Lecture 8 Radiographic Exposures (Module 6)

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Topic 3 Selection of kV
• High kV scattered radiation is energetic.
• Angle of rays is same as those of primary
beam and is not deflected very much.
• Follows path of primary beam in forward
direction adding to film density.
• Use of relatively high grid ratio, e.g. 12:1
or higher, improves image quality as
scattered radiation in high kV techniques
would be more efficiently absorbed.
Low kV results in high absorption of
x rays.
High kV results in less absorption of
x rays. More x rays penetrate the tissue
being examined and reach the film. This
results in good visualization of structures
required in pattern recognition.
Poor patient positioning
Entire knee
architecture not
included. Lack
of penetration of
knee joint due
to low kV
selection.
Topic 4 Selection of mAs
• The higher mAs, the darker the image,the higher
the patient dose.*
• Factors affecting mAs selection:
 Focus film distance: For the same image
density, if new FFD is equal to a factor
multiplied by the old FFD, the new mAs equals
the old mAs multiplied by a number equal to the
square of this factor.**
Selection of mAs
Speed of film: Fast films need less mAs
than slow films.
Size, thickness, and type of phosphors in
intensifying screens:
Fast screens require less mAs than slow
ones. When changing from a 200 speed
system to a 400 speed one, reduce
original mAs by 50%. This reduces dose
to patient.
Selection of mAs
Intensification factor of intensifying
screens determines selection of mAs.
Large phosphors require less mAs than
slow ones. Fast screens need less mAs
than slow ones. Single intensifying screen
requires more mAs than two screens in
cassette.
Selection of mAs
Slow screen systems produce good image
detail, but more mAs needed for film
blackening resulting in higher patient dose.
Selection of specific film-screen
combinations depends on information
needed for pattern recognition.
Selection of mAs
 Use of grid: mAs must be increased
based on grid factor.
 Degree of collimation: Reducing area of
interest needs an increase in mAs. *
Output of generator/capability of unit.**
Using a 3-phase generator instead of a
single phase one requires mAs reduction.
Relationship to kV used.
Size of focus.
Very black film
of distal femur
due to high
mAs. Patient
subjected to
unnecessary
dose. Film
retaken with
reduced mAs.
Grossly over exposed radiograph
AP pelvis of child. Too
high mAs. Poor
collimation – gonads in
primary beam which
means dose to
reproductive organs.
Repeat radiograph
done.
Grossly overexposed skull
radiograph
Too high mAs. Poor
radiographic technique –
patient's skull rotated. No
evidence of beam
restriction – unnecessary
dose to patient's eyes and
thyroid.
Lecture 8 Radiographic
Exposures (Module 6)
•
•
•
•
Topic 1
Topic 2
Topic 3
Topic 4
Introduction
Exposure Chart
The step system
Tasks 26 – 28
Topic 1 Introduction
• Objectives:
On completion of this module the student
will
Understand the effect each exposure
factor has on the resultant radiograph.
Be able to use this knowledge effectively.
Be able to select the correct exposure
factors required.
Topic 1 Introduction
Understand and use the step system of
exposure calculation.
Be able to manipulate exposure factors
using the step system.
Be able to establish and maintain a
reliable exposure chart.
Be able to modify an existing exposure
chart.
Topic 1 Introduction
Exposure:
 Refers to the quantity of radiation to which
the patient is exposed.
In radiography, the term "exposure" is
used as the collective exposure factors
kV, mA and time, which together produce
the radiation exposure that will give the
required penetration, density and contrast
on the radiograph.
Topic 1 Introduction
Other factors influence film quality but
here we will only consider kV, mA, time
and FFD (SID)
kV
Controls largely the penetrating power
(better described as beam quality), and to
a lesser degree, the radiation intensity and
threfore film density and patient dose.
Topic 1 Introduction
Affects contrast. The higher the kV, the
lower the contrast. The lower the kV, the
higher the contrast.
mA: Controls radiation intensity and
therefore film density and patient dose.
The higher the mA, the higher the film
density and patient dose.
Topic 1 Introduction
Time: Controls the length of time of
radiation flow, and therefore film density
and patient dose. The longer the exposure
time, the higher the film density and
patient dose.
mAs:
The product of mA and time (in seconds).
Many modern x-ray units use mAs rather
than mA and time separately.
Topic 1 Introduction
FFD ( SID):
The greater the distance an x-ray beam
travels, the less effective it will be. The
less the distance it travels, the more
effective it will be.
Exposure compensation is necessary with
changes in FFD (SID).
