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CHAPTER 13 Special Considerations on Safety in Computed Tomography
257
the patient. A miscentering of 2 cm can produce as
much as a 25% unnecessary increase in patient dose.9
Automatic patient miscentering correction software is
becoming available to alleviate this problem.10 Still,
there is no substitute for proper positioning.
COMPUTED TOMOGRAPHY DOSE PARAMETERS
To approximate and characterize the effective radiation
dose to a patient who has undergone a CT study, the
values of two CT-specific dose markers or quantities
need to be determined for each scan series. The following discussion introduces and briefly defines all the relevant parameters, describes how their values can be
obtained and shows their relationships to one another.
The relevant dose parameters are as follows:
• Computed tomography dose index (CTDI)
• CTDIW (weighted CTDI)
• CTDI
• Effective milliampere-second (mAs)
• Dose length product (DLP)
DLP and CTDI
ers and have quantitative values that directly depend on
the technical details of the performed CT scan. Both
quantities are displayed by the scanner’s software for each
completed patient scan. There is a progressive relationship
among all of the listed parameters, as demonstrated in the
following paragraphs.
CTDI is determined by an ionization measurement
using a 1 cm diameter and 10 cm (100 mm) long,
(CTDI volume)
vol
VOL
are the two specific CT dose mark-
1 cm diameter pencil
chamber
Peripheral hole
16-cm head PMMA phantom
Fig. 13.6 Demonstrates the measurement process for obtain-
ing CTDIw for both the body and head phantoms. The pencil
chamber will be successfully inserted into each of the four peripheral cavities and then into the central cavity and ionization
measurements obtained. PMMA (poly methyl methacrylate) is
an organic compound made up of carbon, hydrogen, and oxygen. It is the chemical name for what is commonly referred to
as acrylic or Lucite or Plexiglas.
32-cm body PMMA phantom
Center hole
PMMA plug
pencil-like ionization chamber inserted into a cylindrical acrylic phantom (Figs. 13.5 and 13.6).
The phantom is similar in diameter to either an average human head (16 cm diameter) or human abdomen
(32 cm diameter) and is scanned utilizing a single axial
scan with technical factors (kVp, mAs, and slice thickness)
that are equivalent to those used for an actual patient
study that could be either helical or axial. The irradiated
pencil chamber is attached to an electrometer whose ionization charge reading, when multiplied by several correction factors, will then be given in milligray (mGy) units.
Dividing this number by the scan direction collimation*
will make this ionization measurement representative of
the localized dose from a multiple slice examination.
is simply a weighted average of two mea-
CTDI
W
sured CTDI values: one that is obtained with the pencil
chamber placed in the central cavity of the acrylic
phantom and the other derived from the average of
four peripheral (1 cm from the phantom cylindrical
Fig. 13.5 Acrylic body and head CTDI dosimetry phantoms.
*Scan direction collimation is equal to the product of the number
of slices (N) used during one axial acquisition and the nominal
slice width (T). For example, in a single detector row or singleslice scanner for a 10-mm slice thickness, N 5 1 and T 5 10, so
NT 5 10. In a multislice CT scanner with selected scan thickness
parameters 4 3 5 mm, then N 5 4, T 5 5, and NT 5 20.
258
CHAPTER 13 Special Considerations on Safety in Computed Tomography
surface) cavity measurements that are located at the 3,
6, 9, and 12 o’clock positions (see Fig. 13.6). The mathematical expression for CTDIW is given by:
CTDI
1
CTDI (CTDI ) (CTDI )
W
3
is the average absorbed dose within the
VOL
center
2
peri ph
3
scanned volume. It takes into account whether the scan
is axial or helical, and its value is directly related to
CTDIW by the following expression:
5CTDI CTDI /Pitch
VOL W
For purely axial scans, such as those typically used in
adult head scan sequences, the pitch (P) equals 1, and
therefore CTDI
5 CTDIW. For helical scans, how-
VOL
ever, the pitch is usually greater than 1 (e.g., 1.375 is
quite common) but sometimes can be less than 1 when
overlapping is desired, so the magnitude of CTDI
VOL
could be either less than the CTDIW or greater than
the CTDI
W.
Effective mAs is simply the ratio of applied mAs to
selected pitch:
≡Eff mAs mAs/pitch
Box 13.2 provides an example of the determination
of effective mAs.
At the conclusion of each patient’s CT scan sequence, built-in computer software accesses a detailed
BOX 13.2 Example of the Determination
of Effective mAs
Which of the following five CT scan protocols would be
expected to result in the highest CTDI
and beam collimations are all the same.)
