Dose Information - Siemens SOMATOM Manual

Dose Information

CTDI and CTDI
Vol
W
The average dose in the scan plane is best described by
the CTDI for the selected scan parameters. The CTDI
W W
is measured in dedicated plastic phantoms – 16 cm
diameter for head and 32 cm diameter for body (as
defined in IEC 60601 – 2 – 44). For scan modes with z-
Sharp the CTDI100 is calculated using the single num-
ber of tomographic sections (not doubled by z-Sharp)
to remain within the terms of IEC 60601-2-44. The z-
coverage with and without z-Sharp is the same and so
is the dose. This dose index gives a good estimation of
the average dose applied in the scanned volume, as
long as the patient size is similar to the size of the
respective dose phantoms.
Since the body size can be smaller or larger than
32 cm, the CTDI value displayed can deviate from the
W
dose in the scanned volume.
The CTDI definition and measurement are based on
W
single axial scan modes. For clinical scanning, i.e.scan-
ning of entire volumes in patients, the average dose
will also depend on the table feed between axial scans
or the feed per rotation in spiral scanning. The dose,
expressed as the CTDI , must therefore be corrected by
W
the pitch factor of the spiral scan or an axial scan series
to describe the average dose in the scanned volume.
For this purpose the IEC defined the term "CTDI " in
Vol
September 2002:
= CTDI
w
CTDI
Vol
Pitch factor
This dose number is displayed on the user interface for
the selected scan parameters.
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