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CHAPTER 12 Management of Patient Radiation Dose During X-Ray Procedures
247
if needed, but also suitable entertainment and distraction items such as cartoon posters and puppets. The
examination efficiently progresses with the best
chance of patient cooperation when the child feels less
intimidated.
Collimation
Precise collimation is especially significant in pediatric
studies because of the increased possibility of late tissue
effects occurring in these young patients as compared to
in older patients. The automatic collimation system is
designed to reduce the radiation field size to the dimensions of the image receptor, but because many pediatric
patients are significantly smaller than the image receptor, further manual adjustment of collimation is often
necessary. As in any other radiographic study, limiting
the field size to the anatomic features of interest not
only reduces patient exposure but also enhances the
quality of the completed image by decreasing scatter.
Wise selection of projection orientation is also essential.
Female patients who may be imaged in either a PA or
AP projection will receive significantly lower doses to
the breast tissue if imaged in a PA projection.
21
Patient Protection in Computed Tomography for Adults and Children: Similarities and Necessary Changes
Unfortunately, many facilities have, in the past, routinely used the same technical exposure factors for both
adults and small children and continue to do so today.
There has been an apparent unwillingness to develop
new scanning protocols because of the conflicting demands of multiple pediatric protocols. Mindful of the
overall enhanced vulnerability of children to ionizing
radiation, all facilities and imaging personnel must
make every conscious effort to develop and use lowdose pediatric protocols that are in the best interest of
the children entrusted to their care.
IMAGE GENTLY CAMPAIGN
An initiative of the Alliance for Radiation Safety in
Pediatric Imaging (as discussed in Chapter 1) is the
Image Gently Campaign. The goal of this campaign is
to change long-established practice by raising awareness
about methods for lowering radiation dose during pediatric medical imaging examinations (Alliance for Radiation Safety in Pediatric Imaging, 2016.)22 The Image
Gently website, www.imagegently.org, provides infor-
mation about pediatric imaging examinations for RTs,
medical physicists, radiologist, pediatricians, and par-
ents. Also, of value to RTs, the site includes protocols for
reducing pediatric radiation dose during digital radiog-
raphy, fluoroscopy, computed tomography examina-
tions, interventional radiology procedures, and nuclear
medicine studies. Radiographers and imaging facilities
can “pledge” to Image Gently (see Appendix B for the
pledge). As of 2020, over 64,000 RTs have taken the
pledge.
IMAGE WISELY CAMPAIGN
A second initiative of the Alliance for Radiation Safety
in Pediatric Imaging (as discussed in Chapter 1) is the
Image Wisely Campaign. This campaign promotes
lowering the amount of radiation used in medically
necessary imaging procedures and eliminating unnec-
essary procedures in adult medical imaging. The Im-
age Wisely Campaign includes on its website, www
.imagewisely.org, information on radiography, fluo-
roscopy, computed tomography, and nuclear medicine
examinations for radiologic technologists, medical
physicists, radiologists, referring physicians, and all
adults. Radiographers can pledge to Image Wisely (see
Appendix B for the pledge). Over 40,000 RTs have
taken the pledge.
DUAL ENERGY X-RAY ABSORPTIOMETRY
(DEXA, or DXA SCAN)
Of all naturally occurring materials in the human body,
bone has the highest physical density. Bone is composed
of numerous layers of tissues of various densities and
contains a high calcium content that accounts for its
overall mass and general strength. Cortical, or compact,
bone that comprises the outer layer of bone is exception-
ally dense, while trabecular, or cancellous, bone is com-
posed of a spongy, less solid, or more porous inner layer.
The size and shape of bones change as a person grows.23
A marked loss of bone mass, however, commonly occurs
in many individuals as aging progresses. This condition
is termed, osteoporosis, a systematic metabolic bone dis-
order.24 Women older than 50 years are primarily af-
fected by bone density loss, often resulting in pathologic
fractures. Lately, it has also been noted that the condition
unexpectedly has become more prevalent in younger
248
CHAPTER 12 Management of Patient Radiation Dose During X-Ray Procedures
adults and children. Methods have been developed to
determine the degree of bone loss, by measuring bone
mineral density (BMD). The most widely used test to
accomplish this is called Dual-Energy X-Ray Absorpti-
ometry (DEXA, or DXA Scan).25 The DEXA scan is a
noninvasive x-ray procedure that can quantitatively predict the risk of bone fracture(s). For this scan two different low-energy level x-ray beams are utilized. Since
denser bone absorbs more of the low-energy radiation
than does less dense bone, the ratio of the transmission
of the lower and higher energy beams in various osseous
(bony) locations is used to calculate average scores for
the densities of the patient’s bone in those locations.25
The results can then be evaluated by comparison with
normal value ranges. An example of a DEXA scan is
shown in Fig. 12.10.
