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ORIGINAL RESEARCH
Sarabjeet Singh, MBBS, MMST
Mannudeep K. Kalra, MD
Jiang Hsieh, PhD
Paul E. Licato, MS
Synho Do, PhD
Homer H. Pien, PhD
Michael A. Blake, MD
Purpose:
To compare image quality and lesion conspicuity on abdominal computed tomographic (CT) images acquired with
different x-ray tube current–time products (50–200 mAs)
and reconstructed with adaptive statistical iterative reconstruction (ASIR) and filtered back projection (FBP)
techniques.
Materials and
Methods:
Twenty-two patients (mean age, 60.1 years 6 7.3 [standard deviation]; age range, 52.8–67.4 years; mean weight,
78.9 kg 6 18.3; 12 men, 10 women) gave informed consent
for this prospective institutional review board–approved
and HIPAA-compliant study, which involved the acquisition of four additional image series at multidetector CT.
Images were acquired at different tube current–time products (200, 150, 100, and 50 mAs) and encompassed an
abdominal lesion over a 10-cm scan length. Images were
reconstructed separately with FBP and with three levels
of ASIR-FBP blending. Two radiologists reviewed FBP and
ASIR images for image quality in a blinded and randomized manner. Volume CT dose index (CTDIvol), dose-length
product, patient weight, objective noise, and CT numbers
were recorded. Data were analyzed by using analysis of
variance and the Wilcoxon signed rank test.
Results:
CTDIvol values were 16.8, 12.6, 8.4, and 4.2 mGy for 200,
150, 100, and 50 mAs, respectively (P , .001). Subjective noise was graded as below average at 150 mAs and
average at 100 and 50 mAs for ASIR images, as compared
with FBP images, on which noise was graded as average
at 150 mAs, above average at 100 mAs, and unacceptable
at 50 mAs. A substantial blotchy image appearance was
noted in four of 22 image series acquired at 4.2 mGy
with 70% ASIR. Lesion conspicuity was significantly better at 4.2 mGy on ASIR than on FBP images (observed
P , .044), and overall diagnostic confidence changed from
unacceptable on FBP to acceptable on ASIR images.
Conclusion:
ASIR lowers noise and improves diagnostic confidence in
and conspicuity of subtle abdominal lesions at 8.4 mGy
when images are reconstructed with 30% ASIR blending and at 4.2 mGy in patients weighing 90 kg or less
when images are reconstructed with 50% or 70% ASIR
blending.
1
From the Department of Radiology, Massachusetts General Hospital, 25 New Chardon St, Suite 400B, Boston, MA
02114 (S.S., M.K.K., S.D., H.H.P., M.A.B.); and GE Healthcare,
Waukesha, Wis (J.H., P.E.L.). Received December 7, 2009;
revision requested January 12, 2010; revision received
February 1; accepted February 10; final version accepted
June 7. Address correspondence to S.S. (e-mail:
[email protected] ).
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RSNA, 2010
Supplemental material: http://radiology.rsna.org/lookup
/suppl/doi:10.1148/radiol.10092212/-/DC1
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n GASTROINTESTINAL IMAGING
Abdominal CT: Comparison
of Adaptive Statistical Iterative
and Filtered Back Projection
Reconstruction Techniques1
GASTROINTESTINAL IMAGING: Reconstruction Techniques at Abdominal CT
I
mage reconstruction algorithms play a
critical role in the quality and appearance of tomographic images (1–4).
Although iterative image reconstruction algorithms were used to generate
images with the very first commercial
clinical computed tomographic (CT)
scanner (4) and underwent substantial
improvements in the 1980s, especially
in the context of emission tomography
(1–3), filtered back projection (FBP)
algorithms were used for CT image reconstruction owing to their faster image reconstruction and ease of implementation (5). Over the past decade,
the desire for finer resolution, greater
volume coverage, and faster scan times
and the desire to concurrently lower
radiation dose have pushed the performance of FBP reconstruction to its
limits. Alternative techniques of image
reconstruction with iterative reconstruction algorithms have recently become available for clinical use. In contrast to FBP, iterative reconstruction
uses a forward reconstruction model
and a more precise modeling of scanner geometry and the underlying physics. Results of prior studies (6–13) have
shown that iterative reconstructions
produce higher-resolution images with
greater robustness for various imaging
artifacts with a longer computational
processing time than FBP reconstruction. The increase in reconstruction time
Advances in Knowledge
n Abdominal CT images acquired
at 100–200 mAs and reconstructed with adaptive statistical
iterative reconstruction (ASIR)
have acceptable image noise and
enable acceptable diagnostic
confidence.
n Higher blending factors of 70%
ASIR and 30% filtered back projection (FBP) are required to
obtain acceptable image noise
and diagnostic confidence at
lower radiation doses.
n Abdominal CT images tended to
have a blotchy pixilated appearance at the higher blending proportion of 70% ASIR and 30%
FBP.
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Singh et al
and the need for costlier computation
hardware presently makes iterative reconstructions unacceptable for clinical
use. To circumvent the issue of the prolonged reconstruction time and the need
for higher computational power, the
adaptive statistical iterative reconstruction (ASIR) algorithm has been developed. This algorithm takes into account
precise modeling of the x-ray photon
statistics and electronic noise, all of
which are less accurate in FBP (Appendix E1 [online]). To decrease reconstruction time, ASIR utilizes the information
contained in the FBP-reconstructed image as an initial “building block” in the
reconstruction process (14). Reduction
of noise and artifacts in low-radiationdose images are especially helpful in
young and obese patients, in clinical
situations like multiphase CT examinations, and in patients who have undergone multiple prior examinations. The
ASIR technique has been approved by
the U.S. Food and Drug Administration
for clinical use.
The purpose of our study was to
compare image quality, lesion detection,
and lesion conspicuity on abdominal CT
images acquired at different x-ray tube
current–time products (50–200 mAs)
and reconstructed with ASIR and FBP.
