The AAPM/RSNA Physics Tutorial for Residents Radiation Safety

IMAGING & THERAPEUTIC TECHNOLOGY
The AAPM/RSNA Physics
Tutorial for Residents
Radiation Safety Considerations for
Diagnostic Radiology Personnel1
Libby Brateman, PhD
regarding policies for
pregnant workers in the
fluoroscopic work environment.
In radiation protection, the guiding philosophy is ALARA (as low as reasonably
achievable), and states have regulatory authority. Dose limits are in part based
on effective dose equivalent and differences in tissue sensitivities. In diagnostic
radiology, the main source of occupational dose is scattered radiation from the
patient—particularly from fluoroscopically guided procedures. Personnel stand
near patients for long times, and angulated geometries with C-arm equipment
may result in high personnel doses from backscatter. For all procedures, judicious applications of time, distance, and shielding affect dose. Appropriate use
includes collimating properly, optimizing beam-on time, minimizing distances
between image intensifier and patient, ensuring sufficient distance between patient and x-ray tube, and optimizing exposure rates for image quality and dose.
Although dose limits typically regulate maximum whole-body dose, protective
clothing worn by fluoroscopists reduces personnel risks; weighting factors
can be applied to estimate effective dose equivalent. Pregnant personnel have
lower limits, which apply only with voluntary declaration of pregnancy. With
appropriate precautions, fetal doses can typically remain within recommended
limits without changes in occupational tasks. Radiation workers in each state
must ensure that regulations are appropriate. Then, for protection of both employees and employers, the rules can and must be followed.
• Understand the inter-
■
LEARNING
OBJECTIVES
After reading this article
and taking the test, the
reader will:
• Know the fundamental
principles of radiation
safety as applied to medical x rays, particularly in
fluoroscopy.
• Be familiar with revised
and traditional quantities
and units used in quantifying radiation and radiation risks.
• Know the salient issues
play among regulatory
authorities governing
medical x-ray safety.
• Be familiar with applications of ALARA in diagnostic radiology.
INTRODUCTION
Radiation safety is important in diagnostic radiology, not only because of regulatory requirements but also because of personnel and patient considerations. This article, the
first in a series on radiation safety in radiology, focuses on the major issues related to
personnel x-ray safety in radiology, primarily in fluoroscopy; personnel shielding; and
Abbreviations: ALARA = as low as reasonably achievable, CRCPD = Conference of Radiation Control Program Directors,
NCRP = National Council on Radiation Protection and Measurements, NRC = Nuclear Regulatory Commission, SSRCR =
Suggested State Regulations for Control of Radiation
Index terms: Radiations, exposure to patients and personnel • Radiations, protective and therapeutic agents and devices • Radiology and radiologists, design of radiological facilities
RadioGraphics 1999; 19:1037–1055
1From
the Department of Radiology, University of Florida College of Medicine, 1600 SW Archer Rd, Gainesville, FL 326100374. From the AAPM/RSNA Physics Tutorial at the 1998 RSNA scientific assembly. Received March 4, 1999; revision requested April 9 and received May 6; accepted May 11. Address reprint requests to the author.
©RSNA,
1999
1037
personnel dosimetry (occupational monitoring).
The philosophy of radiation protection and associated regulations (with an overview of jurisdictions of various agencies), as well as the policies related to medical x-ray workers, including
pregnant x-ray workers, are discussed. The article also introduces basic terminology for radiation safety from x rays, including definitions of
quantities and units. (The Appendix lists several
publications that are useful sources of information on their respective topics.)
■
RADIATION SAFETY TERMINOLOGY
Quantities and Units for Quantifying
Ionizing Radiation
●
The quantities of concern relate to (a) measurements of the ionizing radiation field, (b) the energy absorbed by objects (perhaps people) in
the radiation field, and (c) the biologic effects
associated with the type of ionizing radiation
and the associated risk of the exposed individual. The rate of irradiation may also be a factor in addition to the energy absorbed.
Exposure.—Ionizing radiation interactions
cause ionization in the medium being irradiated. For diagnostic x rays, the instrument typically used for quantifying the radiation field is
an air ionization chamber, which consists of air
inside a measurement volume. When the chamber is exposed to ionizing radiation, some of
the air molecules are ionized and the charges
are collected. Instruments may thus be calibrated to measure the radiation field intensity in
the chamber volume by expressing the collected charge as coulombs per kilogram of air.
Many instruments determine and display measurements of exposure in units of roentgen,
where 1 R is equivalent to 2.58 × 10−4 C/kg (1).
Exposure is defined strictly for air as the interacting medium. However, the term entrance
skin exposure is frequently used in comparing
techniques for various radiologic procedures,
and it refers to the exposure at the location in
space at which the central ray of the radiation
beam enters the patient. Entrance skin exposure is not equivalent to entrance skin dose, be-
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cause it does not include the contributions
from radiation scattered within the patient. It
is, however, a quantity that can be easily measured and compared among facilities.
Kerma.—To characterize an x-ray field, the
quantity exposure is being replaced by the
quantity air kerma. Kerma (an acronym for kinetic energy released per unit mass) is defined
for air as well as for other interacting media
such as tissue (2) and quantifies the energy
transferred to charged particles from ionizing
photon radiation. The unit of kerma is the gray
(formerly, rad; 1 Gy = 100 rad), and 1 Gy of
kerma is equivalent to 1 J of energy transferred
to charged particles per kilogram of irradiated
medium. For air, 1 Gy of kerma is equivalent to
115 R of exposure; therefore, 1 cGy is approximately equal to 1 R.
Absorbed Dose.—Absorbed dose refers to the
energy that is deposited locally in an absorbing
medium from ionizing radiation. For the energies typically used in diagnostic x-ray procedures, absorbed dose and kerma are equivalent.
By contrast, for high-energy photon interactions, the more energetic charged particles may
deposit energy at sites distant from the initial
interaction site, with a corresponding loss of
electronic equilibrium. In this case, absorbed
dose and kerma are not equivalent (2). The unit
of absorbed dose is the gray, which is the same
unit as for kerma, and 1 Gy of absorbed dose is
equivalent to 1 J of energy absorbed by the medium per kilogram of absorbing medium. One
gray of absorbed dose is equivalent to 100 rad,
the traditional unit of absorbed dose. Doses, including skin dose, include the contributions
from scattered radiation in addition to the primary radiation. Organ doses consist of the total
energy absorbed by an organ per unit mass of
the organ.
Quantities and Units for Quantifying
Biologic Risks
●
Radiation Weighting Factor.—Biologic effects associated with ionizing radiation depend
not only on the absorbed dose but also on the
type of radiation that deposits its energy, that
is, on the density of the energy deposited as the
radiation passes through the absorbing me-
Volume 19 Number 4
Table 1
Radiation Weighting Factors
Type and Energy Range
WR
X and gamma rays, electrons
High-energy protons
Neutrons (depending on energy)
Alpha particles, fission fragments
1
5
5–20
20
Note.—Adapted from ICRP report no. 60 (3).
