Key words
thorax - CT-spiral - radiation safety
Introduction
Multidetector computed tomography (MDCT) has become an essential technique in daily
practice [1]
[2]. Over the years, CT imaging essentially accounted for a trend of increasing medical
radiation exposure [3]
[4]. The use of CT in pediatric patients has been among the most rapidly increasing
diagnostic procedures [5]. As CT involves significantly higher radiation doses than conventional X-ray, possible
radiation-induced side effects have to be considered [6]. In a pediatric setting, the high radio-sensitivity of the developing tissues and
the remaining years of life in which cancer might occur underline the significance
of an effective patient care program with careful management of pediatric patient
dose. Several cohort studies have predicted an increased cancer risk associated with
CT scans, particularly concerning leukemia and also some solid cancer types [2]
[7]
[8]
[9]
[10].
In this context, the gradually increasing awareness of radiation exposure, mainly
from CT examinations, has motivated manufacturers to develop techniques to manage
radiation dose [11]. In this context, optimized protocols for routine CT examinations, state-of-the-art
technique and the indication for CT in the light of possible adverse effects are of
special concern [2]. Different methods of reducing radiation dose have been developed in recent years,
such as low dose protocols [12] with automated attenuation-based tube current modulation and tube voltage selection
[13]
[14] or the use of tin filtration [15]. Moreover, reconstruction methods – like iterative algorithms [16] – and high-pitch technique have been evaluated in several studies [17]
[18]
[19]. In our study, iterative reconstruction was not applied. However, state-of-the-art
CT scanners with well managed dual-source CT protocols (e. g. low kV protocols) and,
to a lesser degree, the corresponding techniques of dose reduction (e. g. iterative
reconstruction) were not widely available in routine clinical practice for the examination
of pediatric patients, especially until the middle of the 2010’s [20]. We believe that our study is still relevant for wide ranges of pediatric examinations
worldwide, where these technological innovations are not available.
The aim of this retrospective observational study was to identify trends in state-of-the-art
pediatric CT concerning radiation dose management in a hospital with maximum care
facilities including a specialized pediatric radiology department.
Materials and methods
All CT examinations were performed in a single center with a board-certified division
of pediatric radiology in the period from January 2007 to March 2014. The inclusion
criterion was clinically indicated CT including the chest at the age of less than
18 years.
Patient characteristics
1695 CT examinations were performed in 768 patients (mean age ± standard deviation,
9.8 ± 5.6 years; median and 95 % confidence level, 10.4 years [9.6; 10.1]; 1004 male
(10.1 years ± 5.6; 10.4 years, [9.8; 10.5]) and 691 female (9.4 years ± 5.5; 10.2
years, [9.0; 9.8])). Based on the relation between children age and body stature [21] and according to the diagnostic reference levels of the Federal Office for Radiation
Protection [22], patients were divided into the following age groups: neonates (n = 16), ≥ 1 month
and ≤ 1 year (infants; n = 195), 2 – 5 years (n = 285), 6 – 10 years (n = 405), 11 – 15
years (n = 531), 16 – 17 years (n = 263). Because of the small sample size, neonates
were excluded from the subgroup analyses. The underlying indications for performing
CT are listed in [Table 1]. As [Fig. 1] illustrates, the number of CT examinations did not change significantly in the observation
time.
Table 1
Indications for chest CT scans.
Tab. 1 Indikationen für Thorax-CTs.
|
indication
|
number (n)
|
|
infection process
|
852
|
|
monitoring of progress or response under therapy
|
239
|
|
non-malignant and non-infectious processes
|
205
|
|
cardiac and vascular imaging
|
149
|
|
staging
|
147
|
|
follow-up
|
40
|
|
trauma management
|
21
|
Fig. 1 Number of scans and patient age per year. Number of scans performed (n) and boxplots
of patient age (in years) in the different years of the observation time. It should
be noticed that in the year 2014 only three months were included in the study. In
the year 2007 79 CTs were performed. However, the number of scans remains above 200
examinations per year in the main years of observation from 2008 to 2013. The mean
patient age decreased from 10.5 to 9.1 years in 2013.
