The document evaluates the automatic exposure control (AEC) systems from four major CT scanner manufacturers in terms of their ability to reduce radiation dose while maintaining image quality. By scanning an anthropomorphic chest phantom with and without AEC activated on 16- and 64-slice CT scanners, the authors found that: 1) Tube current modulation dynamics were similar between GE and Toshiba systems and between Philips and Siemens systems. 2) Radiation dose was reduced by 35-60% with AEC activated, but image noise generally increased, especially in low-dose regions like the lungs. 3) Image noise became more uniform along the scanning direction with AEC activated compared to a fixed tube current.

1. ORIGINAL ARTICLE
Automatic exposure control in computed tomography – an evaluation
of systems from different manufacturers
MARCUS SÖDERBERG & MIKAEL GUNNARSSON
Department of Medical Radiation Physics, Malmö, Lund University, Skåne University Hospital, Malmö, Sweden
Background: Today, practically all computed tomography (CT) systems are delivered
with automatic exposure control (AEC) systems operating with tube current modulation in
three dimensions. Each of these systems has different specifications and operates somewhat
differently.
Purpose: To evaluate AEC systems from four different CT scanner manufacturers: General
Electric (GE), Philips, Siemens, and Toshiba, considering their potential for reducing radiation
exposure to the patient while maintaining adequate image quality.
Material and Methods: The dynamics (adaptation along the longitudinal axis) of tube
current modulation of each AEC system were investigated by scanning an anthropomorphic
chest phantom using both 16- and 64-slice CT scanners from each manufacturer with the AEC
systems activated and inactivated.The radiation dose was estimated using the parameters in the
DICOM image information and image quality was evaluated based on image noise (standard
deviation of CT numbers) calculated in 0.5 cm2
circular regions of interest situated throughout
the spine region of the chest phantom.
Results: We found that tube current modulation dynamics were similar among the different
AEC systems, especially between GE and Toshiba systems and between Philips and Siemens
systems. Furthermore, the magnitude of the reduction in the exposure dose was considerable,
in the range of 35–60%. However, in general the image noise increased when the AEC systems
were used, especially in regions where the tube current was greatly decreased, such as the lung
region. However, the variation in image noise among images obtained along the scanning
direction was lower when using the AEC systems compared with fixed mAs.
Conclusion: The AEC systems available in modern CT scanners can contribute to a signifi-
cant reduction in radiation exposure to the patient and the image noise becomes more uniform
within any given scan.
Key words: Radiation exposure; image quality; anthropomorphic chest phantom; computed
tomography; automatic exposure control
Marcus Söderberg, Department of Medical Radiation Physics, Malmö, Lund University, Skåne
University Hospital, SE-205 02 Malmö, Sweden (tel. 46 40 33 86 51, fax. 46 40 96 31 85,
e-mail. marcus.soderberg@med.lu.se)
Submitted August 19, 2009; accepted for publication February 11, 2010
Although computed tomography (CT) is not the most
common radiological examination, it is responsible
for the largest contribution to the collective effec-
tive dose to patients in radiology. According to the
United Nations Scientific Committee on the Effects
of Atomic Radiation, CT constituted approximately
5% of all radiological examinations in the world but
contributed to about 34% of the collective effec-
tive dose to the population (period 1991–1996) (1).
A recent report from the Swedish Radiation Safety
Authority showed that CT accounts for 58% of the
total radiation dose from medical radiological exam-
inations in Sweden (2). The challenge to radiolo-
gists and medical physicists is to establish adequate
image quality with the lowest radiation exposure to
the patient, in agreement with the ALARA (As Low
As Reasonably Achievable) principle.