WR = radiation weighting factor.
dium. Technically, these values are somewhat
different for different energies of x rays and
electrons, as well as for other types of particulate radiations. For radiation protection purposes, such as for the determination of dose
equivalent, equivalent dose, effective dose
equivalent, and effective dose, these values
have been standardized and simplified to the
tabulated values for radiation weighting factor
(WR) (Table 1) (3). For ionizing photons (ie, x
rays and gamma rays) and electrons, this value
is equal to 1, but it is much higher for other
particulate ionizing radiations.
Dose Equivalent and Equivalent Dose.—
Dose equivalent is a quantity that was defined
for radiation protection purposes and that takes
quality factors into account: Dose equivalent is
the product of the absorbed dose and the quality
factor. Quality factors include biologic effects
related to contributions from the different absorbed energy densities associated with different
types (and energies) of ionizing radiations. The
quality factor for x rays is equal to one; that is,
for x rays, it is numerically equivalent to the radiation weighting factor (4). The traditional unit
for dose equivalent is the rem, and although this
unit has been replaced by the sievert (1 Sv = 100
rem), it is still used in radiation protection regulations, and dosimetry reports present data in
units of millirem. Although dose equivalent has
been replaced in the scientific communities by
the quantity equivalent dose, radiation protection regulations continue to specify limits for occupational doses from x rays in quantities based
July-August 1999
on dose equivalent. For radiation protection
purposes, the deep-dose equivalent is defined
at a depth of 3 mm in tissue (5).
Equivalent dose is the product of the average absorbed dose in a tissue or organ and its
associated radiation weighting factor (WR). The
unit of equivalent dose is the joule per kilogram, with the special name of sievert (Sv).
Effective Dose Equivalent and Effective
Dose.—In addition to weighting factors associated with different types of radiation (ie, WR),
additional weighting factors are associated with
different tissues because of different tissue sensitivities to ionizing radiation. Both effective
dose equivalent (HE) and effective dose (E) have
tissue weighting factors associated with them.
These quantities are neither numerically equivalent—because of differences in their associated tissue weighting factors—nor interchangeable—because of differences in the underlying
assumptions associated with their definitions.
Tissue Weighting Factor.—Different tissue
weighting factors (WT) have been defined for
the determination of effective dose equivalent
and effective dose. Both sets of weighting factors relate to the risks associated with cancer
for different organs and the risks of creating
heritable effects from future offspring as a result of radiation dose. Effective dose equivalent
was defined earlier than effective dose, and
rules of the Nuclear Regulatory Commission
(NRC) were modified to incorporate some aspects of this quantity in its revisions to 10 CFR
Part 20; therefore, this quantity is used as the
basis for radiation protection limits in existing
regulations (6). Effective dose equivalent includes consideration of the risks to a population of radiation workers, and hereditary effects
are included for two generations of offspring of
these radiation workers. Risks to extremities
and deterministic effects, such as to the lens of
the eye, were excluded from effective dose
equivalent.
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Reassessment of radiation risk was made in
the late 1980s after a revised determination of
the radiation doses and subsequent radiation
biologic effects resulting from the atomic bombs
in World War II in Japan. Analysis of these revised doses and their associated effects on survivors and their offspring led to revisions of the
assessed risks associated with different organ
doses. Effective dose was defined as a new
quantity to take these revised risks into account
(3,7,8). These risks for radiation workers include both fatal and nonfatal cancers, 4.0 ×
10−2 Sv−1 (4.0 × 10−4 rem−1) and 0.8 × 10−2 Sv−1
(0.8 × 10−4 rem−1), respectively, and severe genetic risks of 0.8 × 10−2 Sv−1 (0.8 × 10−4 rem−1). In
addition, risks for members of the general public are included so that these risks may be used
for the assessment of patient risks, for example,
in research protocols. The list of organs considered for tissue weighting factors to determine
effective dose was expanded from the list for
effective dose equivalent to include the esophagus, stomach, liver, and bladder and thereby reduced the fraction considered as “remainder.”
In both cases, the total risk is 1.00.
Table 2 presents the tissue weighting factors
for both effective dose equivalent and effective
dose. Current scientific understanding of the
risks associated with exposure to radiation is
described by effective dose. However, current
radiation protection regulations are based on
the older quantity, effective dose equivalent.
Therefore, both sets of values need consideration at the present time.
RADIATION SAFETY PHILOSOPHY
AND ALARA
■
The goal of a radiation safety program is to provide regulatory oversight, education, and radiation safety consultation services that serve to
minimize exposure to ionizing radiation while
promoting the safe and effective use of radiation sources in diagnosis, therapy, and research.
One goal of a radiation safety program is to
keep the risks to radiation workers to levels
that are comparable with those of other safe occupations. The pervading philosophy is that of
“as low as reasonably achievable” (ALARA).
ALARA also includes the concept of including
economic and societal considerations (9).
In most environments, radiation safety means
minimizing all radiation exposure to all indivi-
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Table 2
Comparison of Tissue Weighting Factors
Organ or Tissue
Gonads
Red bone marrow
Colon
Lung
Stomach
Bladder
Breast
Liver
Esophagus
Thyroid
Skin
Bone surfaces
Remainder
WT for HE*†
WT for E†‡
0.25
0.12
…
0.12
…
…
0.15
…
…
0.03
0.01
0.03
0.30
0.20
0.12
0.12
0.12
0.05
0.05
0.05
0.05
0.05
0.05
0.01
0.01
0.05
Note.—E = effective dose, HE = effective dose
equivalent, WT = tissue weighting factor.
*From ICRP report no. 26 (4) and NCRP report
no. 116 (8).
†
Total tissue weighting factors equal 1.
‡
From ICRP report no. 60 (3) and NCRP report
no. 116 (8).
duals; however, medical care presents a unique
situation in which patients are intentionally irradiated. Protection philosophy requires consideration of the benefits of the radiation to which
a person is exposed. The goal is to perform diagnostic procedures that optimize both radiation exposure and diagnostic information.
Patients and personnel must be considered
separately: Regulatory dose limits for radiation
workers do not apply to medical irradiation received by patients. Therefore, when a radiation
worker becomes a patient, only patient considerations are applicable.
BASIC PRINCIPLES OF X-RAY
SAFETY
■
●
Beam-on Time Considerations
The duration of exposure of the individual is directly proportional to the occupationally received radiation dose. Therefore, personnel
doses may be directly linked to patient doses.
In fluoroscopy, typically both the peak kilovoltage (kVp) and milliamperage (mA) are adjusted
automatically, and the operator can control
only the time that the x-ray beam is on. Reducing both personnel and patient doses may be
accomplished effectively by minimizing the to-
Volume 19 Number 4
dose. Similarly, bringing patients to radiographic rooms with control booths rather than bringing mobile units to patient beds whenever possible is also desirable.
Definitions for X-ray Interactions
and Safety
●
Figure 1. Schematic provides a graphical representation of definitions of radiation safety terms as
they are used in radiation protection (with an x-ray
tube as the radiation source).
