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CHAPTER 15 Management of Imaging Personnel Radiation Dose
307
A
B
Scattered
C
Collimator
X-ray
tube
Fig. 15.16 When the protective factors of distance and shield-
ing have been accounted for, the radiographer will receive the
least amount of scattered radiation by standing at a right angle
(90 degrees) to the scattering object (the patient) (in position A).
The most scattered radiation would be received at point C
because of backscatter coming from the patient. (Intensity, or
quantity, of x-ray exposure at any given point is indicated in this
picture by the number of scattered x-rays reaching that point.)
radiation
Primary
(useful)
beam
Image
receptor
If possible, the radiographer should also attempt to
stand at a right angle (90 degrees) to the x-ray beam–
scattering object (the patient) line. This is the location
at which the least amount of scattered radiation is received (Fig. 15.16). However, because distance and
shielding have much more influence on the reduction of
exposure to the technologist, those factors should be
addressed first.
PROTECTION DURING C-ARM FLUOROSCOPY
Personnel Exposure Resulting From Scattered Radiation
Safety procedures are particularly important when mobile fluoroscopy (C-arm) systems are used. Because patterns of exposure direction are less predictable and the
equipment is frequently operated by physicians whose
training and experience in radiation safety may not
match those of an experienced radiologist, the radiographer should exercise special vigilance. For C-arm devices
LATERAL X-RAY TUBE ORIENTATION
IMAGE INTENSIFIER
LOW
SCATTERED
RADIATION
AREA
Fig. 15.17 Cross-table exposure during use of a C-arm fluoro-
scope. The exposure rate caused by scatter near the entrance
surface of the patient (the x-ray tube side) exceeds the exposure
rate caused by scatter near the exit surface of the patient (the
image intensifier side). The location of lower potential scatter
dose is on the side of the patient away from the x-ray tube (i.e.,
the image intensifier side). (From Mark Rzeszotarski.)
LOW
SCATTERED
RADIATION
AREA
HIGH
BACKSCATTER
REGION
HIGH
BACKSCATTER
REGION
X-RAY
TUBE
with similar fields of view, the dose rate for personnel
located within a meter of the patient is comparable to
that in routine fluoroscopy—approximately several milligray in air (mGya) per hour. The exposure of personnel
is primarily caused by radiation scattered from the patient. During operating room procedures in which crosstable exposures are used (Fig. 15.17), an understanding
of the patterns of x-ray scatter is particularly useful. The
exposure rate caused by scatter near the entrance surface
of the patient (the x-ray tube side) exceeds the exposure
rate caused by scatter near the exit surface of the patient
(the image intensifier side). The difference in the amount
of scatter, typically a factor of 2 or 3, is caused by the
higher radiation intensity at the entrance surface of the
patient. Thus, the location of the lower potential scatter
dose is on the side of the patient away from the x-ray
tube (i.e., the image intensifier side). The radiographer
should never encounter the actual useful beam.
Need for Protective Apparel for All Personnel and Monitoring of Imaging Personnel
The C-arm fluoroscope can be manipulated into almost
any position and remain energized for lengthy periods
to accommodate, for example, an orthopedic surgeon
performing an open reduction of a fractured hip in the
operating room or a vascular surgeon performing an
interventional procedure. When radiographers and
308
CHAPTER 15 Management of Imaging Personnel Radiation Dose
other medical personnel participate in procedures that
require this unit to be energized for significant durations, they are subject to increased radiation exposure.
Also, the physical configuration of a C-arm fluoroscopic
unit limits the methods that can be used to achieve protection from scattered radiation. For this reason, personnel who routinely operate a C-arm fluoroscope or
those who are in the immediate area of the unit when it
is energized must wear a lead apron instead of crowding
behind a portable lead–acrylic shield. This garment
should be 0.5 mm lead equivalent to ensure adequate
protection. A neck and thyroid shield of 0.5 mm lead
equivalent should also be worn. Appropriate monitoring of imaging personnel who are typically involved in
C-arm fluoroscopic procedures is mandatory.
Positioning of the C-Arm Fluoroscope
The positioning of a C-arm fluoroscope influences the
radiation exposure to the patient and the radiographer.
From the perspective of increased radiation safety, it is
best to reverse the C-arm to place the x-ray tube under
the table and the image intensifier over the table (see
Fig. 11.23).