Abb. 1 Untersuchungszahlen und Patientenalter pro Jahr. Anzahl der durchgeführten Untersuchungen
(n) und Boxplots des Patientenalters (in Jahren) in den verschiedenen Jahres des Beobachtungszeitraums.
Dabei ist zu bemerken, dass im Jahr 2014 nur drei Monate in die Studie eingeschlossen
wurden. Im Jahr 2007 wurden 79 CT-Untersuchungen durchgeführt. In den Hauptjahren
des Beobachtungszeitraums von 2008 bis 2013 verbleibt die Anzahl der durchgeführten
Untersuchungen über 200 pro Jahr. Das mittlere Patientenalter sank von 10.5 auf 9.1
Jahre im Jahr 2013.
CT devices
Six different CT scanners (Siemens Healthcare GmbH, Forchheim, Germany.; [Table 2]), each in line with the currently available CT generation, were used in our study,
depending on the date of implementation in our institution ([Fig. 2]): Somatom Sensation 64 (05/2007 – 03/2014; 42 % of all examinations), Somatom Definition
Flash (09/2010 – 03/2014; 26 %), Somatom Sensation Open (01/2007 – 09/2009; 23 %),
Somatom Definition (01/2007 – 05/2010; 7.5 %), Somatom Definition AS+ (10/2009 – 07/2013;
1 %), Somatom Sensation 16 (09/2008 – 04/2013; 0.5 %).
Table 2
CT devices available in our study.
Tab. 2 Verfügbare CT-Geräte in der Studie.
|
device
|
somatom sensation 64
|
somatom definition flash
|
somatom sensation open
|
somatom definition
|
somatom definition AS+
|
somatom sensation 16
|
|
availability
|
05/2007 – 03/2014
|
09/2010 – 03/2014
|
01/2007 – 09/2009
|
01/2007 – 05/2010
|
10/2009 – 07/2013
|
09/2008 – 04/2013
|
|
number of examinations
|
716
|
444
|
388
|
124
|
16
|
7
|
|
detector
|
UFC detector
|
2 × Stellar detector
|
UFC detector
|
UFC detector
|
UFC detector
|
UFC detector
|
|
number of slices
|
64
|
2 × 128
|
40
|
128
|
128
|
16
|
|
rotation time (s)
|
0.33
|
0.28
|
0.5
|
0.33
|
0.3
|
0.5
|
|
kV steps (kV)
|
80, 100, 120, 140
|
70, 80, 100, 120, 140
|
80, 100, 120, 140
|
80, 100, 120, 140
|
70, 80, 100, 120, 140
|
80, 100, 120, 140
|
|
max. pitch
|
2.0
|
3.2
|
2.0
|
1.5
|
1.5
|
2.0
|
|
AEC
|
CARE Dose4D
|
CARE Dose4D
|
CARE Dose4D
|
CARE Dose4D
|
CARE kV
|
CARE Dose4D
|
UFC = Ultra-fast ceramic; AEC = Automatic exposure control, automatische Belichtungssteuerung;
CARE Dose4D = User-specific adjustment of image quality reference mAs and real-time
mAs modulation for each rotation, benutzerspezifische Anpassung von Bildqualitäts-Referenz-mAs
und Echtzeit-mAs-Modulation für jede Rotation; CARE kV = additional automatic kV selection
technique, zusätzliche automatische Modulation der Röhrenspannung.
Fig. 2 Dose parameters per year and availability of different scanners. Median volume CT
dose index (CTDI; mGy), effective dose (Eeff, mSv) and size-specific dose estimate
(SSDE; mGy) in the different years of the observation time. Linear regression was
performed for each parameter and regression bands are shown. Additionally, the period
of availability for each scanner is outlined at the top.
