RAD 214 Chapter 4

Higher grid ratio =

higher contrast

Automatic collimators are also called

PBL-positive beam limitation

Recommended SIDs that can be used with a focused grid is the

focal range

Focused grids lead strips are angled to match

the angle of the X-rays in the primary beam

The overall quality of the radiographic image is determined by

photographic properties visibility of recorded detail geometric properties

Spatial resolution is improved with

increased pixel density and decreased pixel pitch

As kVp increases, amount of scatter hitting the IR

increases

Decreasing collimation =

increasing field size

Medium frequency grid

120-150 lines/inch used with stationary grid holders

High frequency grid

150-170+ lines/inch used with stationary grid holders avoids aliasing

Grid that provides excellent scatter clean up but requires close to perfect alignment is

16:1

Long dimension grid

lead strips run parallel to long axis portable chest length wise

Short dimension grids

lead strips run parallel to short axis ei. portable chest crosswise

Purpose of automatic collimator is

limiting patient exposure

Higher grid frequency =

lower contrast

To compensate for the reduction of radiation reaching the IR when a gird is used, ___ is typically increased

mAs

Grid used for air gap technique should be high frequency

false

Grids absorb almost all of the scatter radiation and allow all of the transmitted photons to pass through to the IR

false

The grid limits the amount of scatter produced in the patient

false

With automatic collimation, collimating to anatomy of interest is unnecessary because it is already collimated

false

Xray beam field size and light field size are exactly the same

false

Absorption of transmitted photons by a grid caused by misalignment is

grid cutoff

mAs/relative speed formula

mAs old/mAs new = RS new/RS old

Grid with lead strips that run parallel to each other is a

parallel nonfocused

Volume of tissue is determined by

part thickness and field size (collimation)

When should you use a grid?

part thickness is 10 cm or more kVps over 60/70

Anyone in the room during an exam must wear a lead apron to absorb the scatter radiation produced in the

patient

As collimation increases,

patient dose decreases scatter radiation decreases radiographic contrast increases quantum noise increases radiation reaching IR decreases

As collimation decreases,

patient dose increases scatter radiation increase radiographic contrast decreases quantum noise decreases

What is used to determine the grid used for an exam

patient exposure level of contrast improvement needed focal range

Restriction of primary beam to anatomy of interest

reduces patient exposure reduces amount of scatter

Lateral L-spine, placing a lead shield on the table behind the area of exposure

reduces scatter reaching the IR

The unrestricted primary X-ray beam is

round

Focused grids are more effective than parallel grids in absorbing scater and allowing transmitted radiation to reach the IR

true

Scatter is of no value in xray

true

To overcome the decrease in sharpness caused by air gap technique, increase in SID is required

true

Collimators have

two or three pairs of shutters two way mirror light source face plate with crosshair

Cutoff that appears as a strip of appropriate density in the middle of the image and a significant loss of density on both sides is

upside-down focused

Grids are typically used when the part measures more than ___ cm and ___kVp is needed

10, 60

Low frequency grid

100-120 lines/inch used with moving grid assemblies (bucky)

What type of interaction would occur between a 50kev X-ray photon and an atom with inner shell electrons that have a binding energy of 60 kev or more?

Classical

Scatter is primarily from which interaction?

Compton, 90%

Beam restriction devices are located above tube housing

False

Who invented the grid?

Gustav Bucky in 1913

Who invented the moving grid?

Hollis Potter in 1920

Is it possible for a photoelectric interaction to occur between a 62 kev xray photon and an atom with inner-shell electrons that have a binding energy of 60 kev or more?

No

Air gap technique is based on creating gap by increasing

OID

Image intensification steps

Remnant radiation reaches IR Latent image is processed by the computer Manifest image is displayed on a monitor Manifest image can be adjusted because it is composed of digital data

To compensate for some of the loss of recorded detail due to large OID with air gap, ___ should be increased

SID

Is attenuation the reduction in energy or amount of radiation in the primary beam?