Exposure Reduction for Personnel
At the start of each procedure, the unit’s cumulative
timer should be reset to zero so that it will be possible to
be aware of the actual beam-on time.3 When an image
storage device (e.g., last image hold) is used, beam-on
time decreases, and therefore exposure reduction increases. If the image intensifier (II) is positioned as close
to the patient as possible, the required fluoroscopic x-ray
beam intensity is minimized. This equipment–patient
arrangement also permits the image intensifier to function more effectively as a scatter barrier between the
patient and the person operating the C-arm fluoroscope.
All these methods can lead to significant exposure reduction to both personnel and patient.
During a C-arm procedure, it is imperative that the
patient’s anatomic region of interest be oriented correctly with minimal use of “positioning” the fluoroscope.3 Furthermore, collimating the x-ray beam to the
smallest anatomical area possible will decrease the
amount of scattered radiation produced from the interaction of the x-ray beam with the patient, the actual
scattering object.
Because distance from the source of radiation is the
simplest method of protection for occupationally exposed
personnel, C-arm operators should use it to their advantage whenever possible. Usually, this can be accomplished
by using the foot pedal or the handheld exposure switch
with the cables extended away from the machine as far as
possible when making x-ray exposures.
For better visualization of small body parts, C-arm
fluoroscopes can magnify the image. However, the use
of magnification usually requires higher mA, which
produces additional radiation exposure. Mag mode
should be used only upon request by the physician performing the procedure.
3
PROTECTION DURING HIGH-LEVEL CONTROL INTERVENTIONAL PROCEDURES
Increased Importance of Radiation Safety Techniques
All of the previously mentioned standard precautions
and procedures for the reduction of dose to personnel
are applicable during interventional procedures. Here,
these techniques take on increased importance because
of the extended length of some of these procedures,
the large number of digital images that may be taken,
and, in particular studies, the frequent use of the highlevel control (boost) mode of operation. In boost mode,
the exposure rate may significantly exceed the rate used
in routine fluoroscopy (e.g., maximum allowed entrance
exposure rate dose to a patient in regular fluoroscopy is
8.8 cGY/minute [or 10 R/min exposure rate], whereas in
high-level or boost mode this value can range upward to
20 to 40 cGy/minute).
Knowledge of Dose-Reduction Techniques Required by the Radiographer
Although the duration of the procedure and the number
of the exposures taken are under the control of the radiologist or other interventional physician, the radiographer
should verify that all dose-reducing features are available
and in good working order. These include the presence of:
• High-quality, low-dose fluoroscopy mode
• Pulsed beam operation (e.g., using 7.5, 15, or 30 radia-
tion pulses/second in place of continuous fluoroscopy
radiation)
• Manual collimation
• Correct beam filtration
• Removable grids
CHAPTER 15 Management of Imaging Personnel Radiation Dose
309
• Road-mapping*
• Time interval differences
†
• Last image hold mode
As previously discussed, using the last image hold
mode feature, the operator does not need to be exposed
again to review the position of a catheter in relation to
landmarks when no new information is required. Also,
if possible, the patient beam entry side could be changed
during the procedure to reduce the total dose to any one
area of skin.
High-level control is to be used sparingly and only
when increased visualization is necessary during a critical maneuver such as the deployment of stents4 (tubular
support device that can be inserted into a blood vessel,
canal, or duct to relieve obstruction). Many standard
and C-arm interventional fluoroscopic systems now
possess the technical capability for standardized dose
*Road-mapping is a method of digital image subtraction in
which the frame that contains the greatest amount of contrast
material in vessels is identified and is then subtracted from all
subsequent images. Live fluoroscopic images of the catheter
moving through the vasculature can then be seen even after
the vessels contain less contrast. By using this equipment feature, overlaying of two images can be accomplished (e.g., a
stored image and a current image). Thus, there is a decrease of
procedure time because fewer mask images (masking refers to
a two-step imaging process that can be used to remove or
obscure unwanted areas from the final or useful image by
subtracting from a current image an initially or subsequently
acquired reference image) are needed. This leads to some reduction in radiation dose. Choosing the road-mapping feature in place of cineradiography (a technique in which a camera is used to record continuous images, of nonstationary
internal body structures, that are generated during selected
intervals [e.g., 10 seconds] of nonstop radiation exposure) can
also result in a lower radiation dose.
