physics ch 15,18 Flashcards
what are two ways we can minimize scatter to the IR?
restrict the x Ray beam and use a grid
what is the purpose of “restricting the beam”?
to reduce the amount of primary photons that are emitted from the tube and collimator (less scatter photons )
what 2 principal factors affect the amount of scatter produced?
KV
material irradiated (volume )
kilovoltage affects beam ___________.
penetrability
KV is mostly selected based on _______and ________
size of the part and desired contrast
The amount of scatter created during an interaction is affected by the ________and ________
volume and atomic number of the material
The volume of irradiated material is controlled by ___________and ________
Field size and patient thickness
as the volume of irradiated tissue increases the amount of scatter _____________.
increases
volume increases as the ___________or_____________ increases
Field size or patient thickness
________field size equals __________scatter
decreased, decreased
how is KV and the field size controlled?
by the technologist
True or false
when the beam is restricted, less scatter radiation will reach the image receptor so the technical factors may need to be increased to compensate for the reduction in the overall IR exposure
true
what is the most common type of beam restrictor?
collimator
collimation decreases the amount of _______irradiated
tissue
what is an aperture diaphragm?
A flat sheet of lead with an opening cut in the middle
_________are rarely used but most effective at scatter control.
cones
what is the downside of using cones?
they have a fixed field size
what are two examples of ancillary devices?
lead blockers, lead masks
what is the most common beam limitation device used ?
variable aperture collimator
True or false
when using cones MAS must be increased to account for photons absorbed by the cone
true
The first set of shutters in a collimator, the upper shutters, are mounted as close as possible to the tube window to reduce the amount of______
off focus or stem radiation
define off focus or stem radiation
x-rays emitted from parts of the tube other then the focal spot
how far from the collimator should the patient’s skin surface be to minimize skin exposure from off focus
15cm
why is collimation not used in mammography?
Low technical factors are used
how does scatter affect an image?
impairs image quality
what is the role of the bottom shutters in collimators?
to reduce penumbra along the beam periphery
when is a lead mask most commonly used
during cerebral angiography
what is a lead blocker
A sheet of lead impregnated rubber
A sensing device that activates motor to drive shutters into proper position
positive beam limitation device
what type of grid is designed so that the central grid strips are parallel but become more inclined as they move away from the central axis
focused
The greatest source of scatter comes from?
The patient
what are the two things to remember when using a focused grid?
you must be at a particular SID and the central ray must be aligned perpendicular to the grid
high ratio grids have ______ positioning latitude than low ratio grids
less
True or false
as Bucky factor increases, radiographic technique and patient dose increase proportionately
true
as the grid ratio gets higher contrast ________
improves
what causes grid cut off?
misalignment or improper SID
what happens on the image if the grid is upside down?
severe grid cut off toward the image edge
what is the formula for grid ratio?
H/D
The height of the grid strip divided by the width of the interspace material
what are the three types of grids?
parallel, cross hatched and focused
describe a focused grid
The central grid strips are parallel and as the strips move away from the central axis they become more and more inclined
True or false
The focused design results in a grid with lead strips designed to match the divergence of the x-ray beam
true
in a focused grid, if the lead strips were extended the strips would intersect along a line in space known as the__________
convergence line
what is the grid radius?
The distance from the face of the grid to the points of convergence of the ledge strips
For the grid to be properly focused, the x-ray tube must be located along the_______
convergence line
parallel grid’s function best at _____SID
long
grid conversion factors
5:1 = 2
6:1 = 3
8:1 = 4
10/12:1 = 5
16:1 = 6
what are the two criteria for measuring a grids performance?
selectivity and contrast improvement ability
what is the only positioning error possible with a parallel Grid?
off level grid error
what will a radiograph look like with an off level grid error
decreased exposure across the entire image (a lot of white)
The greater the degree of lateral decentering the ________ the grid cut off
greater
describe the airgap technique
place the patient at a greater OID creating an air gap between the patient and the IR. The amount of scatter reaching the IR will be reduced, contrast improved even though a grid is not used. this is equal to a 6:1 grid but the image will be magnified
what is the primary disadvantage of the airgap technique
loss of sharpness due to increased OID
The contrast improvement factor is dependent on the amount of scatter produced which is controlled by ______ and __________
KV P and volume of irradiated tissue