1. mA5 400, rotation time 5 0.5 sec, pitch 5 1
2. mA5 100, rotation time 5 1 sec, pitch 5 0.5
3. Effective mAs5 200, pitch 5 0.5
4. Effective mAs5 200, pitch 5 1
5. There is no difference in CTDI
Solution: The value of CTDI
the scan technique applied and this includes both mA,
time, and pitch. The combination of the three is the Eff
mAs. It is seen that this value is the same (200) for all
four cases. Therefore the CTDI
and choice #5 is the correct answer
From Cagnon C, DeMarco J, Angel E, et al: Estimating patient
radiation dose from computed tomography, UCLA David Geffen
School of Medicine, 2008. https://www.aapm.org/meetings/
amos2/pdf/34-9723-93678-499.pdf.
VOL
ultimately depends on
VOL
will be the same also
VOL
? (Note: KVP
vol
for any of these
measurement database that supplies a numeric value of
the patient’s CTDI
for that particular scan sequence.
VOL
Also provided is the value of the quantity dose length
product (DLP).
DLP represents the product of the CTDI
VOL
irradiated scan length. DLP is expressed in mGy-cm and
characterizes the volumetric extent along the patient’s
body that has been irradiated with an average absorbed
dose and therefore is representative of the total patient
absorbed radiation energy. As such, it has a greater significance than just the CTDI
alone for estimating
VOL
future cancer risk as a result of radiation doses unavoidably delivered to sensitive organs from CT examinations. Mathematically:
DLP* CTDI irradiated scan length
Scan length, which may be considered as approxi-
VOL
mately the superior-inferior extent of the patient’s irradiation, can be derived from the product of slice width
times the number of slices times pitch. Thus a helical
scan that is composed of sixty 5-mm slices with a pitch
of 1.5 has an approximate scan length of 60 3 0.5 3 1.5
5 45 cm. If, instead, this was a contiguous axial scan, the
scan length would be just 60 3 0.5 5 30 cm. The length
of the scan shown or calculated from the console is the
distance between the centers of the starting and ending
images. The irradiated length for such a set of images
can vary from this distance, how much depending on
the acquisition mode.
EFFECTIVE COMPUTED TOMOGRAPHY DOSE
Table 13.1 lists body scan region–specific conversion
factors generated by the European Union11 that when
combined with the dose information supplied by the
CT software for each delivered scan sequence will yield
an effective dose (EfD) value for that CT scan. The
*Although CTDI
the central slices within a long scan length, it is not an accurate
measure of dose on the edges of the scan. DLP particularly
implies that the CTDI
whole scan length. However, to be exact, because of the sharp
dose gradients between the “on” and “off” values at the edges
of the volume of interest, DLP just serves as an acceptable and
useful first approximation and guide to the entire actual
physical situation.
is an accurate representation of the dose in
vol
values are exactly the same for the
vol
and the
CHAPTER 13 Special Considerations on Safety in Computed Tomography
259
TABLE 13.1 Scan Region Specific
Conversion Factors
Body Region
Scanned
Head 0.0023
Neck 0.0054
Chest 0.017
Abdomen 0.015
Pelvis 0.019
Normalized Effective
Dose (EfDLP) Factor
simple expression to be used for the calculation of a CT
scan EfD is given by the following equation:
EfD DLP EfDLP
where EfDLP is the normalized EfD associated with a
specific scan region of the body. It is expressed in millisieverts per milligray-centimeter (mSv/mGy-cm) and
represents a conversion factor from a patient’s scan DLP
to the effective dose received by the patient as a result of
that scan. EfDLP values are given in Table 13.1.
Using those values and the information displayed on
the dose page printout for a patient’s CT scan, the EfD
from that scan can be calculated. Several examples of
using such data from actual patient CT scans are demonstrated in Cases 13.1 to 13.3. Typical EfD value ranges
for several frequent CT examinations can be found in
Table 13.2.
CASE 13.1 Head Scan (Axial)
Scan data: CTDI
(portion of scan series at 140 kVp)
CTDI
5 49.1 mGy, DLP 5 351 mGy-cm (portion of
VOL
scan series at 120 kVp)
From Table 13.1, EfDLP 5 0.0023.
Therefore the EfD to this patient is given by the following
equation:
(375 351) 0.0023 1.67 mSv (0.167 rem)
5 60.8 mGy, DLP 5 375 mGy-cm
VOL
CASE 13.3 Abdominal Scan (Helical)
Scan data: CTDI
From Table 13.1, EfDLP 5 0.015.
Therefore the EfD to this patient is given by the following
equation:
TABLE 13.2 Typical Effective Dose Values
5 10 mGy, DLP 5 460 mGy-cm
VOL
460 0.015 6.9 mSv (0.69 rem)
for CT Examinations
Examination Effective Dose (mSv)
Head 1–2
Chest 2–6
Abdomen 5–8
Pelvis 3–6
Coronary artery calcification 0.1–3
Coronary angiography 1–18
In the early 2000s, various publications calculated individual and population risks of cancer from CT procedures
based upon risk estimates derived from the atomic bomb
survivors’ studies. These studies were used to predict the
number of cancers in large populations due to medical
procedures such as CT by multiplying the small risk estimates for individual procedures by the large population of
patients undergoing such studies every year.