†
Time-interval difference (TID) is a method of digital image
subtraction in which each TID image results from subtracting
a previously acquired regular image from one that is a few
frames in advance (e.g., if a time-interval difference of five
images is chosen, the first TID image to appear will be that
obtained when frame one is subtracted from frame six. The
second image will contain the subtraction of frame two from
frame seven, etc.). This technique progressively reveals vessels
containing material and suppresses soft tissue in the images. It
is less sensitive to patient motion than when the first image is
selected as the “mask” or reference image and is then subtracted from all successive images.
structure reporting.5 These systems automatically furnish reports that yield an equivalent dose record for
every examination.
How the Radiologist or Other Interventional Physician Can Reduce Radiation Exposure
The radiologist or other interventional physician can
reduce radiation exposure by the following means:
• Decreasing the duration of the procedure, thereby
shortening fluoroscopic beam-on time
• Obtaining fewer digital images
• Reducing the use of continuous fluoroscopic mode
relative to the pulsed mode of operation
• Retaining the protective curtain, if available, on the
image intensifier or leaving the scatter shield in place
during a procedure
• Regularly using the last image hold feature to view
the most recent fluoroscopic image
These practices will substantially decrease exposure
not only to all participating personnel but to the patient
as well.
Extremity Monitoring
Because the hands and forearms of physicians performing interventional procedures can be subjected to significant radiation exposures if the safety protocol is not
carefully followed—and sometimes this may not be
possible—it is essential that extremities be monitored.
Physicians need to be aware of the recommended dose
limits that have been established for extremities. The
NCRP currently recommends an annual EqD limit to
localized areas of the skin and hands of 500 mSv (50 rem).
To avoid remotely approaching this limit and consequently significantly increasing the possibility of future
adverse effects, protective gloves should be worn whenever feasible by any physician whose hands will, of necessity, often be close to the fluoroscopic beam.
DIAGNOSTIC X-RAY SUITE PROTECTION DESIGN
Requirement for Radiation-Absorbent Barriers
To reduce the EfD to radiographers, nonoccupationally
exposed personnel, and the general public to levels
deemed statistically safe by both federal and international bodies, every radiography room must be equipped
310
Leakage
CHAPTER 15 Management of Imaging Personnel Radiation Dose
BOX 15.3 Radiation-Absorbent Barrier
Design Considerations
• The mean energy of the x-rays that will strike the barrier
• Whether the barrier is of a primary or a secondary
nature
• The distance from the x-ray source to a position of
occupancy 0.3 m from the barrier
• The workload of the unit
• The use factor of the unit
• The occupancy factor behind the barrier
• The intrinsic shielding (e.g., tube housing attenuation)
of the x-ray unit
• Whether the area beyond the barrier is “controlled”
or “uncontrolled”
with radiation-absorbent barriers. The design of these
barriers is based on considerations listed in Box 15.3.
Reason for Overshielding
The shielding design for radiographic rooms must include all of the factors identified in Box 15.3 to meet
necessary radiation protection standards. In addition,
the designer should plan conservatively to satisfy future
regulatory limits that may be more stringent. These
considerations and the ALARA principle are why many
diagnostic x-ray facilities are overshielded. Spending
extra money up front for additional shielding is far
easier and much less expensive than adding shielding
after the suite has been completed.
Radiation Shielding Categories
As mentioned in previous sections, three categories of
radiation sources are generated in an x-ray room. They
are classified as follows:
1. Primary radiation
2. Scatter radiation
3. Leakage radiation
The last two categories are collectively known as
secondary radiation.
Primary Radiation. Primary radiation emerges directly
from the x-ray tube collimator (Fig. 15.18) and moves
without deflection toward a wall, door, viewing window,
and so on. Because of this property, primary radiation,
also is known as direct radiation, is energy that has not
been degraded by scattering, and substantial portions of
Steel
housing
Insulating oil
Fig. 15.18 Primary radiation emerges directly from the collimator
and spreads throughout the room.
Leakage
Fig. 15.19 Scatter radiation emerges from the patient and
spreads in all directions.
Primary
beam
Useful
beam
radiation
Scatter
the initial beam may not have been attenuated. Therefore, a wall in the path of direct radiation requires the
most protective shielding to ensure the safety of personnel and the public. In a typical x-ray suite, the most
crucial primary radiation barrier is that behind the wall
Bucky unit.
Scatter Radiation. Recall that scatter radiation results
whenever a diagnostic x-ray beam passes through matter. Compton interactions between the x-ray photons
and the electrons of the atoms within the attenuating
object deflect x-ray photons from their initial trajectories. As a result, photons emerge from the object in all
directions (Fig. 15.19). Scattered radiation is greatly reduced in intensity relative to the incident beam. It also
is substantially weakened in energy and consequently in
penetrating power. The amount of shielding required to
protect against scatter radiation is, therefore, usually
much less than that for primary radiation. In general,
the patient is the major source of scatter radiation.