Figure 2. Schematic illustrates the inverse-square
law. As the surface area of a sphere is proportional
to the square of its radius, the radiation intensity decreases proportionately as the square of the distance
from a point source. The ray spacing is related to the
radiation intensity.
tal “beam-on” time for a procedure through judicious use of the exposure switch to ensure
that irradiation is occurring only when the fluoroscopist is actively viewing the image. Use of a
last-image hold feature can be helpful in reducing the overall beam-on time, so that scrutiny of
anatomic features can be accomplished without
a competing concern about additional radiation
dose. Teaching hospitals in particular should
provide fluoroscopes with a last-image hold feature, and all new fluoroscopes should be purchased with this feature.
Another consideration in personnel dose is
the time that the worker is inside the fluoroscopy room or near a mobile radiography unit
when exposures are being made. Optimizing
the number of images in an exposure sequence
for serial radiography (including digital radiography) or cinefluorography by controlling both
the exposure rate and duration of the sequence
allows both patient and personnel doses to be
optimized. Use of a control booth whenever
possible is desirable to minimize occupational
July-August 1999
The x-ray beam and its interactions and their
implications for safety considerations are illustrated in Figure 1. A small amount of radiation
is emitted through the shielded x-ray tube housing (ie, leakage). The useful or primary beam
exits the tube and interacts with the patient:
Some radiation is absorbed (the source of patient dose), some is transmitted (the source of
the image), and some is emitted from the patient as primary scatter (the source of potential
hazard). The patient attenuates the primary
beam so that 5%–15% is transmitted; the beam
transmitted through the patient interacts with
one or more barriers—preferably only the image receptor. Primary scatter is emitted from
the patient in all directions, which is the major
source of hazard to personnel in mobile radiography and fluoroscopy. The patient thus becomes the source of scattered radiation but
only during the time of the primary radiation
exposure. Primary scattered radiation may also
interact with other objects (including people)
and produce secondary scattered radiation. The
intensity of secondary scatter is much lower
than that of primary scatter. Stray radiation is
defined as the sum of the scattered radiations
and x-ray tube housing leakage, with scattered
radiation from the patient being the major component.
●
Inverse-Square Law
Radiation is emitted from a point source in all
directions about that point, and the point can
be considered to be in the center of a sphere.
As seen in Figure 2, the radiation intensity decreases proportionately as the square of the distance from a point source. However, when the
source is larger than a small point, the radiation
intensity decreases less rapidly. That is, when a
patient is exposed to most reasonably large xray fields and personnel are standing very close,
the inverse-square law does not hold precisely.
This variability depends on the energy of the
primary beam, the beam area, and patient dimensions. Because increasing the distance decreases the radiation field intensity, it is prudent to back away from the source.
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●
Fluoroscopic Geometry
The inverse relationship between the source
and the radiation intensity some distance from
the source implies that small changes in distance have the largest impact on radiation intensity very close to the source. Therefore,
keeping the skin of a patient at least a minimum
distance from the x-ray tube focal spot is a regulatory requirement in addition to prudent technique. In particular, many mobile C-arm fluoroscopes have a removable spacer assembly for
the x-ray tube that can become misplaced;
when the spacer is not used, the patient skin
dose increases dramatically. NCRP Report No.
102 states that the minimum distance shall be
no less than 30 cm and should be not less than
38 cm (1).
Keeping the image intensifier or imaging assembly as close to the patient as possible minimizes the overall distance between the focal
spot and image receptor. This geometry keeps
the fluoroscopic beam intensity as low as possible, allows the image intensifier to serve as a
scatter barrier between the patient and operator, and minimizes image blur simultaneously.
In Figure 3, appropriate fluoroscopic geometry
is compared with two poor geometries.
Scattered Radiation.—Diagnostic x-ray energies, which are low on the scale of ionizing radiation energies, scatter in all directions with
more uniformity in intensity than higher energy
radiations. The scattered rays are slightly lower
in energy than the primary x rays. The number
of scattered x rays is proportional to the number of incident rays—that is, to the entrance
dose—and to the volume of material that is irradiated. For diagnostic x-ray beams, the radiation
field intensity scattered through 90° is reduced
by a large factor that depends on field size and
beam energy. As a rule of thumb, this value may
be estimated at 1 m to be approximately 1,000,
or to 0.1%–0.2%, compared with the incident
radiation. Therefore, for a primary beam air
kerma rate of 20 mGy/min where the beam enters the patient, corresponding to an exposure
rate of 2.3 R/min, the rate of scattered radiation
at 1 m from the patient is approximately 20–40
µGy/min (2.3–4.6 mR/min).
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Figure 3. Schematics illustrate three fluoroscopic
geometries. In (a), the x-ray tube is too close to the
patient, which is indicated by the large change in
ray spacing between the entrance to and exit from
the patient. (Ray spacing is related to the dose rate.)
In (b), the x-ray tube is farther from the patient, and
the dose distribution is more uniform; however, the
image intensifier tube is too far from the patient,
which causes the patient entrance dose to be increased. In (c), both tubes are placed appropriately.
Diagnostic radiology health care personnel
typically receive little to no radiation dose
while working in control booths, a small
amount while working with mobile radiographic units, and most of their radiation dose
from fluoroscopy. The main source of this dose
is scattered radiation emitted from the patient.
Thick body parts and large patients, as compared with thinner anatomy, require more primary radiation to allow sufficient transmitted radiation for the formation of an adequate image,
because the requirement for adequate image
quality is based on the radiation that enters the
image intensifier. This requirement depends on
the quality of the imaging system components,
as well as the type of study being performed
and the fluoroscopist’s willingness to accept
noisier images from a larger aperture in the
camera optics (10).
Several clinical use choices impact scattered
radiation dose. Exposure rates at the location of
the beam entrance to the patient depend on the
patient’s anatomy and the amount of contrast
medium in the imaged volume. Maximum limits
are set by federal regulations at 2.6 mC/kg/min
(corresponding to air kerma rates of approximately 4 Gy/min) for normal image-intensified
fluoroscopy and twice that for equipment with
optional high-level controls manufactured after
Volume 19 Number 4
Figure 4. Graph depicts the effects of collimation,
distance, and peak tube potential (kVp) on scattered
radiation intensity. Measurements were made at 3
mA with a thin plastic cube phantom 22 cm on each
side and filled with water for scattering. The location of measurements was at a height of 1.5 m (5 ft)
above the floor to simulate dose rates to the head of
many individuals. The scattered radiation from the
smaller field, shown as stars for the three peak tube
potentials, is much lower at all distances from the
table and particularly so at the table side. The fluoroscopist can back away one-third of a meter (approximately 1 foot) from the table by taking one
step backward.
May 1995 (11). Although the high dose rate
mode in fluoroscopy produces images with
lower levels of noise, use of this mode may potentially increase radiation exposures to personnel and patients, because higher levels of primary radiation correspond to production of
more scattered radiation. For older equipment,
federal regulations have no limit to the high
dose rate mode, and some units have been set
by manufacturers to allow very high dose rates
(five to 10 times the new equipment limit).
On the other hand, pulse-mode fluoroscopy
typically corresponds to an overall decrease in
cumulative beam-on time by limiting the irradiation time to a portion of the procedure. The reduction in dose rate is likely not to be as great
as one might expect from the fraction of the
beam-on time, because the tube current is typically increased to reduce the noise level because of perception considerations; however,
significant reduction in dose, perhaps by a factor of two, is achievable, compared with that
incurred in continuous fluoroscopy (10).
