Low-Dose MDCT and CT Enterography of Patients With Crohn Disease: Feasibility of Adaptive Statistical Iterative Reconstruction

Abstract

The purpose of this study was to evaluate the image quality and diagnostic performance of low-dose MDCT and CT enterography with adaptive statistical iterative reconstruction (ASIR) in the evaluation of Crohn disease.
Forty-eight patients (20 men, 28 women; mean age, 33.3 years; range, 17-83 years) with known or suspected Crohn disease who underwent low-dose MDCT and CT enterography with ASIR between December 2008 and December 2009 were included in the study. Twenty-seven patients had previously undergone standard-dose 64-MDCT and CT enterography with filtered back projection (FBP), and those images were used for comparison. The weight-based i.v. contrast protocol and scan parameters (120 kVp, 5-mm section thickness, 0.5-second rotation, pitch of 1.375, 64 × 0.625 mm detector configuration) were constant for the two techniques except for a higher noise index (×1.3) in the ASIR group. Two blinded readers reviewed 75 randomized MDCT-CT enterographic scans of 48 patients to assess image quality and diagnostic performance in the evaluation of Crohn disease, and the radiation dose for the studies was estimated.
All 75 MDCT and CT enterographic scans had acceptable quality for diagnostic interpretation. Findings of Crohn disease were seen on 63 of 75 scans (84%). Low-dose scans in the ASIR group had optimal image quality and were rated comparable to or better than standard-dose FBP images (mean score, 4.2 vs 3.87; p = 0.007). The subjective image noise score (mean, 1.43 vs 1.58; p = 0.2) and objective image noise measurements were lower for ASIR images (p < 0.001). Low-dose studies with ASIR allowed average dose reduction of 34.5% compared with standard-dose scans with FBP (volume CT dose index for ASIR, 7.7 ± 2.1 mGy; for FBP, 12 ± 5.5 mGy; p < 0.01).
Low-dose MDCT and CT enterographic studies reconstructed with ASIR were of appropriate quality for confident evaluation of the manifestations of Crohn disease while allowing approximately 34% dose reduction in comparison with FBP technique.