CHAPTER 15 Management of Imaging Personnel Radiation Dose
311
Leakage Radiation. Leakage radiation is radiation gener-
ated in the x-ray tube that does not exit from the collimator
opening but instead partially penetrates the protective tube
housing and also, to some degree, the sides of the collimator (see Fig. 15.18 and 15.19). Leakage radiation is there-
fore always present in some amount. When shielding is
planned for a secondary barrier, the potential contributions from leakage radiation must be added to those from
the scatter radiation reaching that barrier.
Calculation Considerations
Workload. Because a diagnostic x-ray unit does not pro-
duce radiation 24 hours a day, 7 days a week, a parameter
that reflects the unit’s radiation-on time has been used in
the determination of barrier shielding requirements. The
quantity, workload (W), is essentially the radiation output-
weighted time that the unit is delivering radiation during
the week. It has also been called the unit’s duty cycle. Work-
loads are specified either in units of milliampere-seconds
(mAs) per week or milliampere-minutes (mA-min) per
week. The following example illustrates this concept.
Example: A radiographic x-ray suite is in operation
5 days per week. The average number of patients per day
is 20, and the average number of images per patient is 3.
The mean technical exposure factors are 90 kVp, 300 mA,
and 0.1 sec. Find the weekly workload.
Solution:
( ) ( )
W 300 mA 0.1 sec 5 days/wk
( ) ( )
20 patients/d a y 3 images/patient
9000 mAs/wk
150 mA min/wk
Note that the kVp is not used in the workload calculation. It is, however, an essential parameter in the calculation of barrier-shielding thickness. (This will be
demonstrated later on in an example illustrating the
calculation of shielding for a wall in an x-ray suite.)
Inverse Square Law. Just as the perceived brightness of
a light source decreases with distance from its origin, the
intensity of an x-ray beam is lessened as the displacement from its source increases. The ISL, introduced
earlier in this chapter, is the mathematical relation describing this property and is a fundamental component
of radiation protection. As such, the ISL plays a significant role in the design of radiation safety barriers. An
example of its use is shown here.
Example
: At a distance of 1 m from an x-ray tube
target, the dose rate measured by a radiation survey
meter was 4.5 mGy per hour. What would that instrument read if it were moved back an extra 2 m?
Solution: ISL is mathematically given by the follow-
ing proportion:
5
(d )
2
2
1
II(d )
122
If the given data are substituted into the relation and
cross-multiplied, the following result is obtained:
2
3 I 4.5 1
2
9 I 4.5
2
I 0.5 mGy/hr
2
2
This result demonstrates a large reduction in radiation intensity. Its direct consequence is a greatly reduced
barrier shielding thickness requirement.
The inverse square law is built into the combined
mathematical and empiric (i.e., experimentally derived)
formulas that determine primary barrier thickness values
and secondary barrier thickness values. Because these
relations are formulated to give answers for broad-beam
attenuation* rather than just for a very localized area, the
ISL effect is slightly less than it would be for an idealized
situation. A short discussion with an example illustrating
the usage of this for a primary barrier is presented in the
following pages. Before this discussion proceeds, however,
several other concepts of fundamental importance in the
design of appropriate shielding have to be introduced.
Use Factor. If radiation, whether primary or secondary,
is never directed at a particular wall or structure, then
conventional or existing construction is sufficient. Most
structures in a diagnostic x-ray suite, however, are
struck by radiation to some degree for some fraction of
the weekly beam-on time. The use factor (U) was introduced to delineate this fractional contact time.
For primary radiation, the use factor represents the
portion of beam-on time that the x-ray beam is directed
at a primary barrier during the week. Consider a typical
radiographic suite with a wall Bucky unit. If 50% of the
*As defined in NCRP Report 147 (see below), broad-beam
attenuation refers to that effect occurring when the field area
is large at the barrier and the point of measurement is near the
barrier’s exit surface.
312
CHAPTER 15 Management of Imaging Personnel Radiation Dose
x-ray examinations involve this device, the wall behind
the Bucky unit has a U (primary) 5 1/2.
Because scatter and leakage radiation emerge in all
directions in the x-ray room, every wall, door, viewing
window, and other surface will always be struck by some
quantity of radiation. Therefore U (secondary) 5 1 for
all radiation-accessible structures. Furthermore, if a
particular wall is considered a primary barrier and its
required shielding is designed on that basis, then in
virtually all situations no supplementary shielding need
be added for the secondary radiation that may also be
striking this barrier.
