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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5196_Библиотеки_им_академика_М_И_Перельмана.pdf
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
Fig. 14.10 Operational views of a DBT system. (From Siemens.)
TABLE 14.1 Physical Characteristics of Five Manufacturer’s Digital Breast Tomosynthesis
10
Units
Manufacturers: Fuji GE Hologic IMS Siemens
Anode Material: W Mo or Rh W W W
Filter Material: Al or Rh Mo or Rh Ag Ag Rh
Detector: a-Se FPD* CsI FPD** a-Se FPD a-Se FPD a-Se FPD
Pixel size (mm): 150/100 100/50 100/140 85 85
Pixel shape: Hexagonal Square Square Square Square
Tube motion: Continuous Step and shoot Continuous Step and shoot Continuous
Sweep angle (°): 15/ 40 25 15 40 50
No. of projections: 15 9 15 13 25
Dose/projection: Uniform Uniform Uniform Variable Uniform
Antiscatter grid: No Yes No No No
*amorphous selenium flat panel detector
**Cesium iodide flat panel detector
287
BOX 14.1 Siemens Mammomat DBT
Twenty-five views are acquired in an angular range from
-25° to 125°. The exposure release button on the control
box or on the foot or hand switch must be pressed and
held during all exposures. Following proper positioning
and compression of the patient’s breast, the steps of the
image acquisition process are:
1. Initially, the swivel arm is in the 0° position.
2. The first view is acquired for automatic exposure control
settings.
3. Swivel arm then moves to -25° and a second exposure
is taken.
4. The swivel arm subsequently covers the entire angular
range from -25° to 125° while an exposure is taken at
every 2° for a total of 50 projections.
5. Swivel arm returns to the start position.
288
Face
shield
Compression
plate
Fig. 14.11 Closeup of breast positioning and detector assemblies.
(From Siemens.)
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
To ensure radiation safety, a restricting face shield must
be used during tomographic examinations (Fig. 14.11).
The face shield is present because there is a risk that the
patient’s head can drift into the x-ray beam path if not
prevented from doing so. Note that the face shield also
moves when the swivel arm moves. The patient’s head
must therefore not lean against the face shield during the
arc rotation.
RADIATION DOSAGE
The x-ray spectrum used for tomosynthesis is generally
similar to that employed in standard digital mammography, and the selected peak kilovoltage (kVp) depends
on the thickness and density of the compressed breast.
If the x-ray energy is increased slightly, thereby sacrificing some degree of image quality, then it is possible to
reduce the absorbed dose just by using less mA. Another option, especially for thicker breasts, for reducing
dose while maintaining image quality, is to have additional filtering of the x-ray beam. This inherently increases the mean beam energy and consequent net ra-
diation penetration. It can be achieved by selecting in
place of the standard molybdenum/molybdenum (Mo/
Mo) or molybdenum/rhodium (Mo/Rh) x-ray target/
filter combinations, other pairings such as tungsten/
rhodium (W/Rh), tungsten/aluminum (W/Al), or
tungsten/silver (W/Ag).
For mammography, a useful and practical implementation of tomography was not possible until the development
of digital flat panel wide field detectors which initiated full
field digital mammography (FFDM). Subsequently, FFDM
was coupled with tomographic motion. With this, any desired image slice and slice thickness could be computer reconstructed from the stored multiple angle projection images,
and all of this was obtained with just one angular sweep of
the x-ray tube.
12,19
The breast is composed of three types of tissue: glandular, adipose (fatty tissue), and skin. Because statistics
stemming from a large database assembled from breast
cancer screenings by many institutions has repeatedly
shown that the development of breast cancer in adipose
tissue is rare, mammographic radiation dosimetry is predominantly concerned with the dose deposited in the
glandular tissue of the breast. Therefore, the parameter
that has been chosen for both standard and 3D digital
mammography to estimate and represent effective dose in
x-ray breast imaging is the mean glandular dose (MGD).