Full text

AJR :196, June 20 11 W743
ing [10, 11]. These risks are particularly rel-
evant in Crohn disease because individuals
with this disease are vulnerable owing to
their young age. They often need multiple
imaging investigations and interventions and
therefore are at increased lifetime risk of de-
velopment of cancer [12, 13].
Several approaches have been proposed to
lower the radiation dose from MDCT exami-
nations. The use of lower tube current or low-
er voltage alone or in combination has been
found effective [10, 14–16]. Aggressive dose
lowering, however, is often affected by the
undesirable consequences of excessive im-
age noise, which degrades image quality and
therefore diagnostic performance [15, 17].
To some extent this is a limitation of the im-
Low-Dose MDCT and CT
Enterography of Patients
With Crohn Disease:
Feasibility of Adaptive Statistical
Iterative Reconstruction
Avinash R. Kambadakone1
Naueen A. Chaudhary1
Gaurav S. Desai1
Deanna D. Nguyen2
Naveen M. Kulkarni1
Dushyant V. Sahani1
Kambadakone AR, Chaudhary NA, Desai GS,
Nguyen DD, Kulkarni N, Sahani DV
1Depar tment of R adiology, M assachuse tts Gener al
Hospi tal, Har vard Medical S chool, Wh ite 270 , 55 Frui t St,
Bos ton, MA 0 2114. Addre ss corresp ondence to
D. V. Sahan i (dsahani@ partner s.org).
2Depar tment of M edicine, Gas troent erology U nit,
Massac husett s General Ho spital, B oston, M A.
Gastrointestinal Imaging • Original Research
WEB
This i s a Web exclusiv e article .
AJR 20 11; 196:W 743 –W7 52
0361–803X/11/1966–W743
© Amer ican Roent gen Ray Socie ty
C
rohn disease, a chronic inflam-
matory disease of the gastroin-
testinal tract characterized by re-
current remissions and relapses,
predominantly affects young people, the
peak of occurrence being the second and
third decades of life [1, 2]. MDCT, particu-
larly CT enterography, has been established
as a key imaging modality not only in the di-
agnosis of Crohn disease but also in monitor-
ing treatment effects and complications [3–
5]. Several studies have validated the
importance of MDCT findings in decision
making regarding the treatment of patients
with Crohn disease [3, 5–9]. Despite the
known diagnostic benefits of MDCT, con-
cern about the radiation risk of CT is increas-
Keywords: adaptive statistical iterative reconstruction,
Croh n disease, C T enterogr aphy, radiat ion dose
DOI:10.2214/AJR.10.5303
Recei ved July 11, 20 10; accepte d after re vision
November 4, 2010.
OBJECTIVE. The purpose of this study was to evaluate the image quality and diagnostic
performance of low-dose MDCT and CT enterography with adaptive statistical iterative re-
construction (ASIR) in the evaluation of Crohn disease.
SUBJECTS AND METHODS. Forty-eight patients (20 men, 28 women; mean age,
33.3 years; range, 17–83 years) with known or suspected Crohn disease who underwent low-
dose MDCT and CT enterography with ASIR between December 2008 and December 2009
were included in the study. Twenty-seven patients had previously undergone standard-dose
64-MDCT and CT enterography with filtered back projection (FBP), and those images were
used for comparison. The weight-based IV contrast protocol and scan parameters (120 kVp,
5-mm section thickness, 0.5-second rotation, pitch of 1.375, 64 × 0.625 mm detector configu-
ration) were constant for the two techniques except for a higher noise index (×1.3) in the ASIR
group. Two blinded readers reviewed 75 randomized MDCT–CT enterographic scans of 48
patients to assess image quality and diagnostic performance in the evaluation of Crohn dis-
ease, and the radiation dose for the studies was estimated.
R E S U LTS . All 75 MDCT and CT enterographic scans had acceptable quality for diagnostic
interpretation. Findings of Crohn disease were seen on 63 of 75 scans (84%). Low-dose scans
in the ASIR group had optimal image quality and were rated comparable to or better than stan-
dard-dose FBP images (mean score, 4.2 vs 3.87; p = 0.007). The subjective image noise score
(mean, 1.43 vs 1.58; p = 0.2) and objective image noise measurements were lower for ASIR im-
ages (p < 0.001). Low-dose studies with ASIR allowed average dose reduction of 34.5% com-
pared with standard-dose scans with FBP (volume CT dose index for ASIR, 7.7 ± 2.1 mGy; for
FBP, 12 ± 5.5 mGy; p < 0.01).
CONCLUSION. Low-dose MDCT and CT enterographic studies reconstructed with
ASIR were of appropriate quality for confident evaluation of the manifestations of Crohn dis-
ease while allowing approximately 34% dose reduction in comparison with FBP technique.
Kamba dakone et al .
MDC T and CT Ent erograp hy of Crohn D isease
Gastrointestinal Imaging
Original Research

