Page 33 - A Practical Guide to Equine Radiography

14 A PRACTICAL GUIDE TO EQUINE RADIOGRAPHY

It is influenced by the following. still achieve an acceptable result. CR and DR
VetBooks.ir • Scatter: in the horse, the major factor caus- systems have a wide latitude, meaning that

over- and underexposures can still result in a
ing ‘noise’ in an image is scatter radiation,
Of course, following the ALARA principle
which is in turn influenced by the amount diagnostic image thanks to post-processing.
and radiodensity of the tissues. This is espe- one should always aim for the lowest expo-
cially a problem in the proximal area of the sure necessary for radiation safety reasons.
horse. • Overexposure on CR and DR systems does
• Collimation: the easiest way to decrease not result in a black image like on conven-
scatter is to collimate as tightly as possible tional film–screen systems and to assess
around the area of interest. exposure one needs to be attentive to certain
• Exposure values: discussed below. numeric values (e.g. in the Fuji CR system
• Detector and post-processing system: some sys- they are labelled S and L value and they must
tems are more sensitive to scatter than others. be within a certain range). These values are
• Filter: this can be in the form of mechani- recommended by the manufacturer.
cal grids or digital filters. Grids can be used • Overexposure can result in an artificial black-
to reduce scatter to a certain degree. DR sys- ness of borders of bones (‘blackout’). This
tems often reach the same scatter reduction may obscure lesions at bone margins and
as a grid through inbuilt filters. Grids need to care should be taken that the soft tissue
be carefully chosen for the system used and envelope is visible on radiographs to ensure
require perfect alignment of the beam and an that the true margins of the bones are visible
exact distance between the grid and the X-ray (Fig. 3.3).
machine (required distance is usually shown • Underexposure often results in a very noisy
on a label on the grid). image, which influences our ability to see tra-
becular detail and may also obscure lesions;
in these cases, an increase in mAs is indicated
Exposures (Fig. 3.4).
There are three parameters that determine radi- • Portable X-ray machines are low-output
ation exposure: kVp (peak kilovoltage), mA generators. The maximum output they
(milliamperes) and time (seconds). are capable of should be displayed on the
machine (usually on a little sticker some-
• kVp determines the energy of the X-ray beam where). Many of these machines will only
and its penetration ‘power’. Decreasing the generate the maximum output for a few
kVp settings increases the image contrast and times in a short time period before they
decreases the latitude. A kVp setting under 70 either stop working for a while or image
is desirable for good bone radiographs. The quality will deteriorate because the machine
higher the kVp, the lower the contrast and generates lower outputs.
the more scatter is generated.
• mAs is the product of time and mA and
determines the number of electrons and Image distortion and centring
hence photons. It influences the ‘blackness’ • Image distortion is influenced by the angle of
of radiographs, but not the contrast. As a rule the X-ray beam to the object and the angle
of thumb, it is a good idea to keep the time to of the X-ray beam to the plate. Ideally, the
under 0.2 seconds to avoid motion artefacts. X-ray beam should be aligned at 90 degrees
• Exposure latitude is the extent to which a to the area of interest and the plate to avoid
radiograph can be over- or underexposed and image distortion.

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