Use of photofluorospot film, cinefluorographic film, and digital recording of fluoroscopic images requires higher dose rates per
July-August 1999
image than doses required for fluoroscopic
viewing. The maximum exposure rates described above for fluoroscopy do not apply for
these uses, nor do they apply for conventional
spot film (ie, radiographic) imaging. Each image
requires a patient entrance air kerma of 30–400
mGy, and serial imaging involves many images
taken at a high repetition rate (12). Clearly, the
total dose depends on the number of images,
which is lowered by a reduction in the repetition rate. As for all radiologic procedures, only
essential personnel should be in the room during the exposures.
The effect of equipment quality control is of
particular significance in fluoroscopy. As the
image intensifier ages and loses efficiency, the
automatic brightness control automatically increases the primary beam technique factors and
exposure rates to keep an adequate light level
to the camera. Monitoring equipment performance over time is necessary to identify this
source of potential hazard.
When the collimation is inadequate, the radiation field can be larger than suspected, with
an increase to patient and personnel doses. Collimation to a smaller beam area reduces two dimensions of the scattering volume and also improves image contrast. Figure 4 shows the effects of field size (ie, collimation) and distance
on scattered radiation intensity for a constant xray tube current and a fixed phantom. It also
shows the effect of peak tube potential, which
typically increases automatically with patient
thickness. As peak tube potential increases, this
effect is large because (a) the primary beam intensity increases and (b) the scattered radiation
is more penetrating. The effect of collimation is
dramatic (Fig 4).
The decrease in area from collimation might
be expected to decrease the rate of scattered radiation to the operator, an appropriate assumption when the technique factors (kilovoltage
and milliamperage) are kept constant. The issue
is complicated, however, because the automatic brightness control feature in fluoroscopic
equipment typically adjusts the technique factors automatically, and the reduced scatter almost always results in an increase in patient entrance dose rate. Therefore, even though the
scattering volume is smaller, the scatter rate may
not decrease accordingly. Another confounding
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Figure 5. Schematics show the effects of clinical
field location on radiation scatter. In (a), the primary beam is close to the midline of the patient and
is associated with less intense radiation scattered
to the fluoroscopist because it is attenuated more
through a greater thickness of overlying tissue and is
farther away. In (b), there is more scatter to the fluoroscopist, because the primary beam is closer to the
operator and less attenuated by the patient. Because
of these factors, the fluoroscopist may not be fully
aware of the intensity of either the primary beam or
the scattered field.
Figure 6. Schematics show the effect of C-arm geometry on backscatter. In (a), the fluoroscope has
an overhead x-ray beam orientation, with the image
intensifier below the patient. The face of the fluoroscopist is not shielded from scattered radiation and is
fairly close to the x-ray tube housing, from which
leakage radiation emerges (typically, but not always,
a small amount). In (b), the overhead image intensifier acts as a barrier to protect the face of the fluoroscopist, with the backscattered radiation directed
toward the floor.
factor affecting the scatter rate to the fluoroscopist is the location of the beam within the patient. When the field is closer to the proximal
side of the patient, the scattered radiation penetrating the patient is less attenuated by the patient (Fig 5).
ly 1 mGy) in 1 hour at a distance of 1 m, when
the x-ray tube is operating at a specified high
kilovoltage and low milliamperage (13). Frequently, with certain overhead x-ray tube C-arm
geometries, personnel are standing for long periods of time closer than 1 m from the x-ray
tube. Any leakage radiation transmitted through
the tube housing is more penetrating than
might be expected for radiation scattered from
the patient at a comparable kilovoltage. In particular, one fluoroscopic x-ray tube was found
to be a source of personnel overexposures because part of the shielding for the tube housing
had not been installed. In overhead and angulated x-ray tube geometries, fluoroscopists
should pay attention to the possibility of exposure to this source (ie, leakage radiation) in addition to scattered radiation, that is, to stray radiation, which is the sum of leakage and scatter.
Backscattered Radiation.—Backscattered radiation is radiation that is scattered back from
the surface at the beam entrance. It is of high
intensity because the entrance surface of the
patient has the highest dose rate because the
incident beam has not been attenuated. In
some orientations of a C-arm fluoroscope, the
backscattered field is potentially hazardous to
the operator for procedures of extended duration (Fig 6).
All attenuators reduce transmitted radiation
to some fraction of the radiation incident on
them. When the incident radiation is a spectrum of energies, the radiation transmitted is
also a spectrum of energies, with a hardening
of the spectrum: That is, lower energy radiation
is absorbed preferentially, leaving a greater proportion of higher energy radiation in the transmitted spectrum. X-ray tube housings contain
lead as shielding, and they are designed to emit
no more than 100 mR (870 Gy or approximate-
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Stray Radiation.—Stray radiation fields for
two orientations of an x-ray unit in a cardiac
catheterization suite are illustrated in Figures 7
and 8 (14). Figure 7 shows the stray radiation
measured for a phantom with isokerma curves
for the 90° left anterior oblique cardiac projection, that is, with the image intensifier to the
patient’s left and with the x-ray beam horizontally directed approximately 1 and 1.5 m above
the floor. Figure 8 shows the stray radiation
Volume 19 Number 4
7a.
7b.
8a.
8b.
Figures 7, 8. (7) Diagrams show stray radiation fields measured in a cardiac catheterization suite for a 90° left
anterior oblique projection, with the x-ray beam approximately 1 m above the floor. Scale bar is 0.5 m. Curves
represent the kerma rates at 1.5 m above the floor (approximating eye level) (a) and 1.0 m above the floor (approximating waist level) (b). (Reprinted, with permission, from reference 14.) (8) Diagrams show stray radiation fields measured in a cardiac catheterization suite for a 60° left anterior oblique projection, with the x-ray
beam approximately 1 m above the floor. Scale bar is 0.5 m. Curves represent the kerma rates at 1.5 m above
the floor (approximating eye level) (a) and 1.0 m above the floor (approximating waist level) (b). (Reprinted,
with permission, from reference 14.)
with isokerma curves for the 60° left anterior
oblique cardiac projection for the same two
heights above the floor. It is clear from Figure 8
that both the x-ray tube and image intensifier
provide shielding and that the angulation of the
x-ray tube to the floor provides additional protection.
●
Principles of Shielding
Shielding involves the use of protective barriers. Protective barriers may be structurally affixed, movable, or worn by individuals. The
fundamental concept is to provide shielding
interposed between the radiation source and
personnel so that radiation is first heavily at-
July-August 1999
tenuated by the barrier. The best type of shielding protects the whole body of an individual.
Such barriers include walls, windows, and
doors in general and control booths in particular, as well as the x-ray tube housing. The closer
the shielding is to the source, the smaller and
less costly it can be. Barriers are designed to
protect individuals by keeping their exposures
below certain regulatory limits or lower, as
ALARA permits. These limits depend on who is
being exposed behind the barrier and how long
they are expected to be there during irradiation.
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Figure 9. Photograph shows a
typical set-up in a radiography-fluoroscopy suite. The table provides a
flip-up side shield to cover the
Bucky slot, and the imaging assembly
has protective drapes to protect the
fluoroscopist from patient scatter.