MGD is used to characterize the absorbed dosage to
the radiosensitive fibroglandular breast tissue. Quantitively, it is determined from measurements which employ a special design (thin window circular parallel
plate) ionization chamber to measure the air kerma or
exposure incident on the breast or on a breast-equivalent
phantom. To obtain the MGD from such measurements, the ion chamber readings must be multiplied by
special conversion factors derived from data generated
by complex randomized computer modeling of radiation interaction processes in the breast. These factors
are referred to as the normalized glandular dose coeffi-
cients. For each ion chamber measurement, there is a
particular coefficient value that is directly associated
with the x-ray beam quality (HVL) employed as determined by the x-ray tube target/filter combination and
the selected kVp. Tables for the latest determined coefficients are listed in the current American College of
Radiology Mammography Quality Assurance Manual.
The angular range and number of exposures taken
over the x-ray tube arc during the DBT scan are additional variables that need to be optimized with respect
to balancing patient radiation dose and acceptable image quality. In general, it would seem that taking more
exposures during a DBT procedure will generate reconstructions with fewer artifacts. However, doing this
must be weighed against the consideration that for a
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
289
patient receiving a full mammographic examination*,
it is still desired to limit the total MGD to 3 mGy
(0.3 cGy). More DBT angles will then mandate lesser
irradiation per exposure and consequently smaller detector signals for each of the individual tomographic
projections. For low enough exposures, image receptor
inherent noise will start to become a non-negligible
component of the raw image and thereby may noticeably degrade reconstructed image quality. A greater
number of exposures also increases raw data size which
produces longer reconstruction times.
During a DBT acquisition process, the total delivered
radiation dose is apportioned among the multiple single
x-ray views with every projection contributing about
only 5% to 10% of a normal single-view mammogram
absorbed dose.
12,19
Because each image voxel is ulti-
mately overall probed or irradiated by essentially the
same number of X-ray quanta as in a standard 2D mammography acquisition, a quality tomosynthesis scan, in
total, should deliver approximately the same MGD as a
conventional mammogram. A major prerequisite for
this to be achieved, however, is that the image receptor
has a high detective quantum efficiency** (DQE). Rapid
imaging processing is also another requirement for the
DBT detector.
Usually it is assumed that within a normal breast
there is a homogeneous mixture of adipose and glandular tissue surrounded by a layer of skin. In reality, it is
*A total or full mammographic examination in a facility that
has a DBT unit usually includes the standard stationary 2D
x-ray projections as well as the DBT series of projections.
**Detective quantum efficiency, or DQE, refers to how efficiently a detection system translates incident x-ray photons
into a useful signal relative to random noise within that image.
In medical radiography, the listed DQE describes percentagewise the degree to which an x-ray imaging system can produce
an image with a high signal-to-noise ratio (SNR) relative to
that delivered by an ideal detector whose efficiency is by definition taken to be 100%. This specification can also be considered an alternate measure of the radiation dose efficiency of a
detector, since the needed amount of radiation exposure to a
patient decreases as the system DQE is increased for the same
image SNR and physical exposure conditions. Compared with
film/screen imaging, a digital detector with high DQE has the
potential to deliver significant object-detectability improvements at the same equivalent dose or an identical degree of
detectability at a lower patient dose.
BOX 14.2 Typical Mean Glandular
Dose Values From Breast Phantom
Measurements for Individual and Combined
DBT and Standard 2D Exposures
• Radiation dose for a single standard 2D mammogram is
about 1.2 to 1.4 mGy or 1.3 mGy (130 mrads) on average.
• Radiation dose for a 3D or DBT sequence of projec-
tions is about 1.3 to 1.5 mGy or 1.4 mGy (140 mrads)
on average (no grid)
• Total MGD for the combination is thus about 2.7 mGy
or 270 mrads. (ACR recommends that total MGD not
exceed 3 mGy (300 mrads) per examination)
found that the glandular dose deposited in various regions can differ considerably and that, especially, is why
the mean glandular dose was adopted as a useful and
practical parameter of mammographic absorbed dose.
Some typical numerical values for mammographic procedures are shown in Box 14.2.
DBT SUMMARY
Breast tomosynthesis is a three-dimensional imaging
technology that involves acquiring a dozen or more images of a stationary compressed breast at multiple angles
of incidence during a partial rotational scan of the x-ray
tube about an axis located within the breast. With these
multiple diverse x-ray projections or views, objects at
varying depths in the breast will be projected onto different locations on the detector. At any desired depth in the
compressed breast, a reconstructed plane can be obtained
by using the Shift and Add method or other techniques to
properly combine all registered x-ray projection views for
that location. Thus, in a particular reconstructed or synthesized tomosynthesis plane, a structure that is actually
located at the corresponding depth will be in-focus,
whereas structures lying above or below that plane are
blurred.
