Re: CT Dose calculation

[Philip Hypes <phypes@lanl.gov>]

My first question would be, when was the unit last calibrated
by maintenance personnel and tested by a medical physicist?
If there is a fairly recent medical physicist's report on the unit,
it should be able to give you exposure measurements made
with a pencil probe in the AAPM phantom, hopefully for the 
slice width and other techniques that were used on the patient.

Also, the manufacturer should be able to supply some information
about the exposure levels that should be observed when a
properly functioning unit is tested.  "Properly functioning" is
important, of course.  How well has this unit been maintained,
and has there been any sort of QA program to ensure that
everything is still working as intended?

I happen to have a physicist's report on a GE 9800 Quick
scanner with Highlight/HTD system unit in my office.  I'm not
sure that this is exactly the same system, so you should
assume that there may be some differences in the exposure
levels you'd expect from your scanner.  Just for a ballpark
values, though, here are some of the data:

10mm cut:  3.92 rads
5mm cut:  3.99 rads
3mm cut:  4.26 rads

The techniques were 120 kVp, 400 mAs, 360 degree scan of
a 16cm acrylic head phantom.  All values are at isocenter of
the phantom/beam and were taken with a pencil probe calibrated
for the high aluminum filtrations used in CT scanners.

You did not specify if the scans were of the abdomen and
pelvis or another region of the body.  If the scans were not of
the abdomen and pelvis, you need to consider scatter the
primary source of exposure.  Typical scatter values using
120 kVp and 600 mAs would be about 2-4 mR a short distance
along the bench (say, 0.5m or so).

Hope this helps,

Phil

At 04:58 PM 3/9/99 -0600, you wrote:
>Dear Radsafers: Does any one can help me or refer me to some literature to
>calculate dose to abdomen and pelvis ( a female patient) using a 120 kVp,
>170 mA,  32 slices each of 2 seconds  technique- GE 9800 Quick CT scanner.
>We are trying to determine the dose as the  subject is found to be pregnant
>and hence the need to counsel on the risk.
>
>
>Vaidy Bala
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___________________________________________________________
Philip Hypes
Los Alamos National Laboratory
Safegaurds Science and Technology Group (NIS 5)
(505) 667-1556  phypes@lanl.gov

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