Siemens Software version syngo CT 2005C User Manual

28

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 sin-

gle 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 cor-

rected 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 “CTDIVol“ in 
September 2002:

This dose number is displayed on the user interface for 
the selected scan parameters.

C2-025.630.01.01.02_APPLICATIONGUIDE_SPIRIT.book  Page 28  Friday, April 8, 2005  9:55 AM