Materials and Methods
M.K.K. received research funding from
GE Healthcare (Waukesha, Wis). J.H.
and P.E.L. are employees of GE Healthcare. The remaining authors (S.S., S.D.,
H.H.P., and M.A.B.) have no pertinent
disclosures and had full control of the
study data during the study.
Implications for Patient Care
n Reduction of CT radiation dose
down to 8.4 mGy is feasible for
abdominal CT images reconstructed with ASIR without compromising image quality, lesion
detection, and lesion conspicuity;
for patients weighing 90 kg
or less, radiation dose reduction
down to 4.2 mGy is possible with
the ASIR technique.
Patients
This prospective clinical study was approved by the Human Research Committee of our institutional review board
and was conducted in compliance with
Health Insurance Portability and Accountability Act guidelines. Written informed consent was obtained from all
22 patients (mean age, 60.1 years 6
7.3; age range, 52.8–67.4 years; mean
weight, 78.9 kg 6 18.3; 12 men [mean
age, 60.9 years 6 7] and 10 women
[mean age, 59.3 years 6 8]) for the acquisition of four extra sets of images,
in addition to their clinically indicated
standard-of-care abdominal CT examinations. All consecutive eligible patients
who gave informed consent for participation in the study were included in
the study, which was performed between February 2, 2009 and August 4,
2009.
Only those patients who met all of
the following inclusion criteria for the
study were recruited: age greater than
50 years, scheduled for CT of the abdomen, able to provide informed written
consent, able to hold their breath for at
least 10 seconds, able to follow verbal
commands for breath holding and remain still for the duration of scanning,
and hemodynamic stability (conscious
and oriented, with a regular respiration rate of 12–40 breaths per minute,
a pulse rate of 60–90 beats per minute
Published online before print
10.1148/radiol.10092212
Radiology 2010; 257:373–383
Abbreviations:
ASIR = adaptive statistical iterative reconstruction
CTDIvol = volume CT dose index
FBP = filtered back projection
MTF = modulation transfer function
Author contributions:
Guarantor of integrity of entire study, M.K.K.; study
concepts/study design or data acquisition or data analysis/
interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors;
manuscript final version approval, all authors; literature
research, S.S., M.K.K., J.H., H.H.P., M.A.B.; clinical studies,
S.S., M.K.K., M.A.B.; experimental studies, S.S., M.K.K., S.D.,
H.H.P.; statistical analysis, S.S., M.K.K., H.H.P.; and
manuscript editing, S.S., M.K.K., J.H., P.E.L., H.H.P., M.A.B.
See Materials and Methods for pertinent disclosures.
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GASTROINTESTINAL IMAGING: Reconstruction Techniques at Abdominal CT
without dysrhythmia, and a systolic blood
pressure of 100–140 mm Hg). To avoid
variations in the postcontrast timing of
various abdominal CT protocols and to
minimize radiation dose, only patients
who were undergoing single-phase abdominal CT with image acquisition were
included in our study.
We excluded patients from our study
if they were younger than 50 years of
age, were hemodynamically unstable,
or were not scheduled for routine CT
of the abdomen. Patients were also excluded if they were unable to provide
informed written consent, follow verbal
commands for breath holding, or remain still for the duration of scanning.
To recruit patients for the study,
we checked our radiology information
system to identify patients scheduled
for standard-of-care clinical CT examinations with the scanner used in our
study. Ten to 15 such patients were
identified per day. We then applied our
inclusion and exclusion criteria to find
eligible patients. Most of the patients
included in our study underwent abdominal CT scanning for clinical indications that included staging of known
malignancy (primary sarcoma, breast,
colon, prostate carcinoma) or suspected
malignancy (chest or abdominal), renal stones, change of bowel habits, and
ruling out abdominal pain. Next, we
requested permission from the physicians of the eligible patients (five or six
per day) for participation in the study
protocol. Of these eligible patients, only
25 gave their informed consent for participation in the study. Only one patient
refused to undergo this research study
after signing the consent form but prior
to the acquisition of research images.
Therefore, no research CT images were
acquired in this patient. Thus, our final
study group, in whom qualitative and
quantitative image quality was assessed
for comparison of the ASIR and FBP
techniques, consisted of 22 patients.
Scanning Techniques
First, a clinically indicated standardof-care abdominal CT examination was
performed with a commercial CT scanner (Discovery CT750 HD; GE Healthcare) and administration of 80–100 mL
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n
of an intravenous contrast medium
(Iopamidol 370; Bracco Diagnostics,
Princeton, NJ). Subsequently, four additional sets of images were acquired
in each patient for the purpose of this
study. Each of the additional image data
sets was acquired through an identical
part of the abdomen over a scan length
of 10 cm. The location of the acquisition
of these additional image data sets was
based on a review (M.K.K., a radiologist with 8 years of experience) of the
patient’s prior and current standard-ofcare abdominal CT images to include
the most subtle or smallest lesion in the
abdomen. All patients were scanned at
the level of either the lower liver (at or
below the porta hepatis) or the kidneys,
as all the assessed lesions of interest
were located at these levels. None of the
image series was acquired through the
lung bases. As per the standard of care,
scanning in the portal venous phase
was triggered by using a bolus-tracking
technique. The time between completion of the standard-of-care abdominal
CT examination and the initiation of
scanning for research image acquisition was 10 or fewer seconds in most
patients. All research images were acquired within 1 minute of the standardof-care abdominal CT examination. The
interval between the acquisitions of the
four research image series was 6–10
seconds. Although these measures minimized the time between acquisitions of
research images and standard-of-care
dynamic contrast material–enhanced CT
images, most research images were acquired in an early equilibrium phase, as
it was not possible to perform focused
dynamic image acquisition at four different dose levels.
To compare the FBP and ASIR techniques, abdominal CT images were acquired at four radiation dose levels by
selecting four fixed tube current–time
products (200, 150, 100, and 50 mAs).
