# Accessory Equipment

```University of Guyana
BMI 2202 – Science and Instrumentation II
Lecturer – Ms. Schimze Sagon
Presented by – Patrick Fung 1013992

Explain what is a radiographic grid, including materials used, grid ratio
and grid frequency

Describe and differentiate between grid type and grid pattern

Explain the relationship of grid selection to patient dose and image
receptor exposure

Describe methods for evaluating grid performance.

Grid cut off and the effects of these errors on the radiographic image

Discuss the physical characteristics and uses of immobilization devices
and how they help to reduce patient dose and/or improve image quality
• Radiographic grids or grids, invented by Dr. Gustave Bucky in 1913, are “sheets”
material of choice, separated by spaces filled with low attenuating material, e.g.
Aluminum or carbon fiber.
• They are placed between the patient and the x-ray film to improve image
contrast by reducing scattered radiation, mainly via Compton Effect.
• It is most useful when imaging thicker patients where there is a more probability
Grid Ratio
• The grid ratio is the working ability of the grid.
• It is calculated as the height of the lead strips divided by the distance between
them .i.e., strips 1.9mm high; 0.2mm apart = 19:2 grid ratio.
Grid Frequency
• Grid frequency is the amount of grid lines or radiopaque strips per inch or
centimeter.
• In general as the radiopaque content increases, the ability of the grid to
remove scatter and improve contrast increases.
Grid Pattern
• This is the orientation of the lead strips in their longitudinal axis as viewed
from the top
• Some of the main patterns are;
1. Crossed
2. Linear or Parallel
3. Focused
FOCUSSED
GRIDS
A grid in which the
absorbing strips are
slightly
angled
towards the focal
spot. The grid can
therefore be used
only at a specified
focal
distance.
Otherwise the grid will
absorb the primary
the film are barely
exposed.
Focused
grids may be linear or
crossed
LINEAR OR
PARALLEL GRIDS
A grid where the lead strips
are parallel to each other in
their longitudinal axis. Most
linear grids are focused, i.e.
their strips are slightly tilted,
converging at a line in space
(the convergent line).
A non-focused linear grid
also has strips that are
parallel when viewed in
cross-section; this is called a
parallel grid.
Many X-ray tables are
equipped
with
linear,
focused grids, and the strips
in these grids are parallel
with the long axis of the
table allowing the X-ray tube
to be tilted in this direction.
CROSSED GRIDS
A grid consisting of
two
superimposed
parallel grids having
the same focusing
distance. Such grids
are very efficient in
removing scattered
however, be arranged
at exactly 90 degrees
to the beam. This
makes the use of
crossed
grids
limited.
There are two types of grids used for radiographic purposes
1. Moving Grids
The moving grid was invented by Hollis E. Potter in 1920. A grid which is set
into motion (one way or oscillating) just before an X-ray exposure starts. The
intention is to remove or reduce the shadows produced by the lead strips in the
grid. This method increases patient dose.
2. Stationary Grids
In this grid, the X-ray opaque strips are so thin and so close together that the
grid can remain stationary without the shadows of the strips being sufficiently
visible to interfere with the image detail of the film
and decreases the overall contrast. Thus, the higher the efficiency of 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 technique using below 90kVp 8:1 grid recommended. Techniques
using above 90kVp 12:1 grid recommended.
There are three main method of grid performance;
1. Primary Transmission
This is the measurement of the percentage of primary radiation transmitted through a grid .
1. Made with the grid in place to determine the intensity of the radiation through the grid .
2. 2. Made after removal of the grid to determine the intensity of the radiation directed at
the grid .
Primary Transmission (Tp) = Intensity with grid/intensity without grid X 100 (%)
• The ideal Primary transmission is 100%, however typical values are ~55-75%
• The theoretical calculation accommodates for the fraction of the grid that is interspace.
Theoretic Cal. Tp (%)= 100 X W / (W+w) where
W = Interspace Thickness
2. Bucky Factor
Bucky Factor is the ratio of the incident radiation on the grid to the transmitted
radiation passing through the grid .
• Indicates the actual increase in exposure dur to the grid’s presence.
• Measure of the Grid’s ability to absorb scatter radiation
• Unlike primary transmission , Bucky factor indicates the absorption of both
• Higher Grid Ratio = Higher Bucky Factor
3. Contrast Improvement Factor
This is the ultimate test of a grid’s performance.
It is the ratio of the contrast with a grid to the contrast without a grid, and is a
measure of the grid’s ability to improve contrast, which is its primary function.
• It depends on :
1. kVp
2. Field size
3. Phantom thickness
Higher Grid Ratio = Higher Contrast Improvement Factor
• Grid cut off is the loss of primary radiation that occurs when the images of the
lead strips are projected wider than they would be with ordinary magnification.
• Cut off is complete and no primary radiation reaches the film when the projected
images of the Lead strips are thicker than the width of the interspaces.
• Amount of cut off is always greatest with high ratio grids and short grid focus
distances.
Situations that produce grid cut off are;
1. Focused grids used upside down
2. Lateral Decentering or grid angulation
3. Distance decentering
4. Combined decentering
When a focused grid is used upside down, there is severe peripheral cut off with a
dark band of exposure in the center of the film with no exposure at the periphery. The
higher the grid ratio, narrower the exposed area .
When the X-ray tube is positioned lateral to the convergent line but at the correct
focal distance. There is uniform loss of radiation over the entire surface of the grid,
When a linear grid is tilted , there is uniform loss of primary radiation across the entire
surface of the grid . Effect on the film being same as that of lateral decentering
This occurs when the target of the X-ray tube is correctly centered to the grid , but it is
positioned above or below the convergent line . The central portion of the film isn't
affected but the periphery is light. The loss of primary radiation is directly proportional to
the grid ratio and the distance from the center line.
This causes an uneven exposure resulting in a film that is light on one side and dark on
the other side. The effect is directly proportional to the grid ratio and decentering
distance and inversely proportional to the focal distance of the grid .
• One of the many factors that affect diagnostic quality is motion. For instance the wiggly
hand of a 3 year old or the shaking hand of a badly injured patient. The resultant image
for these instances would not be of appropriate diagnostic quality
• Forms of immobilization or restraint can be simple or intricate.
• The simplest techniques involve the use of a radiolucent positioning sponge to support
the anatomic area of interest or gently laying a sandbag across a patient’s forearm to
minimize shaking caused by patient anxiety.
• More complex techniques might involve completely wrapping an infant or small child in
a sheet (often referred to as a mummy wrap) or securing an accident victim to a
backboard
Curry TS, Dowdey JE, Murry RE. Christensen ́s Physics of
Diagnostic Radiology 4 Ed. Lippincott Williams &amp; Wilkins.
(1990) ISBN:0812113101.
2. JPI Healthcare Korea, X-Ray Grids
3. Bushberg et al, The Essentials of Physics and Medical
Imaging, Williams &amp; Wilkins Publisher
4. Upstate Medical University,