Abb. 2 Dosiswerte pro Jahr und Verfügbarkeit der verschiedenen CT-Geräte. Medianwerte für
Volumen-CT-Dosisindex (CTDI; mGy), effektive Dosis (Eeff, mSv) und size-specific dose
estimate (SSDE; mGy) in den verschiedenen Jahres des Beobachtungszeitraums. Es wurde
eine lineare Regression mit Angabe der Konfidenzbereiche für jeden Parameter durchgeführt.
Zudem ist im oberen Bildabschnitt die zeitliche Verfügbarkeit jedes CT-Gerätes dargestellt.
CT examination
All procedures were in accordance with the 1964 Helsinki declaration and its later
amendments or comparable ethical standards. For this retrospective study formal consent
was not required. All acquisitions were performed with specific pediatric CT protocols
determined from the division of pediatric radiology. Scan parameters were adapted
to patient body weight and age. The CT settings followed the current standard operating
procedures at the time of the examination. Accordingly, parameters also depended on
the technical conditions present at the time of the scan (as mentioned above; CT devices).
Iomeprol (Imeron 400®) or Iopromid (Ultravist 370®, both Bayer Healthcare Deutschland, Leverkusen, Germany) were applied as intravenous
contrast agent, either via a manual injection (body weight < 10 kg) or by a dual head
power injection device. The injected volume was adjusted to the patients’ body weight.
The injection of an amount of up to 20 ml of contrast agent was performed manually.
Otherwise, the automatic injection device was used.
Patients who were not older than 6 years were examined breathing freely, whereas children
above the age of 6 received a breathing command. A support cushion was used to ensure
a stable position of infants on the moving CT table. In clinical care, diagnostic
reading was carried out by board-certified pediatric radiologists.
In 95 % of all examinations, a slice thickness of 3 mm and an increment of 3 mm were
applied. A slice thickness of 5 mm and an increment of 5 mm were chosen in only 5 %
of cases. For high-resolution (HR) images of the lung parenchyma, a high-resolution
kernel (B60f) with a slice thickness of 1 mm and an increment of 5 mm was used. Further
coronal and sagittal reformations were performed. The typical scan range was chosen
from the jugular notch (fossa jugularis sternalis) as the cranial starting point and
in the caudal direction including the adrenal glands as a landmark. The mean scan
range was 22.9 cm, ranging from 8.6 – 33.6 cm.
Most examinations were performed with 100 kV (n = 1104; 65 %) or 120 kV (n = 509;
30 %). Less frequent kV values were 80 (n = 78; 4.5 %) and 140 kV (n = 4; 0.5 %).
The patients scanned with 140 kV were among the age groups “11 – 15 years” and “16 – 17
years”. In these examinations angiography of the thoracic vessels was included, which
may contribute to the relatively high dose parameters (effective mAs range: 122 to
128 mAs; CTDIvol range: 11 to 15 mGy). Most scans (n = 1259) were non-enhanced. Intravenous contrast
agents were used in 436 acquisitions. The injected volume was adjusted to the patients’
body weight. 403 of them were primarily contrast-enhanced and 33 examinations included
non-contrast and contrast-enhanced series. No iterative reconstruction algorithms
were available during the observation time.
Radiation dose
Tube voltage (kV), volume CT dose index (CTDI; mGy) and dose length product (DLP;
mGy ∙cm) were recorded for all CT acquisitions and extracted from the examination
protocol.
In accordance with the European Guidelines on Quality Criteria for Multislice Computed
Tomography, the effective radiation dose (Eeff; mSv ) was calculated for all examinations by multiplying DLP with a conversion coefficient,
depending on the body region scanned and the patient age (newborns 0.039; 1 year 0.026;
5 years 0.018; 10 years 0.013 mSv/(mGy·cm)) [23]. The CT scanners used in our study reported a CTDIvol referring to a 32-cm body phantom. However, for pediatric patients the conversion
coefficient mentioned above refers to a 16-cm body phantom so that our DLP values
had to be converted to a 16-cm phantom. For this purpose we used an additional scanner-specific
conversion factor provided by the manufacturer ([Table 3]) to convert DLP to a 16-cm phantom suitable for dose estimation in pediatric patients.