More recent analysis, however, has shown no increase
in the risk of cancer or any other measures of early death
from effective doses below 100 mSv (10 rem).
12,13
Routine
head and body scans fall in the 1 to 10 mSv EfD range,
and CT angiography rarely exceeds 15 mSv (1.5 rem).
Because there is considerable uncertainty in estimating
risk below 100 mSv, many of the major scientific and
advisory bodies concerned with radiation bioeffects
(UNSCEAR, ICRP, NCRP) have discredited this prac-
14–16
tice.
In 2011, the American Association of Physicists
in Medicine issued the following position statement17 :
CASE 13.2 Chest Scan (Helical)
Scan data: CTDI
From Table 13.1, EfDLP 5 0.017.
Therefore the EfD to this patient is given by the following
equation:
5 8.5 mGy, DLP 5 323 mGy-cm
VOL
323 0.017 5.49 mSv (0.549 rem)
“Discussion of risks related to radiation dose from
medical imaging procedures should be accompanied
by an acknowledgment of the benefits of the procedures. Risks of medical imaging at effective doses
below 50 mSv for single procedures or 100 mSv for
multiple procedures over short time periods are too
low to be detectable and may be nonexistent. Predictions of hypothetical cancer incidence and deaths in
260
CHAPTER 13 Special Considerations on Safety in Computed Tomography
patient populations exposed to such low doses are
highly speculative and should be discouraged.”
In summary, the goal of CT imaging should be to
obtain the best image possible while delivering an acceptable level of ionizing radiation to the patient. In the
absence of specifically designed scan protocols, the fulfillment of this responsibility lies with the technologist
performing the examination.
MULTIDETECTOR COMPUTED TOMOGRAPHY SCANNING (MDCT)
X-ray beam collimation for a single-slice CT (SSCT)
scanner is accomplished by employing physical pre and
post size restriction of the CT x-ray beam. Collimation
is used to reduce scatter radiation to the patient and
thereby decrease patient dose and improve image sharpness. Prepatient collimation controls the aperture at the
x-ray tube, directly limiting the beam to the desired
width. Postcollimation can be used to limit penumbra
effects. Therefore, in SSCT, slice width is very much determined by x-ray beam collimators. The slice number
refers to the amount of slice images the CT scanning
machine can obtain per gantry rotation. It is equal to
one for SSCT scanners.
Multidetector scanners employ numerous rows of
CT detectors, unlike SSCT scanners, which only use
one. MDCT’s most direct advantages over SSCT scanners are faster scanning procedures with improved
spatial resolution. For both SSCT and MDCT scan-
ners, there is a “fan-beam” geometry for the utilized
x-ray beam (Fig. 13.7). The z-axis dimension is along
the length of the patient and illustrates the longitudinal
or axial extent of the scanner detectors, whereas the x-y
plane corresponds to the cross-patient scan coverage.
To reconstruct an accurate image from a patient’s
scan, at least 180 rotational degrees of data must be acquired. The remaining 180 degrees of projections for a
full 360° rotation are simply a mirror image of the first
180° because in whichever opposed direction a photon
travels through a particular path length through the
body, it will be overall attenuated the same amount. Due
to the fan beam geometry of the x-ray beam, however, it
is necessary to measure an extra amount equal to the fan
angle, or transverse spread, of the x-ray beam to wholly
acquire all of the data needed. An element of tissue must
be included in all of the 180 degrees of projections to be
reconstructed correctly.
18
Fig. 13.7 Fan beam geometry for two detector configurations.
Single-slice computed tomography (left) versus multislice computed tomography (right). The arc expanse of the x-ray beam is
what constitutes the “fan-like” geometry. The fan angle of the
emitted beam determines the extent of detector coverage in
the transverse (cross-plane or x-y) dimension. (From Goldman
LW: Principles of CT: multislice CT, J Nucl Med Technol,
36(2):57–68, 2008, ©SNMMI. This research was originally published in the Journal of Nuclear Medicine Technology.)
MDCT Collimation, Slice Width, and Slice Number
In MDCT systems, the total number of data channels is
simply equal to the total number of detector rows. The
product of the selected detector configuration or the
number of involved detector rows multiplied by the effective detector row thickness is equivalent to the beam
collimation. For example, with a detector configuration
of 60 detector rows, each row having a z-axis dimension
0.5 mm (i.e., 60 3 0.5 mm), the beam collimation
would be 30 mm. Concerning the patient, the physical
manifestation of the beam collimation is considered to
be the x-ray beam size as defined at the gantry isocenter.