Table 15.2 presents the most current recommended
use factor values. U also can be referred to as the beam
direction factor.
Occupancy Factor. Radiation barriers are installed to
protect personnel and the general public from radiation
exposure. If no one will ever be present beyond an existing wall in a particular area while the x-ray unit is being
operated, the addition of extra shielding to that wall is
unnecessary. For that location the shielding design
TABLE 15.2 Use Factors Recommended
by the International Commission on
Radiological Protection
Use Factor Primary Barrier
Full use (U 5 1) Floors of radiation rooms except
dental installations, doors, walls,
and ceilings of radiation rooms
exposed routinely to the primary
beam
Partial use
(U 5 ¼)
Occasional use
1
(U 5
)
16
From International Commission on Radiological Protection
(ICRP): Report of Committee III on protection against x-rays
up to energies of 3 MeV and beta and gamma rays from
sealed sources, ICRP Publication No. 3, New York, 1960,
Pergamon Press.
Doors and walls of radiation rooms
not exposed routinely to the
primary beam; also floors of
dental installations
Ceilings of radiation rooms not
exposed routinely to the primary
beam; because of the low use
factor, shielding requirements for
a ceiling usually determined by
secondary rather than primary
beam considerations
statement would be that existing construction is sufficient. An example of this is an outside wall facing a
courtyard that “always” has no occupancies. The opposite extreme is an area in which someone is always present. When planning radiation protection shielding for a
diagnostic x-ray suite, the designer must consider not
only zero and full occupancy cases but also the more
common partial occupancy situations. The occupancy
factor (T) is used to modify the shielding requirement
for a particular barrier by taking into account the fraction of the work-week during which the space beyond
the barrier is occupied. Table 15.3 lists the latest recom-
mended values for T.
Controlled and Uncontrolled Areas. If a region adjacent
to a wall of an x-ray room is to be used only by occupationally exposed personnel (e.g., radiographers), that location
is designated a controlled area. Conversely, a nearby hall
or corridor that is open to and frequented by the general
public, is classified as an uncontrolled area. For the latter,
the weekly maximum permitted equivalent dose (MPED)
is equal to 20 microsieverts (20 µSv or 2 mrem); for
controlled areas, it is a much more significant amount:
1000 µSv or 1 mSv (100 mrem). The main reason for
allowing this huge disparity lies in the fact that the occupationally exposed population makes up only a tiny fraction
of the overall population. Therefore, the potential for detrimental radiobiologic effects in the general public as a
whole (i.e., its genetically significant dose [GSD] value) as
a result of the higher MPED to occupationally exposed
personnel is statistically negligible. Whether the area beyond a structure is designated as controlled or uncontrolled is clearly very significant in determining the amount
of radiation shielding to be added to that structure. The
following sections discuss the use of these concepts in the
determination of radiation shielding requirements.
Calculating Barrier Shielding Requirements
For each wall, door, and other barrier in an x-ray room
that is to provide protection from radiation, the product
of W 3 U 3 T must be determined. The number of
mA-minutes per week or workload is generally determined by overall utilization of the x-ray unit. In contrast, the use and occupancy factors are typically different among the various barriers. The shielding designer
also must conclude whether the barrier is primary or
secondary and whether the area beyond the barrier is
controlled or uncontrolled.
CHAPTER 15 Management of Imaging Personnel Radiation Dose
313
TABLE 15.3 Suggested Occupancy
Factors*
Location
Administrative or clerical offices; laborato-
ries, pharmacies, and other work areas
fully occupied by an individual; receptionist areas, attended waiting rooms,
children’s indoor play areas, adjacent
x-ray rooms, image reading areas,
nurses’ stations, x-ray control rooms
Rooms used for patient examinations and
treatments
Corridors, patient rooms, employee
lounges, and staff rest rooms
Corridor doors
Public toilets, unattended vending areas,
storage rooms, outdoor areas with
seating, unattended waiting rooms,
patient holding areas
Outdoor areas with only transient pedestri-
ans or vehicular traffic, unattended parking
lots, vehicular drop-off areas (unattended),
attics, stairways, unattended elevators,
janitors’ closets
*For use as a guide in planning shielding where other
occupancy data are not available.
†
When using a low occupancy factor for a room immediately
adjacent to an x-ray room, care should be taken to also
consider the areas farther removed from the x-ray room.
