i. Due to the possibility of scatter radiation in x-ray facilities, it is important to
limit the size and shape of the x-ray beam. This is the concept of collimation. Of equal
importance here is that only the area of interest in the patient should be exposed to the
beam. Collimation is an aid in limiting the patient's dose from x-rays.
j. In considering filtration, we use the concept of half value layers. A half-value
layer (HVL) is that thickness of a specified material which, when placed in the path of a
given beam of radiation, reduces the exposure rate by one-half. Materials used are
lead or aluminum and copper equivalents of lead. This concept is also used when
considering the shielding required for x-ray facilities.
Section II. RADIATION PROTECTION PROGRAMS
2-6. EARLY RADIATION PROTECTION MEASURES
a. X-rays and radioactivity were discovered less than 100 years ago. In less
than a year, some of their adverse biological effects had been observed and
precautions taken to avoid them. The essence of those protective measures are
essentially the same today.
(1) Shielding. The idea is to surround the radiation source with a material
that will reduce radiation exposure. Sometimes we can put shielding on the individual.
(2) Distance. We can also reduce exposure by increasing the distance from
the radiation source.
(3) Time. Limiting time of exposure will limit the actual exposure.
b. The first radiation protection standards, as they were then called, were
simply specified thicknesses of lead necessary to reduce a radiation beam to some
unspecified level thought to be acceptable. The reason for this rather vague
specification was that until 1928, there was no acceptable or agreed upon means for
measuring radiation quantitatively.
c. The individuals at risk during this period were the medical doctors and
radiologists. They were essentially the only users of x-rays and radioactive material.
They could recognize the damage caused to themselves by tremendous overdoses and,
until the late 1930s, members of the medical professions were the pioneers who strove
hard and actively toward the development of protective procedures.
d. Their concern was accented by one of the great landmarks in radiation
history--namely, the development of the Coolidge hot cathode x-ray tube, which could
be run for long periods at very large outputs. By 1920, there were substantial numbers
of badly injured physicians and, more or less simultaneously, the United States, United