WEEK 7--Scatter Control Flashcards
Scatter radiation is always undesirable
.
1–It does NOT contribute to the diagnostic value of the image.
2–It occurs primarily as a result of
___________________________.
•Also occurs somewhat from coherent scatter.
3–It makes the image less visible by reducing
_____________
COMPTON EFFECT
CONTRAST
Effects of Scatter Radiation on Image Contrast
- ____________is the degree of difference in OD between areas of an image (shades of gray)
a –An ideal image would be created with transmittance and absorption only (no scatter) - Scatter reduces image contrast by exposing parts of the film, which are not representative of the anatomy.
Contrast
Scatter Radiation
There are 3 main factors that contribute to
scatter radiation:
1 –kVp (energy)
2–Patient (object) ___________
3 –X- ray field size
thickness
How does kVp affect scatter
We know that as the energy (kVp) increases BOTH
compton and photoelectric ____________ the absolute
number of interactions.
Since photoelectric decreases much faster than compton, the end result is relatively _______ compton than photoelectric at higher energies
INCREASE IN ENERGY = INCREASE IN SCATTER!!
decrease
more
So why would we ever raise the kVp?
2 Reasons:
1: Patient dose
•Low kVp techniques (giving a lot of photoelectric effect) increase the patient absorption, which in turn __________the patient radiation dose.
2: Transmittance
• If the kVp is to low, not enough x-rays will go through the patient to create an image
increases
Another factor that affects the level of scatter
radiation is the __________ ______________.
•Imaging thick parts of the body results in
more scatter radiation than thin parts.
–The more chances for interaction, the more
compton is produced.
___________ of anatomy reduces scatter and
reduced patient dose by redistributing tissue.
–The more chances for interaction, the more compton
is produced.
PATIENT THICKNESS
Compression
More interactions = More scatter
KNOW
Another factor that affects the amount of
scatter radiation but is controlled by the technologist is the x-ray beam field size.
As the field size is __________ scatter radiation also ___________
–The more chances for interaction, the more compton
scatter is produced.
increased,
increases.
How do we limit the beam size?
Beam restricting devices—–Three main types:
- Aperture diaphragm
- Cones or Cylinders
- Variable-aperture collimator
KNOW
Aperture Diaphragm
•The ___________of all beam-restricting devices
•Lead or lead-lined metal diaphragm attached to the x-ray tube head
•The opening in the diaphragm is usually designed to cover just less than the size of the image receptor
used
simplest
Cones & Cylinders
_________ is one difficulty when using cones/cylinders
Now mostly used with teeth & skulls
Alignment
Variable Aperture Collimator
The most common beam-restricting device is the
light- localizing variable aperture collimator
•The first part of the collimator serves to control
off-focus __________
radiation
Collimation
•An ________ collimator allows for variable x-ray beam
sizes.
•Lead (Pb) leaves are at least 3 mm thick
•They work in pairs and are independently controlled
adjustable
Beam-restricting devices
•Are helpful to improve ______ resolution however the inherent problem is they are placed between the source and the patient.
•Even under the most favorable conditions, most of
the remnant x- rays are scattered.
contrast
Grids
•Are very effective devices for reducing scatter radiation.
•The grid is designed to transmit only x-rays that are traveling in a \_\_\_\_\_\_\_\_ \_\_\_\_\_\_ from the source to the Image Receptor (IR).
•Scatter radiation is absorbed in the grid material
straight line
Grids “clean up” scatter radiation
The grid is a series of sections of radiopaque material (grid strips) alternating with sections of radiolucent material (interspace material)
•A high quality grid can attenuate ___- _____% of scatter radiation
80-90
Grid Strips
Should be very thin and have high photon absorption properties
_________ is most common
Lead
Interspace Material
- Used to maintain precise ________ between the delicate lead strips.
- Allow for x-rays to pass through, unaltered.
- Made of aluminum or plastic fiber
separation
Grid Ratio
3 important dimensions on a grid:
1–The height of the grid
2–The thickness of the grid strips
3–The width of the ______________ material
interspace
The_______ _______ is defined as the HEIGHT of the grid divided by the INTERSPACE WIDTH:
–Grid ratio = h/D
grid ratio
Grid Ratio
- High-ratio grids are more effective in cleaning up scatter radiation than low-ratio grids.
- However, the higher the ratio the more _________ ________ necessary to get a sufficient number of x-rays through the grid to the image receptor.
radiation exposure
The higher the ratio the ___________ the photon
must travel to reach the IR
Most systems use 8:1 to 12:1
A 16:1 grid may clean up as much as 97%
straighter
Grid Performance
•The principal function of a grid is to improve image contrast by absorbing _________
scatter
Bucky Factor
Using grids require more________ _________
When a grid is used, it stops a number of x-rays that would have exposed the film.
The amount of increase is expressed by the Bucky
factor (B)–[aka:grid factor}
The higher the grid ratio or grid frequency the higher the
B
patient dose
HIGHER PATIENT DOSE is a limiting factor and major disadvantage to using grids.
Grid Types
•________Grid–simplest type of grid
•All the lead strips are parallel
•Only clean up scatter in one direction (along the axis of the grid)
•Easy to make, however can cause grid cutoff with short
SID.
Parallel
Grid Cutoff
With ___________ SID, there is more potential for grid
cutoff
X-ray field size will also influence grid cutoff
decreasing
Crossed Grids
•Clean up more than twice as much _________ as linear
grids.
–A 6:1 crossed will clean up more scatter than a 12:1 parallel grid.
•However, central ray vs. grid placement is critical
scatter
Focused Grid
Designed to minimize grid _________
•Lead strips are aligned with the divergence of the
x-ray beam
•Each focused grid must be identified with the appropriate SID
•Wrong SID = Grid cutoff
cutoff
Moving Grids
Moving grids were designed to eliminate ____ _______
Grid Lines are made when non-scattered x-rays are absorbed in the grid strips.
grid lines.
Potter-Bucky Diaphragm
- The grid is placed in a holding mechanism that begins moving just before the x-ray exposure and continues moving after the exposure ends
- The motion of the grid blurs the lines on the image, so they are_______noticeable.
- Most grids in diagnostic imaging are moving
less
The biggest problem with grids is ___________
GRID PROBLEMS RESULT IN: UNDEREXPOSED IMAGE
OR UNDEREXPOSED EDGES OF IMAGE
- Grid Problems–Off Center
- Grid Problems–Off Level
- Grid Problems–Off Focus
- Grid Problems–Upside-Down
misalignment.
Grid Cutoff
•Grids can present issues with x-ray quality if NOT _________ correctly
installed
Air-Gap Technique
- Involves increasing the OID (object to image receptor distance)
- Can be used instead of a grid.
- An alternate method of reducing scatter.–Main disadvantage
is____________________ ____________________
4–mAs still needs to be increased
•approximately 10% for every cm of air gap.
• Occasionally is unavoidable
IMAGE MAGNIFICATION