With MDCT, slices can be composed of a single detector row thickness or multiple adjacent detectors, and
the effective slice thickness or slice width is therefore
determined by the chosen detector configuration and
not by physical x-ray beam collimation. Thus, for a
60-row, 0.5 mm MDCT, combining every two adjacent
detectors would allow a set of thirty 1 mm slices (30 3
1 mm) per rotation of the gantry. In contrast, a combination of four detector rows instead could yield fifteen
2 mm slices (15 3 2 mm). The slice number thus
CHAPTER 13 Special Considerations on Safety in Computed Tomography
261
correlates to how many images the CT scanning machine can obtain per gantry rotation.
The selection of the detector imaging configuration
should always be related to the type of study to be performed, the required slice thickness for diagnostically
useful multiplanar reconstructions, and whether there is
also a need for three-dimensional (3-D) images. For routine acquisitions, without the need for 3-D imaging, very
thin effective detector thicknesses (0.5 mm or 1 mm) are
unnecessary. In such cases, if 3 mm thick images in the
axial, coronal, and sagittal planes are sufficient for radiologist review, the effective detector thickness can be enlarged to a combination of six 0.5 mm units without a
marked impact on image quality.
19
MDCT Advantages
A significant difficulty that arose with early single-slice
spiral (helical) CT scanning was the inverse relationship
between scan length (z-axis extent) and spatial resolution along the head to toe axis (i.e., axial resolution) of
the patient. Thus, because of the requirement for small
scan lengths for adequate spatial resolution, using SSCT
scanners for a volumetric acquisition with acceptable
spatial resolution in all directions (i.e., isotropic) would
be very difficult since only limited regions could be imaged during each breath-hold. A solution to this issue
was to design a CT scanner with a detection system that
would permit the acquisition of multiple thin (1.5 mm
and less) simultaneous slices and also have an increased
x-ray tube rotational speed. For example, even a very
early four-detector row scanner with a 0.5 second gantry rotation yielded a performance that was up to eight
times greater than a 1-second single-slice scanner. These
scanners allowed higher spatial resolution to be achieved
over a longer scan range. Currently, most institutions
employ scanners with 16, 64, or even greater numbers of
detector rows instead of just four, faster gantry rotational times, and sub-millimeter length detector rows.
Therefore faster and more varied slice acquisitions can
be achieved along with much-improved z-axis resolution. Box 13.3 lists the multiple benefits of MDCT scan-
ners over previous generation SSCT scanners.
20
Slice Thickness and Reconstruction Interval
Detector configuration is the number of data channels
being used in the z-axis direction and the effective detector thickness of each data channel. For example, a detector configuration of 128 3 0.5 mm would signify the use
BOX 13.3 Advantages of MDCT
Shorter Scan Duration
Reduced movement artifacts for:
• Children
• Trauma patients
• Acutely ill or dyspneic patients
Improved Contrast-Enhanced Scans
• Well-defined phase of contrast enhancement
• Reduced contrast volume for CT Angiography (CTA)
• More-homogeneous enhancement
Longer Scan Ranges Permitting CT
• Thoraco-abdominal aorta
• Carotids from arch to intracerebral circulation
Trauma
• Full spine examinations
Thinner Sections
Near Isotropic Imaging (any application):
• Arbitrary imaging planes
• Multiplanar reformations
• Three-dimensional rendering
From Prokop M: Multislice CT: technical principles and future
trends. Eur Radiol 13:3–13, 2003; Prokop M: General principles
of MDCT. Eur J Radiol 45(suppl 1): S4–S10, 2003.
of 128 data channels in the z-axis, each of which has an
effective thickness of 0.5 mm. As mentioned previously,
MDCT slices can be composed of single detector row
thicknesses (for highest resolution) or from combinations of multiple adjacent detector rows. Images can also
be acquired at selected intervals or separations.
The reconstructed slice width or thickness can be varied independently of the data set acquisition as long as
the recreated width is not less than the slice collimation
(SC). In other words, if slices of a certain thickness are
used for scanning, it is not possible to later reconstruct
thinner slices to obtain smaller detail. However, it is simple to reconstruct broader slices for more convenient
clinical viewing if thin slices were used for scanning.
21
The reconstruction interval is the selected spacing
between adjacent processed image slices of the patient. It
is configured by the software from all of the raw data obtained during the patient scan. Reconstructed images can
be obtained at any desired and reasonable separation. The
slices remain the same thickness (z-axis dimension) for
whatever partition is chosen, but their individual relative
262
CHAPTER 13 Special Considerations on Safety in Computed Tomography
Slice interval:
5 mm
Slice Thickness:
5 mm 5 mm 5 mm 5 mm 5 mm
Contiguous
5/5 mm
Fig. 13.8 Several slice reconstruction intervals for 5 mm slices. In the figure, the “mm” values represent the
center to center spacing of the slices. (From Tristan Charles, RadTrain. Available at www.radtrain.com.au.)
positioning changes. This simply means that the produced
slice images can overlap or be contiguous (no spacing) or
be non-contiguous (gapped) (Fig. 13.8).
