Keywords
Effective aortic diameter - Indian pediatric population - thoracic aorta
Introduction
The normal standards for the aortic diameter at various levels have been established
for the adult population and can be used to determine aneurysm formations or stenosis.[[1]] In contrast, similar standards for infants (1 month to 1 year old), children (1–12
years old), and adolescents (13–17 years old) are not as well established,[[2]] and such standards pertaining to Indian pediatric population have not yet been
published in the literature.
A prerequisite for identifying abnormal is to establish the normal. Effective aortic
diameter assumes significance in early detection of diffuse aortic hypoplasia in conditions
such as Williams syndrome or aneurysmal dilatation in children with connective tissue
disorder.[[3]] Anomalies of the aorta in pediatric age group include coarctation, residual findings
after catheter-guided interventions or surgery, connective tissue diseases such as
Marfan syndrome and dilatation of the aortic root associated with aortic valve anomalies,
non-specific aorto-arteritis or post-surgical in patients with congenital heart diseases.[[4]]
Though echocardiography is the standard method for determining the size of the thoracic
aorta in children, a recent review of echocardiographic methods showed a general lack
of standardization in technique.[[2]] The evaluation of thoracic aorta on echocardiography relies on planar measurements
rather than on transverse measurements. Cross-sectional imaging using computed tomography
(CT) and magnetic resonance imaging (MRI) with multiplanar reconstructions does overcome
these limitations.
Effective aortic diameter is average of the transverse and anteroposterior diameters
of the aorta, this method of measurement nullifies errors due to obliquity and has
been previously used for similar studies.[[2], [5]]
Subjects and Methods
This was a single institutional cross-sectional observation study. The study included
children and young adults of age group ‘zero’ to eighteen years who underwent contrast-enhanced
CT (CECT) thorax scans at our institute during the 13-month period between January
2016 and January 2017. Exclusion criteria included a) History of congenital heart
disease/dysmorphisms. b) History of cardiovascular disease or cardiothoracic surgery.
c) Patients who are being evaluated for cardiac diseases. In patients where multiple
CT examinations were performed during the study period, only the first of such CT
scans were included in the study. The study was approved by our Institutional Ethics
Committee.
During the study period of 13 months, a total of 321 CT studies were evaluated, out
of which 207 satisfied the above-described inclusion and exclusion criteria. Out of
the excluded studies (n = 114), prior history of cardiovascular diseases or surgeries and subjects under
evaluation for cardiac diseases (n = 68) formed the majority followed by repeat examinations in the study window (n = 27) and scans excessively degraded by motion artifacts (n = 19).
All the CT scans included in the study were performed on a 64-slice CT scanner (Philips
Brilliance 64-slice CT, Koninklijke Philips N.V). Non-ionic iodinated contrast agent
with an administration rate of 1-2.5 mL and a dose of 1-2 mL/kg [not exceeding 100
mL] along the peripheral venous route, followed by a saline chaser of 10-20 cc was
used. CT data were obtained in keeping with the as low as reasonably achievable (ALARA)
principle with a weight-based variable dose parameters (80-120 kVp, 20-150 mAs) with
scans performed from thoracic inlet to the level L1-L2. Image data were analyzed on
a workstation (Tera-recon AQI viewer) after image reconstruction of 1-mm slice thickness.
Multiplanar reformations (MPR) were created using a workstation. All MPR with double-oblique
reconstructions were obtained perpendicular to the aorta [[Figure 1]].
Figure 1: Coronal and sagittal oblique multiplanar reconstructions perpendicular to the aorta
created to determine the cross section of the descending aorta
The effective diameter at each level was determined by averaging the anteroposterior
and lateral diameter measurements. Measurements were obtained by using an electronic
cursor at the outer widest diameter of the vessels. The measurements were obtained
at the following five predefined locations: Aortic root, ascending aorta at the level
of the right pulmonary artery, aortic arch, proximal descending aorta (distal to the
aortic arch where the descending aorta obtains a cranial-caudal orientation), and
aorta at the level of diaphragmatic hiatus.
Statistics
The effective diameters at various levels were tabulated against the subject’s age.
Descriptive statistics were employed to calculate the mean, standard error (SE), and
standard deviation (SD) of the aortic diameter at various levels for different age
groups separately for both boys and girls.
Regression analysis was used to describe the relationship between the aortic diameter
(dependent variable) and the subject’s age (independent variable). Multiple regression
models as described by previous studies[[2], [5], [6]] were analyzed to determine the best fit model. Linear, logarithmic, exponential,
and polynomial regression models with quadratic, cubic, and linear terms were evaluated
using the R2 value to determine the best fit functional form. The intercepts for linear,
cubic, and quadratic terms were determined and were tested for significance. Scatter
plots were used to determine the equation of independent variables at various levels.