As mentioned previously, in terms of radiation safety,
the benefit of exposing a patient to radiation in terms of
diagnostic information obtained must outweigh any possible risk of biologic damage resulting from the exposure.
When compared with other imaging procedures (e.g.,
conventional x-ray images, computed tomography), the
A B
Fig. 12.10 DEXA images of the spine and hips. (From Lupsa BC,
Insogna K: Bone health and osteoporosis, Endocrinol Metab Clin
North Am 44(3): 517–530, 2015.)
DEXA scan results in substantially less radiation exposure for the patient. It is therefore considered a nonharmful radiation procedure.26 As a result, DEXA scanning
does not require protective shielding for the technologist
or patients undergoing this procedure.
23
S U M M A R Y
• Effective communication with the patient is the first
step in holistic patient care.
• Imaging procedures should be explained in simple
terms.
• Patients must have an opportunity to ask questions
and receive truthful and clear answers within ethical
limits.
• Adequate immobilization of the patient is neces-
sary to eliminate or at least minimize any voluntary
motion.
• Restraining devices are available to immobilize either
the whole body or the individual body part to be
imaged.
• Involuntary motion can be compensated for by short-
ening exposure time with an appropriate increase in
mA and by using very-high-speed image receptors.
• Protective shielding may be used to reduce or elimi-
nate radiation exposure of specific radiosensitive
body organs and tissues.
• The guidelines for use of protective shielding with
newer digital equipment in light of more recent
analyses of radiation risks have been recently reexam-
ined by professional societies and scientific advisory
groups. Current recommendations now discourage
routine use of patient gonadal shielding and fetal
shielding.
• Appropriate technical exposure factors for each examination that ensure a diagnostic image of optimal
quality with minimal patient dose must be selected.
• Standardized technique charts must be available
for each x-ray unit to help provide a uniform selection of technical exposure factors. High kVp and
lower mAs should be chosen whenever possible to
reduce the amount of radiation received by the
patient.
• When digital images are acquired, correct postprocessing is essential to produce a high-quality diagnostic image.
• Imaging departments should establish a quality control program that ensures standardization in the acquisition and postprocessing of digital images.
• An air gap technique can be used as an alternative to
the use of a mid-ratio grid (8:1).
• Repeat radiographic exposures must be minimized
to prevent the patient’s skin and gonads from receiving a double dose of radiation.
CHAPTER 12 Management of Patient Radiation Dose During X-Ray Procedures
249
• Radiographic examinations are to be performed only
when patients will benefit from useful information
gained from the procedure. Nonessential radiologic
examinations should not be performed.
• The amount of radiation received by a patient from
diagnostic imaging procedures may be specified as
entrance skin exposure (ESE) (including skin and
glandular), gonadal dose, or bone marrow dose.
• ESE is the most straightforward quantity to obtain
and thereby the most widely used.
• The estimated genetically significant dose (GSD) for
the population of the United States is approximately
0.20 mSv.
• Fluoroscopically guided positioning is an unethical
and unacceptable practice that leads to increased
patient radiation dose.
• “Abdominal radiologic examinations that have been
requested after full consideration of the clinical status
of a patient, including the possibility of pregnancy,
need not be postponed or selectively scheduled.”
16
• The NCRP recommendations for elective abdominal
examinations of women of childbearing years states
that such examinations should be performed during
the first few days after the onset of menses to minimize the possibility of irradiating an embryo.
17,18
• A radiographer must carefully question female
patients of childbearing age regarding any possibility of pregnancy before they undergo an x-ray
examination.
• If irradiation of an unknown pregnancy occurs, a
calculated estimate of the appropriate equivalent
dose to the embryo-fetus as a result of the examination should be obtained. A radiological physicist
performs this estimation.
• Children are much more vulnerable to late effects of
radiation than are adults.
• Use a PA projection to protect the breasts of female
patients.
• Adequate collimation of the radiographic beam to
include only the area of clinical interest is essential,
and effective immobilization techniques should be
used when necessary. The use of a high mA station
and a short exposure time also helps minimizes patient motion effects.