Although automatic exposure control
techniques are routinely used for dose
modulation in typical clinical practice,
we did not use these techniques to
achieve radiation dose reductions in
our study for several reasons. First, to
obtain four levels of radiation dose with
automatic exposure control with the CT
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Singh et al
scanner used in our study, we would
have been required to adjust the noise
index (a descriptor for desired image
noise used on the scanner to adjust tube
current), minimum tube current, and
maximum tube current for each dose
level. Second, these factors would have
had to be tailored to individual patient
size because at a constant noise index,
automatic exposure control increases
the radiation dose to larger patients and
decreases the dose to smaller patients.
Furthermore, this increase or decrease
in radiation dose with automatic exposure control is intentionally kept as
nonlinear in the actual CT scanners to
avoid the inadvertent use of high tube
current in larger patients (15) and, paradoxically, to prevent the use of too low
a tube current in smaller patients. Also,
results of prior studies (10) have indeed
shown that other iterative reconstruction techniques can maintain a similar
noise level regardless of radiation dose
levels at up to a ninefold reduction in
dose.
Compared with the use of a fixed
tube current, it is more difficult to control the extent of dose reduction with
an automatic exposure control technique with adjustment in the desired
image quality metric such as noise index (GE Healthcare) or reference tube
current–time product. To avoid contrast
enhancement bias due to the delay in
scanning from the start of injection, the
acquisition sequence of the four research CT data sets was randomized.
Because it was not possible or practical to inject contrast medium four times
in a patient to acquire images at identical contrast enhancement phases, no
additional intravenous contrast medium
was administered for acquisition of the
research image series. All scanning parameters, with the exception of tube
current, were held constant and included
120 kVp, a pitch of 0.984:1, a 39.37-mm
table speed per gantry rotation, a helical
acquisition mode, a detector configuration
of 64 detector rows with 0.625-mm-thick
sections, a 0.5-second gantry rotation
time, a reconstructed section thickness
of 5 mm, a reconstruction section interval
of 5 mm, and a standard reconstruction
kernel.
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GASTROINTESTINAL IMAGING: Reconstruction Techniques at Abdominal CT
Singh et al
Figure 1
Figure 1: Flowchart shows CT image reconstruction in different radiation dose levels and the varying levels
of FBP and ASIR blending (ASIR 30, ASIR 50, and ASIR 70) for each radiation dose level.
Image Reconstruction
Each of the four raw research CT data
sets was reconstructed by using FBP to
generate four FBP image series (one
each at four tube current–time products: 200, 150, 100, and 50 mAs) (Fig 1).
The four research raw scan data sets
were also reconstructed by using the
ASIR technique. ASIR image reconstruction was selected for three different levels of blending (30%, 50%, and
70%) for each of four radiation dose
levels to obtain 12 sets of research ASIR
image data sets (30% ASIR at 200 mAs,
50% ASIR at 200 mAs, 70% ASIR at
200 mAs, 30% ASIR at 150 mAs, 50%
ASIR at 150 mAs, 70% ASIR at 150
mAs, 30% ASIR at 100 mAs, 50% ASIR
at 100 mAs, 70% ASIR at 100 mAs,
30% ASIR at 50 mAs, 50% ASIR at 50
mAs, and 70% ASIR at 50 mAs). Thus,
352 image series (22 patients times 16
image data sets) were generated. There
was a delay of about 5 seconds in the
processing and display of the first ASIR
image in the series, but the frame rate
of display of the ASIR images was quite
similar to that of the FBP images, at
up to 12 frames per second. To enable
double-blinded evaluation, each image
data set was coded, deidentified, and
randomized by a study coauthor (S.S.,
with 3 years of experience).
The blending of ASIR and FBP techniques was preferred to pure ASIR, as
376
the ASIR technique alone causes substantial changes in noise texture, which
was found to be undesirable by a group
of radiologists in an advisory board meeting of the vendor (GE Healthcare).
Blending of the two reconstruction techniques helped preserve the image texture, or the “look,” of FBP images, while
the use of ASIR reduced image noise.
As higher blending proportions of ASIR
to FBP (ie, .70%) would have substantially changed the texture and characteristics of the images, on the basis of the
vendor’s recommendation, we limited
assessment of image quality to 70% ASIR.
Subjective Image Quality
All randomized CT image data sets were
reviewed at a picture archiving and communication systems diagnostic workstation (Impax ES; Agfa Technical Imaging Systems, Ridgefield Park, NJ) for
assessment of subjective image quality.
All image data sets were presented in
blinded and randomized manner to two
experienced radiologists (M.A.B. [with
12 years of experience] and M.K.K.)
for independent assessment of image
quality. Both radiologists were trained
on two image data sets for the grading
of different aspects of subjective image
quality and lesion assessment so that
they would understand the evaluation
system, as well as to improve interobserver agreement. These two image data
sets belonged to the first two patients recruited in our study and were not used
for the subsequent statistical analyses.
Both radiologists assessed these two image data sets just before the evaluation of
the rest of the CT studies that were used
in the statistical analyses.