Over the past years, there has been increasing interest
in the addition of automatic exposure control (AEC)
systems to CT scanners for optimizing CT examinations
(3–7). Today, practically all CT systems are delivered
with AEC systems operating with tube current modula-
tion in three dimensions (3D). Each of these systems
has different specifications and operates somewhat dif-
ferently. However, their main principle is to manage the
required image quality and radiation dose in a repro-
ducible manner by adapting the tube current to the
patient’s shape, size, and attenuation. If theAEC system
DOI 10.3109/02841851003698206 © 2010 Informa UK Ltd. (Informa Healthcare, Taylor & Francis AS)
ACTA RADIOLOGICA
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2. M. Söderberg & M. Gunnarsson626
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is properly optimized, it can reduce the radiation dose
to the patient by about 20–40% while still producing an
image of sufficient quality for confident diagnosis (8).
AEC systems have a number of benefits: better control
of the dose absorbed by the patient, improved consis-
tency of image quality among patients, reduction of
certain image artifacts, and reduced load on the X-ray
tube, which increases its lifetime (9). Several reports
have demonstrated the efficacy of AEC systems by
using homogeneous phantoms and performing clinical
work (9–17). However, few studies have investigated
3D AEC systems in terms of dose versus image quality
using anthropomorphic phantoms (18–21).
The purpose of this study was to evaluate the poten-
tial for reducing radiation exposure to the patient
while maintaining adequate image quality using the
AEC systems from four different CT manufacturers:
GE Healthcare (Milwaukee, Wisc., USA), Philips
Medical Systems (Best, The Netherlands), Siemens
Medical Solutions (Erlangen, Germany), and
Toshiba Medical Systems (Tokyo, Japan).
Material and Methods
AEC is a technique that performs automatic modulation
of the tube current in the x-y plane (angular modula-
tion), along the scanning direction (z-axis; longitudinal
modulation), or both (combined modulation; Table 1)
(22–24). The modulation is performed according to the
patient’s size and shape, and the attenuation of the body
parts being scanned. The operator selects an indicator
of the image quality that is required and then the system
adjusts the tube current to obtain the predetermined
image quality with improved radiation efficiency.
AEC systems
The combined tube current modulation system from
GE is AutomA 3D (9, 25, 26), which consists of
two parts: AutomA provides longitudinal AEC and
SmartmA provides angular AEC. The two parts can
be used separately or in concert. The image quality
is specified in terms of a selected noise index (NI),
defined as the standard deviation (SD) of pixel values
in the central region of an image of a uniform water
phantom. Based on each patient’s attenuation values
measured on the scan projection radiograph (SPR), the
tube current is adjusted to preserve the same level of
noise in each image. The system allows the operator to
define the range within which the tube current can be
modulated by selecting minimum and maximum mA
limits.
The Philips AEC system, DoseRight, has three parts:
Automatic Current Selection (ACS), which provides
patient-based AEC; D-DOM, which provides angular
AEC; and Z-DOM, which provides longitudinal AEC
(9, 25, 27). Currently, it is not possible to use all three
tube current modulation tools together; instead, ACS
can be applied with Z-DOM or with D-DOM. To set the
required image quality level, the Philips system uses
a reference image concept. The operator chooses a
protocol-specific mAs value and, based on each patient’s
attenuation information from the SPR, the mAs is auto-
matically adjusted to achieve approximately the same
noise level as in a predefined reference patient (start
value: 33 cm in diameter).
The Siemens system uses a combined tube current
modulation system called CARE Dose 4D (9, 25, 28).
The system works with automatic tube current modu-
lation according to the patient’s size and attenuation
changes together with real-time, online, controlled
tube current modulation during each tube rotation.
The image quality is defined by the operator-selected
image quality reference mAs value and the adapta-
tion strengths (weak, average or strong). From the SPR
the algorithm determines whether the patient sections
are slim or obese relative to an internally stored X-ray
attenuation of a standard sized patient. Based on the
preselected adaptation strengths the extent of change in
tube current (image quality and radiation dose) can be
controlled. The tube current will be weak, average or
strong decreased for slim sections and weak, average
or strong increased for obese sections. By default the
adaptation strengths are set by the manufacturer to
average decrease for slim and average increase for
obese sections.