The operator is wearing a protective
skirt and vest and a heavy leaded
glove to protect her hand when it is
exposed to the primary beam.
The most recent NCRP report on shielding
was written in 1976 (15), before the current
guidelines for occupational exposure to radiation were written. NCRP Report No. 116, written in 1993, recommends that new facilities be
designed to limit occupational doses to a fraction of the annual cumulative dose limit of 10
mSv (1 rem) and to limit doses to others (ie, not
occupationally exposed persons) so that they
do not exceed 1 mSv (100 mrem) annual effective dose (8,16). This guidance is in stark contrast to the design criteria in NCRP Report No.
49 (15) of 50 mSv (5 rem) per year for controlled areas occupied by radiation workers and
5 mSv (500 mrem) per year for uncontrolled areas occupied by others. In addition, the ionizing radiation workload in diagnostic radiology
has changed substantially since 1976, particularly in interventional procedures and computed tomography. Therefore, guidance for
shielding design is in transition, and revisions to
the guidelines are under development.
Types of Shielding.—One aspect of shielding
design that should be considered for all modern
work areas is control booths that are sufficiently large to shield all the personnel who do not
need to be in the examination area during spot
radiographic, fluorospot radiographic, cine radiographic, or digital exposures. Booths with
large windows and additional monitors for re-
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Imaging & Therapeutic Technology
mote viewing are particularly useful in teaching
hospitals.
Mobile barriers may be rolled into position
inside radiography-fluoroscopy suites to protect
individuals (eg, anesthesiologists and nurses)
who need to be near patients for extended periods during the procedures. Ceiling-suspended
barriers are available as transparent shielding
during interventional procedures to protect the
entire upper body of the fluoroscopist. In addition, the x-ray equipment may offer shielding
options: Fluoroscopic tables may be purchased
with optional shielding and leaded drapes that
protect personnel from patient scatter.
Flexible protective clothing such as aprons,
vests, skirts, thyroid shields, and gloves should
be available for the times when an individual
must work in an unshielded environment. The
necessity for wearing protective items depends
on an individual’s work habits and environment,
facility policies, and applicable regulations. The
typical flexible material for protective clothing
is lead-impregnated rubber. Special composite
materials that may be lightweight and easier to
wear are also available but usually more expensive. Eyeglasses with side shields are available
with or without prescription lenses for eye protection for personnel who need them.
Personnel who may have their backs exposed to the x-ray beam should wear wraparound shielding. In wraparound garments, the
back shielding is typically only one-half the
thickness of the front shielding. For skirts and
vests, the shielding is typically one thickness
Volume 19 Number 4
Figure 10. Photograph shows a biplane C-arm set-up in a neuroangiography suite. The room contains a mobile floor-standing (mostly transparent)
barrier for use by other personnel and a
ceiling-suspended transparent barrier
with a suspended drape to protect the
upper body of the interventionalist from
scattered radiation (mostly backscatter).
This shield is not always used; therefore, the physician is wearing protective eyeglasses. The importance of side
shields for eyeglasses is shown: As the
fluoroscopist observes the monitor, his
face is rotated so that the lens of his left
eye would be exposed to scatter if the
side shield were absent. He is also wearing a thyroid shield to protect important organs and tissue (eg, thyroid and
sternum).
and is doubled in front and held in place with
straps. Frequently, wraparound items are not
completely doubled when they are worn, and
the shielding may not be as protective as expected. That is, for a wraparound apron that is
specified to be equivalent to 0.5 mm of lead,
one thickness of the material is only 0.25 mm
lead equivalent in the back and wherever the
material is not overlapped. Personnel who need
to wear wraparound aprons or their equivalent
are typically support personnel who move
around the room during procedures and therefore cannot be protected by mobile barriers.
Others may wish to wear them for reasons of
comfort.
Figure 9 shows an operator working in a
typical radiography-fluoroscopy suite. The radiographic table and its side shield protect personnel from stray radiation that originates below the table; this shielding is absent for C-arm
units. Figure 10 shows a fluoroscopist wearing
leaded eyeglasses with side shields in a neuroangiography suite with a biplane C-arm unit. In
this room are a mobile floor-standing (mostly
transparent) barrier for use by other personnel
such as anesthesiologists and a ceiling-suspended transparent barrier for interventionalists. Because this shield is not always
used, the fluoroscopist is wearing protective
eyeglasses.
All protective clothing items should be evaluated both at acceptance for attenuation properties and integrity of the shielding and periodically for shielding integrity (ie, to check for
cracks or holes). The evaluations can be performed fluoroscopically with fixed technique
July-August 1999
factors, and defects can be radiographed. The
required frequency of testing, which is typically
annual, depends on the item used and the care
that it receives.
Principles of Attenuation.—The amount of
radiation attenuated by a material depends on
the elemental composition of the material, its
thickness, and the energy of the radiation passing through it. Each element has attenuating
properties that are related to its atomic number
and the binding energies of its electrons. Lead
has historically been used as an effective shield
because of its high attenuation. However, all elements, including lead, have discontinuities
that create transmissive notches in their attenuating properties as a function of energy; these
discontinuities (most frequently, the K-edge)
have to do with the binding energies of the
electrons in various orbitals. Therefore, the
amount of lead required for safety purposes
(typically 0.5 mm of lead equivalent thickness
for aprons) is related to the overall transmission
of scattered radiation, and the discontinuities affect the overall effectiveness of the attenuation.
The transmission through 0.5 mm of lead is
3.2% at 100 kVp and 0.36% at 70 kVp (17).
Lead aprons are somewhat heavy to wear,
and they can become uncomfortable at times
for individuals wearing them. Aprons of 0.5 mm
lead equivalent have been reported to range
from 2.5 to 7 kg (18). Aprons can be custom
made, so that they are only as long or as wide
as they need to be for protection purposes.
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Well-fitting aprons cover much of the red bone
marrow, and for women they should cover
breast tissues (thyroid shields can also be used
for this purpose). Different styles (eg, belted,
two-piece, shoulder-supported) are available
and were designed to improve comfort by taking some of the weight off the shoulders and
upper back and sharing it with the hips and
lower back.
Some aprons are made of composite materials of lead combined with elements of somewhat lower atomic numbers so that the weight
of the apron is less than for lead alone (17,19).
The elements that make up these composite
materials are chosen so that their discontinuities in attenuation occur at somewhat lower energies than lead and so that they absorb more
effectively than lead in the transmissive notches
for lead. Because the combination provides a
more uniform attenuation than for one element
alone, composite materials can be essentially
equivalent in effectiveness to lead with a lower
total mass or weight by 15%–25%. Figure 11
shows the attenuating properties of three elements (tungsten, barium, and lead) used in a
composite material described by Yaffe and
Mawdsley (17).
■
RADIATION SAFETY REGULATIONS
Regulatory Authority for Radiation
Safety
●
Many agencies in the federal and state governments have responsibilities for regulating certain aspects of protection against radiation—
and sometimes they overlap.
Occupational safety in general comes under
the jurisdiction of the U.S. Occupational Safety
and Health Administration. However, inspection for, compliance with, and enforcement of,
both occupational safety and radiation control
regulations are typically performed by state
regulatory agencies.
