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.