Because there is a practical and technical limit to the
angular extent of the projection acquisitions, DBT does
not have isotropic spatial resolution, i.e., there will be a
very high spatial resolution within the planes parallel to
the detector (x-y), and a considerably less fine plane
by plane discrimination in the perpendicular direction
(z-axis). The depth resolution, however, that has been
achieved in present 3D systems is deemed to be good
290
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
enough to substantially reduce the 2D mammography issue of tissue superposition, thereby noticeably lowering its
negative impact on sensitivity and specificity.
23
To keep the radiation dose to the breast from a single
DBT procedure at levels comparable to a standard 2D
mammography breast examination (e.g., a mediolateral
view), the total x-ray tube radiation output is spread out
over multiple projections, so that each individual projection delivers only a small fraction of the radiation
dose of a standard 2D mammogram. This necessitates
the usage of detectors with a high DQE at low air kerma
or exposure levels and has led to the development of
detectors specifically designed for DBT imaging.*
*Besides meeting the ordinary requirements for 2D digital mammography, detectors for DBT need to have additional capabilities.
These include: (i) faster reading time, to keep the total acquisition
time of all projections to a minimum; (ii) minimal ghosting (the
reduction of sensitivity caused by previous exposure history of
the detector) and minimal lag (the carryover of signal from a
previous image); and (iii) minimal reduction in detective quantum efficiency at much lower exposures (a consequence of the
need to divide the total exposure over multiple projections).
23,24
Following irradiation, the individual images are
reconstructed using sophisticated algorithms into a
large number of thin (,1 mm) or much fewer slab-like
(,1 cm) high-resolution slices that can be displayed
individually or in a ciné mode. Note that additional
acquisitions at a different focal distance are not required to enhance the visibility of objects at any desired
depth—one set of acquired data can be reprocessed to
generate the entire 3D volume set. It should always be
kept in mind that “there is no free meal in tomosynthesis -
imaging parameters are a trade-off between z-axis
accuracy, spatial resolution and importantly, radiation
exposure to the breast.”16 In conclusion, digital breast
tomosynthesis provides the following benefits relative
to other modalities:
• Enhances conspicuity of abnormalities by minimiz-
ing the obscuring effects of overlying structures
• Permits cross-sectional imaging (i.e., planes of view)
with high resolution
• A lower radiation dose as compared to CT mam-
mography
• Lower cost as compared with CT and magnetic reso-
nance imaging
S U M M A R Y
• Nonpalpable breast cancer may be detected through
mammography.
• Flattening and reducing breast thickness prior to a
standard mammographic radiation examination is
necessary for producing a sharper image and reducing patient dose.
• Protocols founded upon compression pressure which
takes into account breast surface area will be a more
realistic guide to better and less uncomfortable
mammograms.
• Two x-ray views of each breast (craniocaudal direction
[CC] and mediolateral oblique direction [MLO]) are
taken in a standard screening mammogram evaluation.
• The American College of Radiology, the American
Cancer Society, and the American Medical Association advocate annual mammography screening or
mammography screening at least every other year for
women age 40 to 49.
• Federal regulations state that the mean dose to the
glandular tissue (MGD) of a 4.5-cm compressed
breast using a digital mammography system should
not exceed 3 mGyt per view.
• Studies have shown that well-calibrated mammographic systems are capable of providing excellent
imaging performance with an average glandular dose
of not more than 2 mGyt.
22
• Dose reduction in mammography can be achieved
by limiting the number of anatomical projections
taken.
• Axillary projections in mammography should only
be done on request of the radiologist.
• Metallic elements such as molybdenum (Z 5 42)
and rhodium (Z 5 45) are commonly employed as
filters in mammography enabling lower incident
x-ray energy ranges that are more effective for breast
imaging.
• Beryllium (Z 5 4) takes the place of glass in the window of the low-kVp mammographic x-ray tube.
• Digital mammography units with image gray-level
manipulation offer visualization improvement for
patients with dense breasts.
• The method used for generating an in-focus twodimensional image of a slice or cross-section through
a three-dimensional object is called Tomography.
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
291
• Standard digital mammography can lead to both
false positives and false negatives because the multiple tissues and structures of the compressed breast
appear overlapped in the acquired projection image.