These areas may have significantly higher occupancy factors
than the adjacent room and may therefore be more important
in shielding design despite the larger distances involved.
‡
The occupancy factor for the area just outside a corridor door
can often be reasonably assumed to be lower than the
occupancy factor for the corridor.
Adapted from National Council on Radiation Protection and Measurements (NCRP): Structural shielding design for medical x-ray
imaging facilities, Report No. 147, Bethesda, MD, 2004, NCRP.
†
Occupancy
Factor (T)
1
½
1
5
‡
1
8
1
20
1
40
With the publication of NCRP Report No. 147,6 entitled Structural Shielding Design for Medical Imaging
Facilities, the objective of a shielding calculation is now
described as determining the thickness of a barrier sufficient to reduce the air kerma* in a full or partially occupied area to a value that is less than or at most equal
*Recall that air kerma (Ka) is essentially absorbed dose in air
resulting from the passage of an x-ray beam through it. Its
numeric value is specified in units of gray.
to the ratio P/T where the quantity P refers to the permissible weekly radiation dose (note: for diagnostic
x-rays, dose, and equivalent dose are numerically equal)
to that location, and T is the area’s occupancy factor.
Primary Barrier Calculation. Using material from
NCRP Report No. 147, the combined mathematical and
empiric relation that was briefly mentioned in the section on the ISL is introduced. For primary, or direct,
radiation only, the mathematical expression is given by:
B P d / K NUT
5
2
(
(
)
p
)
r
where B is by definition the broad-beam x-ray trans-
mission factor and is the ratio of air kerma (Ka) behind
a barrier of material thickness “x” to the value of Ka at
the same location with no intervening barrier; dp is the
distance from the x-ray source to a representative location and distance behind the direct radiation barrier
(e.g., one-third of a meter beyond the barrier is typical);
Kr is the average unshielded air kerma per patient at a
reference distance of 1 m from the source; N is the expected
number of patients examined in the room per week; U
and T are, as noted, previously, respectively, the use and
occupancy factors; and P depends on whether the barrier is for a controlled or uncontrolled area. Once the
value of B has been calculated for a particular situation,
then plots (Fig. 15.20) of transmission factors (values
of B) versus attenuating material thickness supplied in
Appendix B of NCRP Report No. 147 may be used to
obtain the required shielding thickness for the barrier.
6
The primary radiation intensity for a selected kVp at a
barrier location for an x-ray suite may be determined by
making air kerma measurements on the suite’s x-ray unit
at a reference distance (e.g., 100 cm) from the x-ray tube
target with the aid of a calibrated ionization chamber. This
information can then be used to determine the amount of
shielding necessary to attenuate the radiation to permissible levels for that x-ray energy. The following example
demonstrates determination of the primary barrier shielding requirement associated with a wall Bucky from an average-usage radiographic room with an uncontrolled area
existing behind the barrier (e.g., a public corridor).
Example:
Given the following specifications, determine the required
shielding:
• The average kVp for the x-rays striking the barrier 5
100.
• P 5 0.02 mGy/wk because of the uncontrolled area.
314
CHAPTER 15 Management of Imaging Personnel Radiation Dose
1.0
8
6
4
2
−1
10
8
6
4
2
−2
10
8
6
4
2
−3
10
8
6
4
Transmission
2
−4
10
8
6
4
2
−5
10
8
6
4
2
−6
10
0.0 0.5 1.0 1.5
Primary Transmission Through Lead
Chest Room
Rad Room
(all barriers)
Lead thickness (mm)
(chest bucky)
Rad Room
(floor or other
barriers)
Rad Room
Rad Tube
(R&F Room)
2.0 2.5 3.0
Fig. 15.20 Assorted plots of primary broad-beam transmission
through lead. (From National Council on Radiation Protection and
Measurements [NCRP]: Structural shielding design for medical
x-ray imaging facilities, Report No. 147, Bethesda, MD, 2004.)
• dp 5 3 m
• 100 patients per week in this x-ray room.
• U 5 1/2
• T 5 1/4
• The measured value of Kr is 6 mGy per mA-min at 1 m
at 100 kVp.
Solution:
Substituting the foregoing information into the expression for the transmission factor “B” gives:
B (0.02) (3) /[(6) (100) (0.5) (0.25)]
2
0.0024
From the graph in Fig. 15.20, using the curve for ra-
diographic room chest Bucky, a shielding requirement of
approximately 1.3 mm lead is obtained. Currently, in the
United States shielding is specified in fractions of an inch
of lead, most commonly, 1/32 and 1/16. The standard
1/32 inch is approximately 0.8 mm, and the standard
1/16 inch is equal to 1.58 mm. Because the calculation of
shielding for the primary barrier required 1.3 mm lead,
then in the United States the conservative choice would
be to install 1/16-inch lead shielding for this barrier.