The majority of multiplanar reconstructions will generally be contiguous, ensuring that there are no missing
areas of anatomy, which could occur with non-contiguous,
or gapped, intervals. With overlapping scanning, more
images than necessary are created, and more radiation
dose to the patient is delivered. Overlapped images, how-
ever, can be useful in certain situations. They are acquired when it is needed to track in fine detail small and/
or tortuous objects (arteries, veins) such as are present in
cardiovascular studies.
As the reconstructed slice thickness decreases, the
number of photons within each voxel also decreases,
resulting in increased relative image noise. Unfortunately, to maintain acceptable noise levels within an
image with a smaller slice thickness, the radiation dose
to the patient must then be increased. Ultimately, slice
thickness determines the trade-off in image quality between spatial resolution (how accurately small physical
changes in the image can be differentiated) and image
noise (the standard deviation of the image data). De-
creased slice thickness 5 increased spatial resolution and
increased image noise. It is also important to note that
reducing the reconstruction interval (i.e., smaller gaps)
can increase the visibility of small lesions that may otherwise be obscured by volume averaging. If the lesion is
centered in the slice, there will then be less healthy tissue
2.5 mm 7.5 mm
5 mm
Overlapped
5/2.5 mm
Non-contiguous
5/7.5 mm
to average or smooth it out. Table 13.3 presents a sum-
mary of some (but not the latest) existing CT hardware
and their characteristics.
22
COMPUTED TOMOGRAPHY CARDIOVASCULAR IMAGING (CT CVI)
Complications in the usual functioning of the heart
brought on by various anatomical changes or defects,
collectively known as coronary heart disease (CHD),
is a major cause of morbidity and mortality. In the field
of diagnostic radiology, cardiac imaging has become a
commonplace procedure. In conjunction with a cardiologist, a trained cardiac radiologist supervises and
then interprets various types of medical imaging to
diagnose disorders of the heart such as leaky heart
valves, defects in the size and shape of the heart, and
potential ruptures or tears or blockages in various
structures. A cardiac radiologist can employ a variety of
imaging techniques such as interventional fluoroscopy,
ultrasound (e.g., echocardiograms), high speed x-ray
CT scans, and magnetic resonance imaging (MRI)
scans. All of these can be used to screen for heart disease, determine the cause of various symptoms, monitor the heart’s performance status, and, especially, to
ascertain if a particular treatment is yielding positive
results. The following sections focus only on CT imaging
methods and systems to discover and examine cardiovascular maladies.
CHAPTER 13 Special Considerations on Safety in Computed Tomography
Right pulmonary
TABLE 13.3 Summary of CT Hardware and Parameters for State-of-the-Art CT Scanners
From the Major CT Manufacturers
Total
Detector-row
X-ray
sources,
Scanner
GE Discovery CT750
HD
Hitachi SCENARIA 1 64 0.625 40 0.35 0.175
Phillips Brilliance iCT 1 128 0.625 80 0.27 0.135
Siemens SOMATOM
Definition FLASH
Toshiba Aquilion ONE 1 320 0.5 160 0.35
*Values measured at scanner isocenter
†
Values do not reflect the use of multi-segment reconstruction
From Halliburton S, et al: State of the art in CT hardware and scan modes for cardiovascular CT, J Cardiovasc Comput Tomogr
6(3):154–163, 2012.
number
1 64 0.625 40 0.35 0.175
2 (95° apart) 64 0.6 40 0.28 0.075
Detector
rows,
number
z-axis
dimension,
mm*
nominal
beam
width,
mm
Fastest
gantry
rotation
time, sec
Temporal
resolution
for each
cross-sectional
image, sec
†
263
Basic Heart Anatomy and Processes
The heart is a multi-chambered muscle about the size of
an average human fist. Essentially, it is a sophisticated
pumping mechanism that consists of four major sections or compartments, named the right atrium, left
atrium, right ventricle, and left ventricle. As shown in Fig.
13.9, the atria are the two upper or more cranially posi-
tioned chambers. The right atrium receives and holds
oxygen-depleted blood emerging from venous channels.
