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