Subjective image quality was assessed in terms of subjective image
noise by using a five-point scale (1 =
minimal image noise, 2 = less than average noise, 3 = average image noise, 4 =
above average noise, and 5 = unacceptable image noise). Each artifact was
graded by using a four-point scale (1 =
no artifacts, 2 = minor artifacts not interfering with diagnostic decision making,
3 = major artifacts affecting visualization
of major structures but diagnosis still
possible, and 4 = artifacts affecting diagnostic information). The following artifacts were assessed: helical or windmill
artifacts; streak artifacts due to metals
and leads; beam hardening artifacts due
to the patient having his arms by his side;
truncation due to large body size or off
centering (this was rare); and a blotchy,
pixilated appearance. The visibility of
small structures (small blood vessels,
adrenal glands, small lymph nodes) was
ranked by using a five-point scale (1 =
excellent visualization, 2 = above average visibility, 3 = acceptable visibility,
4 = suboptimal visibility, and 5 = unacceptable visualization), while lesion size was
measured by using a four-point scale
(1 = focal and , 1 cm, 2 = focal and
1–5 cm, 3 = focal and . 5 cm, and
4 = diffuse lesion), Subjective visual lesion
conspicuity was assessed by using a fivepoint scale (1 = well-seen lesion with
well-visualized margins, 2 = well-seen
lesion with poorly visualized margins,
3 = subtle lesion, 4 = probably an artifact
mimicking a lesion, and 5 = definitely an
artifact mimicking a lesion), image contrast was assessed by using a five-point
scale (1= excellent image contrast, 2 =
above average contrast, 3 = acceptable
image contrast, 4 = suboptimal image
contrast, and 5 = very poor contrast),
and diagnostic confidence was assessed
by using a four-point scale (1 = completely
confident; 2 = probably confident; 3 =
confident only for a limited clinical entity
such as a kidney stone, a calcified lesion,
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Table 1
Percentage Agreement between the Two Radiologists for Each of the Assessed Subjective Image Quality
and Lesion Assessment Parameters
Reconstruction Technique and Tube
Current–Time Product (mAs)
FBP
200
150
100
50
30% ASIR
200
150
100
50
50% ASIR
200
150
100
50
70% ASIR
200
150
100
50
Confidence in Diagnosis
Image Contrast
Lesion Conspicuity
Noise
Visibility of Small Structures
95.4 (21/22)
100 (22/22)
95.4 (21/22)
86.4 (19/22)
100 (22/22)
100 (22/22)
100 (22/22)
72.2 (16/22)
95.4 (21/22)
90.9 (20/22)
81.8 (18/22)
81.8 (18/22)
100 (22/22)
90.9 (20/22)
72.2 (16/22)
95.4 (21/22)
100 (22/22)
100 (22/22)
100 (22/22)
100 (22/22)
100 (22/22)
100 (22/22)
100 (22/22)
86.4 (19/22)
95.4 (21/22)
95.4 (21/22)
95.4 (21/22)
72.2 (16/22)
100 (22/22)
90.9 (20/22)
81.8 (18/22)
77.2 (17/22)
100 (22/22)
95.4 (21/22)
86.4 (19/22)
95.4 (21/22)
100 (22/22)
100 (22/22)
100 (22/22)
86.4 (19/22)
100 (22/22)
100 (22/22)
100 (22/22)
86.4 (19/22)
95.4 (21/22)
95.4 (21/22)
95.4 (21/22)
77.2 (17/22)
100 (22/22)
90.9 (20/22)
81.8 (18/22)
77.2 (17/22)
100 (22/22)
100 (22/22)
100 (22/22)
81.8 (18/22)
100 (22/22)
100 (22/22)
100 (22/22)
77.2 (17/22)
100 (22/22)
100 (22/22)
100 (22/22)
86.4 (19/22)
95.4 (21/22)
95.4 (21/22)
100 (22/22)
72.2 (16/22)
100 (22/22)
90.9 (20/22)
81.8 (18/22)
77.2 (17/22)
100 (22/22)
100 (22/22)
100 (22/22)
100 (22/22)
95.4 (21/22)
95.4 (21/22)
95.4 (21/22)
77.2 (17/22)
Note.—Raw data are in parentheses.
or a large lesion; and 4 = poor confidence). The image quality attributes assessed in our study have been described
in the European Guidelines on Quality
Criteria for Computerized Tomography
document (16) and have been used in
multiple prior studies in the radiology
literature (17).
To avoid bias in lesion detection at
four dose levels in 16 image series, radiologists were first asked to grade the subjective image noise for individual image
series and then to assess lesion detection,
beginning with the image series with the
highest image noise. Neither radiologist
knew about the presence of lesions in
the patients. A standardized template for
lesion assessment could not be used, as
different patients had different body regions imaged and images were acquired
through limited scan lengths.
Objective Measurements
Objective image noise (in Hounsfield
units) 6 standard deviations and CT
numbers (Hounsfield units) were measured for 352 CT image series. Circular
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n
regions of interest (20–30 mm) were
drawn in the abdominal aorta, without
touching the lumen walls, to cover at
least two-thirds of its lumen. Circular regions of interest (20–30 mm) were also
drawn in the homogeneous area of the
right lobe of the liver. The skin-to-skin
maximum transverse diameter of the
abdomen was measured from localizer
radiographs, as transverse images are
often reconstructed with a smaller field
of view and may not include the skin.
Each patient was weighed by using a digital weighing machine just prior to the CT
examination. CT radiation dose descriptors such as volume CT dose index (CTDIvol, described in milligrays) and doselength product (described in mGy · cm)
were recorded after completion of the
CT examination for all image data sets.
Estimation of Modulation Transfer
Function
To compare the effect of ASIR and FBP
reconstructions on spatial resolution, we
performed a phantom study to estimate
the modulation transfer function (MTF)
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by using the same scanner used for imaging patients that is described above.
A phantom (Catphan600; The Phantom
Laboratory, Salem, NY) with 28-mmdiameter tungsten wire was scanned at a
tube current of 120 kVp, with 200 mA,
a helical acquisition mode, a section
thickness of 0.625 mm, a 0.5-second
gantry rotation time, and a standard
reconstruction algorithm. Images were
reconstructed with the FBP technique
and with 30%, 50%, and 70% ASIR techniques. The MTF was measured as the
angular average of the two-dimensional
Fourier transform of the point spread
function in each of the reconstructed
image data sets.
Statistical Analysis
Data were analyzed by using analysis
of variance for objective metrics such
as objective image noise and CT numbers and by using the Wilcoxon signed
rank test for subjective image quality and lesion assessment parameters.
