Download with a grid

RADIOGRAPHIC GRIDS
Abstract
Grids are devices that are used to improve contrast on a
radiographic image. This improvement of contrast is
achieved by absorption of scatter radiation produced by
the patient as the primary beam interacts with the
patient’s tissues. This contrast improvement also comes
at a cost. As the grids’ ability to improve contrast
increases, so does the patient’s radiation dosage per
radiographic exposure. In general when a radiographic
exposure is performed using a grid device, the primary
photons will either: 1. Pass through the body tissue
unaffected. 2. Become absorbed by the tissues within the
body or 3. Interact with body tissues and change direction
(Compton’s scatter).
Objectives and Outline
Objectives:
At the completion of this lecture, the participant will:
1.
Be able to describe the rationale for using a grid.
2.
Know the history of radiographic grids.
3.
Explain grid construction.
4.
Explain grid efficiency.
5.
Employ grid conversion factors in setting techniques.
Objectives and Outline
Outline
A.
History
1. Grid invention
a. 1913
1. Dr. Gustav Bucky
b. 1920
1. Dr. Hollis Potter
B.
Grid construction
1. Grid material
2. Inter-space material
3. Grid design
a. Parallel
b. Focused
1. Grid radius
c. Cross Hatched
C.
Grid Efficiency
1. Grid ratio
2. Grid frequency
D.
Grid conversion factors
1. Factor assignment re. Grid ratio
What is a Grid?
A device placed between the patient
and film for the purpose of
absorbing scatter radiation before it
can interact with the imaging
receptor.
History
1913-
First grid was
made by Dr. Gustav
Bucky.
Consisted of wide strips of
lead approx. 2cm apart
in a crisscross pattern.
Despite the crudeness, it
removed enough
scatter to improve
contrast.
History
1920 – Dr. Hollis Potter
improved the grid device.
Realigned the lead strips to
run in one direction.
Made the lead strips thinner.
Designed the Potter-Bucky
diaphragm which allowed
the grid to move during
the exposure.
Grid Construction
Grid Materials
A series of radiopaque lead strips which alternate
with radiolucent materials.
a. Strips are held firmly together then sliced
into flat sheets.
b. Lead is the radiopaque material of choice.
Interspace materials are radiolucent.
a. Aluminum
b. Plastic fibers
Grid Construction
•
•
•
Aluminum is more common than plastic
fiber b/c of ease to manufacture,
durability and provides additional
absorption of low energy photons.
Disadvantage when using low kVp
technics.
Fiber Interspace grids are preferred when
using low kVp technics (pediatric
radiography).
Grid Construction
3. Grid design
a. parallel grids
1. All lead strips are straight up and down.
2. Less commonly employed than focused grids.
3. Best used with longer SID’s b/c beam is straighter and
more perpendicular at longer SID’s.
b. focused grids
1. Lead strips are tilted toward the center to correspond
with the divergence of the X-ray beam.
2. Convergence lines:
A distance in space where if the grid lines were extended
above the grid surface they would intersect.
Grid Construction
Grid radius:
The distance from the grid face to the points of convergence of
the lead strips.
Each focused grid will identify the focal range within which the
tube should be located.
Grid patterns
Linear
All lead strips run in the same direction & are straight up
and down.
Crisscross
Contains two sets of lead strips at 90 degrees from one
another.
Cross-hatched
Equivalent of two linear grids not quite at 90 degrees.
Grid Efficiency
The ability of a grid to clean up scatter and
improve contrast.
Criteria for efficiency measurement:
1. Selectivity
2. Contrast Improvement Ability.
Grid Efficiency
Selectivity measures a grid’s ability to
absorb a greater percentage of scatter
than primary radiation.
Measured by:
% of primary radiation transmitted
% of scatter radiation transmitted.
Thus, a grid with high lead content would
have a greater selectivity.
Grid Efficiency
Contrast Improvement Ability is measured
by how well a grid functions to improve
contrast in the clinical setting.
Grid Efficiency
This is measured by: K=
Radiographic contrast with the grid
Radiographic contrast w/o the grid.
Note that this is dependent upon the kVp used and
the volume of tissue irradiated.
Most grids have a K of 1.5 to 3.5.
Thus the higher the K factor, the greater the
contrast improvement.
Grid Ratio
Calculated by the
height of the lead
strips divided by
the distance
between them.
i.e., strips 1.2mm
high; 0.1mm apart
= 12:1 grid ratio.
Grid Ratio
Grid ratio plays a major
role in the grid’s
ability to improve
contrast
Thus if the height of the
grid is constant, the
distance b/w the lead
strips was decreased,
this would result in an
increase in the grid
ratio.
Grid Frequency
The number of grid lines per
inch or centimeter.
Grid frequency ranges from
60 to 196 lines/inch (2578 lines/cm)
Most commonly used grids
have a frequency of 85103 lines/inch (33-41
lines/cm).
In general as the lead
content increases, the
ability of the grid to
remove scatter and
improve contrast
increases.
Grid Selection/Conversion
Note: Grids absorb scatter radiation, scatter
adds density to the radiographic image and
decreases the overall contrast. Thus, the
more efficient is the grid the less density is
produced on the image receptor.
As a general rule, any anatomical part
measuring 10cm or greater should be
imaged with a radiographic grid. Any
radiographic technic using more than 70 kVp
should employ the use of a grid device.
Grid Selection/Conversion
Grid conversion factor is best used
when it is necessary to change grids
while maintaining a similar density
on a subsequent radiographic image.
Grid conversion Factor
GCF = mAs with a grid
mAs w/o a grid
Grid conversion Factor
Example:
A CXR is produced using
5 mAs at 85kVp w/o a
grid. A second film is
to be produced using
a 12:1 grid. What
mAs is needed to
produce a satisfactory
radiographic image
with a similar density?
X
5 =----5
Answer: 25 mAs
Grid conversion Factor
When converting from one radiographic grid to
another, the following formula should be used.
mAs1
GCF1
------ = -----mAs2
GCF2
mAs1 = original mAs
mAs2 = new mAs
GCF1 = Original grid conversion factor
GCF2 = new grid conversion factor
Grid conversion Factor
Example:
A satisfactory
radiographic image of
the abdomen is
produced using an
8:1 Grid, 35mAs and
86kVp. A second film
is requested using a
12:1 grid. What mAs
is needed to produce
a second satisfactory
image?
35
4
------- = ------X
5
4X = 175
X = 43.75
Grid Conversion Factors
Grid Ratio
no grid
5:1
6:1
8:1
10:1
12:1
16:1
Conversion Factor
1
2
3
4
5
5
6
Grid Errors