This blood is then sent down to the right ventricle
(through the tricuspid valve), which in turn, via a muscular contraction, pumps it through the pulmonary artery for recirculation through the lungs and consequent
reoxygenation. The left atrium receives the newly oxygenated blood by transport from the lungs through the
left and right pulmonary veins and immediately by contraction pushes the blood into the left ventricle (through
the mitral valve). Subsequently, passage across a oneway valve (aortic valve) into the aorta artery followed by
another contraction or “beat” of the heart muscle moves
the blood into whole-body circulation. This is known as
the cardiac cycle. The cardiac cycle comprises a complete
relaxation and contraction of both the atria and ventricles and lasts approximately 0.8 seconds.23 Box 13.4
contains explanations of the anatomic and medical
terms that are used for CT CVI.
artery
TO LUNG
FROM LUNG
Superior vena
cava (from
head and arms)
Pulmonary
semilunar
valve
Right atrium
Tricuspid
valve
Right ventricle
Inferior vena cava
(from trunk and legs)
Fig. 13.9 Anatomy and blood flow through the heart. In this
figure, the blue lines and blue arrows show the direction in
which oxygen-poor blood flows through the heart to the lungs
while the red lines and arrows show the direction in which
oxygen-rich blood flows from the lungs into the heart and then
out to the rest of the body through both the ascending (superior circular red segment) and the descending aorta. (From
Stein L, Hollen CJ: Concept-based clinical nursing skills,
St. Louis, 2021, Elsevier.)
Descending
Left pulmonary
artery
TO LUNG
Left atrium
FROM LUNG
Aortic semilunar
valve
Mitral valve
Left
ventricle
Interventricular
septum
aorta
264
CHAPTER 13 Special Considerations on Safety in Computed Tomography
BOX 13.4 Explanation of Various Anatomic and Procedure Terms
Aneurysm: a rupture or tear in the outer wall of an organ.
Angina: chest pain (such as squeezing or unusual pres-
sure) or general discomfort caused when the heart
muscle doesn’t get enough oxygen-rich blood.
Aorta: the largest and most important artery in the body.
The aorta begins at the top of the left ventricle, the
heart’s muscular pumping chamber. The heart pumps
blood into the aorta through the aortic valve. The aorta
ascends a little ways, bends over like a cane (i.e.,
arches) and then goes downward.
Cardiovascular disease: this most often refers to condi-
tions that involve narrowed or blocked blood vessels
that can lead to a heart attack, chest pain (angina), or
stroke. Other heart conditions, such as those that affect
its muscle, valves, or rhythm, also are considered forms
of heart disease.
Cardiovasculature: the circulatory system which supplies
the heart with nutrients and oxygen.
Coronary arteries: the network of lesser blood vessels
that branch off the aorta to nourish the heart muscle itself with oxygen-rich blood. They are mainly composed
of the left and right coronary arteries, both of which give
off smaller branches. They are called the coronary arter-
ies because they encircle the surface of the heart in the
manner of a crown encircling royalty. The main arteries
and all of their branches are often described as the
“Coronary Tree”.
Coronary sinus: a collection of veins joined together to
form a large vessel that collects blood from the heart
muscle (myocardium). It delivers less-oxygenated blood
to the right atrium, as do the superior and inferior vena
cava.
CT angiography (CTA): a CT procedure that is used to diag-
nose and evaluate blood vessel disease or related conditions, such as aneurysms or narrowing or blockages.
Diastisis: the middle stage of diastole during the cycle of
a heartbeat, where the initial passive filling of the ventricles has slowed down, but before the atria contract to
complete the active filling.
Left ventricle: it is located in the bottom left portion of the
heart below the left atrium, separated by the mitral
valve. The left ventricle is the thickest of the heart’s
chambers and is responsible for pumping reoxygenated
blood to the aorta which delivers the blood to tissues all
over the body.
Myocardial infarction (heart attack): a failure of the heart
muscle which occurs when a portion of the heart is deprived of oxygen due to decreased blood flow as a result
of blockage of a coronary artery. An artery supplying the
heart with blood and oxygen becomes blocked by fatty
deposits building up over time, forming plaques in the
heart’s arteries. If a plaque ruptures, a blood clot can
form and significantly obstruct the arteries, leading to a
heart attack.
Myocardium: the muscle tissue which forms the wall (or
most of the wall) of the heart; also known as the heart
muscle. The myocardium establishes a thick middle
layer between the outer layer of the heart wall (the epicardium) and the inner layer (the endocardium), with
blood supplied via the coronary circulation.
Perfusion: the passage of fluid through the circulatory
system or lymphatic system to an organ or a tissue. As
related to cardiac processes, perfusion is measured as
the rate at which blood is delivered to tissue, or volume
of blood per unit time per unit tissue mass
Pulmonary arteries: the vessels carrying oxygen depleted
blood from the right ventricle of the heart to the left and
right lungs for reoxygenation.
Pulmonary embolism (PE): a blockage of an artery in the
lungs by a substance that has moved from elsewhere in
the body through the bloodstream (an embolism).
Pulmonary veins: the vessels carrying reoxygenated blood
from the lungs to the left atrium of the heart.
Right atrium and left atrium: akin to household foyers,
these are entrances to the heart. Blood enters the heart
from different portions of the body through the two
atria. Deoxygenated blood enters the right atrium
through the inferior and superior vena cava and reoxygenated blood from the lungs enters the left atrium
through the pulmonary vein.