However, even if the concept of Eeff has not been developed for patients and its applicability in children is indeed severely
limited, it may serve as a rough estimation of the dose delivered.
Table 3
Scanner-specific conversion factors from DLP Ø 32 cm to DLP Ø 16 cm.
Tab. 3 Scanner-spezifische Konversionsfaktoren von DLP Ø 32 cm auf DLP Ø 16 cm.
|
pediatric body
|
somatom sensation 16
|
somatom definition AS+
|
somatom definition
|
somatom definition flash
|
somatom sensation open
|
somatom sensation 64
|
|
80 kV
|
n. a.
|
n. a.
|
2.5
|
2.3
|
2.2
|
2.2
|
|
100 kV
|
2.0
|
2.4
|
2.4
|
2.2
|
2.1
|
2.1
|
|
120 kV
|
2.0
|
2.3
|
2.3
|
2.2
|
2.1
|
2.0
|
|
140 kV
|
n. a.
|
n. a.
|
n. a.
|
2.2
|
2.0
|
2.0
|
DLP = Dose length product, Dosislängenprodukt.
In accordance with the American Association of Physicists in Medicine, size-specific
dose estimates (SSDE; mGy) were calculated for all examinations [24]. Axial images were used to measure the widest transverse and anteroposterior skin-to-skin
diameters of each patient and to determine the conversion factor.
To quantify the development of the dose descriptors (CTDI, SSDE, Eeff) within the observation time, linear regression was performed for each age group. Using
the equations of the resulting lines, the percentage reduction in the CTDI, Eeff and SSDE per year was calculated. In addition, median dose parameters were compared
between the starting year and the final year of our study for each age group.
High-pitch protocols
The use of high pitch values was documented in our database. High-pitch protocols
were only available on Somatom Definition Flash (Siemens Healthcare GmbH, Forchheim,
Germany), which was established in 09/2010 in our institution. Thus, the evaluation
of a potential impact of high-pitch protocols on dose estimates was limited to scans
within this period (n = 944 scans in total).
Dose modulation technique
Dose modulation technique was recorded if used. It was available during the entire
observation period. Depending on the CT device, an automatic exposure control (AEC)
algorithm (CARE Dose 4 D) and, where available, an automatic tube voltage adjustment
technique (CARE kV; both Siemens Healthcare GmbH, Forchheim, Germany) were used.
CARE Dose4D enables a user-specific adjustment of “image quality reference mAs” (mAsref) for each examination, depending on the required image quality. The parameter expresses
the mAs applied on an average-sized phantom. However, the system also performs real-time
mAs modulation for each rotation around the patient, depending on the attenuation
profile. For this purpose, a single topogram was performed prior to each scan. Finally,
the term of effective mAs (mAseff) takes the dependence on pitch factor into consideration and is defined as the quotient
of mAs and pitch factor.
In addition, CARE kV adjusts the tube voltage (kV settings) suitable for the patient
size and the chosen examination protocol. Tube current is then adapted automatically
to maintain a constant contrast-to-noise ratio and to reach an appropriate combination
of voltage and current [13]. The two techniques (CARE Dose4D and CARE kV) can be used simultaneously.
Diagnostic image quality
To assess the non-inferiority in diagnostic image quality of acquisitions with different
radiation dose levels, non-enhanced and i. v. contrast-enhanced examinations (n = 20)
with the highest and the lowest CTDI in each age group were analyzed. Image quality
was assessed independently on a 3-point Likert scale (from 1 = lowest to 3 = highest
image quality) by three readers (one senior physicians with seven years of experience
and two assistant physicians each with two years of experience) on a standard PACS
workstation (Centricity RA 1000, GE Healthcare, Waukesha, Wisconsin, USA). In non-enhanced
scans, the pulmonary intersegmental septum of the lower lobe, subsegmental pulmonary
arteries and bronchi, as well as motion and breathing artifacts were evaluated. In
enhanced scans coronary arteries and sinus, the aortic valve and pulsation artifacts
were assessed additionally. Readers were blinded to all identifying data and technical
details.
