15
General
Dose Information
CTDI
W
and CTDI
Vol
The average dose in the scan plane is best described by
the CTDI
W
for the selected scan parameters. The CTDI
W
is measured in the dedicated plastic phantoms – 16 cm
diameter for head and 32 cm diameter for body (as
defined in IEC 60601-2 -44). This dose number gives a
good estimate for 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
W
value displayed can deviate from the
dose in the scanned volume.
The CTDI
W
definition and measurement is based on
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 in between axial scans
or the feed per rotation in spiral scanning. The dose,
expressed as the CTDI
W
, must therefore be corrected by
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
Vol
“ in
September 2002:
CTDI
Vol
= CTDI
W
/pitch-factor
This dose number is displayed on the user interface for
the selected scan parameters.
Please note: Up to now the dose display on the
Somaris\5 user interface was labeled “CTDI
W
“.
This displayed CTDI
W
was also corrected for the pitch.
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