Motion
artefact,
artefact occurring whenever image acquisition takes
longer than the time over which physiological motion occurs in the
body region of interest. Motion artefacts are usually not a problem
in imaging the brain and the extremities, except when the patient
cannot lie still during the examination, but they can be prominent
when imaging the trunk. Typical periods over which physiological
motion occurs and an approximate severity scale for the motion
effects on image quality are given in table 1.
Motion artefact, Table 1
Examples of physiological motion, its duration and effect on imaging
Body region Type of motion Severity of effect Period of motion
Brain Cerebrospinal fluid (cardio-sync.) + 100 ms
Blood flow + 100 ms
Spine Cerebrospinal fluid (cardio-sync.) + - ++ 100 ms
Neck Glutition + suppressible
Respiration ++ 4 s
Blood flow ++ 100 ms
Thorax Respiration +++ 4 sec
Cardiac motion ++++ 50 ms (sys.) - 400 ms (diast.)
Blood flow +++
Upper abdomen Peristalsis ++ 10 s
Respiration ++ - +++ 4 s
Blood flow ++ 100 ms
Lower abdomen and pelvis Peristalsis + 30 s
Blood flow + 100 ms
Extremities Blood flow + 100 ms
The best way of suppressing image motion artefacts is to acquire
data faster than the typical periods given in Table 1. However this
does not work for all imaging modalities. For different imaging
techniques, and particularly for MRI, various ingenious ways have
been devised to suppress motion artefacts such as cardiac gating,
respiratory gating and motion compensation.
Photgraphic
unsharpness
Photographic unsharpness factors in Film screen
radiography
Film
emulsion grain size
thickness of the emulsion layer
single vs double emulsion film
cross-over in double emulsion
Screens
thickness of the phosphor layer
size of the phosphor crystals
reflective layer
absorbing layer
dye tint
Screen unsharpness, the contribution to image blurring or
unsharpness due to spreading or diffusion of light within the
intensifying screen and between the screen and film surfaces.
Because the screen has a finite thickness, the X-ray absorption
event which generates the emission of light within the screen may
occur at some small distance from the film. The light diverges from
that point and has spread a small distance, related to the screen
thickness, by the time it reaches the film surface
Parallax unsharpness, an
image unsharpness seen only in double emulsion film. In principle,
there is an image in both emulsions, separated by the thickness of
the film base, about 0.1-0.2 mm. If the film is looked at from an
angle, these two images do not overlap exactly causing parallax
unsharpness. Its influence to total image unsharpness is negligible.
Parallax,
the apparent displacement of an object when
viewed from two different angles, e.g. when observing an object
first with the right eye and then with the left eye (Fig.1). In
Figure 1, the apparent position of object A with respect to object B
changes when the view shifts from one eye to the other. Due to the
shorter distance to object A than to object B, the convergent angle
from object A (a) is larger than that from object B (b). The
difference in angles (a - b) is called the angle of instantaneous
parallax.
Geometric unsharpness,
unsharpness in the image caused by the fact that the X-rays are
emitted from an area rather than from a point. Regions at the edges
of an object will be formed in which the X-ray intensity will be
gradually increasing (or decreasing), causing unsharpness (see
geometric magnification (I), Fig. 1). These regions are generally
referred to as penumbra. The magnitude of the penumbra is dependent
on the focal spot size and the ratio focus-object
distance/focus-film distance.
Geometric magnification,
the (theoretical) magnification in an X-ray image that occurs when
the focal spot is assumed to be a point and not an area. The
magnification of an object is easy to calculate, given the
focus-object and focus-film distances, respectively, and assuming
that the focal spot is a point (Fig. 1, left). The magnification M,
is then:
M = d/c = (a+b)/a
However, if the actual size of the focal spot is taken into account,
the geometry is not the same. The image will now be slightly more
magnified having, however, a more diffuse edge due to the penumbra
present . The magnification is now:
M = [(a+b)/a] + {[(a+b)/a] - 1}(f/c)
where f is the diameter of the focal area. When the focal spot size
is accounted for, the magnification is referred to as "true
magnification".
Turbidity
image unsharpness due to radiation scattered
by the photographic emulsion.
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