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The Estimation of Exposure
The aim of the
radiographer must always be to select those exposure factors which in
conjunction with the radiographic apparatus and technique used will produce a
radiograph showing optimum visualization of the part under examination.
The
factors involved in an X-ray exposure
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X-ray machine
(Make and type-input voltage-kilovoltage-iamperge-Time- Distance-Collimation of
X-ray beam).
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Patient
(Thickness of part - Nature of part - Pathological changes within part -
Movement - Dressing and casts).
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Cassette
(Film type and speed - Intensifying screen factor - Grid factor).
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Dark room
(Correct and incorrect development).
Patient Thickness and Exposure Time
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When using small apparatus in
which the kilovoltage is also limited, exposure differences in relation to body
thickness must usually be made by varying the time (m A will already be at the
maximum value).
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Obviously, the penetrating power of the
maximum k V can traverse only a certain volume of tissue before becoming
completely absorbed.
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Any increased in exposure time after this stage
has been reached will be useless (and will have also resulted in an exposure
time that is too long to be participles for veterinary purpose).
Collimation of X-ray beam
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The use of cones or other
means of limiting the field of radiation is valuable method of reducing scatter.
However, this also reduces the amount of blackening of the radiograph and when
a particularly narrow cone is employed some increase in the exposure factors
will be necessary to compensate for this.
Movement
Dressing and casts
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Wherever possible these
should be removed. Wood and fabric need only a slight increase in exposure.
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Soft metals such as aluminum need about 5 k V
more but steel splints are completely radio-opaque to the output of small sets.
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Dry plaster of Paris needs twice the usual
exposure. Wet casts need four times the exposure.
Grids
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