Off Level Grid Error


This occurs when the
CR is angled across the
long axis of the grid
strips/across the
radiographic table.
This most commonly
occurs during bedside
radiography.
• E.g. pt’s body weight
is not evenly
distributed on the grid
device.
Grid Errors

When this occurs, there
is an undesirable
absorption of the
primary beam which
results in a radiograph
with a decreased
density across the
entire image.
Grid Errors

Off Center Grid
Error


This occurs when
the CR is not
properly centered
to the grid device.
It results in a
decrease in density
across the entire
film.
Grid Errors

Off Focus Grid Error


This error occurs when
a grid is used at a
distance other than
that is specified as the
focal range.
This results in grid cutoff along the peripheral
edges of the film.
• This is especially
common when using
higher grid ratio grids.
Grid Errors

Upside-Down grid
error



This type of grid error
occurs when the
radiographic grid is
used upside-down.
Severe peripheral grid
cut-off occurs.
The radiation will pass
through the grid along
the central axis where
the grid strips are most
perpendicular.
Air Gap Technique




An alternative to the use of a grid.
Its primary usage is magnification radiography.
Results in an increased OID, thus creating an
air gap b/w the patient and the film.
The end result is that less scatter radiation
reaches the image receptor.



Contrast is is improved.
Primary disadvantage is a loss in detail and sharpness
of the image.
It has been demonstrated by Gould and Hale
that a 25cm air gap is equivalent to a 15:1 grid
for a 10cm body part.
References

Bushberg et al, The Essentials of Physics and Medical
Imaging, Williams & Wilkins Publisher.
Bushong, S., Radiologic Science for Technologists, Physics,
Biology and Protection, 8th Edition, C.V. Mosby Company.
Cullinan, A., Producing Quality Radiographs, Lippincott,
New York, 1987.
Carlton et al, Principles of Radiographic Imaging, An Art
and Science, Delmar Publishing.
Selman, J., The Fundamentals of X-Ray and Radium
Physics, 8th Edition, Charles C. Thomas Publisher.