Page 32 - A Practical Guide to Equine Radiography

IMAGE QUALITY 13

Figure 3.2 This
VetBooks.ir figure shows the same
radiograph in different
window levels. The image
on the right is windowed
to highlight the soft
tissue envelope, whereas
the image on the left
shows more bone detail.
The ability to change
brightness and contrast
of a radiograph is a huge
benefit of digital images
and enhances our ability
to assess these.

− Post-processing: many systems have edge − Detector: different systems have different
enhancement options. Beware that these contrast and this should be considered
can also lead to a false impression of when buying a new system
sharpness. − Object: size and radiodensity; this is obvi-
− Size of the focal spot: the smaller, the ously out of the control of the operator
sharper, something to consider when buy- − Exposure values: this is a major factor and
ing a new X-ray machine. exposure settings are discussed in detail
− Poor screen–film contact in conventional below
radiography. − Image processing: DR and CR systems
offer many post-processing and viewing
options. You should work with an appli-
Image contrast cation technician to get the most out
• Image contrast is the difference in radioden- of it
sity that makes an object distinguishable − Image display: make sure your viewing
from another structure. screen has an adequate range of contrast.
• It is expressed as contrast resolution and is a When you view an image, changing con-
measure of the grey values. The more ‘greys’ trast and brightness often enable you to
an image displays, the more structures of dif- gain additional information (Fig. 3.2).
ferent radiodensities it can distinguish.
• The pixel depth (in bits per pixel) is a mea-
surement for contrast resolution (grey value) Signal-to-noise ratio
of the image. For example, a 1-bit image The signal-to-noise ratio is the ratio between
shows only black and white, an 8-bit image the wanted information (for example, the
shows 256 greys and a 12-bit image, 4096. image of a bone) and unwanted interference
• Image contrast depends on: ‘noise’ (anything that hinders us from seeing
the bone clearly).

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