The best regression model was used to plot the trend line in the scatter plot. From
the slope estimates of the best fit model, formulas were specified for the predicted
diameters along with R2 for each of them. The effect of gender on the aortic diameter
was determined by comparing the means of the diameter in male and female subjects.
From these regression formulas, estimated mean squared error (MSE) was calculated.
Predicted estimates of the aortic diameter were then calculated using the regression
models. The z scores were calculated and then used to plot charts that can be used
to determine the normal aortic diameter within the confidence interval of 95% (z =
2).
These statistical analyses were performed on Excel (Microsoft) and Statistical Package
for the Social Sciences (SPSS) (IBM) software.
Results
The age and sex distributions are summarized in [[Table 1]]. The youngest patient included in the study was a 10-dayold infant, and the oldest
patient was 18 years old. The median age of the study population was nine years.
Table 1
Age and sex distribution of the subjects
|
Age group
|
Male
|
Female
|
Total
|
|
≤ 2 years
|
14
|
18
|
32
|
|
>2 years to ≤5 years
|
18
|
13
|
31
|
|
>5 years to ≤9 years
|
22
|
19
|
41
|
|
>9 years to ≤14 years
|
23
|
28
|
51
|
|
>14 years to ≤18 years
|
28
|
24
|
52
|
|
Total
|
105
|
102
|
207
|
The descriptive statistics data of the effective diameter of aorta and multiple locations
has been summarized in [[Table 2]], and further subgroup analyses have been made and specified. [[Table 3]] summarizes the mean, SD, and the SE of the effective aortic diameter at different
levels in gender subgroups.
Table 2
Summary of descriptive statistics
|
Effective diameter
|
Group
|
Mean
|
Standard Deviation (SD)
|
Standard error of Mean (SEM)
|
P
|
|
Aortic Root
|
0-2
|
12.09
|
2.45
|
0.45
|
<0.01
|
|
3-5
|
18.02
|
1.28
|
0.23
|
|
|
6-9
|
21.32
|
1.74
|
0.27
|
|
|
10-14
|
24.58
|
1.90
|
0.27
|
|
|
15-18
|
27.27
|
1.37
|
0.19
|
|
|
Ascending Aorta
|
0-2
|
9.82
|
1.90
|
0.35
|
<0.01
|
|
3-5
|
15.06
|
1.46
|
0.27
|
|
|
6-9
|
17.69
|
1.78
|
0.28
|
|
|
10-14
|
21.00
|
1.84
|
0.26
|
|
|
15-18
|
23.68
|
1.38
|
0.19
|
|
|
Arch of aorta
|
0-2
|
8.97
|
1.83
|
0.33
|
<0.01
|
|
3-5
|
12.70
|
1.65
|
0.30
|
|
|
6-9
|
15.22
|
1.53
|
0.24
|
|
|
10-14
|
18.28
|
1.91
|
0.27
|
|
|
15-18
|
20.13
|
1.38
|
0.20
|
|
|
Proximal descending
|
0-2
|
7.08
|
1.18
|
0.22
|
<0.01
|
|
Aorta
|
3-5
|
10.18
|
1.08
|
0.20
|
|
|
6-9
|
11.95
|
1.28
|
0.20
|
|
|
10-14
|
14.37
|
1.54
|
0.22
|
|
|
15-18
|
16.41
|
1.32
|
0.19
|
|
|
Diaphragmatic hiatus
|
0-2
|
6.71
|
1.15
|
0.21
|
<0.01
|
|
3-5
|
9.64
|
1.03
|
0.19
|
|
|
6-9
|
11.53
|
1.25
|
0.20
|
|
|
10-14
|
13.96
|
1.51
|
0.21
|
|
|
15-18
|
15.92
|
1.30
|
0.18
|
|
Table 3
Summary of the mean, standard deviation, and the standard error of mean of the effective
aortic diameter in gender subgroups
|
Effective diameter at
|
Group
|
Mean
|
Standard Deviation
|
Standard Error of Mean
|
|
Aortic Root
|
Female
|
21.72
|
5.41
|
0.54
|
|
Male
|
21.76
|
5.38
|
0.54
|
|
Ascending Aorta
|
Female
|
18.39
|
4.86
|
0.49
|
|
Male
|
18.49
|
5.03
|
0.50
|
|
Arch of aorta
|
Female
|
15.86
|
4.15
|
0.42
|
|
Male
|
15.93
|
4.24
|
0.42
|
|
Proximal
|
Female
|
12.56
|
3.38
|
0.34
|
|
descending Aorta
|
Male
|
12.79
|
3.47
|
0.35
|
|
Diaphragmatic
|
Female
|
12.14
|
3.33
|
0.33
|
|
hiatus
|
Male
|
12.31
|
3.48
|
0.35
|
On regression analysis, the best model was the polynomial regression model of an effective
diameter that included linear, quadratic, and cubic terms as independent variables.