Intraobserver variability was not estimated, as each radiologist assessed
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GASTROINTESTINAL IMAGING: Reconstruction Techniques at Abdominal CT
Singh et al
Table 2
Subjective Image Quality Scores and Frequency of Scores for Image Noise, Lesion Conspicuity, and Diagnostic Confidence
Reconstruction Technique and Tube
Current–Time Product (mAs)
FBP
200
150
100
50
30% ASIR
200
150
100
50
50% ASIR
200
150
100
50
70% ASIR
200
150
100
50
Image Noise
Lesion Conspicuity
Diagnostic Confidence
2 (16/22), 3 (6/22)
3 (16/22), 4 (3/22), 5 (3/22)
4 (16/22), 3 (3/22), 5 (3/22)
5 (19/22), 4 (2/22), 3 (1/22)
1 (18/22), 3 (3/22), 2 (1/22)
1 (17/22), 3 (4/22), 2 (1/22)
1 (15/22), 3 (4/22), 2 (3/22)
3 (14/22), 1 (6/22), 2 (1/22), 4 (1/22)
1 (22/22)
1 (20/22), 2 (1/22), 3 (1/22)
2 (17/22), 1 (4/22), 3 (1/22)
4 (18/22), 1 (3/22), 2 (1/22)
2 (19/22), 3 (2/22), 1 (1/22)
3 (14/22), 2 (5/22), 4 (3/22)
3 (15/22), 2 (1/22), 4 (6/22)
4 (15/22), 5 (5/22), 3 (2/22)
1 (18/22), 3 (3/22), 2 (1/22)
1 (17/22), 3 (4/22), 2 (1/22)
1 (16/22), 3 (4/22), 2 (2/22)
1 (13/22), 3 (5/22), 2 (3/22), 4 (1/22)
1 (22/22)
1 (22/22)
2 (16/22),1 (6/22)
2 (18/22), 1 (2/22), 4 (1/22), 3 (1/22)
1 (19/22), 2 (3/22)
2 (18/22), 3 (2/22), 1 (2/22)
2 (16/22), 3 (4/22), 1 (1/22), 4 (1/22)
4 (16/22), 3 (4/22), 2 (2/22)
1 (17/22), 3 (3/22), 2 (2/22)
1 (17/22), 3 (4/22), 2 (1/22)
1 (16/22), 3 (4/22), 2 (2/22)
1 (14/22), 3 (5/22), 2 (2/22), 4 (1/22)
1 (22/22)
1 (20/22), 2 (2/22)
1 (20/22), 2 (2/22)
2 (14/22), 1 (4/22), 3 (3/22), 4 (1/22)
1 (22/22)
1 (19/22), 3 (2/22), 2 (1/22)
2 (17/22), 1 (4/22), 3 (1/22)
3 (16/22), 2 (3/22), 1 (2/22), 4 (1/22)
1 (17/22), 3 (3/22), 2 (2/22)
1 (17/22), 3 (4/22), 2 (1/22)
1 (16/22), 3 (4/22), 2 (2/22)
1 (15/22), 3 (5/22), 2 (1/22), 4 (1/22)
1 (21/22), 2 (1/22)
1 (19/22), 2 (3/22)
1 (19/22), 2 (3/22)
1 (13/22), 2 (4/22), 3 (3/22), 4 (2/22)
Note.—Data are scores, with the frequency of each score in parentheses.
the images only once. Interobserver
variability was estimated by using both
k statistics and percentage agreement
between the two radiologists for each
of the assessed subjective image quality and lesion assessment parameters.
Definitions of levels of agreement on
the basis of k values were as follows:
k , 0.20 indicated slight agreement;
k = 0.20–0.40, fair agreement; k = 0.41–
0.60, moderate agreement; k = 0.61–
0.80, substantial agreement; and k =
0.81–1.0, almost perfect agreement.
To determine the effect of patient size
on subjective image quality and objective image noise on ASIR and FBP images, we arbitrarily classified patients
into two groups (those weighing ⱕ 90 kg
and those weighing . 90 kg). P , .05
was considered to indicate a statistically significant difference.
Results
There was no significant difference in
size between the 12 male patients (mean
age, 59.5 years 6 6.8; mean weight,
90.3 kg 6 17.5; and mean transverse
378
diameter, 43.1 cm 6 4.8) and the 10
female patients (mean age, 65.9 years 6
7.3; mean weight, 71.7 kg 6 11.5; and
mean transverse diameter, 41.3 cm 6
3.8) (observed P = .3).
There was variable interobserver
agreement between the two radiologists (k = 0.12–1.0). However, as summarized in Table 1, the percentage
agreement between the two radiologists
ranged from 72.2% (16 of 22 scores in
agreement for visibility of small structures) to 100% (22 of 22 scores in
agreement for criteria such as image
noise, lesion conspicuity, and diagnostic
confidence).
Subjective Image Quality
Detailed image quality and lesion detection scores are summarized in Table 2.
Both radiologists ranked subjective image noise as suboptimal or unacceptable
in FBP images obtained at 50 and
100 mAs (k = 0.12 at 50 mAs), while noise
was ranked as average or acceptable in
30% ASIR images at 100–200 mAs (k =
0.59 at 100 mAs; k = 0.8 at 150 mAs),
in 50% ASIR images ( k = 1.0 at
100 mAs), and 70% ASIR images. Only 70%
ASIR images were rated as acceptable for
image noise at 50 mAs. No major artifacts were seen in any of the ASIR or FBP
image series acquired at the four dose
levels. Minor beam hardening or photon
starvation artifacts were noted in both
ASIR and FBP images in three of 22 patients (mean weight, 97.1 kg 6 23.4).