Table 1. Automatic exposure control (AEC) techniques available in the modern CT systems
Parameter
AEC modulation
Angular Longitudinal Combined
Principle The tube current is adjusted during
each gantry rotation, according to
the size, shape, and attenuation of
body region being scanned
The tube current is adjusted along the scanning
direction of the patient, according to the size and
attenuation of the anatomic region being scanned
and the predetermined image quality
The tube current is adjusted both
during each gantry rotation and for
each slice position
Direction x, y z x, y, z
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3. Evaluation of automatic exposure control in CT systems 627
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Toshiba uses a combined system called Sure
Exposure 3D (9, 25, 29). The image quality is speci-
fied in terms of selected SD of CT numbers measured
in a patient-equivalent water phantom. Attenuation
information from the SPR of each patient is used
to map the selected image quality to tube current
values. The system makes use of the frontal and lat-
eral patient diameter and the detector intensities to
account for the oscillating tube current modulation
during each gantry rotation. The system allows the
operator to define the range within which the tube
current can be modulated by selecting minimum and
maximum mA limits.
Phantom
An anthropomorphic chest phantom (Chest Phantom
PBU-X-21, Kyoto Kagaku Co. Ltd, Kyoto, Japan) was
used (Fig. 1). The phantom is based on a skeleton of
a 160 cm tall male and consists of epoxy resins, ure-
thane, calcium hydroxyapatite, and other substances to
achieve variations in contrast in the phantom images
similar to those from a human body.
During CT acquisitions, the phantom was centered
as in a routine clinical CT examination, that is, supine
position with the sagittal midline and mid-thickness of
the phantom at the isocenter of the gantry. The scan-
ning direction was chosen according to the recommen-
dations of the individual system manufacturers.
Testing approach
Measurements were performed using 16- and 64-slice
CT scanners from each manufacturer (Table 2).
Because the method for adjusting the tube current dif-
fers among the individual systems, it was not practical
to use a common standard protocol for all systems.
Furthermore, for a given manufacturer, the system
operation differed depending on whether a 16- or
64-slice scanner was used; therefore, an individual
protocol was created for each examination by modi-
fying an existing standard clinical thorax protocol for
each CT scanner.As many scanning parameters as pos-
sible were set equal for all examinations (Tables 2 and
3). The SPR view was as regularly performed on the
respective CT scanner (Table 2). Image quality param-
eters used for the various AEC systems were typical
for a routine adult thorax CT protocol (Table 4). All
tests were performed in helical scan mode with a com-
bined AEC system activated and inactivated (fixed
mAs), with the exception of the Philips system, which
currently has no combined AEC system. Instead, their
longitudinal AEC system Z-DOM was used together
with ACS, as recommended for a thorax scan by
Philips. The manually selected tube load values (AEC
off) for Philips and Siemens were set as the manufac-
turers recommend (Table 5). These two AEC systems
also use the mAs value (AEC off) as an indicator of the
image quality when the AEC system is activated. GE
and Toshiba AEC systems do not use an mAs value as
an input for the tube current modulation. The selected
tube load values (AEC off) for GE and Toshiba were
instead selected with the purpose to be clinically rel-
evant for a thorax CT protocol (Table 5).
Fig. 1. The anthropomorphic chest phantom (Chest Phantom PBU-X-
21, Kyoto Kagaku Co. Ltd, Kyoto, Japan).
Table 2. Individual scanning parameters for thorax protocol and used scan projection radiograph (SPR) views
Manufacturer Model Collimation (mm) Pitch Kernel SPR
General Electric LightSpeed 16 16 0.625 0.938 Standard Lateral frontal
LightSpeed VCT 64 0.625 0.984 Standard Frontal lateral
Philips Brilliance CT 16 16 0.75 1.063 Standard B Frontal
Brilliance CT 64 Power 64 0.75 1.078 Standard B Frontal
Siemens SOMATOM Sensation 16 16 0.75 1 B31f Frontal
SOMATOM Sensation 64 64 0.6 1 B31f Frontal
Toshiba Aquilion 16 16 0.5 0.938 FC10 Frontal lateral
Aquilion 64 64 0.5 0.828 FC10 Frontal lateral
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Details of the radiation dose were obtained for each
CT scan from the DICOM image information.The dose
reduction (DR) was calculated from the dose length
product (DLP) values using equation 1:
DR
DLP DLP
DLP
.100%AEC off AEC
AEC off
(1)
To characterize the tube current modulation
(dynamic) of each AEC system, the mean mAs value
for each image slice was plotted for the image slices.