The U.S. Center for Devices and Radiological Health under the Food and Drug Administration sets standards for new equipment that produces radiation. However, this agency does not
set occupational dose limits for workers who
use the equipment.
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Figure 11. Graph shows attenuation characteristics of three elements used in a composite material,
demonstrating that the three elements provide more
uniform attenuation than lead alone. (Adapted from
reference 17.)
The NRC regulates the production of radioactive materials that come from nuclear reactors as well as the safety of people who are exposed to the radiations from these materials.
X-ray personnel who are also occupationally
exposed to these radioactive materials are regulated by NRC rules.
States have authority to regulate radiation
safety for all sources, including x-ray tubes, and
all persons within their boundaries.
At the local level, institutions can create
their own policies that are more strict than federal and state regulations for the purpose of
keeping radiation exposures ALARA.
The interplay among the regulatory authorities is intricate and is made somewhat uniform for different radiation sources and users
through the Conference of Radiation Control
Program Directors (CRCPD), a nonregulatory
organization. The CRCPD, whose members are
mostly the heads of the state-controlled radiation control programs, has created the Suggested State Regulations for Control of Radiation (SSRCR) (20). The SSRCR are model regulations that combine all the federal radiation
standards into one document, and the states
can adopt any, all, or none of them. Most states
Volume 19 Number 4
use at least some of the SSRCR, and they are required to adopt NRC rules for safety associated
with nuclear by-product radiation (ie, with radiation from reactors and from radioactive materials created in the reactors).
●
Dose Limits for X-ray Workers
The 1995 revisions to NRC rules 10 CFR Part 20
include some, but not all, aspects of the newer
quantity effective dose equivalent (6). In these
revisions, NRC used effective dose equivalent
for evaluating the doses to individuals exposed
to internal sources; however, the agency kept
its previous policy that doses received from external sources were to be evaluated by the
older quantity deep-dose equivalent. Such externally received doses are considered to have
been received from whole-body irradiation
when certain parts are exposed (such as the
head or upper arm), without consideration of
the scientific basis underlying the concept of effective dose equivalent. Because of a need to
combine values to set limits for individuals exposed to both external and internal sources, the
NRC defined new quantities, the total effective
dose equivalent (TEDE) and the total organ
dose equivalent (TODE) for specific organs.
The TEDE and TODE consist of the sum of
the deep-dose equivalent from external radiation sources and a quantity related to effective
dose equivalents from internally deposited radioactive sources (5). Only annual limits are
set, and there is no allowance for a lifetime limit. For x-ray workers, the annual TEDE limit is
0.05 Sv (5 rem), which is equal to the previous
whole-body limit; this is the limit against which
the value from the required whole-body dosimeter is assessed. Some states have quarterly limits in addition to the annual limit.
The lens of the eye is limited to 0.15 Sv (15
rem), and any other organ or tissue is limited to
0.50 Sv (50 rem). Because of the high dose limit
to the eye, it might be argued that protective
eyeglasses are unnecessary. Certainly they are
not warranted for routine fluoroscopy or when
structural shielding is employed. However,
wearing glasses is justified and certainly follows
the ALARA philosophy for fluoroscopists who
perform many fluoroscopically guided interventional procedures without upper-body shielding
from interposed transparent barriers.
In NRC rules, the deep-dose equivalent for
an individual does not allow consideration of
partial-body shielding. That is, the NRC requires
July-August 1999
that a radiation worker wear a dosimeter, called
an individual monitoring device, at the location
at which the individual receives the highest
deep-dose equivalent. This value (ie, the highest deep-dose equivalent) is considered to have
been received uniformly by the whole body,
without consideration of protective shielding
that covers most of the body (eg, an apron
worn by a fluoroscopist). However, one must
keep in mind that NRC rules apply to users of
nuclear by-product materials and do not necessarily apply to x-ray workers who do not use radioactive sources unless the individual state
regulations incorporate the NRC rules and apply them to the latter individuals as well.
The SSRCR are required to be identical to
NRC rules for users of nuclear by-product materials. Because of concerns that personnel in
interventional radiology suites were not being
offered consideration for wearing protective apparatus (eg, aprons, thyroid shields, eyeglasses)
and that NRC rules were too restrictive in this
situation (for which the rules were not intended), the CRCPD accepted optional special
considerations for fluoroscopists. That is, the
SSRCR have combined the NRC rules with a
special addition for fluoroscopists, which allows consideration for determining the effective dose equivalent based on a fluoroscopist
wearing an apron (21). However, each state
must determine whether this feature (or another substitute) is adopted, and it is up to the
affected x-ray workers in each state to provide
input to the state to ensure that its rules are reasonable so that they can be followed.
The optional SSRCR rules for handling partial-body exposures from external radiation
sources are written specifically for fluoroscopists. These rules were written as a way of estimating effective dose equivalent for fluoroscopy personnel who are likely to exceed the
NRC-defined whole-body deep-dose equivalent
limit of 0.05 Sv (5 rem) in a year in the normal
performance of their work. The individuals involved are typically those who perform interventional procedures on a regular basis and, in
particular, those who do not use transparent
barriers to shield their upper bodies during procedures. The rules require a dosimeter to be
worn at the neck as an estimate of the dose received by the unshielded portion of the body,
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and they allow an additional dosimeter to be
worn under an apron of 0.5 mm lead equivalence to give a more realistic estimate of the
dose received by the major portion of the body.
When the two dosimeters are worn in this way,
the effective dose equivalent for fluoroscopists
may be estimated from the relationship HE =
0.04(N) + 1.5(W), where HE is the effective
dose equivalent, and N and W are the unshielded
neck and shielded waist deep-dose equivalent
values, respectively (22). This equation was derived by Webster (23) from data reported by
Faulkner and Harrison (24), who evaluated the
relationships among effective dose equivalent,
peak tube potential, and apron thickness for a
fluoroscopic geometry.
Because an individual might not wear two
dosimeters, the SSRCR also allow an estimation
of effective dose equivalent from a single unshielded neck dosimeter from the relationship
HE = 0.3(N). This 0.3 factor overestimates the
value determined by weighting the values for
two dosimeters; however, it provides a better
estimate of the dose than the value reported for
the unshielded dosimeter worn at the neck. This
neck dosimeter can also be used to estimate eye
dose when protective eyeglasses are not worn. A
ring dosimeter is worn to estimate the extremity
dose, with the dosimeter facing the beam (ie, toward the palm of the hand) on the hand that is
likely to be exposed (25).
Considerations for Pregnant
Workers
●
Regulations.—Because of the relationship between the NRC and the CRCPD in creating the
SSRCR, NRC rules regarding pregnancy have
found their way into state regulations for x-ray
workers. Foremost in consideration is the right
to privacy for the individual: She is not required
to make known that she is pregnant to her employer, even if it is obvious that she is. Special
rules about dose limits for the conceptus apply
only after a written, voluntary, official declaration that includes the estimated date of conception.