Statistical analysis
For statistical analysis, IBM SPSS Statistics (version 22 for Windows, Ehningen, Germany)
and (SAS jmp, version 11.1.1 for Windows, SAS Institute Inc., Cary, NC, USA) were
used.
For all statistical tests, a significance level of p < 0.05 was set. Continuous variable
data are presented as means ± standard deviations. Data that did not follow a normal
distribution are presented as median with 95 % confidence interval (CI). By using
univariate multifactorial analysis of variance, the influence of age, sex, CT scanner,
pitch factor and dose modulation on dose descriptors was evaluated.
A non-inferiority analysis was performed for the 3-point image quality score using
a 95 % confidence interval [25]. A non-inferiority margin of 0.2 image quality score points for the difference between
the examinations was predefined. To evaluate the interrater reliability, a two-way
intraclass correlation coefficient (ICC) was calculated.
Results
Radiation dose
Dose descriptors depend on patient age; see the appendix for detailed numerical information
about the dose values.
[Fig. 2] demonstrates that the median dose-specific values generally decreased within the
observation time. Especially between the years 2009 and 2010, a major leap can be
observed. In the period from 2011 to 2013 only minor differences can be found. In
addition, [Fig. 3] confirms the dose reduction in all age groups. Following linear regression, dose
parameters dropped by an average of 12 % per year in the age groups ≤ 1 year, 2 – 5
years, 6 – 10 years and 11 – 15 years. Only in the age group “16 – 17 years” was a
minor annual decrease of 10 % observed. Comparing the median dose values from the
starting year 2007 to the final year of our study reveals a percentage decrease of
74 % in the age group ≤ 1 year and a reduction of 90 %, 76 %, 82 % and 82 % in the
following age groups.
Fig. 3 Dose parameters in the different age groups. Median CT volume dose index (CTDI; mGy),
effective dose (Eeff, mSv) and size-specific dose estimate (SSDE; mGy) in the different
age groups and through the years of the observation time.
Abb. 3 Dosiswerte in den verschiedenen Altersgruppen. Medianwerte für Volumen-CT-Dosisindex
(CTDI; mGy), effektive Dosis (Eeff, mSv) und size-specific dose estimate (SSDE; mGy)
in den verschiedenen Altersgruppen und Jahren des Beobachtungszeitraums.
High-pitch protocols
In the period from 09/2010 to 03/2014, 414 of 944 examinations were performed with
high pitch values (≥ 3.0) on Somatom Definition Flash. [Fig. 4] illustrates the distribution of dose descriptors in the different age groups. All
indicators showed lower values when high-pitch protocols were used, independent of
contrast agent use. When age, sex, and the use of AEC were included in univariate
analysis, these differences between acquisitions with and without high-pitch mode
were statistically significant (p < 0.05).
Fig. 4 Dose parameters depending on the application of high-pitch mode. Median volume CT
dose index (CTDI; mGy), effective dose (Eeff, mSv) and size-specific dose estimate
(SSDE; mGy) in the different age groups depending on the application of high-pitch
mode (Y = yes, N = no) and contrast agent use (left, non-enhanced; right CM = contrast
agent). This figure is limited to scans in the period from 09/2010 to 03/2014 (n = 944).
Abb. 4 Dosiswerte in Abhängigkeit von Verwendung des High-pitch-Modus. Medianwerte für Volumen-CT-Dosisindex
(CTDI; mGy), effektive Dosis (Eeff, mSv) und size-specific dose estimate (SSDE; mGy)
in den verschiedenen Altersgruppen in Abhängigkeit der Verwendung des High-Pitch-Modus
(Y = ja, N = nein) und einer Kontrastmittelapplikation (links, nativ; rechts, kontrastangehoben).
