| In certain imaging situations, the
attenuation of the X-rays by the human body varies extremely
within the body sections to be imaged on one and the same film.
The lateral views of the lumbar spine and of the transition
between the thoracic spine and the cervical spine are prominent
examples for this dramatic variation in attenuation. It is
virtually impossible to image these areas adequately without
special aids.
One way to overcome the problem caused by the extreme
variation in transparency is the use of a shaped filter with
varying thickness. The thicker area of the filter is placed in
the beam where the object is more transparent. Thus, the beam
intensity at the film will be more uniform.
Another approach to achieve a more uniformly exposed image is
the use of a screen set with varying sensitivity. This type of
screens is called gradient screens. Two different basic designs
are available: Either the thickness of the phosphor layers
varies across the screens, or the screens have a uniform
phosphor layer thickness, but are covered with a laquer layer of
varying transparency to the light emitted by the screen. In
order to obtain a more uniformly exposed image, the less
sensitive screen areas have to be placed under the more
transparent sections of the object. |