Secondary Barrier Calculation. Secondary barriers
intercept both scatter and leakage radiation. As has been
mentioned previously, no additional shielding against
secondary radiation is needed for areas already protected
against primary radiation. Because scatter and leakage
radiation emerge in all directions, the use factor U for
these is always 5 1.
Scatter radiation. The intensity and energy of the
scatter radiation at the location of a barrier are generally
unknown because of all the potential variables involved.
Therefore, the following properties, typically, are assumed for the determination of scatter radiation barrier
shielding:
1. The energy of the scatter radiation is conservatively
considered to be equal to that of the primary radiation.
2. The intensity of radiation scattered at 90 degrees at a
distance of 1 m from its source is reduced by a factor
of 1000 relative to the primary radiation for a field
size of 400 cm2 (20 cm 3 20 cm).
The larger the x-ray field dimension at the source of
the scatter radiation (usually the patient), the more significant will be the amount of generated scatter radiation. Also, notably of importance are the primary beam
photon energy and the incident location of the x-ray
beam on the patient. The ISL again plays a vital role in
shielding requirements, but regarding scatter radiation,
however, the distance is measured from the center of the
irradiated portion of the patient (the prime source of
scatter) rather than from the x-ray tube target.
Leakage radiation. Leakage radiation does not
emerge directly from the collimator opening but instead
seeps out or leaks through the x-ray tube housing walls
and also partially penetrates the sides of the collimator
when the x-ray beam is on. Leakage radiation is therefore an additional radiation output component that
shielding designers must consider. Regulatory standards
mandate that the maximum permissible leakage exposure rate at 1 m from the target of a diagnostic x-ray
tube in all directions cannot exceed 100 millitoentgens
(mR) per hour or 0.88 milligray (mGy) air kerma per
hour when the tube is being operated continuously at its
maximal permitted kVp and mA combination.
Because of the attenuation that occurs when leakage
radiation penetrates the tube housing walls, any emerging
radiation is essentially monoenergetic; thus, the basic concept of half-value layer (HVL) may be applied at barriers
to reduce leakage radiation levels to permissible values.
Data tables incorporating this concept have been developed to specify the amount of shielding needed to attenuate leakage radiation sufficiently at various distances from
CHAPTER 15 Management of Imaging Personnel Radiation Dose
315
the x-ray tube. This value should be compared with that
necessary to attenuate scatter radiation satisfactorily. Traditionally, if both shielding requirements do not differ
substantially (i.e., are less than three HVLs of shielding
material apart), then the conservative decision would be
to install a composite that is the sum of the required shield-
ing for each radiation source. However, if the needed barrier shielding for each of the two differs substantially (i.e.,
more than three HVLs), then, again conservatively, the
larger value alone can be used for the shielding. The
following example illustrates the method used in most
existing diagnostic x-ray rooms for determining leakage
radiation shielding requirements.
Example: If shielding must be added to a wall that is
subject only to secondary radiation to protect a controlled area, then find the total thickness of lead needed,
given the following information:
HVL for scatter and leakage radiation: 0.2 mm lead
(Pb) for each. Shielding requirement for scatter radiation alone for a particular barrier: 0.75 mm Pb
Shielding requirement for leakage radiation alone for
that barrier: 0.3 mm Pb
Solution: The difference in barrier shielding requirements for scatter and leakage 5 0.45 mm lead, which is less
than 3 HVL, which equals 3 3 0.2 mm lead, or 0.6 mm lead.
Therefore, the conservative total shielding thickness amount
for the barrier would be 0.75 1 0.3 5 1.05 mm Pb.