An example of regression models employed for selection of the best fit model is provided
in [[Table 4]] along with respective R2 scores.
Table 4
Regression analysis of aortic root diameter to patients age
|
Model
|
Equation
|
R
2
|
|
(Ao Rt- Effective diameter of the aortic root, age in years). Polynomial regression
model of third order was selected as it had the highest R
2 value as the best fit model among all the regression models analysed
|
|
Exponential
|
Ao Rt=13.552e0.0464 (age)
|
0.7897
|
|
Linear
|
Ao Rt=0.8909 (age) + 13.405
|
0.8876
|
|
Logarithmic
|
Ao Rt=4.2768ln (age) + 13.866
|
0.8827
|
|
Polynomial
|
|
|
|
Order 2
|
Ao Rt= -0.0428 (age)2+1.6724 (age) + 11.22
|
0.9334
|
|
Order 3
|
Ao Rt=0.0045 (age)3-0.1647 (age)2+2.5385 (age) + 10.09
|
0.9443
|
|
Order 4
|
Ao Rt= -0.0006 (age)4+0.0279 (age)3-0.4384 (age)2+3.609 (age) + 9.2832
|
0.933
|
For all levels, the intercept and linear, quadratic, and cubic terms were significant
(all P < 0.05). The formulae for calculating the predicted diameters along with R2 for the
model used (polynomial regression model of order three) are tabulated in [[Table 5]].
Table 5
Formulae to calculate the predicted effective aortic diameter as a function of subjects
age
|
Aortic level
|
Formulae
|
R
2
|
|
EAD: Effective aortic diameter in mm, age in years
|
|
Aortic root
|
EAD=0.0045 (age)3-0.1647 (age)2+2.5385 (age) + 10.09
|
0.9443
|
|
Ascending aorta
|
EAD=0.0035 (age)3-0.1278 (age)2+2.0796 (age) + 8.25
|
0.937
|
|
Arch of aorta
|
EAD=0.0021 (age)3-0.0831 (age)2+1.5785 (age) + 7.579
|
0.903
|
|
Proximal descending Aorta
|
EAD=0.0024 (age)3-0.0796 (age)2+1.2802 (age) + 6.064
|
0.9122
|
|
Aorta at diaphragmatic hiatus
|
EAD=0.0023 (age)3-0.0743 (age)2+1.2337 (age) + 5.71
|
0.9195
|
Predicted diameters were calculated for each level and age group using the polynomial
regression models with cubic terms determined previously. MSE was also calculated
for each of the models. z scores were then calculated using the following formula,
z = (observed diameter-predicted diameter)/√MSE. The z scores calculated are of approximate
normal distribution, they have a mean of zero and SD of one. They represent how many
SDs above or below the observation is in relation to the mean (predicted regression
line). A z value of 1 signifies that the observed value is 1 SD above the estimated
mean of that level at that age-group, whereas a z of -1 signifies that the value is
1 SD below the mean. Assuming a normal distribution, approximately 68.3% of the population
will fall within the mean ±1 SD interval. Whereas 95.4% of the population is within
the mean ±2 SDs.
This data has been plotted in the form of graphs [[Figures 2], [3], [4], [5], [6]] that do not require any complex calculations to determine the normal. These graphs
contain the mean for age group and ±2 z score barricade lines.
Figure 2: Effective diameter of the aorta at the aortic root (in mm) versus age (in years).
Central line represents the mean (predicted normal diameter), above and below are
the z = ±2 score lines
Figure 3: Effective diameter of the ascending aorta (in mm) versus age (in years). Central
line represents the mean (predicted normal diameter), above and below are the z =
±2 score lines
Figure 4: Effective diameter of the aortic arch (in mm) versus age (in years). Central line
represents the mean (predicted normal diameter), above and below are the z = ±2 score
lines
Figure 5: Effective diameter of the proximal descending aorta (in mm) versus age (in years).
Central line represents the mean (predicted normal diameter), above and below are
the z = ±2 score lines
Figure 6: Effective diameter of the aorta at diaphragmatic hiatus (in mm) versus age (in years).
Central line represents the mean (predicted normal diameter), above and below are
the z = ±2 score lines
Discussion
It is imperative to acquire a complete and thorough knowledge of normality and its
variants to study and diagnose abnormalities and pathologies with certainty. Although
the normal standards for the diameter of thoracic aorta have been established for
adults, such standards are not well established in pediatric population. Though echocardiography
is the standard method for determining the size of the thoracic aorta in children.