Die Abbildung beschränkt sich auf die Untersuchungen in der Zeitspanne von 09/2010
bis 03/2014 (n = 944).
Dose modulation
A dose modulation technique was used in 409 of all 1695 examinations. These scans
were performed with a median effective mAs of 74 mAs [95 % CI, 77; 85]. 80 kV were
chosen in 39 cases, 100 kV in 161 scans and 120 kV in 206 scans. Three patients were
scanned with 140 kV (Methods and materials, CT examination). These parameters accounted
for a median CTDI of 4.3 mGy [95 % CI, 5.2; 6.0], a median Eeff of 4.1 mSv [95 % CI, 5.1; 6.0] and a median SSDE of 7.2 mGy [95 % CI, 8.2; 9.4].
In non-enhanced acquisitions dose parameters were higher in all age groups when AEC
was used compared to a manual adjustment of the examination protocol ([Fig. 5]). These differences were statistically significant in the age groups 6 – 10 years,
11 – 15 years and 16 – 17 years (p < 0.001) – even when sex and the use of different
CT devices was taken into account – and not statistically significant in the age groups
≤ 1 year and 2 – 5 years (p = 0.08 – 0.70).
Fig. 5 Dose parameters depending on the application of dose modulation. Median volume CT
dose index (CTDI; mGy), effective dose (Eeff, mSv) and size-specific dose estimate
(SSDE; mGy) in the different age groups depending on the application of a dose modulation
technique (Y = yes, N = no) and contrast agent use. This figure includes all scans.
Abb. 5 Dosiswerte in Abhängigkeit von Verwendung einer automatischen Dosismodulation. Medianwerte
für Volumen-CT-Dosisindex (CTDI; mGy), effektive Dosis (Eeff, mSv) und size-specific
dose estimate (SSDE; mGy) in den verschiedenen Altersgruppen in Abhängigkeit der Verwendung
einer automatischen Dosismodulation (Y = ja, N = nein) und einer Kontrastmittelapplikation.
Die Abbildung berücksichtigt alle Untersuchungen.
In 298 of 436 contrast-enhanced chest CTs, AEC was carried out. The age groups “≤ 1
year” and “16 – 17 years” showed a lower median CTDI, Eeff and SSDE when AEC was applied. However, only in the age group “16 – 17 years” was
the difference statistically significant (p = 0.04; age group “≤ 1 year”, p = 0.5)
([Fig. 5]). The median values of age groups 2 – 5 years, 6 – 10 years and 11 – 15 years presented
higher dose parameters in scans with dose modulation. For the two older age groups,
these differences were statistically significant (p < 0.001; age group “2 – 5 years”,
p = 0.2).
Image quality
The mean image quality scores were 2.18 [95 % CI, 2.04; 2.32] in the subgroup with
the highest CTDI values and 2.14 [95 % CI, 1.98; 2.30] in acquisitions with the lowest
radiation dose. Thus, differences in image quality between the examinations with the
highest and lowest CTDI values resided within the predefined non-inferiority margin
([Fig. 6]). The overall inter-rater reliability was found to be ICC 0.889, (95 % CI: 0.784,
0.950; p < 0.001), indicating almost perfect agreement.
Fig. 6 Non-inferiority analysis of image quality scores. Non-inferiority analysis with the
mean image quality score of the subgroup with high radiation dose (standard method;
2.18) and the resulting non-inferiority range (2.18 – 1.98). Two sided 95 % confidence
interval (CI) of the test method (subgroup with low radiation dose; CI 95 %, 1.98;
2.30) resides within the margin.
