Hi Nancy,

Radiation exists everywhere. Your spouse lying next to you emits
radiation. The banana you eat emits radiation. We receive radiation
when we walk under the sun. The radiation exposure of a 6 hour flight
equals that of taking 2 chest X rays. A CT head scan's radiation
exposure is equal to half to 1 year's background radiation exposure of
a normal person, or 12-24 flights from Hong Kong to London. The yearly
cumulative dose clearly linked to increased cancer risk is 100mSv:
that equals 1200 flights from Hong Kong to London or taking 100 CT
head scans in 1 year. So long as we try to avoid unnecessary radiation
exposure and do not exceed 50 mSv cumulative dose per year then we
should be fine.
(Ref: https://twistedsifter.com/wp-content/uploads/2016/08/radiation-dose-chart-infographic-xkcd.png
)

Now back to your question. A head and neck CT with contrast gives you
about 1-2 mSV of radiation; an abdomen and pelvic CT with contrast
gives you about 8-14 mSv. (Ref
https://www.openaccessjournals.com/articles-images/imaging-in-medicine-dose-values-1-1-65-t001.png
). The required radiation varies among individuals, depending on how
much is necessary in order to achieve a good quality image. Factors
affecting radiation dose include the fat content of the patient,
target organ motion, the CT image reconstruction algorithm, the size
of the area involved, and the resolution required. This list is not
exhaustive, but it can give you somewhat an idea why there is a
difference in radiation dose in different situations.

Sincerely,
Dr. Jason Ko