• A developing embryo-fetus is especially sensitive to
exposure from ionizing radiation.
• Use the smallest technical exposure factors that will
generate a diagnostically useful radiographic image,
carefully collimate the beam to include only the
anatomic area of interest, and cover the lower abdomen and pelvic regions with a suitable contact
shield if they do not need to be included in the examination.
• The goal of the Image Gently Campaign is to change
practice by increasing awareness about methods to
lower radiation dose during pediatric medical imaging examinations.
21
• Radiographers and imaging facilities can pledge to
Image Gently.
• Radiographers can pledge to Image Wisely.
• The most widely used test to measure bone mineral
density is Dual Energy X-Ray Absorptiometry
(DEXA, or DXA scan).
• When compared with other imaging procedures
such as conventional x-ray radiography or computed
tomography, the DEXA scan results in much less radiation exposure for the patient and is therefore
considered a non-harmful radiation procedure.
25
G E N E R A L D I S C U S S I O N Q U E S T I O N S
1. How does the patient benefit from effective commu-
nication with the radiographer during an imaging
procedure?
2. What can the radiographer do to eliminate the
problem of voluntary patient motion, and how can
involuntary motion be compensated for during
radiography?
3. What recent changes have occurred in patient go-
nadal shielding and fetal shielding protocol?
4. Why should a radiographer use a standardized
technique chart to select technical exposure factors
before performing an imaging procedure?
5. Why are correct radiographic image postprocessing
6. How does an air gap technique reduce scattered
7. What are the benefits of clear, concise patient in-
8. When is a radiographic examination considered
and the establishment of a quality control program
important for imaging departments that use digital
imaging display equipment?
radiation?
structions prior to the beginning of a radiographic
examination?
nonessential? Give some examples.
250
CHAPTER 12 Management of Patient Radiation Dose During X-Ray Procedures
9. Describe three ways in which the amount of radia-
tion received by a patient from diagnostic imaging
procedures may be specified.
10. What does the genetically significant dose take into
consideration?
11. Why is it unacceptable to use fluoroscopically
guided positioning?
12. How should irradiation of an unknown pregnancy
be handled?
13. What are the goals of the Image Gently Campaign
and the Image Wisely Campaign?
14. When does the NCRP recommend the scheduling
of elective examinations?
15. How do children compare with adults with regard
to the potential for biologic damage from exposure
to ionizing radiation?
R E V I E W Q U E S T I O N S
1. As a consequence of their anatomic location, the female
reproductive organs receive about _____________
exposure during a given radiographic procedure involving the pelvic region than do the male reproductive
organs.
A. Three times less
B. Three times more
C. Ten times less
D. Ten times more
2. In fluoroscopy, how is the amount of radiation that a
patient receives usually estimated?
A. By having the patient wear an optically stimu-
lated luminescence (OSL) dosimeter during the
procedure
B. By measuring the radiation exposure rate at ta-
bletop and multiplying this by the milliamperage
(mA) and kilovoltage (kVp) settings
C. By measuring the radiation exposure rate at ta-
bletop and multiplying this by the fluoroscopy
time
D. By placing an ionization-type survey meter next
to the patient during the procedure to record the
dose received
3. As part of the Image Gently Campaign, radiogra-
phers and imaging facilities can pledge to:
A. Image Gently
B. Image Wisely
16. What precautions should be taken by a radiographer
who must perform a radiographic examination on a
pregnant patient?
17. What is the position of the American College of
Radiology (ACR) regarding abdominal radiologic
examinations of pregnant or potentially pregnant
patients?
18. What are the benefits of using a pediatric-designed
x-ray room for young children?
19. How is a thermoluminescent dosimeter (TLD) used
to measure skin dose?
20. What is meant by standardized exposure techniques?
21. What accounts for the overall mass and strength of
bone?
22. How is bone mineral density (BMD) determined?
C. Image a patient only once in any given year
D. Image a patient only when that individual is in
danger of expiring
4. In which of the following projections will a young
female patient receive a significantly lower dose to
her breast tissue during a chest x-ray study?