• Digital Breast Tomography (DBT) is an imaging
technique, which generates multiple planar images
acquired while the x-ray tube rotates within a limited
arc above the patient’s breast.
• The introduction of full field digital detector mammography (FFDM) was the primary technical breakthrough that enabled the wide-spread usage of DBT.
• Because in Digital Breast Tomography a third axis
(z-direction) is stereoscopically extracted from the
overall two-dimensional data, DBT is often referred
to as “3D’ mammography when mammographic images are reconstructed at arbitrary angles from the
many different x-ray projections.
• Each DBT arc acquisition sequence may typically
consist of from 15 to 25 separate x-ray projections.
From the data generated by these, many nonblurred
planar views can be obtained either individually or in
a continuous sequence thereby generating a “movie”.
• The ability of DBT to decrease both false positives
and false negatives as well as better image evaluation of dense breasts is due mainly to the enhanced
z-axis resolution achievable by the tomographic
method.
• The conventional method of image reconstruction
used in digital tomosynthesis is the Shift and Add
Algorithm (SAA).
• Artifacts present in DBT are primarily due to patient
motion, the method of image acquisition, and imperfect z-axis or depth resolution.
• With DBT used alone, a compression force only great
enough to securely retain the breast in a stable position during the procedure may suffice.
• For thicker breasts, a method of reducing patient dose
while maintaining image quality is to employ additional filtering of the x-ray beam. This can be achieved
by selecting in place of the standard molybdenum/
molybdenum (Mo/Mo) or molybdenum/rhodium
(MoRh) x-ray target/filter combinations, other pairings such as tungsten/rhodium (W/Rh), tungsten/
aluminum (W/Al), or tungsten/silver (W/Ag).
D I S C U S S I O N Q U E S T I O N S
1. What filters are recommended for use with a molyb-
denum anode when a mammographic examination
is performed on a patient with larger or dense
breasts? Why are they recommended?
2. Why is the age recommendation for screening mam-
mography so controversial?
3. What tissues in the human body compose breast tissue?
4. What is mean glandular dose (MGD) used for?
5. What does detective quantum efficiency (DQE) refer
to?
R E V I E W Q U E S T I O N S
1. Studies have shown that well-calibrated mammo-
graphic systems are capable of providing excellent
imaging performance with an average glandular dose
of not more than:
A. 10 mGy
B. 7 mGy
C. 5 mGy
D. 2 mGy
t
t
t
t
6. How can contrast in the radiographic image be
enhanced during digital mammography?
7. What is digital tomosynthesis?
8. What are the advantages of digital breast tomosyn-
thesis over conventional digital radiography of the
breast?
9. How do artifacts result from the motion of the
x-ray tube during DBT?
10. How can radiation dose for the patient be reduced
during DBT?
2. Digital mammography units, which can enhance
contrast with image gray-level manipulation, offer
substantial improvement for patients with:
A. Thin breast tissue
B. Sparse breast tissue
C. Porous breast tissue
D. Dense breast tissue
292
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
3. Axillary projections of the breast should be done
only at the request of the:
A. Administrator of the imaging facility
B. Patient
C. Radiologist
D. Radiographer
4. What material takes the place of glass in the window
of the low-kVp-producing mammographic x-ray
tube?
A. Aluminum
B. Beryllium
C. Copper
D. Lead
5. For DBT imaging the following is not true:
A. A lesser number of x-ray tube angular projections
leads to a thinner in-focus plane or improved
z-axis resolution
B. Increasing the number of x-ray projections for
image reconstruction increases the blurring of
out of plane structures for a particular plane
C. Using a smaller range of x-ray tube angles yields
an overall sharper breast image
D. All of the above
6. For DBT imaging the following is true:
A. The x-ray portion of the examination procedure
will last at least an hour
B. Because of the movement of the gantry it is nec-
essary that there be greater breast compression
than for 2D mammography
C. The mean glandular radiation dose will exceed
3 mGy because of the many x-ray exposures taken
during the examination
D. None of the above
7. With regard to spatial resolution of breast imaging,
which of the following is correct:
A. DBT is far inferior to CT in x-y plane image
resolution
B. The smaller the DQE value of the unit’s detector,
the better will be the quality of the mammographic image
C. Increasing the number of x-ray tube angular
projections can lead to a more accurate positional display of microcalcifications
D. All the above
8. A discrete image area element is termed a:
A. gantry
B. kerma
C. pixel
D. voxel
9. A 3D image technology that involves acquiring a
dozen or more images of a stationary compressed
breast at multiple angles of incidence during a partial rotational scan of the x-ray tube about an axis
located within the breast is called:
A. Breast tomosynthesis
B. 2D mammography
C. Halo tomography
D. Standard digital mammography
10. Which of the following can reduce dose in standard
mammography?