A minor, blotchy pixilated appearance of the images was seen in two CT
studies at 50 and 100 mAs with 30%
ASIR and in three CT studies at 50–150
mAs with 50% ASIR (Table 3). No such
appearance was seen at 200 mAs. On
the other hand, a blotchy, pixilated
appearance was substantial in four of
the 22 CT image series acquired at 100
and 50 mAs and reconstructed by using 70% ASIR. In three of four image
series, this appearance did not affect
diagnostic interpretation, while in one
series acquired in a patient of average size (weight = 77 kg), it interfered
with the diagnostic confidence of both
radiologists (Fig 2). Visibility of small
abdominal structures was rated as acceptable at all doses with both FBP
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GASTROINTESTINAL IMAGING: Reconstruction Techniques at Abdominal CT
Figure 2
Figure 3
Figure 2: Transverse abdominal CT images. (a)
Image reconstructed with 50% ASIR is diagnostically
acceptable, but (b) image reconstructed with 70% ASIR
shows a blotchy, pixilated image texture and irregular
edges, rendering it diagnostically unacceptable.
Figure 3: Transverse abdominal CT images in 51-year-old woman weighing 63 kg with a right hepatic lobe
enhancing lesion (hemangioma) reconstructed with FBP and three levels of ASIR (30%, 50%, and 70%) at
four tube current–time products (200, 150, 100, and 50 mAs). Image noise and artifacts were reduced with
ASIR, and images were diagnostically acceptable even at 50 mAs with 70% ASIR.
Table 3
Scores for Blotchy, Pixilated Imaging Appearance of 22 Abdominal CT Studies
Reconstruction Technique
and ASIR (k = 0.62 for 30% ASIR at
50 mAs; k = 0.29 for 70% ASIR at
50 mAs). Image contrast was found to be
acceptable at all radiation dose levels
and ASIR blending levels (k = 0.24
for FBP at 50 mAs; k = 0.24 for 30%
ASIR at 50 mAs; k = 0.3 for 50% ASIR
at 50 mAs; k = 0.63 for 70% ASIR at
200 mAs; k = 0.23 for 70% ASIR at
50 mAs) (Figs 3 and 4).
No lesions were missed on FBP or
ASIR images. Detected lesions in our
study included subcentimeter focal renal cysts and masses (n = 23 lesions),
adrenal lesions (n = 6), vertebral lesions (n = 6), focal liver lesions (n = 5),
abdominal lymph nodes (n = 3),
parapelvic cysts (n = 2), gallbladder
stones (n = 1), diverticulosis (n = 1),
and a dilated main pancreatic duct
(n = 1). Of the 68 abdominal lesions
detected in 22 patients, 48 were less
Radiology: Volume 257: Number 2—November 2010
Singh et al
n
FBP
30% ASIR
50% ASIR
70% ASIR
200 mAs
150 mAs
100 mAs
50 mAs
22/0/0/0
22/0/0/0
22/0/0/0
22/0/0/0
22/0/0/0
21/1/0/0
19/3/0/0
7/14/1/0
22/0/0/0
20/2/0/0
19/3/0/0
7/11/3/1
22/0/0/0
20/2/0/0
19/3/0/0
7/11/3/1
Note.—Data are numbers of CT studies given pixilation scores of 1, 2, 3, and 4, respectively. Because discrete values cannot be
averaged, the lowest or similar scores for pixilation given by the radiologists are presented.
than 1 cm in size, and two were between
1 and 5 cm. Of the 68 lesions, 65 had
well-visualized margins, while the remaining three were subtle, low-contrast
renal lesions. Lesions with conspicuity
that was graded as well seen with wellvisualized margins on ASIR images at
50–200 mAs (k = 0.41 for 30% ASIR
at 50 mAs; k = 0.25 for 30% ASIR at
100 mAs; k = 0.44 for 30% ASIR at
150 mAs; k = 0.12 for 50% ASIR at 50 mAs;
k = 0.25 for 50% ASIR at 100 mAs;
k = 0.44 for 50% ASIR at 150 mAs; k =
0.11 for 70% ASIR at 50 mAs; k = 0.25
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for 70% ASIR at 100 mAs; k = 0.44 for
70% ASIR at 150 mAs) were graded as
well seen with well-visualized margins
on FBP images at 100–200 mAs (k =
0.25 at 100 mAs; k = 0.44 at 150 mAs)
but as subtle in FBP images at 50 mAs
(k = 0.25) (observed P , .044). Diagnostic confidence was unacceptable
on FBP images at 50 mAs (k = 0.29),
whereas it improved to a grade of “probably acceptable” on 30% ASIR images
(k = 0.29) and 50% ASIR images (k = 0.29)
and “fully acceptable” on 70% ASIR images (k = 0.29).
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GASTROINTESTINAL IMAGING: Reconstruction Techniques at Abdominal CT
Singh et al
Table 4
Objective Image Noise in the Liver in 22 Abdominal CT Studies
Reconstruction Technique
FBP
30% ASIR
50% ASIR
70% ASIR
200 mAs
150 mAs
100 mAs
50 mAs
14.5 6 0.7 (13.8–15.2)
11.3 6 0.6 (10.6–12.0)
9.3 6 0.5 (8.8–9.8)
7.2 6 0.4 (6.8–7.7)
18.7 6 0.8 (17.9–19.5)
14.9 6 0.6 (14.3–15.5)
12.2 6 0.5 (11.7–12.7)
9.5 6 0.4 (9.1–9.9)
23.8 6 0.9 (22.9–24.7)
18.3 6 0.7 (17.5–19.0)
15 6 0.6 (14.4–15.6)
11.7 6 0.6 (11.1–12.3)
32.4 6 1.2 (31.1–33.6)
25.8 6 1.0 (24.8–26.8)
20.7 6 0.9 (19.8–21.6)
16.2 6 0.8 (15.2–16.8)
Note.—Data are mean objective image noise (in Hounsfield units) 6 standard deviation, with ranges in parentheses. Objective image noise decreased in the liver with an increase in ASIR level of image
reconstruction at all four tube current–time products.
Objective Image Quality
Detailed objective image quality values are summarized in Tables 4 and 5.