A. AP
B. AP lordotic
C. PA
D. Lateral
5. A woman who is 3 months pregnant has been in a
motor vehicle accident. The emergency room physician suspects there is injury to her cervical spine and
feels justified in ordering an x-ray examination to aid
in determining the extent of the patient’s injury. Because the patient is pregnant, the radiographer should:
1. Select the smallest technical exposure factors
that will produce a diagnostically useful image
2. Adequately and precisely collimate the radiographic beam to include only the anatomic
area of interest
3. Shield the patient’s lower abdomen and pelvic
region with a suitable protective contact shield
A. 1 only
B. 2 only
C. 3 only
D. 1, 2, and 3
CHAPTER 12 Management of Patient Radiation Dose During X-Ray Procedures
251
6. Pediatric patients require special consideration and
appropriate radiation protection procedures because they are much more vulnerable to which of
the following?
A. The late effects of radiation
B. Only the late somatic effects of radiation
C. Only the genetic effects of radiation
D. Only the early somatic effects of radiation
7. The use of the PA projection during a juvenile scolio-
sis radiographic examination results in which of the
following?
A. Higher entrance exposure dose to the anterior
body surface, thereby significantly increasing the
dose to the breast
B. Lower entrance exposure dose to the anterior
body surface, thereby significantly reducing the
dose to the breast
C. Poorer-quality images that necessitate a repeat
examination
D. Images that do not adequately demonstrate
spinal curvature
8. The most widely used test to determine the degree
of bone loss by measuring bone mineral density is:
A. A Nuclear Medicine bone scan
B. Dual Energy X-Ray Absorptiometry
C. Routine X-Ray with an injection of a positive
contrast medium
D. Routine X-Ray with an injection of a negative
contrast medium
9. Which of the following examinations are consid-
ered unnecessary radiologic procedures?
1. Chest x-ray study as part of a preemployment
physical
2. Screening mammography
3. Whole-body multislice spiral CT screening
A. 1 and 2 only
B. 1 and 3 only
C. 2 and 3 only
D. 1, 2, and 3
10. If a maximum of 500 people inhabited an island
and each person were to receive an equivalent dose
(EqD) of 0.005 Sv gonadal radiation, the gross genetic effect would be _______ the effect occurring
if 50 individual inhabitants were each to receive
0.05 Sv of gonadal radiation and no equivalent dose
were received by other inhabitants.
A. Greatly different from
B. Slightly different from
C. Almost the same as
D. Identical to
13
Special Considerations on Safety in Computed Tomography
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.
• Explain why computed tomography (CT) examinations
are of greater concern in terms of radiation safety in
comparison with routine radiographic examinations.
• List two concerns that relate to patient dose in CT
scanning and explain each.
• Compare the entrance exposure from a CT
examination with the entrance exposure from a
routine fluoroscopic procedure.
• Describe how interslice scatter affects the radiation
dose to a patient.
• State the reason why direct patient shielding is not
necessary during a CT examination.
• For spiral, or helical, CT, state how patient dose is
affected when pitch ratio is adjusted.
• Identify various methods for reducing patient dose
during a CT examination.
• Discuss the concept of computed tomography dose
parameters.
• Explain how to calculate the effective CT dose.
• State the goal of CT imaging from a radiation
protection point of view.
• Identify a simple equation for the calculation of a
CT scan effective dose (EfD).
• Explain how x-ray beam collimation for a singleslice CT scanner (SSCT) is accomplished.
• Identify the difference between detectors used for a
single detector CT scanner versus a multidetector
scanner.
• State the most direct advantages of multidetector
computed tomography (MDCT) scanners over
SSCT scanners.
• With MDCT, identify the composition of slices.
• With MDCT, state the correlation to slice number.
• Describe what occurs as reconstructed slice
thickness decreases.
• Identify the four major sections, or compartments,
of the heart.
• Explain the difference between the diastole phase
and the systole phase of the cardiac cycle.
• Identify what properties modalities must have to
obtain useful images of a rapidly beating heart.
• In electrocardiogram (ECG) gated imaging, explain
the details of the electronic link with the CT
scanner and the final result that occurs.
• In the cardiac cycle, determine what must be
transmitted for heart beats to occur at a regular
pace.
• Identify the two components in spatial resolution in
CT scanning.
• Identify the primary method used to increase
vascular contrast resolution in CT scanning.
• List the factors affecting temporal resolution, spatial
resolution, and contrast resolution for CT scanning.