This made it possible to study how the tube current
varied along the z-axis of the anthropomorphic chest
phantom.
Image quality evaluation
To evaluate how the AEC systems affected image
quality, the image noise values from scans performed
with the AEC system activated were compared to those
obtained with the AEC system inactivated. Images
from GE, Philips, and Siemens systems were evalu-
ated on a Syngo Multimodality Workplace (Siemens
Medical Solutions, Erlangen, Germany). Since the
Syngo Multimodality Workplace could not read images
fromToshiba these were evaluated using Sante DICOM
Viewer Pro 2.1 (Santesoft, Athens, Greece). All images
were evaluated by the same operator to avoid bias.
Circular regions of interest (ROIs) of 0.5 cm2
were
placed in the spine region of the chest phantom; this
region is uniform and available throughout the phantom
(Fig. 2). The SD of the CT numbers was used as a mea-
sure of the image noise. The SDs for the image slices
are presented graphically.
The mean SD ( ) of the CT numbers in the spine
throughout the chest phantom was calculated, as was
the standard deviation ( ) of the measured SD values.
To evaluate whether the image noise became more uni-
form when the AEC systems were activated as com-
pared to inactivated, the coefficient of variation (Cv
)
was calculated using equation 2:
C = .100%v (2)
Results
Dynamics of tube current modulation
Figs. 3 and 4 illustrate the dynamics of tube current
modulation of eachAEC system in 16- and 64-slice CT,
respectively, and show that the tube current increased
for the shoulder region and decreased through the low
attenuating lung region. Through the denser abdomen,
the mean tube current increased again. The character-
istics and dynamic range of the modulation depend on
the image quality settings, which are unique for each
system. However, as shown in Figs. 3 and 4, the char-
acteristics of the AutomA 3D are similar to those of
the SureExposure 3D and the characteristics of the
ACS Z-DOM are similar to those of CARE Dose
4D. GE and Toshiba AEC systems are both based on a
selected noise reference value and Philips and Siemens
AEC systems are both based on a selected mAs value.
Philips and Siemens AEC systems show greater tube
current increases in the shoulder region relative to the
increases in the abdomen region than do the GE and
ToshibaAEC systems.The SiemensAEC system shows
the greatest range of tube current modulation. The mAs
drops from 100% in the shoulders to 16% (Sensation
16) and 18% (Sensation 64) in the diaphragm.
Table 3. General scanning parameters for thorax protocol
Settings Thorax protocol
Tube voltage (kV) 120
Rotation time (s) 0.5
Slice width (mm) 5
Increment (mm) 5
Filter Standard
FOV (mm) 400
Matrix 512 512
FOV, field of view.
Table 4. Image quality parameters for each manufacturer’s automatic exposure control (AEC) system
Manufacturer AEC system Slice Image quality Various
General Electric AutomA 3D 16 NI 12, Min mA 10, Max mA 200 Plus mode
64 NI 12, Min mA 10, Max mA 200 Plus mode
Philips ACS Z-DOM 16 200 mAs/slice ST: Body
64 200 mAs/slice ST: Body
Siemens CARE Dose 4D 16 Average/Average, Q. Ref. mAs 100
64 Average/Average, Q. Ref. mAs 100
Toshiba SureExposure 3D 16 SD 10, Min mA 10, Max mA 500 QDS
64 SD 10, Min mA 10, Max mA 500 QDS
NI, noise index; ST, scanning type; Q. Ref., quality reference; SD, standard deviation; QDS, quantum denoising system.