A recent review of echocardiographic methods showed a general lack of standardization
in technique.[[1]]
Cross-sectional imaging (CT & MRI) standards of the normal aortic diameter in children
are not established. Our study aims to establish the normal aortic diameter at various
levels of the thoracic aorta on CECT thorax studies. We analyzed CECT studies in 207
children who had no history of cardiovascular disease or cardiothoracic surgery. Effective
diameters of the thoracic aorta were measured by double-oblique reconstructions perpendicular
to the aorta. Measurements were acquired at the aortic root, ascending aorta, arch
of the aorta, proximal descending aorta, and the descending aorta at diaphragmatic
hiatus. The effective diameter is the average of anteroposterior and transverse measurements.
The youngest subject of our study was a 10-day-old infant and the eldest was 18 years
of age. Regression analyses of the data were done, multiple regression models like
linear, logarithmic, exponential, polynomial with quadratic, and cubic terms were
analyzed and the best fit functional form for our data was selected by comparing the
R2 values for each model. The best-fit form was found to be polynomial regression
with cubic terms at all the levels studied and had R2 values of more than 0.9.
Fitzgerald et al.[[7]] in 1987 studied thoracic aortic diameter in 97 children aged between 2 weeks and
19 years and found a linear relationship with thoracic aortic diameter at various
levels with age and with thoracic vertebral body width. Like our study, they did not
find a significant distinction between male and female groups. However, they used
only axial CT images with 5-10 mm thick axial sections without multiplanar reconstructions
perpendicular to the aorta.
Akay et al.[[6]] reviewed CECT chest scans of 133 pediatric patients to measure descending and ascending
thoracic aortic diameter. They found that the ratio of the aortic diameter to that
of the thoracic vertebral diameter is a constant, about 1.1 at the level of ascending
aorta.
Wolak et al.[[8]] determined the aortic diameter at various levels on non-contrast cardiac CT and
defined the normal limits in relation to age, sex, and body surface area (BSA). However,
pediatric population was not included in the study.
Kaiser et al.[[4]] assessed the normal values for aortic diameters in 53 children and adolescents
by contrast-enhanced cardiovascular magnetic resonance (CMR)-angiography, with double-oblique
maximum intensity projections perpendicular to the aorta. Their study found a linear
relationship between the cross-sectional aortic diameter with the square root of BSA.
However, their study lacked any data on children aged less than 2 years.
Mohiaddin et al.[[9]] measured the normal dimensions of the thoracic aorta in 70 healthy volunteers on
MR imaging. They used end-diastolic spin-echo images in oblique planes through the
ascending aorta, transverse aorta, and the descending aorta. They correlated these
measurements with the BSA and found a linear correlation. However, the youngest subject
of the study was 10 years old, and the study had no information on children aged younger
than 10 years. The youngest subject of our study was 10 days old.
Hegde et al.[[2]] determined the normal effective diameter at various levels of the aorta on CECT
studies in children. They included 88 thoracic and 110 abdominal scans in the study.
They measured the average of the antero-posterior and the lateral diameters of the
thoracic and abdominal aorta at various levels on 1 mm collimation double oblique
reconstructions perpendicular to the course of the vessel. They calculated the z scores
at each level for a particular age group. As with our study, they derived a polynomial
regression model with cubic terms relating to the aortic diameters and log BSA. They
found a significant sex difference in the study population.
Bayindir et al.[[10]] evaluated thoracic CECT studies, and measured the diameters of ascending aorta,
descending aorta, main pulmonary artery, and right and left pulmonary arteries. They
concluded that the diameters of the thoracic vascular structures increased with age
and found a significant statistical difference among the age groups and genders, with
higher dimensions in male children. However, the study measured aortic dimensions
at two locations and did not attempt regression analysis of the statistical data.
Limitations of our study
As the scans included in the study were done for non-cardiac indications, electrocardiographic
gating was not routinely performed. This resulted in significant cardiac motion artifacts
in some cases, which could have introduced error in measurements, especially at the
aortic root.
The measurements acquired are neither end-systolic nor end-diastolic measurements.
The measurements were neither the true maximum nor minimum but rather intermediate
effective diameters.
A major limitation of the study was the small number of the study population and the
fact that it was carried out at a single institution which might not be a true representation
of the normal population.
Conclusions
Effective aortic diameter increases with age, however, their relationship with age
is not linear. A polynomial regression model with cubic terms is the best fit functional
form to describe the relation between aortic diameter and age, at all the levels studied.
The R2 values of the study model were high (>0.9) and significant at all levels.
The range of normal effective diameters of the aorta at multiple levels, the predicted
mean and the ±2 SDs values were determined and plotted on graphs. The knowledge of
these normal ranges and the use of graphs can aid the radiologist in diagnosing abnormalities
like ectasia, aneurysm, stenosis, hypoplasia, etc.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms.
In the form the patient(s) has/have given his/her/their consent for his/her/their
images and other clinical information to be reported in the journal. The patients
understand that their names and initials will not be published and due efforts will
be made to conceal their identity, but anonymity cannot be guaranteed.