Yes

Function of a grid

absorbs scatter radiation

Grid lead content

affected by grid frequency and ratio

The reason the grid in the bucky reciprocates is to

blur out the grid lines

Reciprocity law

any combo of mA and time that has the same mAs will produce the same quantity of radiation exposure

Beam restriction device that results in images with the most unsharpness around the edge of the image is

aperture diaphragm

Simplest type of beam restricting device is a flat piece of lead with a hold in it is called

aperture diaphragm

Volume of tissue irradiated is determined by

area of collimation patient thickness field size

Automatic collimator

automatically collimates to the size of the IR

What does PBL do?

automatically collimates to the size of the IR and for changes in SID

Higher the grid ratio, ___ the scatter clean up and the ___ the contrast

better, higher

Grid is located

between patient and IR

The grid is part of a device located just below the table top that also includes a tray to hold the IR. This device is the

bucky

Beam restriction device that includes a visible light projecting the xray field size on the patient is

collimator

Most easily adjusted beam restriction device is

collimator

Most effective and often used beam restriction device is

collimator

Flat piece of lead with a hole in it that has a flange directed toward the patient is a

cone or cylinder

If all the convergent points were connected along the length of the gird they would form the

convergent line

Location is space where all the lines extended from the lead strips meet is the

convergent point

Grid cutoff

decrease in the number of transmitted photons reaching the IR because of some misalignment of the grid

Increasing collimation results in

decreased scatter and higher contrast

Diseases that decrease absorption characteristics

destructive

Tube angulation results in

elongation and reduced IR exposure

Distance between grid and convergent line is the

focal distance

Number of lead lines per inch or cm is the grid

frequency

Type of grid that is permanently mounted but includes a channel for the IR to slide into is the

grid cap

An IR that has a grid permanently attached to its front surface is a

grid cassette

To determine the amount of mAs to be increased when making grid changes, it is necessary to use the GCF, aka

grid conversion factor

A short-dimension grid

has lead strips running perpendicular to the long axis of the grid

A linear grid

has lead strips that travel in one direction, most common type of grid

Formula for grid ratio

height of lead strips/distance between lead strips

Grid ratio

height of the grid strip divided by the thickness of the interspace material

What produces the greatest amount of scatter exiting the patient?

high kVp and large field size

As compared with lower kVp, using higher kVp will result in scatter photon that is of ___ energy

higher

The probability of a photoelectric interaction is ___ in bone than in soft tissue

higher

Xray of parts having different tissues types has ___ contrast than that of a part with similar tissue types

higher

Scatter reduces ___ and increases ___

image contrast, dose

Purpose of a grid

increase image contrast

When imaging the pelvis, changing from 14x17 to 8x10 requires

increase in mAs

Increased grid ratio =

increased contrast increased patient dose likelihood of grid cutoff increases

In digital imaging, spatial resolution is improved with

increased pixel density and decreased pixel pitch

Larger field size produces___ scatter

more

Thicker tissue produces___ scatter

more

With a higher kVp, you get

more even penetration of all tissues more scatter hitting the IR (both reduce image contrast)

Air gap technique reduces the amount of scatter radiation reaching the IR because

more scatter misses the IR

Grid Conversion formula

new mAs/old mAs = new grid factor/old grid factor

Relationship between spatial resolution and relative system speed is

no relationship

Scatter carries

nonuseful information

Grid frequency

number of grid lines per mm or inch aka grid density

When the X-ray beam is not lined up with the center of the grid you have

off-center cutoff

When the SID is above or below the focal range you have

off-focus cutoff

Only grid cutoff error that can occur with a parallel grid

off-level

When the X-ray beam is angled across the lead strips you have

off-level cutoff

Material between the lead strips of a grid must be

radiolucent

Relationship between height of lead strips and distance between them is grid

ratio

During the exposure, the grid in the bucky

reciprocates, moves slightly from side to side

Field shape produced by collimator is

rectangular

Significant amount of collimation will have what effect of the amount of radiation reaching the IR?

reduced

Grid cutoff results in

reduced exposure to IR a light film-screen image possible quantum mottle

Using a grid to reduce scatter results in

shorter scale contrast and decreased IR exposure

The probability of a Compton interaction occurring is related to

the energy of the X-ray photon

Grid ratio

the height of the Pb strip to the width of the interspace

The higher the lead content,

the more efficient the scatter clean up

The numerical value assigned to each pixel is based on

the relative attenuation of x-rays transmitted through the part

Linear grids are most commonly used because

the tube can be angled in the direction of the lead lines

Quantum noise is a result of

too few x-ray photons reaching the image receptor

Grids absorb scatter because the scattered photons

travel at an angle to the transmitted photons

Adding a grid or increasing grid ratio results in an increase in patient dose

true

Grid conversion factor or bucky factor

used to determine the adjustment in mAs needed when changing grid ratios

Moire effect can be caused by

using a grid with frequency similar to CR laser scanning frequency using a grid cassette in the bucky

The amount of scatter produces is dependent on

volume of tissue exposed and kVp used

Two major factors that affect amount of scatter produced and exiting the patient are

volume of tissue irradiated and kVp

Type of grid that is usually taped to the front of an IR is

wafer grid

Moire effect is also known as

zebra pattern