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5. Evaluation of automatic exposure control in CT systems 629
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Dose reduction
The dose reduction achieved with the AEC system in
use was determined relative to the dose delivered with
the AEC system inactivated. The dose savings ranged
from approximately 35% to 60%, depending on the
system and the AEC settings (Table 5).
Noise measurements
The image noise (SD) measurements from the spine
are shown in Figs. 5 and 6. In each of the different
systems, the image noise increased when the AEC
system was used as compared with when constant
tube load was used (AEC off). This noise increase was
more significant in regions where the tube current was
greatly decreased, such as in the lung region. In addi-
tion, different regions in the chest phantom showed
large variations in noise level when constant tube load
was used. Despite the use of tube current modulation,
the level of image noise varied among the different
anatomic regions. However, because the image quality
requirements differ from organ to organ, organ-specific
variations in noise level do not necessarily limit the
diagnostic utility of an image. In the abdominal region,
it is very important to detect low contrast lesions;
therefore, a lower noise level is desirable.
Figs. 5 and 6 show that the measured SD values
were slightly below the selected noise indices (NIs)
in the GE scanners and the selected SD values in the
Table 5. Mean mAs value for the respective scan, estimated dose reduction (DR), and estimated coefficient of variation (Cv
) for respective
CT scanner
Manufacturer Slice AEC setting Mean mAs DLP (mGycm) DR (%) Mean SD SD Cv
(%)
General Electric 16 AEC off 100 508.3 – 6.07 2.02 33.3
AutomA 3D 64 331.9 34.7 8.44 1.41 16.6
64 AEC off 100 403.8 – 7.59 2.54 33.5
AutomA 3D 54 210.6 47.9 10.22 2.39 23.4
Philips 16 AEC off 200 706.8 – 4.97 1.52 30.6
ACS Z-DOM 81 286.6 59.5 7.39 1.92 25.9
64 AEC off 200 605.1 – 4.93 1.45 29.5
ACS Z-DOM 93 296.7 51.0 6.75 1.69 25.0
Siemens 16 AEC off 100 366 – 6.54 1.82 27.8
CARE Dose 4D 59 205 44.0 8.98 2.31 25.8
64 AEC off 100 358 – 8.05 2.22 27.6
CARE Dose 4D 60 204 43.0 10.50 2.55 24.3
Toshiba 16 AEC off 100 769.3 – 5.23 1.60 30.6
SureExposure 3D 44 333.1 56.7 7.92 1.59 20.1
64 AEC off 100 720.4 – 5.32 1.73 32.6
SureExposure 3D 46 293.8 59.2 7.95 1.72 21.6
AEC, automatic exposure control; DLP, dose length product; SD, standard deviation.
Fig. 2. Cross-sectional views showing ROI placement in the spine region of the chest phantom: (A) slice 10, (B) slice 25, (C) slice 50, (D) slice 75.
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6. M. Söderberg & M. Gunnarsson630
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Fig. 3. Mean mAs values along the longitudinal axis of the chest phantom for each manufacturer on their respective 16-slice CT scanner, overlaid on
the scan projection radiograph. Philips and Siemens systems report mAs as mAs/pitch.
Fig. 4. Mean mAs values along the longitudinal axis of the chest phantom for each manufacturer on their respective 64-slice CT scanner, overlaid on
the scan projection radiograph. Philips and Siemens systems report mAs as mAs/pitch.
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7. Evaluation of automatic exposure control in CT systems 631
Acta Radiol 2010 (6)
Toshiba scanners. These differences reflect that the
NI/SD settings are only an indicator of image quality
and adjust the tube current so that the detectors
receive similar X-ray fluence rates, that is, the set-
tings regulate the quantum noise in the projection
data. However, image noise also depends on the
reconstruction kernel, reconstructed slice thickness,
and beam filtration.