A. Increasing kVp to a minimum of 70
B. Not applying compression to the breast during
imaging
C. Limiting the number of projections taken
D. Removing filtration from the x-ray tube in a
dedicated mammographic unit
Management of Imaging Personnel Radiation
Dose During Diagnostic X-Ray Procedures
O B J E C T I V E S
After completing this chapter, the reader will be able to
perform the following:
• Define all key terms.
• State the annual occupational effective dose limit
for whole-body exposure of diagnostic imaging
personnel during routine operations.
• Explain why occupational exposure of diagnostic
imaging personnel is limited and state the reason for
allowing a much larger equivalent dose for radiation
workers than for the population as a whole.
• Identify the type of x-radiation that poses the most
significant occupational hazard in diagnostic radiology
and explain the various ways this hazard can be
reduced.
• Explain how the various methods and techniques
that reduce patient exposure during a diagnostic
examination can also minimize exposure for the
radiographer and any other personnel.
• Discuss the responsibilities of the employer for
protecting declared pregnant diagnostic imaging
personnel from radiation exposure.
• Describe the three underlying principles of radiation
protection that can be used for personnel exposure
reduction.
• State and explain the inverse square law, and solve
mathematical problems applying this concept.
15
• Differentiate between a primary and a secondary
protective barrier, and list examples of such
barriers.
• Describe the construction of protective structural
shielding.
• Discuss the protective garments that may be worn
to reduce whole-body or partial-body exposure.
• List and explain the methods and devices that
may be used to reduce exposure for personnel
during routine fluoroscopic and interventional
examinations.
• Specify the techniques that are useful for reducing
the radiographer’s exposure when performing a
mobile radiographic examination.
• Explain the variation in dose rate caused by scatter
radiation near the entrance and exit surfaces of the
patient during C-arm fluoroscopy and discuss dose
reduction methods for C-arm operators.
• List the three categories of radiation sources that may
be generated in an x-ray room, list the considerations
on which the design of radiation-absorbent barriers
should be based, and explain the importance of each.
• Differentiate between a controlled area and an
uncontrolled area.
• Discuss current approaches to shielding design.
• Describe radiation caution signage.
C H A P T E R O U T L I N E
Annual Limits for Occupationally Exposed Personnel
Effective Dose Limits
Annual Occupational and Nonoccupational Dose Limits
Allowance for a Larger Equivalent Dose for Radiation
Workers
ALARA Concept
Dose-Reduction Methods and Techniques
Repeats in Digital Imaging
The Patient as a Source of Scattered Radiation
Scattered Radiation—Occupational Hazard
Filtration of the Diagnostic X-Ray Beam
Protective Apparel
Technical Exposure Factors
Patient Restraint
Protection for Pregnant Personnel
Imaging Department Protocol
293
294
CHAPTER 15 Management of Imaging Personnel Radiation Dose
Acknowledgement of Counseling and Understanding
of Radiation Safety Measures
Protective Maternity Apparel
Work Schedule Alteration
Basic Principles of Radiation Protection for Personnel
Exposure Reduction
Time
Distance
Shielding
X-Ray Tube Housing Cables
Protection During Fluoroscopic Procedures
Personnel Protection
Dose-Reduction Techniques
Remote Control Fluoroscopic Systems
Protective Curtain
Bucky Slot Shielding Device
Rotational Scheduling of Personnel
Protection During Mobile X-Ray Examinations
Use of Protective Garments
Distance as a Means of Protection
Protection During C-Arm Fluoroscopy
Personnel Exposure Resulting From Scattered
Radiation
K E Y T E R M S
Broad-beam x-ray transmission
factor (B)
Bucky slot shielding device
control-booth barrier
controlled area
cumulative effective dose
(CumEfD) limit
distance
inverse square law (ISL)
leakage radiation
occupancy factor (T)
primary protective barrier
primary radiation
scatter radiation
Need for Protective Apparel for all Personnel and
Monitoring of Imaging Personnel
Positioning of the C-Arm Fluoroscope
Exposure Reduction for Personnel
Protection During High-Level Control Interventional
Procedures
Increased Importance of Radiation Safety Technique
Knowledge of Dose Reduction Techniques Required
by the Radiographer
How the Radiologist or Other Interventional Physician
Can Reduce Radiation Exposure
Extremity Monitoring
Diagnostic X-Ray Suite Protection Design
Requirement for Radiation-Absorbent Barriers
Reason for Overshielding
Radiation Shielding Categories
Calculation Considerations
Calculating Barrier Shielding Requirements
Current Approaches to Shielding
Radiation Caution Signs
Beam-On Indicator Sign
General Posting
Summary
secondary protective barrier
shielding
time
uncontrolled area
use factor (U)
workload (W)
Some x-ray procedures increase the radiographer’s
risk of exposure (Box 15.1) due to scatter radiation.