Right ventricle: the lower right chamber within the heart
that initiates the pumping of oxygen-depleted blood to
the lungs.
Systole and diastole: two phases of the cardiac compres-
sion/relaxation cycle. Systole occurs when the heart
contracts to pump blood out, and diastole occurs when
the heart relaxes or returns to its normal volume after
contraction.
Vena cava (superior and inferior): situated on the right side
of the heart, these are the largest veins in the body.
After the body’s organs and tissues have exhausted the
oxygen in the blood, the superior and inferior vena cava
carry the oxygen depleted blood back to the right
atrium. The superior vena cava moves oxygen-poor
blood from the upper parts of the body, including the
head, chest, arms, and neck while the inferior vena cava
retrieves oxygen-poor blood from the lower parts of the
body.
CHAPTER 13 Special Considerations on Safety in Computed Tomography
265
Phases of the Cardiac Cycle
At the beginning of the cardiac cycle, both the atria and
ventricles are relaxed, and the chambers fill with blood.
This is the diastole phase. Blood is flowing into the right
atrium from the superior vena cava and inferior vena
cava, and the coronary sinus. Blood flows into the left
atrium from the four pulmonary veins. The two valves
situated respectively between each atrium and each ventricle, the tricuspid and mitral valves, are both open, so
blood flows unimpeded from the atria and into the ventricles. Approximately 70% to 80% of ventricular filling
occurs by this method, and the remainder is due to atrial
contraction. Following this are secondary contractions
involving the ventricles that the heart experiences while it
pumps blood into circulation via the pulmonary and
aortic valves. These are both one-way valves, thereby preventing the backflow of blood into the right and left
ventricles from the pulmonary trunk on the right and the
aorta on the left. This is the systole phase (see Fig. 13.9)
In summary, both the atria and ventricles undergo
contraction and relaxation processes, and it is essential
that these components be precisely regulated and coordi-
nated to ensure that blood is pumped efficiently to all
critical areas of the body.
with any muscle in the body, the heart’s cardiac cycle is
essentially an electrical process involving depolarization
of a muscle*, in this case, the heart muscle.
CT Cardiovascular Imaging (CT CVI)
ECG Gated Imaging. To effectively image a rapidly beat-
ing heart, most imaging modalities must have very brief
image acquisition times. It is necessary to eliminate or at
least significantly minimize the heart motion to clearly
visualize coronary arteries located close to the heart
muscles. Since the most quiescent part of the heart cycle
is the diastolic phase, cardiac imaging will be optimal if
*Normally, there is a difference in the amount of electrical
charge in the form of alkali ions between the inside and outside
of the plasma membrane of a muscle. This implies the presence
of an electrical potential difference or voltage between inside
and outside. Depolarization is a loss of that potential difference
due to a change in permeability of the muscle wall that allows
migration of sodium ions to the interior, leading to electrical
neutrality. Depolarization occurs in the four chambers of the
heart: right atria first and then left atria, followed by both ventricles. A wave of electrical depolarization originates within the
right atrium (RA) and then spreads through the RA and across
the interatrial wall into the left atrium (LA).
23,24
It is to be noted that as
25
done during this period. Therefore the heart cycle, which
is essentially an electrical process involving a periodic,
repetitive sequence of different magnitude millivolt electrical pulses associated with the different phases of the
cycle, must be precisely monitored throughout the scanning procedure. To do this, an electrocardiogram (ECG),
which is a graphical tracing or readout of these pulses or
electrical activity with time, is acquired. Box 13.5 presents
a brief discussion of ECG that will enhance understanding of the following material, and Fig. 13.10 shows this
graphically.
26,28
In ECG gated imaging, the CT scanner is electronically linked to the patient’s real-time electrocardiogram.
CT image acquisition and reconstruction can then be
coordinated with the quiescent heart phases associated
with particular magnitude electrical pulses, thereby removing or minimizing motion artifacts. Thus, the scanner is programmed to initiate scanning and scan only
through a selected portion of the diastole phase in the
cardiac cycle in which the heart is entirely at rest. As a
result, there will be non-scanning delay periods during
and after contractions of the ventricles (Fig. 13.11). But
whenever the ECG voltage signal level regains a value
associated with a specific part of the diastole phase, the
scanner will be triggered to image the next section of the
heart and scan until another voltage pulse is received
that will stop the radiation and advance the table position.* This technique is called prospective (i.e., delayed
or future) gating and is essentially a step and shoot
process since for most facility scanners the detector’s
limited longitudinal extent mandates that the scanner
advances the table in between several quiescent phases
of the heart.
25,27,28
This process is depicted in some de-
tail in Fig. 13.11.
