Download Sources in diagnostic Rad. – General Radiology - gnssn

Radiation Sources in medicine
diagnostic Radiology
General Radiography
IAEA
International Atomic Energy Agency
Day 7 – Lecture 1(1)
Objective
• To become familiar with the technology used in general
radiographic x-ray systems;
• To know the specific radiation risk linked with these
devices.
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Contents
• Description of a general radiographic x-ray system;
• Influence of exposure parameters on patient dose and
image quality;
• Equipment malfunction affecting radiation protection.
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Conventional systems for general purposes
General purpose radiography equipment:
•
provides static (radiographic) images using
either x-ray film and intensifying screens or
digital image receptors;
•
may be used to examine most parts of
the body such as the chest, abdomen,
pelvis, head, spine, extremities etc.;
•
However, the available power of the x-ray
equipment may be a limiting factor in
determining the range of examinations
that can be best performed while
ensuring optimal image quality;
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Conventional systems for general purposes
General purpose radiography equipment:
• is also used for contrast examinations where contrast media
such as barium sulphate or iodine based compounds are
ingested by, or injected into, the patient. (For chest x-ray
examinations, air is the contrast medium and an important
reason the examination is taken on full inspiration);
• In addition to fixed installations, mobile equipment for general
radiography are also commonly used. However, those with lower
tube currents, longer exposure times and larger focal spots (i.e.
low powered x-ray equipment) may not be suitable for some
thick body sections, e.g. the abdomen, spine, etc.
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Conventional systems for general purposes
Basic system for
general x-ray
examinations
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Conventional systems for general purposes (cont)
Example of a mobile
system for general
radiographic purposes
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Specific Equipment Requirements
• For general radiography, the generator and x-ray tube should
operate in an energy range from 40-50 kV peak to 120 - 150
kV peak. X-ray tube currents from 50 to 1000 mA or more
are not uncommon.
• An adjustable (rectangular) light beam collimator must be fitted
to the x-ray tube assembly so that the operator can restrict the
size and shape of the x-ray beam to the area of clinical interest.
Proper collimation is perhaps the most important means of
minimizing patient (and operator) radiation dose and in improving
image quality.
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Specific Equipment Requirements (cont)
• Additional and variable filtration (added filtration) should be
available to the operator to reduce low energy radiation
which does not reach the image receptor and which
unnecessarily increases patient dose.
However, the operator must not be able to remove any
permanent filtration required to meet the minimum
filtration specifications.
• The light and x-ray beams of the light beam collimator
must be congruent (within a specified error) and indicate
the extent of the radiation field.
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Specific Equipment Requirements (cont)
An anti-scatter-grid is essential
for the examination of most
thick body parts.
It is a (preferably removable)
device positioned after the
patient, (but before and close
to the image receptor) to
reduce the level of scattered
radiation reaching the receptor.
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Fundamentals of Radiography. Kodak
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Specific Equipment Requirements (cont)
However, a grid necessarily increases the exposure
required (and therefore patient dose) by factors ranging
from 2 to 5 times. Grids should only be used when essential
to image quality.
• The use of an automatic exposure control device (AEC) is
recommended. With such devices, the exposure time is
terminated when a pre-set radiation dose to the image receptor
is reached.
However, correct beam centring and collimation are required
for reliable results i.e. users must be properly trained.
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Specific Equipment Requirements (cont)
•
The x-ray tube voltage (kV peak), tube current (mA), and
exposure time (or mAs) are the minimum parameters to be
displayed at the control panel prior to the radiographic
exposure (the mAs also should be displayed after an
exposure with an AEC).
•
Information about radiation field size, focus to image receptor
distance, the selection and position of the AEC detector also
should be available.
•
If practicable, a device informing the operator of the quantity of
radiation delivered to the patient should be integrated with the
equipment (e.g. a Dose-Area Product meter).
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Specific Equipment Requirements (cont)
Dose-area product meter
DAP readout
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Equipment malfunctions affecting
radiation protection
• Filtration inappropriate to the imaging task.
• Lack of congruency between the x-ray and light beams.
• Misalignment between the x-ray beam and image receptor.
• Inappropriate use of an anti-scatter grid (e.g. unnecessary
use, incorrect ratio, alignment errors, etc.) leading to
unnecessarily increased patient doses and poor image
quality.
• AEC malfunction or poor calibration.
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