Because the ROIs were inserted manually in incom-
pletely homogeneous regions, we evaluated whether
region inhomogeneity affected the results. Intra-observer
variability (repeated noise measurements in the spine)
showed that deviation in SD values was 5%. Due to
the quantum statistics, SD measurements in a uniform
phantom would also have resulted in measurement
variability (30).
Table 5 shows the results of the uniformity test
performed by calculating the coefficients of variation
using equation 2. In all systems, the image noise in
the different anatomic regions of the chest phantom
became more uniform when the AEC system was
activated.
Discussion
A clinical CT examination often covers different ana-
tomic regions with variable attenuation values. Because
the tube current is selected based on the region with
the highest attenuation (e.g. shoulder and pelvis) or
the region that requires the highest image quality, the
tube current is usually set to a high level when an AEC
system is not in use. Furthermore, standard protocols
are usually established to generate images of good
quality for average patient sizes. Therefore, if an AEC
system is not used, smaller patients will be exposed
to unnecessarily high doses of radiation and images
of larger patients may be of worse quality. AEC sys-
tems were developed to enable tube current modulation
according to a patient’s shape, size, and attenuation,
and to improve the consistency of image quality among
patients.
There are a number of benefits to using an AEC
system. One is the potential for dose reduction, which
was verified in this study (Table 5). However, it is dif-
ficult to compare the estimated dose reduction values
obtained in this study with values reported in the litera-
ture. The results are strongly dependent on the selected
scanning parameters, the CT scanner/model, and the
specified image quality for the AEC system. Results
from a study performed by PAPADAKIS et al. (31) showed
a 13.9% dose reduction (only angular modulation) for
the thorax and abdomen regions measured using an
adult anthropomorphic phantom. The results of our
study and those from GUTIERREZ et al. (18), which are
Fig. 5. Measured SD (image noise) in the spine region throughout the chest phantom for the respective manufacturer on their 16-slice CT scanner: (A)
GE, (B) Philips, (C) Siemens, (D) Toshiba, when the AEC system was activated and inactivated.
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8. M. Söderberg & M. Gunnarsson632
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valid for an anthropomorphic chest phantom repre-
senting a standard male patient, verify that the com-
bination of angular and longitudinal AEC enables
much greater dose reductions, in the range of 35–60%
(Table 5). The results are also in agreement with those of
MULKENS et al. (16) and Rizzo et al. (13) who studied
patient populations. A recent study by PAPADAKIS et al.
(20) performed with an adult anthropomorphic
phantom found a dose reduction of 45.2% in the thorax
and abdomen region. However, in agreement with our
findings, they reported a significant noise increase and
a significant signal to noise ratio decrease.
The results of our study are valid for a phantom
based on a skeleton of a 160 cm tall male. The full body
weight of this person is not determined, but the torso
represents a typical rather lean Asian male. A potential
weakness of the study is that a significant proportion
of the dose reduction seen could simply be due to the
overall small size of the phantom compared with the
size of a standard Western European patient.
Since the calculated dose reductions are based on the
selected mAs value when the AEC systems were inac-
tivated, it is crucial that these values are representative
for a clinical thorax CT protocol. Consequently, when
considering the calculated dose reductions one must
take into account the DLP values in Table 5. The result
should be interpreted as an indication of potential for
reducing radiation exposure to the patient.
Large variations in the mAs values are evident at
the beginning and the end of some of the scans (Figs.
3 and 4). One possible explanation for this is that the
particular AEC system has a time delay before correct
adaptation of the tube current.
