Pediatric Trauma - Hennepin County Medical Center

Technology in Critical Care
Technology in Critical Care: Radiation risks from
computed tomography
by Gopal Punjabi, MD
Department of Radiology
Hennepin County Medical Center
Introduction
The last 40 years have seen dramatic advances in
medical imaging that have revolutionized clinical
medicine. The benefits include more effective
diagnosis, shorter hospital stays, and elimination of
exploratory surgery and rapid diagnosis of lifethreatening
conditions 1 . For example, the trauma
patient can undergo a highly accurate whole-body
computed tomography (CT) scan within a few
minutes that often replaces multiple invasive
examinations, such as angiography and exploratory
laparotomy. This has also lead to an increase in
radiation dose delivered to the patient population.
There has been considerable interest in the medical
literature and in the lay press regarding radiation
dose, often with alarming headlines. In this article,
the biological effects of low-dose ionizing radiation
and methods to reduce radiation risks in CT scanning
will be discussed.
Background
Computed tomography scan is a relatively recent
intervention, developed in the late 1960s by EMI,
Ltd., a music, electronics and leisure company based
in the United Kingdom, funded by profits from the
Beatles' recordings. Sir Godfrey Hounsfield, a British
engineer and Allan Cormack, a physicist born in
South Africa, received the Nobel Prize for this
invention in 1979 2 . The CT scanner uses ionizing
radiation to produce highly detailed images. There
has been rapid advancement in CT technology since
its invention, with current scanners using multidetector
helical technology. This involves an X-ray
beam going through the patient and detected on the
opposite side by multiple detector rows, enabling
sub-second imaging of large portions of human body.
With rapid innovations, the use of CT scans has
dramatically exploded. While in 1980 about 3 million
CT scans were performed, the projection for 2011 is
72 million CT scans. This trend is likely to increase
with an aging population, as well as multiple new
applications of CT, such as CT virtual colonoscopy,
coronary CT angiography, and CT perfusion
scanning.
Radiation dose
Radiation dose describes the amount of energy
absorbed per unit mass at a specific point, and is
expressed in Grays (1 Gy deposits 1 Joule per
kilogram). This does not reflect risk; for instance, a
100 mGy dose to an extremity would not have the
same biological effect as the same dose to the
pelvis. A more useful measurement is the effective
dose, which takes into account the biological
sensitivity of the tissue or organ being irradiated, and
is calculated by multiplying the radiation dose with
tissue and radiation weighing factors 3 . The unit of
effective dose is the Sievert (usually millisieverts
(mSv) are used in diagnostic radiology). The use of
the effective dose facilitates communication with the
patient, and understanding of the likelihood of
potential harm from the radiological exam. The
effective dose is a theoretical number that cannot be
directly measured.
The annual level of naturally occurring background
radiation in the United States is estimated at about
3.1mSv 4 . In 2006, the annual per capita effective
radiation dose from man-made sources was
estimated at about 3.1 mSv; of this, CT accounts for
about 1.47 mSv 5 . The effective dose from an
individual CT scan varies between different body
parts 6 (Table One). There is also tremendous
variation among scanners, depending on the
scanning technique used. For comparison purposes,
the average effective dose from a two-view chest X-
ray is 0.1 mSv, and that from a nuclear cardiac stress
test using technetium 99m labeled sestamibi is about
12.8 mSv.
Procedure
CT Head
CT Chest
CT Abdomen
CT Pelvis
CT Cervical Spine
CT Lumbar Spine
Average Effective Dose
2 mSv
7 mSv
8 mSv
6 mSv
6 mSv
6 mSv
Table One: Radiation doses in common CT exams 6
12 | Approaches in Critical Care | June 2011