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These rules stipulate that (a) the occupational dose to an embryo or fetus of an occupationally exposed individual must not exceed 5
mSv (0.5 rem) during the entire gestation period; (b) any dose must be received relatively
uniformly over time, so that typical doses are
not high in any one particular phase of gestational development; and (c) the deep-dose
equivalent of the worker must be used as the
dose to the embryo or fetus (ie, the value reported from an unshielded dosimeter worn on
the trunk of the worker) (26). For x-ray workers who wear protective aprons and one dosimeter outside the shielding, use of the deep-dose
equivalent clearly results in a gross overestimate of the dose to the conceptus. NCRP Report No. 116 recommends “a monthly equivalent dose limit of 0.5 mSv [equivalent to 50
mrem] to the embryo-fetus (excluding medical
and natural background radiation) once the
pregnancy is known” (8).
For personnel wearing protective clothing,
the dose to the embryo or fetus is estimated
much more appropriately from a dosimeter
worn beneath the shielding. Some states have
separate rules for pregnant x-ray workers. In
Florida, for example, all radiation workers are
required to wear one dosimeter, and a radiation
worker with a declared pregnancy is required
to wear an additional monitor underneath protective clothing (27). This rule should be followed by all pregnant radiation workers who
sometimes or routinely wear protective aprons.
Supporting Data.—Table 3 shows data correlated between unshielded neck and shielded
waist dosimeters for individuals working in special procedures (ie, angiography and interventional radiology) at Shands Hospital at the University of Florida for a total of 343 personmonths. It can be seen that most shielded waist
dosimeter values were less than 0.05 mSv (50
mrem) in 1 month, even for some individuals
with very high unshielded neck values (0.41–23
mSv [410–2,300 mrem]). Five person-months
exhibited waist values of 0.6 mSv (60 mrem),
and only one exceeded 0.6 mSv (60 mrem).
These high values were received by radiology
fellows who were performing angiographic and
Volume 19 Number 4
Table 3
Correlation of Dosimetry Data in Fluoroscopically Guided Interventional Procedures*
Shielded Waist (mSv [mrem])
Unshielded Neck (mSv [mrem])
<0.1
(<10)
0.1–0.3
(10–30)
0.4–0.5
(40–50)
>0.5
(>50)
<1.0 (<100)
1.1–4.0 (110–400)
4.1–10.9 (410–1,090)
11.0–23.0 (1,100–2,300)
158
52
5
1
51
9
14
0
2
2
6
2
0
6†
2
3
Note.—Values in parentheses are in millirems.
*Data were collected at Shands Hospital at University of Florida, Gainesville, for 343 person-months.
†
Suspicious data.
interventional procedures continuously for
more than 40 hours a week. It can be seen that
typical shielded waist doses are well within limits for pregnant personnel.
Table 3 also shows that some data are suspicious, in that some shielded and unshielded values are more similar than could reasonably be
expected. This fact points to a potential problem with personnel wearing two dosimeters—
that individuals can become confused in wearing them properly. One way to minimize this
confusion is to color-code the dosimeters. This
institution, for example, uses a yellow indicator
exclusively for waist dosimeters, which can be
remembered by the mnemonic “yellow belly.”
Maternity Aprons.—Commercially available
maternity aprons are wide and have a double
thickness of lead in a location that is appropriate for many pregnant individuals. Other workers may want to add additional shielding to an
apron that they prefer wearing. In either case,
the wearer needs to ensure that her unshielded
back is not facing the x-ray source: Not only is
there no shielding, but the dosimeter measures
the radiation field after it has been attenuated
by the conceptus.
Aprons with additional protection may not
be desirable for everyone because the extra
weight may be burdensome. The data for Table
3 indicate that, even for individuals working in
interventional radiology suites, shielded waist
values exceeded 0.02 mSv (20 mrem) only for
some individuals who performed the procedures full-time. In no case did the values for any
July-August 1999
nurse or technologist exceed this value, and no
maternity aprons were worn during the period
of data collection. These data can be compared
with data for personnel who work in typical
fluoroscopy suites and perform more conventional procedures: Monthly unshielded neck values were typically 0.3–0.6 mSv (30–60 mrem),
and shielded waist values were less than 0.1
mSv (<10 mrem).
All workers wore aprons with 0.5 mm lead
equivalence, in accordance with recommendations of NCRP Report No. 102 (1). Some individuals who work farther from the patient than
the fluoroscopist might prefer less shielding
and less weight from a thinner apron. However,
storing aprons of different thicknesses in the
same area might lead to a misunderstanding on
the part of an individual in choosing one to
wear. Therefore, aprons different from the
norm should be clearly identified. In any case,
state regulations regarding minimum apron
thickness need to be followed.
Maternity Policies.—In addition to regulations and guidelines, each employer needs to
have policies in place, not only about personnel
pregnancy but also for family leave in general. A
1986 report from the American Association for
Women Radiologists (AAWR) (now the American Association for Women in Radiology) on
the impact of maternity on radiologists was
published as an editorial (28). This report states
that scientific findings do not support limiting
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occupational fluoroscopy during pregnancy,
and expresses concern that women might be
subject to job discrimination if they require special assignments because of pregnancy.
A 1992 article in the American College of Radiology Bulletin recommends that written policies are necessary for pregnant workers in radiology (29). The AAWR performed a survey in
1993 on maternity policies in radiology departments, and recommendations on policies were
published in a 1995 report (30). Written maternity policies are recommended because workers need to know (a) what is expected of them
and (b) that they will receive unbiased consideration.
Decisions about working in a radiation environment need to be made by both the employer in setting up facility policy and by the
employee in making personal choices. Decisions are best made with an understanding of
the facts, both scientific and regulatory. The following three resources can help with understanding the issues: (a) NCRP Report No. 116
(8), which presents risks and scientific guidance; (b) NRC draft regulatory guide DG-8014,
proposed revision to Regulatory Guide 8.13
(31), which describes risks and rules; and (c) a
1982 review publication by Wagner and Hayman (32), which evaluates various radiologic
work environments.
■
RADIATION SAFETY POLICIES
●
Personnel Dosimetry
Personnel dosimetry policies need to be in
place for all occupationally exposed individuals,
regardless of pregnancy. All employers are required to provide a dosimeter to all employees
who have a likelihood of receiving doses as
large as 10% of the applicable legal limit (eg,
whole body, lens of the eye, extremity). It is assumed that all personnel who perform x-ray
procedures can potentially receive doses sufficiently large to require at least one dosimeter
for monitoring whole-body dose. Fluoroscopists
may need additional dosimeters.
All employees are required to wear their dosimeters: It is up to the employer to see that
these rules are followed. With regulations in
place that allow determination of effective dose
equivalent from weighting factors for partialbody irradiation, personnel are able to stay
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Imaging & Therapeutic Technology
within regulatory limits without fudging (ie, not
wearing dosimeters part of the time). No excuses are acceptable: It is reasonable for a facility not to allow an employee to spend any time
(ie, work) in a radiation area unless a dosimeter
is worn properly. To do otherwise puts both
the employer and the employee at risk.
Occupationally exposed individuals need to
understand that the dosimeter provides a number that has regulatory and legal implications
and that the dosimetry report is a legal document. The data are reliable only when the dosimeters are properly worn, receive proper
care, and are returned on time. Proper care includes not irradiating the dosimeter except during occupational exposure and ensuring proper
environmental conditions.
