RADI 141 Test 3

what is scatter radiation?

x-rays that have interacted with an electron of an atom which causes the direction and energy of the x-ray photon to be altered.

scatter will ____ the contrast on the radiograph

decrease

is scatter part of the useful beam?

no, it serves so purpose

3 things that can occur with an x-ray photon when it interacts with the patient

absorbed, pass through unaffected, scatter

what are the two major contriollers of Scatter radiation?

beam restriction, grids

what is a Grid?

a device that is used a cassette

what is the purpose of the grid?

to absorb scatter

what are the two factors that affect the amount of scatter produced?

kVp, irradiated material

what are some examples of irradiated material?

bone vs tissue, patient size

kVp affects the ____ of the beam

penetrability

an increase in kVp will do what to the energy of the beam?

increase

Higher kVp = ____ the scatter

greater

Higher kVp = highly penetrating, meaning

less photons being absorbed by patient

less absorbed = ____ patient dose

less

the amount of scatter created during an interaction is affected by

the volume, atomic number (z)

the greater the volume = more ____

scatter

volume increases as what increases?

the field size

the larger the body part means

more tissue and more scatter

the higher the atomic number of the irradiated material =

the more electrons around the nucleus

less scatter occurs due to

more of the photons being absorbed

bone radiation absorbs more ___ than soft tissue

radiation

due to higher atomic number more photons are absorbed and creates

less scatter on the radiograph

the larger the field size, the more x-ray photons allowed to interact with the tissue, which

creates more scatter

how do you minimize scatter?

use the smallest field size possible and collimate

what are the 4 types of beam restricting devices

1.aperture diaphragm

2.cones and cylinders

3.collimated

4.lead blocker/mask

what is a aperture diaphragm?

A flat sheet of metal, usually lead, with a hole cut in the center and attached to the x-ray tube.

what beam restricting device is the simplest to use, cheapest to buy, and can be made into any size or shape?

aperture diaphragm

what are the disadvantages of a aperture diaphragm?

different sizes and shaped needed to accommadate different film sizes, increases Penumbra

what is Penumbra?

Geometric unsharpness around the periphery of the image.

what caused Penumbra?

due to x-rays not having a “point source”

when is Penumbra reduced?

when a beam restrictor is used and when using small focal spot rather than large

if you decrease penumbra what happens to detail?

it increases

what can aperture diaphragms cause?

increase in off-focus radiation

what is off-focus radiation?

photons not created at the focal spot but elsewhere in the tube.

what does off-focus radiation produce on a radiograph?

a ghost like shadow

determining the diaphragm field size aids in determining the

projected image on the image receptor

when using the diaphragm the light field is

altered

A diaphragm is placed 5” from the focal spot. The diameter of the opening in the aperture is a 2” circle. What would be the projected image size if the SID is 40”?

16” diameter circle

what are cones/cylinders?

circular aperture diaphragms with metal extensions

a cone has what?

a smaller diameter on top that flares towards the bottom

due to the flared ends on a cone they do not _____

reduce penumbra

what do cones/cylinders do?

only cleans scatter therefore improving contrast

do cones/cylinders increase recorded detail?

NO

is a cylinder the same size diameter all the way down?

Yes

what will a cylinder reduce?

penumbra

what will a cylinder reduce?

off-focus radiation

what is the disadvatage of a cylinder?

difficult to use for x-ray table exams due to weight

The diameter of the lower rim of a cone is 3”. The bottom of the cone is placed at a distance of 16” from the focal spot. What will be the size of the projected image at a 40” SID?

7.5” circle

what is the most common beam restrictor in radiography?

collimater

what beam restrictor is more complex and expensive?

collimater

the collimater allows how many field sizes?

numerous

what does a collimater provide?

a light source for radiographer to aid in positioning

the light source should ____ be opened greater than the field size of Image Receptor

NEVER

how many sets of shutters are there to regulate field size?

2

what are the two types of shutters?

upper and bottom shutters

what does the upper shutters reduce?

off-focus radiation

what do the bottom shutters reduce?

penumbra

light field is created by what?

the mirror

what degree angle is the mirror placed at

45 degrees

does the x-ray beam travel through the mirror?

yes

when was the positive beam limitation (PBL) device required by law?

from 1974 to 1993 the US required that all x-ray equipment have PBL

what is a positive beam limitation?

sensing device that determines the size and placement of cassette in bucky tray.

does a positive beam limitation devide automatically collimate to the size of cassette by moving shutters?

Yes

what are lead blockers?

Sheet of lead-impregnated rubber that can be cut to any size or shape.

what are lead blockers used for?

to absorb scatter from large body parts and aid in positioning

what do lead blockers help?

cleaning up scatter to produce a more diagnostic image

increasing field size refers to _____ the collimator or ______ beam restriction

opening, decreasing

increased field size ____ volume of tissue irradiated

increases

what does increasing field size reesult in?

increased scatter, decrease in contrast

when the image receptor exposure is increased when increasing field size what happens?

more x-ray photons pass through unaffected

what is decreasing field size?

refers to close collimation or increase in beam restriction

A decreased field size decreases beam quantity also does what

decreases scatter, shortens scale of contrast image

when decreasing field size what happens?

fewer photons reach the image receptor and the image receptor exposure is decreased

an INCREASE in mAs must what?

accompany a significant reduction in field size to maintain image receptor exposure

what is a grid?

a series of radiopaque lead strips which alternate with radiolucent interspace material

how does a grid improve contrast?

by absorbing scatter

what is the defintion of radiopaque?

x-rays or light can’t pass through easily (bone, lead)

what is radiolucent?

x-rays easily pass through (sponge)

when photons pass through the body, what 3 things can they do?

1) Pass through the body unaffected

2) be absorbed

3) interact and change direction

when photons pass through the body and reach the image receptor it creates what?

an image

what does a grid also create?

contrast

during what do some photons scatter?

Compton interaction

scatter radiation serves ____ diagnostic purpose

NO

scatter radiation results in a ____ in contrast

decrease (grayer appearance on image)

when does scatter increase with?

1) Increased patient thickness

2) larger firld size

3) increase in kVp

4) decrease in atomic number of tissue

what is the main purpose of a grid?

to clean up scatter

what are the rules for using a grid?

1) Body part exceeds 10 cm

2) kVp is above 60

when was the first grid made?

1913

who made the first grid?

Gustav Bucky

what type of design did the first grid have?

criss-cross design with lead strips running across and down with a gap in between

when was the grid improved?

1920

who improved the grid and how?

Hollis Potter improved the design by putting thinner lead strips in one direction

who invented the Bucky?

Hollis Potter

What are the primary functions of the grid?

1) Absorb scattered radiation before it reaches the film

2) improve the contrast on the radiograph

what are grids made of?

lead strips and interspace material

what are the types of interspace material?

A) Aluminum (Al)

B) plastic fibers

why is it important to have lead strips going in one direction?

allows us to go along the grain so we can use an angle on the tube

what is the most common interspace material?

AIuminum (Al)

what is the major influence on the grid’s ability to absorb scatter and improve contrast?

Grid Ratio

what is the grid ratio?

h/d