Abb. 6 „Non-inferiority“-Analyse der Bildqualität. „Non-inferiority“-Analyse mit Darstellung
des mittleren Punktwertes der Bildqualität in der Untergruppe mit hoher Strahlendosis
(Standardmethode; 2.18) und des entsprechenden „Non-inferiority“-Bereichs (2.18 – 1.98).
Das zweiseitige 95 %-Konfidenzintervall (CI) der Testmethode (Untergruppe mit niedriger
Strahlendosis; CI 95 %, 1.98; 2.30) verbleibt innerhalb dieser Grenzen.
Discussion
In our retrospective observational study over a period of seven years, a significant
reduction in radiation dose in pediatric chest CT was observed without a loss of diagnostic
image quality using state-of-the-art scanners and dose optimization strategies. In
particular, the implementation of high-pitch scanning seems to essentially contribute
to this development. Interestingly in this context, the different AEC methods provided
were not as efficient as expected. In an early patient study dose values were reduced
by an average of 38 % when using tube current modulation [14].
CT scanners and software equipment evolved as decisive influencing factors of CT radiation
dose [19]. Meanwhile the focus shifted from high-quality CT imaging with precise anatomic
information to better manage radiation dose and to maintain a diagnostic image quality
related to the clinical question. However, this fact should be further addressed in
national and international guidelines for radiation protection. During the process
of accepting noisier images without reducing diagnostic capabilities, pediatric imaging
has adopted a pioneering role [7]. Even with regard to adult CT, a maximum of image quality is rarely required to
answer the clinical question.
Comparing our dose parameters (Appendix) to a national survey from 2007 that proposed
reference dose values in pediatric CT for each age group, we found that in the initial
year of the observation time (2007) the median CTDI in our study was at the upper
limit of the corresponding reference values [26], for example in children aged 2 – 5 with a median CTDI of 5.4 mGy and a reference
value of 5.5 mGy. In this period, the CTDI exceeded the reference level in the age
group of infants with only four patients (≥ 1 month and ≤ 1 year; median CTDI, 4.3
mGy; reference value 3.5 mGy). However, the CTDI was clearly below the reference levels
in the following years across all age groups, also compared to the latest announcement
of the Federal Office for Radiation Protection (BfS) [27]. Especially in the last years of the observation time (2013 and 2014), the CTDI
was more than three times lower than the corresponding reference value in some age
groups, for example in children aged 11 – 15 with a median CTDI of 1.8 mGy (reference
value, 6.5 mGy) matching a range of low-dose imaging.
Miglioretti et al. reported mean radiation doses of 744 scans performed between 2001
and 2011 that were more than twice as high as in our study, when comparing the corresponding
age groups [1]. It can be supposed that different local guidelines for radiation protection and
the technical developments made since the ending of this previous study account for
the differences in dose estimation. However, it has to be considered that iterative
reconstruction was not applied in our study and its use may have led to a further
dose reduction [16].
The implementation of dual-source CT devices allowed for high-pitch scanning, which
has been applied to pediatric chest and cardiac CT to reduce motion artifacts [7]. Our data included a significant decline in median dose values from 2009 to 2010,
when Siemens Somatom Definition Flash (Siemens Healthcare GmbH, Forchheim, Germany)
with the possibility of high-pitch protocols was implemented. Providing a large patient
population, the method appears recommendable in all age groups and independent of
contrast agent use. In addition, further settings of the scanner may have contributed
to dose reduction: In contrast to the other CT devices in our study, Somatom Definition
Flash has two detectors combining the photodiode and an analog-to-digital converter,
which may reduce electronic noise, especially in lower dose applications ([Table 2]). In smaller patient populations and – compared to our patient cohort – restricted
age groups, favorable radiation doses have been reported with the high-pitch technique
for pediatric chest CT imaging and CT angiography (CTA) [18]
[28]
[29]. In addition, several previous studies have suggested that high-pitch scanning does
not compromise image quality, which seems reasonable in light of our results [28]
[29].
