efforts to control the amount of scatter radiation from patient striking the IR
What interaction is the primary responsible for producing scatter radiation?
Compton interaction
What shape is the unrestricted primary beam?
Cone shaped; but projected as round (circular) field on IR
Describe the effects of scatter on a digital image.
Decrease contrast/quality, Alter EI #, No useful patient information (unwanted exposure=fog)
Describe the effects of beam restriction of the primary beam on the production of scatter radiation.
Decrease the x-ray beam field size & amount of tissue irradiated= reduces the amount of scatter radiation produced
How does the use of an aperture diaphragm, cones or cylinders and collimators affect scatter radiation?
Decreases scatter (b/c devices are made of metals that absorb x-rays and decreases beam size.)
Grid ratio:
H/D= grid ratio; ratio of height of lead strips to the distance b/w them
What is the purpose of grid and when should this device be used?
Increase contrast by absorbing scatter before it reaches IR
Describe the effect of collimation on radiographic contrast.
Increased collimation (smaller field size)= increases contrast; decreasing contrast (larger field size)=decreasing contrast
Describe the effect of grid ratio on contrast, patient dose and the probability of grid cut-off.
Increased grid ratio= increases contrast, increases patient dose, increases grid cut off; decreased grid ratio= scatter clean up not as good, less patient dose, increased positioning latitude; Increased grid ratio=more lead in the grid & increased exposure factors=increased pt exposure, increased contrast (better clean up scatter), decreased positioning latitude
Using a grid has what type of impact on radiographic contrast?
Increases contrast by absorbing scatter radiation that exits the patient, reducing the amount of scatter reaching IR (but need more mAs)
What is the relationship between using a higher grid ratio and the amount of radiation reaching the IR?
Increasing grid ratio decreases radiation exposure to IR; better clean up of scatter & increased contrast. Decreased positioning latitude.
State the effects of air gap technique on scatter radiation.
Limits the amount of scatter reaching the IR & requires an increased OID, which will allow scatter to disperse in air before it reaches the IR; increase in SID (60 min) is required to compensate for the unsharpness caused by increased OID
Define positioning latitude when using a grid.
Not a whole lot of margin for error
Why is the patient the primary source of scatter radiation?
Primary radiation hits patient and will bounce off them
Describe the effect of tissue thickness on the production of scatter.
Significant amount of thickness=increases scatter; the greater the volume of tissue irradiated b/c of part thickness=greater amount of scatter radiation produced.
Why is it important to consider patient exposure, level of contrast improvement needed and focal range when making a grid selection?
The grid needs to provide sufficient contrast & keep patient dose as low as possible.;All are factors you have to know when you use the grid. Consideration of contrast-- If patient is significantly larger, use larger grid ratio to clean up more scatter. With small patient, use smaller grid ratio. Benefit vs. risk pt exposure. Focal range consideration b/c Dont want to be out of focal range bc will get grid cut off.
Describe how a grid is constructed.
Very thin lead strips with radiolucent interspace material, grid frequency, and grid ratio(h/D);Thin lead strips or lines that have a precise height, thickness, and space between them
Lead mask:
a sheet of lead filled fubber that can be cut to the size and shape to correspond to a particular field size then secured to the end of a collimator. This can be used in place of a cone or cylinder
Off-level error:
angling the tube across the grid lines or angling the grid itself during exposure=decrease in radiation exposure to the IR
Describe the effect of large changes in collimation has on the number of photons striking the IR, and what must be done to compensate for the changes in order to maintain the exposure to the IR.
as collimation increases, must increase mAs. If significantly collimating down, not allowing enough energy hitting IR for that exposure & going to see noise (taking away the good radiation as your decreasing beam size)
What exams do your facilities utilize a portable grid on and what is the SID for those exams?
ask
Name the most effective and often used radiographic beam-restriction device.
collimator/PBL
Grid cutoff:
decrease in number of transmitted photons that reach the IR because some misalignment of the grid
Focal distance:
distance between the grid and the imaginary lines. need to use the correct SID ranges otherwise cut off will occur. also known as grid radius
focal distance
distance between the grid and the imaginary lines. need to use the correct SID ranges otherwise cut off will occur. also known as grid radius
Grid radius (convergent point):
if imaginary lines were drawn from each lead line in a linear focused grid, these lines would meet to from an imaginary point that helps determine the focal range for a grid
What is the overall effect if a technologist restricts the primary beam when a technologist increases collimation then they increase the field size while using a digital imaging system? Pt dose, scatter, contrast & noise (Table 12-1)
increase collimation=decrease patient dose, decrease scatter radiation, increases contrast, increases quantum noise w/o appropriate increase in mAs; increase field size=increase patient dose, increases scatter radiation, decreases contrast, decreases quantum noise
Stationary grid:
no grid movement is made during exposure of a patient therefore a technologist will be able to see the lead lines from the grid on their image if they examine the image closely. Portables we use this type of grid
Grid focus:
orientation of grid lines to one another inside the grid construction
Focused grid upside down:
placing a focused grid upside down on the IR=significant under exposure to the lateral edges of the IR
Describe how the use of higher KV and a larger field size effects scatter
production. Higher kVp= produces more scatter (increases energy of scatter radiation exiting pt.); Larger field size=produces more scatter. (irradiates more tissue vol)
Focal range:
recommended range of SID that can be used with a focused grid, otherwise the image appearance will a loss of exposure around the periphery of the image decreasing image quality
focal range
recommended range of SID that can be used with a focused grid, otherwise the image appearance will a loss of exposure around the periphery of the image decreasing image quality
While using the air gap technique, to ensure the technologist does not decrease
sharpness of their overall image, they must do what to their SID? Increase SID (60 min)
Why should a grid not be used while performing an exam while using the air gap
technique? its an alternative to using a grid; alternating it with SID and OID
Why should a technologist be concerned with the amount of Compton scattering
that can occur when x-ray a patient? Decreases image quality & contrast, affects exposure indicator #, also over-radiates patient (increased scatter radiation produced within patient or higher energy scatter exiting the patient affects exposure to patient and anyone near.)
Off-center or lateral decentering:
the center of the x-ray beam is not aligned from side to side with the center of a focused grid= decreases in radiation exposure to the IR
Cross-hatched grid pattern:
the lead lines that run at right angles to one another (horizontally & longitudinally), intertwining themselves. They remove more scattered photons but technologist is unable to angle their tube across this grid pattern or grid cut off will occur.
Long dimension grid:
the linear grid has lead strips running parallel to the long axis of the grid
Short dimension grid:
the linear grid has lead strips running perpendicular to the long axis of the grid and these grids tend to be more user friendly when centering the CR
Linear grid pattern:
this grid pattern has lead strips or lines that run in only one direction which makes angling cephalic and caudal possible along the length of the lead lines, therefore it is most popular grid pattern to use.
Reciprocating grid:
this type of grid is incorporated in the bucky, when exposure is being made the grid will begin to move (back & fourth laterally) allowing for the grid lines to be blurred for the image
Off-Focus error because incorrect SID:
using SID outside of focal range= loss of exposure at the periphery of the IR