C H A P T E R O U T L I N E
Patient Dose in Computed Tomography
Radiation Exposure
Concerns Related to Patient Dose: Skin Dose and
Dose Distribution
Direct Patient Shielding
Helical, or Spiral, Computed Tomography
252
Methods for Reduction of Patient Dose in CT
Tube Current Modulation
Iterative Reconstruction
Optimization of Tube Voltage
Patient Centering
Computed Tomography Dose Parameters
CHAPTER 13 Special Considerations on Safety in Computed Tomography
253
Effective Computed Tomography Dose
Multidetector Computed Tomography
(MDCT)
MDCT Collimation, Slice Width, and Slice
Number
MDCT Advantages
Slice Thickness and Reconstruction Interval
K E Y T E R M S
axial images
computed tomography (CT)
contrast resolution
coronary heart disease (CHD)
diastole phase
dose distribution
dose parameters
effective detector thickness
Some types of radiographic examinations require special
consideration for radiation safety. This chapter addresses
these concerns for computed tomography.
electrocardiogram (ECG)
filtered back projection
helical, or spiral, CT
image noise
interslice scatter
iterative reconstruction
pitch ratio
prospective gating
PATIENT DOSE IN COMPUTED TOMOGRAPHY
Radiation Exposure
Computed tomography (CT) is “the process of creating
a cross-sectional tomographic planar image of any part
of the body.”1 This computer-reconstructed digital image is formed by combining an activated x-ray tube
coupled with a precise arrangement of multiple detectors rotating around a specific area of anatomy. CT was
previously referred to as computed axial tomography
(CAT) because the first generation of scanners produced only axial images.* The term computed tomogra-
phy is now more appropriate since “images can now be
*Axial images are images of the plane of the body perpendicular to the long (craniocaudal) axis of the patient. In spiral, or
helical, image acquisition, the patient table moves during the
acquisition so that a three-dimensional data set is obtained.
The speed of table advance and the speed of rotation of the
x-ray source are usually different and determine how much
data are acquired (i.e., how densely the patient is sampled).
Slower table speed allows thinner slices of the patient to be
reconstructed.
Computed Tomography Cardiovascular Imaging
(CT CVI)
Basic Heart Anatomy and Processes
Phases of the Cardiac Cycle
CT Cardiovascular Imaging (CT CVI)
CT CVI and Radiation Dose
Summary
reconstruction interval
skin dose
spatial resolution
systole phase
temporal resolution
tube current modulation
recreated in multiple other planes.”1 Because CT has
become a much more frequently employed diagnostic
x-ray imaging modality and is considered to be a relatively high radiation exposure examination, the patient
dose resulting from exposure to CT ionizing radiation is
of great significance. Currently, CT is of even more concern because of the increasing use of multislice spiral
(helical) CT scanners utilizing thin* slice thickness.
Thin or sub-millimeter slices require an increase in the
selected tube milliamperage per slice to reduce the adverse effects on image quality of ever-present random
noise. With higher radiation exposure to the patient,
there is an increased associated cancer risk. For this
reason, physicians ordering such procedures must weigh
the benefits of the procedure for the patient in terms of
medical information gained and determine whether
these benefits outweigh the risk.
Concerns Related to Patient Dose: Skin Dose and Dose Distribution
Two concerns relate to patient dose in CT scanning.
One concern is the skin dose, and the other is the dose
distribution during the scanning procedure. The radia-
tion at the edge of a beam does not decrease to zero
immediately (i.e., there is no sharp cutoff of radiation at
*Some typical very thin slice thickness values are 0.5 mm and
0.625 mm.
254
CHAPTER 13 Special Considerations on Safety in Computed Tomography
the beam boundary); some extra radiation is delivered
at the edge of the slice. Consequently, the skin dose for
a succession of adjacent scans will be higher than the
skin dose from a single scan. Neighboring slices contribute some overlapping dose from both sides.
The skin dose from a CT scan is generally smaller than
that for a radiographic or fluoroscopic image of the same
region of the body. This is because the use of multiple
views from many angles reduces the need for any individual view to acquire enough x-ray exposure to achieve
an acceptable image. Also, the use of smaller “field size” or
collimation of any one view in CT as compared with radiography or fluoroscopy reduces scatter dose.
The dose distribution within the tissue irradiated in a
CT scan is not the same as the dose distribution occurring in routine radiologic procedures (Fig. 13.1). In radi-
ography or fluoroscopy, skin dose at the patient’s entrance surface is much higher than at the exit surface. In
abdominal imaging, for example, the entrance skin dose
is approximately 100 times the exit dose. For CT, because
the tube rotates around the patient, the dose is much
more uniform throughout the patient. Because CT scanners use a tightly collimated x-ray beam, the amount of
scatter radiation generated is lower than the scatter produced by the less tightly collimated radiographic beam.