This chapter presents an overview of methods that
can be used to reduce exposure for imaging professionals during diagnostic x-ray procedures. Also, a
BOX 15.1 Imaging Procedures That
Increase the Radiographer’s Risk of
Exposure
• General fluoroscopy
• Interventional procedures that employ high-level control
fluoroscopy (HLCF)
• Mobile examinations
• C-arm fluoroscopy
brief explanation of the design of a diagnostic x-ray
suite is presented.
ANNUAL LIMIT FOR OCCUPATIONALLY EXPOSED PERSONNEL
Effective Dose Limits
Federal government standards, following a recommendation of the National Council on Radiation Protection and
Measurements (NCRP), permit diagnostic imaging personnel to receive an “annual occupational effective dose
(EfD) of 50 millisievert (mSv)”1 for whole-body exposure
during routine operations. However, in keeping with the
ALARA (as low as reasonably achievable) philosophy and
careful supervision of personnel cumulative radiation
CHAPTER 15 Management of Imaging Personnel Radiation Dose
295
exposure records, no radiographer should approach this
EfD level. The dose level referred to here, includes only
occupational dose and does not include personal medical
exposure that an employee may receive or the background exposure that all people receive.
To ensure that the lifetime risk of occupationally
exposed personnel is not exceeded, an additional recommendation is that the lifetime EfD in mSvs should
not exceed 10 times the person’s age in years. Hence a
cumulative effective dose (CumEfD) limit has been
established for the whole body.
Annual Occupational and Nonoccupational Effective Dose Limits
The annual occupational EfD limit of 50 mSv (5 rem) is
an upper boundary limit. It is much higher than the annual EfD limit allowed for individual members of the
general population not occupationally exposed. That
limit is:
• 1 mSv (100 mrem) for continuous or frequent exposures
from artificial sources other than medical irradiation
and natural background radiation
• 5 mSv (500 mrem) for infrequent annual exposure
The 1 mSv annual EfD limit set for members of the
general public is intended to limit that exposure to reasonable levels of risk that are comparable with risks
from other familiar sources—i.e., about 1024 to 1026
annually1 (1024 to 1026 means an excess cancer risk of 1
chance in 10,000 to 1 chance in 1 million per year). The
5 mSv maximum annual EfD limit recommendation is
made because annual exposures above the 1 mSv recommendation, need not be regarded as especially hazardous, provided the average exposure to individuals in
these groups does not exceed an average annual EfD of
about 1 mSv.1 Both these limits will maintain the annual
equivalent dose to organs and tissues below levels of
concern for tissue reactions.