At 200 mAs, mean objective image
noise 6 standard error of the mean decreased by about 50.3% in liver (to 7.2 6
0.4 from 14.5 6 0.7) and by about
53.5% (to 8.5 6 0.4 from 18.3 6 0.8)
in the aorta for 70% ASIR images as
compared with FBP images (P , .001).
At 50 mAs, image noise decreased by
about 50% (to 16.2 6 0.8 from 32.4 6
1.2) in the liver and by about 52.2% (to
18.9 6 0.7 from 39.6 6 1.3) in the aorta
for 70% ASIR images as compared
with FBP images (P , .001). Contrary
to FBP images, ASIR images had less
objective and subjective image noise,
regardless of patient weight (observed
P , .001 for both comparisons) (Table 6).
Subjective image noise was deemed
unacceptable for patients weighing
greater than 90 kg for FBP images at
50 mAs and 30% ASIR images. Although
image noise was rated as acceptable
for all patient sizes at all tube current–
time products with 50% and 70% ASIR
blending, 50% ASIR images at 50 mAs
were not fully acceptable in terms of diagnostic confidence for patients heavier
than 90 kg, and greater pixilation was
noted in 70% ASIR images at 50 mAs.
There was no significant change in the
average CT numbers regardless of the
tube current–time product (200–50 mAs)
and reconstruction technique (FBP,
30% ASIR, 50% ASIR, and 70% ASIR)
(observed P , .323).
improvement in the spatial resolution of
images reconstructed with ASIR at all
levels compared with the FBP images
at both 10% and 50% MTF. The spatial resolution improved slightly with
increasing strength of ASIR from 30%
to 70%.
MTF Estimation
Results of the estimation of the MTF, or
spatial resolution, are summarized in
Figure 5. There was small but consistent
Radiation Doses
CTDIvol values at 200, 150, 100, and
50 mAs were 16.8, 12.6, 8.4, and 4.2 mGy,
respectively (P , .001). Dose-length pro-
380
Figure 4
Figure 4: Transverse abdominal CT images in 65-year-old man weighing 70 kg with multiple hypointense
renal lesion (cysts) reconstructed with FBP and three levels of ASIR (30%, 50%, and 70%) at four tube
current–time products (200, 150, 100, and 50 mAs). Even images acquired at 50 mAs and reconstructed
with 70% ASIR had lower image noise and artifacts than FBP images at the same tube current–time product
and radiation dose level.
ducts for 200, 150, 100, and 50 mAs were
237.1, 177.8, 118.5, and 59.3 mGy · cm,
respectively (P , .001).
Discussion
Owing to radiation dose concerns associated with CT, several efforts have been
made to reduce radiation dose without
compromising the quality of diagnostic
information; these efforts include lowering the tube current–time product
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Table 5
Objective Image Noise in the Aorta in 22 Abdominal CT Studies
Reconstruction Technique
FBP
30% ASIR
50% ASIR
70% ASIR
200 mAs
150 mAs
100 mAs
50 mAs
18.3 6 0.8 (17.5–19.1)
13.4 6 0.6 (12.8–14.0)
10.9 6 0.5 (10.4–11.4)
8.5 6 0.4 (7.9–8.9)
24.0 6 0.8 (23.2–24.8)
18.6 6 0.7 (17.9–19.4)
15.3 6 0.6 (14.7–15.9)
11.5 6 0.5 (11.0–12.0)
28.6 6 0.9 (27.7– 29.5)
21.5 6 0.7 (20.8– 22.3)
17.6 6 0.6 (17.0–18.2)
13.8 6 0.5 (13.3–14.3)
39.6 6 1.3 (38.3–41.1)
29.8 6 1.3 (28.5– 31.2)
23.1 6 0.9 (22.2–24.0)
18.9 6 0.7 (18.2–19.6)
Note.—Data are mean objective image noise (in Hounsfield units) 6 standard deviation, with ranges in parentheses. Objective image noise decreased in the aorta with an increase in ASIR level
of image reconstruction at all four tube current–time products.
Table 6
Objective Image Noise and Modal Subjective Image Noise Scores for 22 Abdominal CT
Studies according to Patient Weight Group and Image Reconstruction Technique
Patient Weight Group, Tube Current–Time
Product, and Type of Noise Score
ⱕ90 kg
200 mAs
Objective noise
Subjective noise
150 mAs
Objective noise
Subjective noise
100 mAs
Objective noise
Subjective noise
50 mAs
Objective noise
Subjective noise
.90 kg
200 mAs
Objective noise
Subjective noise
150 mAs
Objective noise
Subjective noise
100 mAs
Objective noise
Subjective noise
50 mAs
Objective noise
Subjective noise
FBP
30% ASIR
50% ASIR
70% ASIR
13.3 6 2.6
2
9.9 6 2.0
2
8.4 6 1.5
1
6.4 6 1.5
1
17.6 6 4.1
4
14.2 6 3.0
3
11.6 6 2.3
2
8.8 6 2.0
2
32.1 6 4.8
3
17.3 6 3.8
3
14.1 6 2.6
2
10.8 6 2.4
1
30.7 6 6.1
5
24.7 6 5.1
4
19.7 6 4.8
3
15.3 6 3.8
3
13.9 6 4.0
2
10.8 6 3.6
2
10.7 6 2.8
1
8.5 6 2.7
1
18 6 4.7
3
16.2 6 2.0
3
13.1 6 2.1
2
10.6 6 2.2
1
22.9 6 5.6
3
19.9 6 3.0
2
16.3 6 3.2
2
13.1 6 2.9
1
31.2 6 7.8
5
27.7 6 3.3
4
22.3 6 3.9
3
17.3 6 3.9
3
Note.—Data are mean objective image noise (in Hounsfield units) 6 standard deviation or modal subjective image noise scores.
Objective and subjective image noise was found to be lower in ASIR images than in FBP images for all four tube current–time
products.