Also, because of this, the mass of human tissue exposed
to radiation falls off rapidly outside the plane of interest
during the production of any given scan. Although in a
single-slice scanner, only one cross-sectional tomographic plane (slice) is exposed and imaged at a time,* a
small overlap of the margins of the x-ray beam occurs
*Note: In current scanners, multiple slices (e.g., 4, 8, 16, 32, 64, 128,
256, and even as many as 320)
1
can be acquired simultaneously.
1
2
when every single tomographic section is made. When a
series of adjacent or contiguous slices is obtained, some
radiation will also scatter from the slice being made into
the adjacent slices (interslice scatter). Both of these factors contribute to dose increase and are the reasons why
a succession of adjacent tomographic sections (slices)
imparts a higher absorbed dose than would a single tomographic section.
Direct Patient Shielding
Direct patient shielding is not typically used in CT because the rotational nature of the exposure makes a
shield no more effective than the collimators that already exist on the device. Since the beam is so tightly
collimated to the slice thickness, any significant exposure to the anatomy outside the field of view is usually
caused only by internal scatter. Therefore generally, in
CT, anatomy does not appear in the primary x-ray beam
unless it is part of the intended field of view.
Helical, or Spiral, Computed Tomography
Helical, or spiral, CT presents a more substantial chal-
lenge for assessing patient dose than conventional CT.
Helical CT may be defined as a data acquisition method
that combines a continuous gantry rotation with a selectable speed of continuous table advance, forming a
spiral path of data acquisition (Fig. 13.2).
Such types of scans are often characterized by the
quantity pitch ratio or just pitch. Pitch is the ratio of the
movement or advance of the patient couch during a CT
scan, also known as table increment (I), to the x-ray
beam collimator dimension (Z). Mathematically, it is
expressed as the proportion I/Z. When the spiral scan
pitch ratio is approximately 1, the patient dose is comparable to that produced by non-helical, or axial, CT.
However, when the pitch ratio is higher (e.g., 2:1), the
patient dose is reduced in comparison with axial CT
because the same number of x-rays produced during
each rotation of the tube is spread out over a larger area
of the patient. The reverse also is true: patient dose increases at a pitch less than one (Fig. 13.3).
1
2.5
3 cGy
Fig. 13.1 Typical distribution of the doses deposited in a single-
slice computed tomography examination. For multiple contiguous slices, the doses may be twice these values.
METHODS FOR REDUCTION OF PATIENT DOSE IN CT
Box 13.1 provides a list of several dose reduction meth-
ods that lead to optimization of patient dose in CT.
These methods are discussed in the sections that follow.
CHAPTER 13 Special Considerations on Safety in Computed Tomography
Gantry
Gantry
Gantry
Gantry
Conventional CT Helical CT
Fig. 13.2 Axial versus spiral (helical) CT scanning.
255
rotation 1
Pitch = 1 Pitch = 2
Gantry
rotation 2
Fig. 13.3 Illustration of different pitch value scan sequences.
rotation 1
BOX 13.1 Optimization of Patient Dose
in CT
• Tube current modulation
• Longitudinal
• Angular
• Iterative reconstruction
• Optimization of tube voltage and other scan
parameters
• Correct patient centering
Tube Current Modulation
Recognizing that the x-ray tube current (mA) determines
the rate of x-ray output from the tube, CT manufacturers
utilize patient anatomical information obtained from an
initial “scout view” to vary or modulate the current as the
tube moves along the longitudinal axis of the patient,
usually referred to as the z-axis. Lower tube current is
used in regions where there is decreased attenuation because the anatomy is thinner or less dense (e.g., the thorax), and higher tube current is used in regions where
there is more attenuation due to the anatomy being
thicker (e.g., the abdomen) (Fig. 13.4). This practice will
rotation 2
Pitch = 0.5
Average mA
per rotation
Fig. 13.4 X-ray tube current as a function of position superim-
posed on a CT projection radiograph illustrating the principle of
longitudinal dose modulation. (From McCollough CH, Bruesewitz
MR, Kofler JM: CT dose reduction and dose management tools:
overview of available options, Radiographics 26(2):503–512,
2006.)
rotation 1
Gantry
rotation 2
Longitundinal (z) modulation
Bone
(shoulder)
Air
(lungs)
Scan distance (z)
Soft tissue
(stomach, liver,
spleen)
reduce patient dose in regions where fewer photons are
needed to acquire an image of acceptable quality. Tube
current modulation is sometimes referred to as auto-
matic exposure control (AEC) for CT in comparison with
the AEC systems that are used in radiographic and fluoroscopic imaging systems.