Topic 1 Introduction
NOTE:
It is important that the radiographer sets
correct exposure factors required to
produce a high quality radiograph which
will give the maximum information yet
minimises patient dose.
Systems must be in place to help the
radiographer select and manipulate
exposure factors effectively.
Topic 2 Exposure chart
• Each x-ray unit should have a list of
commonly used exposures for easy
reference (see Appendix B, page
164).This can serve as a guide.
Methods of recording exposures:
Written chart.
Notebook.
Computerised "chart" within the x-ray unit.
Topic 2 Exposure chart
Here we will deal with:
Establishing and modifying an exposure
chart.
Manipulating exposure factors using the
step system.
Topic 2 Exposure chart
 Establishing an exposure chart
 Method:
1. Draw up a blank exposure chart (see Fig
6-1 and Appendix B page 168).
2. Fill in all anatomical areas to be covered,
listing the views for each.
Topic 2 Exposure chart
3. Divide these into groups using the same
set of conditions, e.g. all extremities
using detail screens, no grid, 100cm
FFD(SID).
4. Produce a well exposed set of
radiographs of an extremity, e.g. hand.
(This can be done in the course of a
routine examination.)
Topic 2 Exposure chart
5. Record the exposures used against
HAND on the chart.
6. Measure the thickness of the patient's
hand in all projections used, e.g. PA,
oblique, lateral
7. Measure thickness at the level of entry of
central ray, for each hand position.
Topic 2 Exposure chart
8. Measure all other areas/positions in the
same group (these can be obtained from
a colleague or friend of a similar size
rather than inconvenience the patient).
9. Calculate the exposures for all other
areas/positions in the group, based on
the following chart (patient thickness
related to exposure change using the
hand as your base level:
Topic 2 Exposure chart
Patient thickness related to exposure
change
 1.5 cm increase in thickness requires a
25% increase in exposure (+1 step).
 5.0 cm increase in thickness requires a
100% increase in exposure(+3 steps).
Topic 2 Exposure chart
 5.0 cm decrease in thickness requires a
100% decrease in exposure (-3 steps).
 1.5 cm decrease in thickness requires a
25% decrease in exposure (-1 step).
NOTE: The reference to "+ or – steps" is
explained in the next topic, the step
system.
Topic 2 Exposure chart
Example
PA HAND (2 cm thick) Exposure 50kV
6mAs.
Calculate an exposure for a lateral WRIST
(7cm thick).
Thickness difference between the PA
HAND and lateral WRIST is 5 cm.
Topic 2 Exposure chart
For an increase of 5 cm the exposure is
increased by 100% ( see previous
discussion).
Therefore the lateral WRIST exposure will
be 50kV 12 mAs.
The exposures for all other areas of the
body can be calculated in this way.
Topic 3 The step system
• A simple standardised form of exposure
factor manipulation, designed to remove
some of the guess work and make
exposure setting more accurate.
• Works on a series of step charts allowing
use of standard step changes in exposure.
• Also related to patient thickness and
medical condition (see Fig 6-5).
Topic 3 The step system
Note: Step changes can be applied to
any one of the charts or split between the
charts.
The step system provides step charts for
kV, mA, mAs, time and FFD(SID), (see Fig
6-2, Fig 6-3, Fig 6-4 & Fig 6-5).
To use the step system for exposure
changes, the term "step" is used.
Topic 3 The step system
Each step on any of the charts will alter
the exposure by approximately 25%.
For a noticeable change in film density,
the exposure must be altered by at least
25%.
If a film is considered to be too light or
too dark, change one of the exposure
factors by at least 3 steps up or down.
Topic 3 The step system
Any step change required to modify an
exposure can be applied to any of the
Step Charts, although other factors will
influence this decision (contrast,
penetration, patient dose, movement,
limitations of the x-ray unit).
Topic 3 The step system
Step changes need not be limited to one
chart only. Necessary step changes can
be split up between charts if necessary,
providing that the overall number of
required step changes is made.
Example
Original Exposure: 60 kV 20 mAs
New kV = 70 kV (an increase of 10 kV)
Topic 3 The step system
 An increase of 10 kV = three steps on the kV
step chart.
 To retain the same density film, the mAs must
be reduced by the same number of steps (three
steps) on the mAs chart.
 New exposure = 70 kV 10 mAs
 All Step Charts are from "Course Notes in
Basic Radiography", Radiation Protection
Branch, South Australian Health Commission.
Topic 5 Tasks 26 - 28
• The tasks are in the next four slides.

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