Current Approaches to Shielding
Among the most current approaches to shielding design
detailed in NCRP Report No. 147, a more rigorous
workload analysis incorporates the range of kVps actually used. Also, the true role of leakage radiation in
state-of-the-art equipment is now modeled explicitly,
along with scatter. The traditional rule of adding an
HVL if leakage and scatter barrier requirements are
similar has now been abandoned in favor of exact calculations. Use factors at present reflect an actual percentage of the time that the beam is directed at various
barriers. Some existing shielding that was generally ignored in older design calculations, such as the patient
table, Bucky, and image receptor, is included in the new
designs. Finally, the suggested occupancy factors have
been reevaluated to approximate more closely the percentage of the time that workers are expected to be present (see Table 15.3). In NCRP Report No. 49,7 a minimal
occupancy factor of at least 1/16 was assumed. Under
the revised guidelines, occupancy factors for areas such
TABLE 15.4 Brief Summary of National
Council on Radiation Protection and
Measurements Report No. 147 New
Shielding Guidelines
Item New Approach
Workload More realistic use of contemporary
survey data
Leakage and
scatter
Use factor Adjusted for beam direction data
Occupancy
factor
Adapted from National Council on Radiation Protection and
Measurements (NCRP): Structural shielding design for medical
imaging facilities, Report No. 147, Bethesda, MD, 2004, NCRP.
Explicit barrier calculations
reflecting actual usage patterns
Realistic assumptions of occupancy of
low-occupancy areas (e.g., stairwells)
as closets and stairways may be placed as low as 1/40.
Some of the changes in the revision of NCRP Report
No. 49 are listed in Table 15.4.
RADIATION CAUTION SIGNS
The appropriate deployment of radiation signs or electronic notices is an essential component of safety in a radiology department. Specific rules and regulations vary
somewhat according to the jurisdiction of the facility (i.e.,
state, federal, military, etc.). However, the main points
concerning the type of signs and the circumstances under
which posting is required are essentially the same. Each
radiographer should check with the radiation safety officer in the institution where employed, concerning specific
regulations that govern the facility where they work.
Beam-On Indicator Sign
Some states require specific imaging equipment installations, primarily computed tomography (CT) scanners,
to include warning lights that are conspicuous near the
door to the examination room from any corridor. The
sign should read “x-ray beam on” or the equivalent and
be self-illuminating whenever the x-ray equipment is
energized. Some states require an interlock such that
exposure is terminated if the door is opened.
General Posting
Radiation warning signs are posted within controlled
areas of the hospital or facility. They are typically found
316
CHAPTER 15 Management of Imaging Personnel Radiation Dose
BA
Fig. 15.21 Radiation warning signs typically found in a diagnostic radiologic facility. (A) A self-illuminated sign
used outside the door to a computed tomography (CT) scanner. (B) A general radiation warning sign used to
indicate the potential for exposure that may exceed 0.05 mSv at 30 cm from a source of radiation. (A, From
Brian Struble/DigitalMomentStudios.com, © 2017.)
on the door to CT and interventional x-ray rooms, linear accelerator treatment rooms, and storage areas for
radioactive materials. An example of the latter is shown
on the right in Fig. 15.21. Such signs are required to be
magenta or purple or black on a yellow background.
Further specifications may be found in Recommended
State Regulations of the Conference of Radiation Control Program Directors.8 A sign reading “Caution Radiation Area” is usually adequate for rooms containing
S U M M A R Y
• An annual occupational effective dose of 50 mSv for
whole-body exposure during routine operations and
an annual effective dose of 1 mSv for individuals in
the general population have been established.
• A CumEfd limits a radiation worker’s whole-body
cumulative occupational effective dose to his or her
age in years times 10 mSv.
• Radiation workers can receive a higher equivalent
dose than the general public without altering the genetically significant dose (GSD).
• Occupational exposure must be kept ALARA.
• The following methods of reducing scatter radiation also
reduce the occupational hazard for the radiographer:
• Use of beam-limitation devices, higher kVp and
lower mA techniques, appropriate beam filtration, and adequate protective shielding
fixed diagnostic equipment. A radiation area is generally an area in which radiation exposures may exceed
0.05 mSv (5 mrem) in 1 hour at 30 cm from a source.
Other categories of caution signage exist for areas in
which radiation exposures may exceed those encountered in diagnostic radiology. These signs are required in
radiation oncology or nuclear medicine departments
and include the labels: High Radiation, Very High Radiation, Airborne Radioactivity, and Radioactive Materials.
• Proper utilization of protective apparel (lead
aprons, gloves, thyroid shields)
• Reduction of repeat images
• The basic principles of time, distance, and shielding
are to be employed to minimize occupational radiation exposure.
• Pregnant radiographers can wear an additional
monitoring device at waist level to ensure that
their monthly equivalent dose does not exceed
0.5 mSv.
• Primary and secondary protective barriers must be
designed so that annual effective dose limits are not
exceeded.
• A lead-lined, metal, diagnostic-type protective tube
housing protects the radiographer and the patient
from leakage radiation.