Slice 1
Slice 2
Slice 3
Slice 4
256
CHAPTER 13 Special Considerations on Safety in Computed Tomography
Tube current may also be varied as the x-ray tube
rotates about the patient to reduce dose to radiationsensitive organs such as the breast that reside on the
anterior surface of the patient rather than the midline.
Angular-based tube current modulation reduces tube
current while the x-ray tube is on the anterior side of
the patient and maintains or increases dose as it rotates
on the posterior side. This method has been shown
to underdose the superficial anterior organs without
adversely affecting image quality.
2
In the past, external shields impregnated with a radiopaque material such as bismuth have been used to
reduce the dose to organs near the patient’s surface.
However, tube current modulation techniques have
been found to be more effective and have less effect on
image quality.
3,4
Iterative Reconstruction
Originally, CT scan data were reconstructed from data
acquired during the scanning process through a technique
known as filtered back projection* that was applied only
a single time.
*Filtered back projection: The standard method of reconstructing CT slices is called back projection. This involves reflecting back the projection across the image at the angle it
was acquired. By reflecting back all of the projections associated with a complete revolution of the x-ray tube and detector system, a composite image (i.e., a summary of more and
less intense detector signals (represented by different gray
levels) can be built up, producing a reconstructed image. This
image looks similar to the real picture but is blurry. To correct
the blurring problem created by standard back projection, a
filter is superimposed on all of the back projections, thereby
producing what is known as a filtered back projection image.
Filtering refers to changing the projection data before doing
the back projections to remove blurriness. The simplest filter
employed is a high-pass filter, or a sharpening filter. This type
of filter preferentially picks up sharp edges within the projection (and thus, in the underlying slice) and tends to ignore
flat areas. Because the high-pass filter creates negative pixels
at the edges, it subtracts out the extra smearing caused by
plain back projection. Thus the end result is a more accurate
reconstruction. The high-pass filter, however, accentuates the
noise already present in a CT image. To obtain a “cleaner”
image, other “softer” filters have been developed to replace
the high-pass filter for images that do not have very high
contrast (with appreciation from xrayphysics.com/ctsim.
html).
Recent advances in computer technology allow data
to be acquired with lower patient dose. CT scan data
can also be reconstructed, modified, reconstructed
again, modified, etc., until an image with the lowest
noise possible is obtained. Such a repetitive process is
referred to as iterative reconstruction. With filtered
back projection alone, decreasing tube current may
decrease dose, but at a price of increased quantum
noise in the final image. Iterative reconstruction techniques, however, can be used to reduce dose to levels
below those of filtered back projection while maintaining acceptable noise levels.
5,6
Optimization of Tube Voltage
Changing x-ray tube voltage changes patient dose in CT
as it does in radiography. However, the effect is the opposite. In radiography, increasing kVp tends to decrease
patient dose because exposure time can be decreased. In
CT, all other factors being equal, increasing kVp tends
to increase patient dose.
7
In general radiography, increasing kVp lowers the
number of photoelectric interactions in the patient because the occurrence probability of photoelectric interactions decreases as photon energy is increased (,1/E3).
So more photons pass through the patient to reach the
image receptor, and the exposure time may be decreased.
However, in CT, because of the higher effective energy of
the x-ray beam (due to its greater filtration), both photoelectric and Compton interactions contribute substantially to the image. Thus if kVp is increased, the output
of the x-ray tube increases during a set scan time, and the
patient exposure due to the higher prevalence of Compton interactions actually increases. Controlling the variables of kVp, scan time, and tube current is a major part
of most CT protocols for optimum use of patient dose.
Patient Centering
Verifying that the center of the patient coincides with
the center of the CT gantry is a significant part of dose
reduction strategies that are under the control of the
CT technologist. Miscentering of the patient by only a
few centimeters can result in unnecessarily large differences in patient dose when the tube current modulation system, or AEC, is in use. If the patient is placed
closer to the x-ray tube when the scout view (radiographic) image is obtained, then the image of the patient is magnified, causing the tube current modulation
system to increase the tube current, thereby overdosing
8