1
1
1
Allowance for a Larger Equivalent Dose for Radiation Workers
Valid reasons exist for permitting radiation workers to
accumulate a larger equivalent dose (EqD). Among the
most important of these reasons is that the workforce
in radiation-related jobs is small when compared with
the population as a whole. Therefore, the expectation
of any measurable increase in disease in the population, in individuals, or impact upon the gene pool is
negligible. Thus, the amount of radiation received by
this workforce can be substantially greater than the
amount received by the general public without alteration in the genetically significant dose, the average
annual gonadal EqD to members of the population
who are of childbearing age. Although the radiographer and other diagnostic imaging personnel are
allowed to absorb more radiation, the EqD received
must be minimized whenever possible, reducing the
potential for:
• Somatic damage
• Genetic damage
ALARA CONCEPT
The best manner for radiographers and radiologists to
conscientiously employ the ALARA principle is to use
all appropriate radiation-control procedures to minimize their exposure levels. Personnel should faithfully
employ procedures such as:
• The principles of time, distance, and shielding
• Adequately collimating the radiographic beam
Fig. 15.1)
DOSE-REDUCTION METHODS AND TECHNIQUES
Methods and techniques that reduce patient exposure
can also reduce exposure for the radiographer, thereby
limiting occupational exposure. Whenever a repeat image is performed because of human or mechanical error,
the patient receives a double dose of primary radiation,
while also increasing the radiographer’s potential for
exposure to scattered radiation.
Repeats in Digital Imaging
Because image contrast and overall brightness in digital
imaging can be manipulated after image acquisition, the
need for all repeats as a result of improper technical
exposure factors has been eliminated. However, repeats
necessitated by mispositioning can still occur, causing
additional radiation exposure to both the patient and
possibly the radiographer. The radiographer must take
the needed time to accurately position the patient prior
to the exposure.
The Patient as a Source of Scattered Radiation
During any diagnostic x-ray examination, the patient
becomes a source of scattered radiation as a consequence
of the Compton interaction process. At a 90-degree angle
296
CHAPTER 15 Management of Imaging Personnel Radiation Dose
decreases the number of x-ray photons available to
undergo Compton scatter. Because scatter is reduced,
the radiographer’s potential for occupational exposure
is decreased.
Filtration of the Diagnostic X-Ray Beam
When a radiographic beam is adequately filtered, nonuseful low-energy photons are removed from the pri-
Collimator
Adjustable lead
shutters
Area of clinical
interest
mary beam. Without proper filtration, a relatively high
percentage of the customarily excluded low-energy
photons will interact with the tissues of the patient’s
body. Some of these photons undergo Compton scatter.
The radiographer’s EqD could, therefore, increase as a
result of exposure to this excess scattered radiation.
Most of these low-energy photons, however, are absorbed in the patient, thereby increasing the patient’s
absorbed dose and contributing no useful diagnostic
information to the image. Thus, filtration primarily
benefits the patient.
Image receptor
Fig. 15.1 Radiographic beam collimation (restricting the x-ray
beam to the area of clinical interest) limits the production of
scattered radiation. This radiation-control procedure helps keep
the radiographer’s occupational exposure as low as reasonably
achievable (ALARA).
to the primary x-ray beam, at a distance of 1 m, the scattered x-ray intensity is generally approximately 1/1000th
of the intensity of the primary x-ray beam. This characteristic should always be kept in mind as an additional
method of radiation protection.
Scattered Radiation—Occupational Hazard
Because scattered radiation poses the most significant
occupational hazard in diagnostic radiology, the use of
any device or imaging technique that lessens the amount
of scattered radiation will significantly reduce occupational exposure of diagnostic imaging personnel. Beam
constraint devices, such as automatic collimation, or
positive beam limitation, restrict the dimensions of the
radiographic beam so that its margins do not extend
beyond the image receptor. This reduction in beam size
Protective Apparel
Protective lead aprons (Fig. 15.2A) and, in their absence,
shielded barriers (Fig. 15.2B) function as gonadal
shields for diagnostic imaging personnel. These devices
protect personnel from scatter and leakage radiation,
which are types of secondary radiation.
Similar to protective gloves that are used to cover the
hands of radiologists or radiographers when they must be
in or near the primary x-ray beam, lead aprons are available in various thicknesses such as 0.25, 0.5, and 1 mm of
lead equivalent.2 Higher lead equivalents in protective apparel provide greater protection from radiation exposure.
However, for practical use in the clinical setting, the
weight of the garment and the approximate length of time
that it will be worn must also be considered. An apron
containing 1 mm lead equivalent may weigh as much as
12 kg.2 Wearing this protective device for a lengthy procedure can, therefore, result in considerable back strain.*
*To reduce the possibility of back or neck problems, other
materials may be used in the protective apron to lessen its
weight, Some garments, for example, are impregnated with
tin2 or similar metals because the electron shell structures of
these substances offer advantages in terms of a more probable
photoelectric interaction attenuation than does lead in the
lower diagnostic x-ray energy range.