Gendex GX CB-500 - Dose and Imaging Performance Information

15-12
Gendex GXCB-500
TM
Service Manual
G990710 30 June 2008
Dose and Imaging Performance Information
The Computer Tomography Dose Index (CTDI) was measured
using a 10cm, 3cc, pencil ionization chamber from Radcal
Corporation (10X9-3CT) in conjunction with a Radcal Corporation
16cm diameter cylindrical CT head phantom (20CT6). This
phantom has one hole at the center and four more at 90° intervals,
1cm inside the outer circumference. The procedure used was as
follows:
• The phantom was positioned with its axis perpendicular to
the tomographic plane at the center of rotation, and its
height adjusted such that its middle was at the same height
as the horizontal laser line.
• The pencil ionization chamber was inserted into one of the
holes and the other four were filled with acrylic rods.
• The phantom was scanned using one of the standard
protocols and the exposure E recorded.
• The CTDI
100
at this location was calculated using the
formula:
CTDI
100
= E . f. L / T
Where:
E is the exposure in mR
f is the factor to convert Roentgens to absorbed dose (rad).
A value of 0.87 for the conversion in air was used.
L is the active length of the chamber, 10cm
T is the verical dimension of the beam.
• The derived quantity CDTI
w
(weighted) was calculated as
follows:
CTDI
w
= 1/3.CTDI
center
+2/3.mean(CTDI
periphery
)
• The CTDI in free air was also measured, i.e. without the
phantom in place, at the same location as the central hole
occupied: i.e. the CTDI
Free-air
The CTDI
w
and the CTDI
Free-air
were measured for the CT
scanning modes available and the results are in the table below.