The different AEC systems were designed for dif-
ferent purposes. The makers of the GE and Toshiba sys-
tems claim that their systems were designed to increase
the uniformity of image quality between different ana-
tomic regions in the same patient. These claims were
verified in this study, as shown by the coefficient of
variation estimates (Table 5). For instance, the esti-
mated standard deviation of the measured SD values
increased when SureExposure 3D was activated but the
coefficient of variation was smaller. This means that
there are greater differences in the image noise, but rel-
ative to the mean value, the image noise is more stable
when SureExposure 3D is activated. The estimation
assumes that the measured SD values follow a normal
distribution throughout the chest phantom. We are
aware that this assumption was not completely fulfilled
in this study.The approach for the SiemensAEC system
is that different sized patients require different levels
of noise in order to obtain adequate image quality. The
Fig. 6. Measured SD (image noise) in the spine region throughout the chest phantom for the respective manufacturer on their 64-slice CT scanner: (A)
GE, (B) Philips, (C) Siemens, (D) Toshiba, when the AEC system was activated and inactivated.
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9. Evaluation of automatic exposure control in CT systems 633
Acta Radiol 2010 (6)
user can also control the extent of tube current adjust-
ment for slim and obese patient sections by selecting
weak, average or strong adaptation strengths (21). The
Philips AEC system, ACS, has the same approach as
Siemens; more noise is accepted for obese patients and
less noise is required for small patients (27).
It is difficult to make direct comparisons between
the different systems. Each system has a different solu-
tion for defining the image quality level. The different
scanner models may also differ in the type of X-ray
tube used, software, detector configuration, scanning
geometry, and beam filtration.
There are many studies on AEC systems in which the
AEC performance and image quality were evaluated
using homogeneous phantoms (9–12, 15). Some studies
have also performed clinical evaluations in which radi-
ologists assessed the image quality (13, 14, 16, 17). In
this study, an anthropomorphic phantom was used to
assess the capabilities and limitations of an AEC system
in a clinical situation. The performances of the AEC
systems might have differed if a uniform phantom had
been used. Likewise, performing noise measurements in
a homogeneous phantom is not analogous to noise anal-
ysis in a human body. The Toshiba images were evalu-
ated on a different workstation and to find out whether
this was a source of bias, images from Siemens were
evaluated on both workstations. The results showed no
difference between the evaluations.
It is difficult to determine the optimal image quality
for a clinical diagnosis, since both quantitative mea-
surements and the observer’s perception are involved.
In this study, the quantum noise was used to assess
the image quality. Image noise is the parameter that
is directly influenced by tube current modulation. An
increase in image noise can potentially impair low con-
trast resolution and affect the diagnostic information.
Image noise is affected by differences in phantom posi-
tion and various scanning parameters (4); therefore, the
position of the phantom was held constant and as many
as possible of the scanning parameters were set equal
for each system. Furthermore, the image noise depends
on the reconstruction process, e.g. reconstruction filter.
These parameters differ between the systems, but they
were standardized as much as possible for a routine
thorax examination.
It is essential that radiologists and medical physi-
cists are aware of the performance of their AEC system
and how image quality is affected. It is necessary to
find the acceptable threshold of image quality with the
minimum possible radiation exposure to the patient,
in agreement with the ALARA principle. This study
did not evaluate whether the diagnostic accuracy was
influenced by the AEC-induced increases in image
noise. A clinical subjective image quality analysis
performed by RIZZO et al. (13) showed that the image
noise was significantly higher in examinations per-
formed with combined modulation (CARE Dose 4D)
as compared with a fixed tube current. However, the
study also concluded that the diagnostic utility of the
images was acceptable.
In conclusion, this study established that the use of
AEC systems could significantly reduce the radiation
exposure to the patient. For the anthropomorphic chest
phantom, the magnitude of the dose savings was con-
siderable, ranging from approximately 35% to 60%.
The dynamics of tube current modulation for each of
the AEC systems were similar, especially between GE
and Toshiba and between Philips and Siemens AEC
systems. We also found that use of the AEC systems
increased image noise. However, the variation in image
noise among images obtained along the scanning direc-
tion was lower when using the AEC systems compared
with the scans with fixed mAs.
Acknowledgments
This project was supported by the Swedish Radiation
Safety Authority (SSI P 1579.07).
Declaration of interest: The authors report no conflicts
of interest. The authors alone are responsible for the
content and writing of the paper.
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