Biomedical Engineering Reference
In-Depth Information
charge, or (3) measuring the energy absorbed by matter from the radiation penetrating it.
Using these fundamental concepts, three kinds of radiation units have been established:
the curie (Ci), the roentgen (R), and the radiation-absorbed dose (rad).
It is important to remember that the curie defines the number of disintegrations per unit
time and not the
of the radiation, which may be alpha, beta, or gamma rays. The
curie is simply a measure of the activity of a radioactive source.
The roentgen and the rad, on the other hand, are units based on the effect of the radiation
on an irradiated object. Thus, while the curie defines a source, the roentgen and rad define
the effect of the source on an object. One of the major effects of x-ray or gamma radiation is
the ionization of atoms—that is, the creation of atoms possessing a net positive or net
negative charge (ion pair). The roentgen is determined by observing the total number of
ion pairs produced by x-ray or gamma radiation in 1 cc of air at standard conditions
(at 760 mm Hg and 0
C). Since each ion pair has an electrical charge, this can be related
to electrical effects that can be detected by various instruments. Thus, one roentgen is
defined as that amount of x-ray or gamma radiation that produces enough ion pairs to
establish an electrical charge separation of 2.58
nature
10
4
coulombs per kilogram of air at atmo-
spheric pressure. Thus, the roentgen is a measure of radiation quantity, not intensity. The rad,
on the other hand, is based on the total energy absorbed by the irradiated material. One rad
means that 0.01 joule (the unit of energy in the metric system) of energy is absorbed per
kilogram of material.
Since human tissue is exposed to various types of radioactive materials in nuclear medi-
cine, another unit of measure, the rem (roentgen equivalent man), is often used to specify
the biological effect of radiation. Consequently, the rem is a unit of human biological dose
resulting from exposure of the biological preparation to one or many types of ionizing
radiation.
These radiation units are used in various situations in nuclear medicine. The roentgen, or
more commonly its submultiple the milliroentgen (mr), is used as a value for most survey
meter readings. The rad is used as a unit to describe the amount of exposure received, for
example, by an organ of interest on injection of a radiopharmaceutical. The rem is the unit
used to express exposure values of some personnel-monitoring devices such as film badges.
The classical method of dosimetry, however, has been replaced by a modern method of
radiation dose calculation through the effort of the Medical Internal Radiation Dose
(MIRD) Committee. The internationally accepted unit for radiation dose to tissue is the
gray (Gy); 1 Gy is equivalent to the absorption of energy of one joule per kilogram of
the tissue under consideration. Previously the radiation-absorbed dose was expressed as
radiation absorbed dose (rad); 1 rad is equivalent to the absorption of energy of 100 ergs
per gram or 0.01 J per kilogram, which is equal to 0.01 Gy. Thus, an expression of 1 rad
per millicurie (mCi) is equivalent 0.27027 milligray per megabecquerel (mGy/MBq), or
1 mGy per megabecquerel is equivalent to 3.7 rad per millicurie. Most of the computations
on radiation dosimetry that are available for nuclear medicine procedures, however, have
been expressed in terms of rads.
Exposure of living organisms, mammals in particular, to a high level of radiation induces
pathological conditions and even death. This feature is utilized in therapy for malignant
diseases where the intention is to deliver a localized high-radiation dose to destroy the
undesirable tissue (Figure 15.4). Radiation has in recent years become a common treatment