●
Detectability Thresholds
Dosimetry reports are prepared from dosimeters that are evaluated in dosimetry laboratories. The National Institute of Standards and
Technology has a voluntary accreditation program for dosimetry laboratories that is designed
to ensure that certain standards are met in dose
assessment and reporting. All types of dosimeters have potentially variable responses for
dose assessment. They also have a lower limit
of detectability (LLD), which is the lowest
value that can be reported with a specified certainty. The LLD depends on the type of dosimeter and the type and energy of radiation, and
some new technologies have improved assessment of the LLD.
One concern with the LLD relates to pregnant workers. If the lowest reportable value is
0.01 mSv (10 mrem) and if the dose received
is just below that level, it will be reported as
a minimal value (ie, the smallest reportable
value). This minimal value should not be assumed to be equal to zero. When a more precise number is desired, a more sensitive dosimeter should be selected. This issue is typically
not critical with current work environments
and regulations; however, if dose limits were to
be lowered, technical problems might develop.
In a potentially high-dose environment, a
pregnant radiation worker might wear an electronic self-reading dosimeter for short-term assessment in addition to a monthly dosimeter. In
this way, she can monitor her dose for a particular task and ensure that she terminates her
work or leaves the environment when the instrument value reaches a particular threshold.
Volume 19 Number 4
●
Practices of ALARA
Personnel Dosimetry.—Local policies are
likely to uphold ALARA principles for occupational radiation doses. At many institutions,
ALARA policies include two sets of criteria for
evaluation: An upper level is set to the legal annual or, in some cases, quarterly limit, and a
lower level, and perhaps a monthly level, is set
as a local standard for assessing values against
those that are typical and expected. Investigating causes for unexpected findings is part of an
ALARA program, and investigations should include looking for missing data or values that are
inappropriately low as well as for causes of
high values.
operate fluoroscopes can be an effective method of oversight (33).
Continuing Education.—Personnel need to
be aware of techniques that they can employ to
minimize their doses as well as those of personnel working nearby. Using available shielding,
particularly with certain overhead x-ray tube
configurations, can be important. Collimating
to the patient anatomy of interest not only reduces scatter but also improves image contrast.
As technology develops, new types of procedures offer possibilities of potential benefits as
well as hazards that need to be evaluated. Continuing education is key.
■
Quality Control.—When an unexpectedly
high dosimetry value is returned for an individual, ALARA implementation implies that an
attempt is made to explain the cause: Sometimes the reading is faulty, sometimes it relates
to a change in work habits, and sometimes it relates to equipment malfunction. Therefore, in
addition to monitoring reported dosimetry values, which relate to activities of individual
workers, ALARA implementation includes quality control assessment of equipment and policies on equipment use.
Equipment needs to meet certain standards
for producing radiation, and regulations and
guidelines apply. For the safety of fluoroscopists, the most important criteria are that the
primary beam be limited in size to the primary
barrier and that the exposure rate be limited to
known levels with adequate image quality. Because fluoroscopic imaging systems degrade
over time, ALARA implementation includes periodic assessment of tabletop air kerma (or exposure) rates and associated image quality.
Minimizing the radiation used in each patient
procedure with careful use of the fluoroscopic
exposure switch (ie, beam-on time) and reducing repeated studies simultaneously minimize
personnel doses.
Some radiology personnel (eg, technologists
and nurses) are exposed to scattered radiation
from fluoroscopic procedures that are performed by physicians who are not radiologists.
This situation occurs frequently in operating
rooms and hospital areas not within the radiology department. It is important for all physicians who operate fluoroscopes to be educated
and trained in principles of radiation safety and
equipment operation and about the risks to patients. Hospital credentialing of physicians to
July-August 1999
SUMMARY
In summary, ALARA policies can be effected
through understanding and using the key parameters time, distance, and shielding. Typically, reducing patient dose also reduces dose
to personnel. Therefore, performing optimized
procedures is an important aspect of radiation
protection in diagnostic radiology. Optimization of procedures includes performing only
medically necessary studies and performing
them sufficiently well that they do not need to
be repeated. Optimization also means ensuring
that the patient dose for each study is as low
as is commensurate with the medical charge.
Using the best geometric configuration for fluoroscopic procedures (good collimation and
proper distances among the x-ray tube, patient,
and image intensifier) optimizes image quality
and reduces the hazard from scattered radiation. Using last-image hold and pulsed fluoroscopy reduces exposure times in procedures,
and operating the fluoroscope only while viewing the monitor minimizes wasted radiation.
Similarly, using frame rates and series durations
that are as low as diagnostically acceptable in
serial radiographic and cine studies reduces
the overall time of patient irradiation. In these
ways, both patients and personnel receive
ALARA radiation doses.
In addition, personnel can concentrate on reducing personal doses. One means is through
judicious use of barriers, that is, protective
clothing and structural shielding such as control booths or mobile transparent shielding to
interpose between oneself and the patient
(who is the main source of the exposure). Another is through keeping the distance between
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1053
oneself and the patient as large as is reasonable,
while not interfering with work efficiency.
Valid assessment of personal dose can be
made only by wearing personal dosimeters
properly and by returning them on schedule.
Rules are state-specific regarding dose limits for
x-ray workers, and institutional policies may be
stricter. Wearing aprons is a regulation; wearing
additional protection may be an issue of local
policy, or it may be a personal choice.
Special rules apply for pregnant workers,
and key aspects of these rules include privacy
and fairness. Although facility policies need to
be established for an institution, declaration of
pregnancy is a personal issue that needs to be
decided with appropriate information by the affected individual.
Personnel need to be aware of the hazards
associated with excessive exposure to radiation. As technology develops, work habits shift,
and similarly, regulations change with time.
Continuing education in radiation protection is
an important aspect of maintaining personal
and institutional ALARA policies.
Acknowledgments: The author thanks Stephen
Balter, PhD, for use of his excellent graphics; Manuel
Arreola, PhD, and Debra Neill-Mareci, MA, for assistance in graphic preparation; and Linda Waters-Funk
for excellent manuscript preparation assistance. She
is grateful to James Kereiakes, PhD, for his encouragement during the preparation of this material for
presentation in the 1998 RSNA/AAPM Physics Tutorial for Residents and is especially thankful to have
been able to do so.
■
APPENDIX
The following publications are listed as useful
sources of information on their respective topics.
Quantities and units.—Bushberg JT,
Seibert JA, Leidholdt EM Jr, Boone JM. The essential physics of medical imaging. Baltimore,
Md: Williams & Wilkins, 1994.
Radiation safety philosophy and
ALARA.—National Council on Radiation Protection and Measurements. Implementation of the
principle of as low as reasonably achievable
(ALARA) for medical and dental personnel.
NCRP report no. 107. Bethesda, Md: NCRP,
1990.
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Imaging & Therapeutic Technology
Principles of x-ray safety.—Wagner LK,
Archer BR. Minimizing risks from fluoroscopic
x rays. 2nd ed. The Woodlands, Tex: Partners in
Radiation Management, 1998.
Working with radiation during pregnancy.—National Council on Radiation Protection and Measurements. Limitation of exposure
to ionizing radiation. NCRP report no. 116.
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