Exposure-Ch.18, Part 1

Grids recommended in patients with patient part thickness *greater* than:

10 cm

What is the high frequency range?

178-200 lines/in (70-80 lines/cm)

What is the typical grid ratio range?

5:1-17:1

Grids recommended when kVp is *greater* than:

60

What is the frequency range?

60-200 lines/in (25-80 lines/cm)

What is the most common frequency?

85-103 lines/in (33-41 lines/cm)

Primary radiation that interacts and changes direction is known as scatter and is produced by ___ interaction.

Compton

___ grids consist of two parallel grids, one on top of the other; one has horizontal and lead strips and one has vertical lead strips.

Criss-cross

If grid has interspace of 0.5mm and lead strips that are 3mm high, what is its grid ratio?

GR= 6:1 (GR=3mm/0.5mm)

___ frequency grids commonly used for stationary grid use (portable radiography) because the *thinner* lead strips minimize the grid line appearance.

High

___ grids have grid lines running in *one* direction only.

Linear

___ is primary source of scatter radiation.

Patient

In *focused* linear grids, the further the ___, the more divergent the beam is.

SID

___ grids are used primarily in mobile procedures or for upright or horizontal beam views.

Stationary

The grid ____ scattered radiation *before* it reaches image receptor

absorbs

Responsible for light areas

absorption

Short axis grids, the grid lines run ___ short axis of grid.

across

Higher ratio grids require careful ___ of grid.

alignment

Interspace material is thicker and usually made of ___ because it is more durable.

aluminum

The thinner lead strips moved grid during exposure to ___ out grid lines, making lines invisible on image.

blur

The contrast (differences) exists because some photons pass through the ___ while others are absorbed.

body

When using criss-cross grids, primary beam must be ___ perpendicular to grid and grid must remain ___.

centered; flat

The grids lead content is most important in determining the grid's efficiency at ___ up scatter.

cleaning

Dr. Potter's improvements resulted in a practical grid device which significantly improved ___ without impairing the view of the patient's anatomy.

contrast

The grid improves radiographic ___ in image.

contrast

In *focused* linear grids, if the lead strips were extended, the strips would intersect along a line in space known as the ___ line.

convergence

Grid ___ is the undesirable interception of primary beam photons by the grid strips.

cutoff

If the height of the grid is constant, ___ the distance between the lead strips would result in an *increase* in the grid ratio.

decreasing

Scatter radiation has no ___ value.

diagnostic

The ___ the grid moves is important if it is to accomplish the job of blurring the grid lines.

direction

Aluminum can be a ___ with low kVp techniques where absorption would be greater.

disadvantage

D=

distance between strips (thickness of interspace material)

In *focused* linear grids, lead strips angle to match ___ of beam, the primary beam will align with interspace material and scatter is absorbed by lead strips.

divergence

A *higher* grid ratio is more ___ in removing scatter, and requires more exposure.

efficient

Grid is ___ in aluminum or plastic for protection.

encased

Aluminum has a higher atomic number than ___, so it can provide additional absorption of low-energy scatter; with the higher atomic number, it also ____ the absorption of primary photons.

fiber; increases

For the linear grid to be properly ___, the x-ray tube must be located along the convergence line.

focused

Grid ___ is the number of lead strips per inch or cm.

frequency

Lead content is measured in mass per unit area:

g/cm^2

A ___ is a series of *radiopaque* strips that alternate with *radiolucent* interspace strips.

grid

If the primary beam is angled *into* the lead, the lead will absorb an undesirable amount of primary radiation, resulting in:

grid cutoff

Grid ratio=

h/D

h=

height of radiopaque strips

High ratio, low frequency grids tend to have ___ lead content.

highest

When stationary grids are used, grid lines on the ___ will usually be noticed on close inspection, this is especially true with *low* frequency grids.

image

As the lead content of a grid *increases* the ability of the grid to remove scatter and improve contrast ___.

increases

Percentage of Compton interactions ___ with *increased kVp*.

increases

If the height of the grid is a constant, ___ the distance between the lead strips would result in a *decrease* in grid ratio.

increasing

The photons that pass through the body unaffected will ___ with the image receptor to create the image, these are the photons responsible for creating the __ on the image.

interact; contrast

The higher the grid ratio, the straighter the scattered photon has to be in order to pass through the ___ material.

interspace

Angulation *across* the long axis of a linear grid would result in the primary beam being directed ___ the lead strips.

into

An ___ relationship exists between the distance between the lead strips and grid ratio when the height of the grid strips remains the same.

inverse

Scatter increases as:

kVp increases Field size increases Thickness of part increases Z# decreases

By combining information about grid ratio and frequency, the total quantity of ___ in the grid can be determined.

lead

What is the material of choice for the radiopaque strips and why?

lead because it is inexpensive and easy to shape into very thin foil

The Potter-Bucky Diapraghm is the bucky and mounts a 17 x 19 grid above cassette, it moves the grid during exposure so that grid ___ will be blurred and not evident on the image.

lines

In typical x-ray table, strips run along ___ axis of table allowing for angling of the tube toward the head or feet of the patient.

long

Parallel grid is best employed at ___ SIDs because the beam will be a straighter, more perpendicular one.

long

Fiber interspace grids are preferred when using ___ kVp techniques where their application can contribute to lower patient dose, like mammography.

low

In *parallel* linear grids, lead and interspace strips run parallel to one another, this means if the grid lines were extended into space they would ___ intersect.

never

Electromagnet pulls grid to one side and releases it during exposure, spring activated

oscillating

When an x-ray beam passes through the body, one of three things will occur with the primary photons that originated at the target:

pass through the body unaffected, be absorbed by the body, interact and change direction

Select radiographic exams warrant the use of radiographic grids, such as when:

patient part thickness is greater than 10cm and kVp is greater than 60

Absorption of photons occurs as the result of ___ interaction, this interaction results in the complete absorption of the primary photon and the production of a ___ photon.

photoelectric; secondary

Short axis grids are useful for ___ chest procedures when cassette is placed *crosswise* because the chance of grid cutoff is decreased.

portable

Higher ratio grids require greater accuracy in their ___ and are more prone to grid errors.

positioning

Interspace material must be ___, allowing radiation to easily pass through it.

radiolucent

The ___ strips are needed to absorb the scatter radiation and must be made of a dense material with a *high* atomic number.

radiopaque

Grid ___ has a major influence on the ability of the grid to improve contrast

ratio

Dr. Hollis Potter made improvements to use of grids. What were they?

realigned lead strips to run in one direction and thinner lead strips

Motor drives grid back and forth during exposure

reciprocating

The lead strips of the grid run along the long axis of the table, to blur the lead lines, the grid must move at a ___ angle to the direction of the lines, meaning that it will be moving *back and forth* across the table and NOT from top to bottom.

right

Lowers contrast

scatter

Parallel linear grids absorb large amount of primary beam, resulting in some cutoff, especially when the grid is employed at ___ SIDs.

short

Grid ratio affects the amount of scatter absorbed by determining the maximum angle of scattered ray that can get through the grid, the ___ the angle, the *less* scatter reaches the image receptor.

smaller

Grids were created by radiologists, Dr. Gustav Bucky (1913) but has a very crude design by todays standards; the crude design created an image on the grid that was ___ on patient's image.

superimposed

Linear grids are the most commonly used because:

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

When criss-cross grids are used, no tube __ is permitted, because any angulation would result in grid cutoff because lead strips are running in ___ directions.

tilt; both

Responsible for dark areas

transmission

The secondary radiation created by the photoelectric interaction is very ___ and is *quickly* absorbed in the surrounding tissues (resulting in the light areas).

weak