(18); automatic exposure control (19);
reducing the peak kilovoltage (20);
using a higher pitch (21); and shielding radiosensitive organs such as the
breast, thyroid, and lenses of the eye
(22–24). The ASIR technique assessed
Radiology: Volume 257: Number 2—November 2010
n
in our study represents a new development for potential dose reduction.
Our study shows that reduction of
radiation dose down to 8.4 mGy is possible when abdominal CT images are
reconstructed with 30% ASIR blending
radiology.rsna.org
and reduction of radiation dose down to
4.2 mGy is possible for patients weighing 90 kg or less with 50% and 70%
ASIR blending. Regardless of the dose
levels (4.2–16.8 mGy) and ASIR percentage levels, subjective image noise
was consistently better with ASIR than
with FBP. This trend in subjective image
noise was supported by a corresponding
decrease in quantitative noise in ASIR
images as opposed to FBP images.
Our results are consistent with the
improved image noise described with
the use of iterative reconstruction techniques to reconstruct phantom image
data sets (6–10). These phantom studies have shown substantial improvement in image quality with iterative
reconstruction compared with FBP for
CT image reconstruction at low doses.
Our results are also in agreement with
the dose reduction reported in a recent
phantom and patient study in which
ASIR was used (25). In that smaller
study, Hara et al reported that ASIR
can provide diagnostic quality images
at 32%–65% lower CTDIvol values than
FBP techniques.
Although both image noise and diagnostic confidence were acceptable on
50-mAs images reconstructed with 70%
ASIR, a major change in image appearance due to substantial blotchy pixilation was noted at this level of blending of the ASIR and FBP techniques.
Neither of the radiologists missed any
lesions, despite the presence of these
changes in image texture or appearance. Thus, although this blotchy, pixilated appearance did not interfere with
lesion conspicuity and diagnostic confidence, it did generate a steplike artifact
at tissue interfaces (such as the margins
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GASTROINTESTINAL IMAGING: Reconstruction Techniques at Abdominal CT
Singh et al
Figure 5
Figure 5: Line graph summarizes MTF for 0.625-mm images CT reconstructed with FBP and ASIR at
30%, 50%, and 70% blending. Compared with the FBP images, ASIR images had higher spatial resolution
at both 10% and 50% MTF, with the biggest improvement noted at 70% ASIR. lp/cm = Line pairs per
centimeter, NO ASIR = FBP, STD = standard kernel.
of the liver, spleen, and blood vessels).
The exact reason for this appearance
exclusively on ASIR images and not on
FBP images is not known, but inherent differences in image reconstruction
and a lack or paucity of image noise on
ASIR images may have contributed to
this appearance.
Although quantitative image noise
did increase with both FBP and ASIR
with a decrease in dose and an increase in patient size, quantitative image noise with ASIR remained lower
than with FBP for all dose levels in all
patients, regardless of their weight or
transverse diameter. This may be the
reason that, regardless of patient size
(weight = 62.8–97.4 kg) and transverse
diameter (30.4–41.8 cm), ASIR images
were found to be better in terms of image quality than FBP images in patients
greater than 90 kg.
382
Our study had limitations. Foremost, our study had a small sample
size given the difficulty in patient recruitment in the face of rising concern
regarding radiation associated with CT
scanning in the patient community. It
is, however, possible that a larger study
may provide paradoxical results in
terms of other unknown artifacts or detection of lesions on ASIR images. Second, we did not investigate the effect
of ASIR reconstruction in patients of
different sizes. Presently, the ASIR reconstruction technique is available from
one vendor and works only with that
vendor’s images; therefore, we did not
assess the effect of ASIR on images acquired with CT scanners manufactured
by different vendors. Although it is very
difficult to blind the experienced radiologist between ASIR and FBP images
owing to differences in image texture,
we randomized the image sets acquired
at varying dose levels (4.2–16.8 mGy)
and reconstruction techniques. Another
possible limitation of our study was
the fact that CT images were acquired
in the equilibrium phase and not in an
unenhanced or dynamic phase. While
acquisition in an unenhanced phase
would have limited our ability to detect
subtle lesions and to focus the scanning acquisition to the location of the
lesion, focused dynamic acquisition at
four different dose levels was deemed
impractical. Another consideration with
our study was the fact that we used 16
image data sets in each patient at four
radiation dose levels and reconstruction techniques, and this repetitive reviewing of images may have biased the
radiologists in the lesion conspicuity
assessment component of our study.
However, to minimize this bias in image
assessment, each radiologist was asked
separately to first assess the lesion conspicuity on the image series with the
greatest image noise or on those image
data sets acquired at the lowest dose
levels and then to assess the image data
sets acquired at the other dose levels.
Unfortunately, at present, this technique is available from one CT manufacturer only. A major implication of
our study is the fact that reduction of
the radiation dose can be achieved by
lowering the x-ray tube current time–
product when using the ASIR image
reconstruction technique. Another implication of our study is the need to optimally set the correct ASIR-FBP blend.
For a CTDIvol of 8.4 mGy (x-ray tube
current time–product of 100 mAs),
30% and 50% ASIR blending is appropriate; further dose reduction will
require a higher degree of ASIR blending, with the associated image texture
alterations. Radiologists can reduce the
radiation dose to 4.2 mGy by using 70%
ASIR if they are willing to accept the
possible induction of a blotchy, pixilated
image appearance.
In conclusion, reduction of CT radiation dose down to 8.4 mGy is feasible
for abdominal CT images reconstructed
with the ASIR technique without compromising image quality, lesion detection,
and conspicuity. For patients weighing
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GASTROINTESTINAL IMAGING: Reconstruction Techniques at Abdominal CT
90 kg or less, radiation dose reduction
down to 4.2 mGy is possible with the
ASIR technique. For routine abdominal
CT examinations at 100 mAs in patients
weighing greater than 90 kg, 30% and
50% ASIR-FBP blending provides acceptable image noise and diagnostic confidence, without a substantial change in
image appearance.
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