Fig. 13.11 exhibits a segment of a prospective ECG
gated imaging cycle. The imaging acquisition is composed of four processes in sync with one another. The
top of the figure shows the desired transverse heart
slices to be acquired in the patient during the step and
*Instructions are built into the protocol to start the scanner’s
x-rays at the desired distance from the R wave peak of the
ECG scan, e.g., at 60% or 70% of the R-R interval duration
(see Box 13.5). Thus the scanner, in congruence unity with the
patient’s ECG pulse, starts the scan at a preset point in the
cardiac cycle, just after diastole commences, and the x-ray
projection data are acquired for only part of the complete
gantry rotation (i.e., a partial scan).
266
CHAPTER 13 Special Considerations on Safety in Computed Tomography
BOX 13.5 Electrocardiogram and the Cardiac Cycle
An electrocardiogram (ECG) is a test that measures the
electrical activity of the heart. With each beat, an electrical
impulse (or “wave”) travels through the heart. This impulse causes various portions of heart muscle to squeeze
and pump blood through and from the heart. The ECG is
essentially a graph of voltage versus time of the electrical
activity of the heart received from electrodes placed on
the skin. The amplitude, or voltage, of the recorded electrical signal is expressed on an ECG in the vertical dimension
and is measured in millivolts (mV). Abnormal heart beat
patterns can be easily discerned by this procedure.
The normal cardiac cycle begins with spontaneous depolarization of the sinoatrial (SA) node.* A wave of electrical
depolarization** spreads from the SA node through the
right atrium and across the inter-atrial septum*** into the
left atrium. This activates the atrioventricular node. In a
normal heart the only route of transmission of electrical
depolarization from atria to ventricles is through this node.
The AV node thus acts as an electrical relay station between the upper and lower chambers of the heart. The AV
node lies near the bottom of the right atrium, on the right
side of the septum that divides the atria. When the impulses generated by the SA node reach the AV node, they
are delayed for about a tenth of a second. This delay in the
cardiac pulse is extremely important: It ensures that the
atria have ejected their blood into the ventricles first before the ventricles contract. Then the wave of depolarization spreads down and into the right and left ventricles.
With normal conduction of this electrical signal, the two
ventricles will contract simultaneously. This is significant
for maximizing cardiac efficiency.
In summary, the spread and evolution of this series of
electrical impulses causes the four chambers of the heart
to both contract and relax (i.e., to depolarize and repolarize) in a coordinated fashion. Studying these electrical impulses permits an understanding of how well the heart is
functioning. The electrical impulses are characterized by
distinct waveforms named: P, QRS complex, and T. P and
T occur by themselves while Q, R, and S sequentially
occur in very rapid succession thus behaving essentially
as a unit named the QRS complex (Fig. 13.10).
ECG Waveform Details
The P waveform or pulse results from the depolarization
of the left and right atrium and thus signifies the contraction of the atria. The P wave is small in amplitude and
typically lasts no more than 0.11 seconds.
complex represents the electrical impulse as it spreads
through the ventricles and indicates the ventricular depolarization leading to the contraction of the large ventricular
muscles. The latter causes the pressured outflow of
blood from the heart into the large arteries exiting the
heart. Under normal circumstances, the duration of the
QRS complex in an adult patient will be between 0.06 and
0.10 seconds. The Q and S portions are always negative
while the R pulse is the positive waveform of the complex. It is the most prominent signal spike seen on an
ECG graph, representing the required electrical stimulus
that passes through the main portion of the muscularly
dense ventricular walls.
tricular repolarization or relaxation. A T wave will normally
have the same voltage direction or polarity as the QRS
complex that preceded it. Should a T waveform demonstrate
an opposite polarity or direction to that of the QRS complex,
this generally indicates some sort of cardiac pathology.
veniently used as a reference point. Data acquisition by
the scanner is normally initiated following a brief delay after the R wave and continues only during heart quiescence. To encompass the entire heart, the images are
usually created from data collected over a series of R to R
intervals (the time between QRS complexes). The instantaneous heart rate or time between heart beats can be
calculated from the time between any two QRS complexes. Normal values for RR interval range from 0.6–1.2
seconds.
The second waveform is the QRS complex. The QRS
The T wave follows the QRS complex and indicates ven-
The R waveform or large signal spike of the ECG is con-
*SA node is a small body of specialized muscle tissue, situated in the top tissue wall of the right atrium of the heart, that acts as
a pacemaker by producing a contraction signal at regular intervals.
**Depolarization is an electrical process involving the movement or migration of potassium ions within the heart muscle membrane that causes it as a whole to become less negatively charged approaching a neutral electrical potential. This type of electrical activity physically leads to a contraction of the heart muscles and happens automatically in a functioning heart.
***The wall of tissue that separates the right and left atria of the heart.
From Cardiology Teaching Packages and Cardiac Conduction System Learning Resources, School of Health Sciences, University
of Nottingham, United Kingdom.