Siemens SOMATOM Spirit, Operator'S Manual, Page: 66 / 484

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Information about Personal Safety Safety
A.2-24 SOMATOM Spirit
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Beam hardening
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When X-ray beams pass through tissue, the average energy of
the radiation spectrum is shifted towards higher energy. This is
known as
beam hardening . It depends on the material density
and thickness of the tissue through which the X-ray beam
passes.
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For body parts similar in density to water, for example, soft tis-
sue, the effect on the CT image can be compensated. In areas of
high density such as hips, shoulders or lower skull, hypodense
streaks and areas with reduced CT values can be seen. Along the
sides of the upper part of the skull, the so-called “cupping effect”
can be seen as a brightening of the CT values. The cupping
effect also occurs in rare cases such as infant skull tomograms,
due to residual extra focal radiation.
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Additional corrections and/or special kernel parameter settings
(head modes) help to minimize these effects. Yet, they cannot
fully eliminate them.
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