Our data demonstrate that the use of a dose modulation technique was not always advantageous
and should be considered individually. Karmazyn et al. found that in abdominal CT
with AEC, dose reduction was least effective for body weight less than 20 kg (our
age groups 1 – 3, considering Child Growth Standards of the World Health Organization,
WHO) [30]
[31]. The vast majority of children from ages 2 – 15 received higher radiation exposure
when AEC was used in contrast-enhanced chest CT. However, adolescents (16 and 17 years)
seem to benefit from dose modulation, which agrees with Karmazyn’s statement about
the highest effectiveness of AEC at a body weight range over 60 kg [30]. However, according to our results, dose modulation is applicable in infants, which
contradicts the previous findings. One of the reasons for this phenomenon could be
that manual adjustment is hampered in the youngest and oldest age groups, so that
the stored algorithms of AEC achieve comparatively favorable dose parameters. In infants
a relatively low patient number and little experience with manual adaption in the
early years of the study can be considered as possible reasons. However, the oldest
age group presents a rather heterogeneous patient cohort concerning weight and body
diameter, so that fluctuating dose values may occur. Our results are important because
radiologists should not only take patient size but also the use of contrast agent
into account when considering pediatric chest CT with dose modulation.
The use of dose modulation systems includes potential sources of error, which may
explain the heterogeneous data in the literature and the unfavorable dose parameters
in our study. A possible technical problem refers to an inappropriate configuration
of the scanner for the use of AEC with different patient sizes. In this context, we
suspect that the expertise of the manufacturer and our experience with AEC protocol
utilization were limited in the early years of our study. At that time, it is possible
that inappropriately high reference mAs values were selected, which may partly explain
the elevated dose values and provides some potential for further optimization. After
AEC protocols were adopted from the manufacturer initially, they have been adjusted
in the following years with increasing know-how and growing feedback to the manufacturer.
In addition, patient movement between the localizer and subsequent scans may lead
to application of an inappropriately adjusted tube current profile. Finally, an inaccurate
position on the CT table and insufficient centering of the patient may be relevant
in younger children [32]. In this context, manual adjustment should be considered as a reasonable alternative
to AEC, at least in dedicated pediatric centers with the necessary pediatric CT experience.
Further development on this issue for the pediatric population is recommended.
Some aspects of our study may seem obvious. Nevertheless such a calculation of dose
values and analysis of the dose development in the context of the clinical routine
process have not yet been published to the best of our knowledge. As low kV protocols
and iterative reconstruction are not universally included in pediatric CT today, the
results of our study still seem relevant for a majority of pediatric examinations
worldwide.
Our study has the following limitations: The study was set up at a single center;
consequently it depends on the local standard of care and the on-site technical installations,
for example CT devices of only a single manufacturer were used. Additionally, due
to the lack of availability, iterative reconstruction algorithms and low kV protocols
were not included.
In summary, radiation dose in pediatric chest CT has been considerably reduced in
the last decade without a relevant loss of image quality. High-pitch scanning appears
to be an effective method of dose optimization whenever iterative reconstruction is
not available. However, in our institution the use of dose modulation techniques had
to be considered individually according to patient size and depending on the examination
protocol. In this context, a centralized position of the patient on the CT table is
of basic importance for reliable dose adjustment. Moreover, additional options are
available today to tap the further potential of dose reduction, for example iterative
reconstruction.
-
High-pitch protocols should be used for pediatric chest CT.
-
Acquisitions with fixed parameters should be considered as an alternative to AEC.
-
The use of AEC may be advantageous in adolescents.
Abbreviations
AEC:
Automatic Exposure Control
CI:
Confidence Interval
CT:
Computed Tomography
CTA:
Computed Tomography Angiography
CTDI:
Volume CT Dose Index
DLP:
Dose Length Product
Eeff
:
Effective Radiation Dose
ICC:
Intraclass Correlation Coefficient
IV:
Intravenous
SSDE:
Size-Specific Dose Estimate
WHO:
World Health Organization
