Biomedical Engineering Reference
In-Depth Information
where
d
E
is the energy of the incident photon and
d
l
is travel distance. The LET
depends upon the chemical composition of the absorbing material and the density
of the material. Water is often chosen as the reference absorbing medium to which
the LET of various types of radiation are compared. The LET is only the
average
kinetic energy given up to the absorbing material over the particle's path and it
does describe the effects of added energy on the absorbing material. LET does not
include the effects of radiative energy loss.
In general, the larger LET value indicates more biologically reactive radiation.
Radiation exposure is expressed in several ways to account for different levels of
harm caused by different forms of radiation and the different sensitivity of body
tissues. An absorbed dose (also referred to as exposure dose) is the amount of ra-
diation energy absorbed into a given mass of tissue. Absorbed dose is measured in
“gray” (Gy) units which is equivalent to joules per kilogram; 1 Gy is equivalent
to 100 rad (abbreviated form of radiation absorbed dose). This allows unification
of different types of radiation (i.e., particles and waves) by measuring what they
do to materials. However, not all radiation has the same biological effect even for
the same amount of absorbed dose. To determine the effect of radiation exposure
on human tissue, a quantity called the the equivalent dose is used. This relates
the absorbed dose in a tissue to the effective biological damage of the radiation.
The equivalent dose is measured in an SI unit called the sievert (Sv). One Sv is
equivalent to 100 rem (abbreviated for roentgen equivalent man). To determine the
equivalent dose (Sv), the absorbed dose (Gy) is multiplied by a radiation-weighting
factor (
W
R
), which is unique to the type of radiation. The radiation-weighting fac-
tor takes into account that some types of radiation are inherently more hazardous
to biological tissue, even if their energy deposition levels are the same. For X-rays
and gamma rays,
W
R
is 1 but the
W
R
value changes periodically with advances in
approximation techniques. The more sieverts absorbed in a unit of time, the more
intense the exposure. Alternatively, a
Q
factor is defined based on
LET
. Although
the relation between
W
R
and
Q
is controversial, for
LET
less than 10 keV/mm, the
quality factor is 10; for
LET
between 10 and 100,
Q
is given by 0.32L-2.2 and for
LET
greater than 100,
Q
is 300L
−0.5
.
Actual exposure is expressed as an amount over a specific time period, such
as 5 mSv per year. This is called the
dosage rate
. The effectiveness of the dose is
dependent on the
dosage rate
. The average dose from watching color television is
2 mrem per year. The probability of a harmful effect from radiation exposure de-
pends on what part or parts of the body are exposed. Some organs are more sensi-
tive to radiation than others. One way to account for tissue specific harm caused by
radiation is to use a tissue-weighting factor (
W
T
). When an equivalent dose to an
organ is multiplied by
W
T
for that organ, the result is the effective dose to that or-
gan. The unit of effective dose is the sievert (Sv). If more than one organ is exposed
then the effective dose,
E
, is the sum of the effective doses to all exposed organs.
8.3.2 Scattering of EM Waves
Light scattering is a physical process where the incident EM wave is forced to devi-
ate from a linear trajectory due to localized nonuniformities in the medium. Apart