W74 4 AJR :196, June 20 11
Kambadakone et al.
age reconstruction algorithm currently used
in CT [18–20]. The filtered back projection
(FBP) technique typically used for recon-
structing CT images is an efficient process
for image production but requires a slightly
higher dose for acquisition of images of ap-
propriate quality [20]. Iterative reconstruction
(IR), on the other hand, allows scanning at a
lower dose while providing images with noise
characteristics of high-dose scanning, but IR
is computationally demanding and extreme-
ly time-consuming [18–20]. Adaptive statis-
tical iterative reconstruction (ASIR), unlike
full iterative reconstruction, is computation-
ally less demanding and functions by model-
ing the statistical variations in the distribution
of image noise in the image data, improving
the signal-to-noise ratio and preserving im-
age contrast [19, 20]. Improvements in image
quality in low-dose MDCT examinations of
the abdomen have been found with ASIR [19,
20]. The purpose of this study was to assess
the feasibility of ASIR in low-dose MDCT–
CT enterography in the care of patients with
Crohn disease and to evaluate the effect of
this technique on image quality, diagnostic
acceptability, and radiation dose.
Subjects and Methods
Study Subjects
This prospective study was approved by our in-
stitutional review board and was HI PAA compli-
ant. The board waived the requirement for writ-
ten informed consent. We enrolled patients with
known or suspected Crohn disease referred for
contrast-en hanced MDCT and CT enterography
between December 2008 and December 20 09.
Two of the patients had been included in a previ-
ous study [15] and underwent follow-up imaging
with ASI R.
A cohort of 48 consecutively registered pa-
tients (20 men, 28 women; mean age, 33.3 years;
age range, 17–83 years) participated. Twenty-sev-
en of the 48 patients (12 men, 15 women; mean
age, 32.7 years; age range, 17–50 years) had un-
dergone a previous MDCT examination with FBP
on a 64-MDCT scanner ( LightSpeed VCT, GE
Healthcare) and were referred for repeat MDCT
for follow-up (interval, 1–12 months) of the dis-
ease status. The data on this group were used for
direct comparison of sta ndard-dose FBP (n = 27)
and low-dose ASIR (n = 48) images. The mean
time between standard-dose FBP and low-dose
ASIR imagi ng was 117 days (range, 7–375 days).
The final a nalysis thus constituted a total of 75
MDCT stud ies representing both baseline and
follow-up MDCT performed on the cohort of 48
patients. For optimization of sca n parameters as
practiced in our depar tment, the patients were cat-
egorized into three groups based on body weight:
those weighing less than 60 kg, those weighing
61–90 kg, and those weighing more than 91 kg.
MDCT Technique
Images were acquired with a 64-MDCT sca n-
ner (Discovery CT 750 HDCT, LightSpeed VCT,
GE Healthcare) after administration of oral and
IV contrast media.
Oral contrast protocol—The 48 patients in the
ASIR group received neutral oral contrast medi-
um for an MDCT examination according to a de-
fined protocol, that is, CT enterography. Sixteen
of the 27 patients who underwent the previous
MDCT exam inations with F BP received neutral
oral contrast medium (i.e., CT enterography), and
11 received positive oral contrast medium.
A commercially available low-density bar ium
(0.1% barium suspension, VoLumen, E-Z-EM)
was administered as neutral oral contrast medi-
um, and each patient consumed a total of 1350 mL
of low-density barium (two bottles of 450 mL each
over 40 minutes followed by one half of a bottle
[225 mL] at 50 minutes and the other half on the
scann ing table).
For the positive oral contrast medium, either
2% barium (Readi-Cat 2%, E-Z-EM) or 2% dia-
trizoate meglumine and diatrizoate sodium (Gas-
trografin, Bracco Diagnostics) was used. Two bot-
tles of 450 mL each (total, 900 mL) of 2% barium
solution were administered gradually over 50– 60
minutes. Pat ients re ceiving diatrizoate meglumine
and diatr izoate sodium consumed two glasses (7.5
mL diatrizoate meglumine and diatrizoate sodium
diluted in 450 m L of liquid each) gradually over a
period of 40 m inutes before the scan.
IV contrast protocol—Nonionic iodinated con-
trast medium (iopamidol, Isovue 370, Bracco Di-
agnostics) was power injected (Empower CTA,
E-Z-EM) at a rate of 2.5 –3 mL/s, and t he volume
of the contrast agent was calculated on t he basis of
patient body weight, the total dose being 80–120
mL. Scanning was initiated 60–65 seconds after
administration of the contrast bolus.
Scanning parameters —The 75 MDCT scans
included in the study were reconstr ucted with ei-
ther ASIR (n = 48) or F BP technique (n = 27) . The
scanning parameters identical for the scans recon-
structed with the two tech niques were as follows:
tube potential, 120 kVp; section thickness, 5-mm;
standard reconstr uction kernel; gantry rotation
time, 0.5 second; pitch, 1.375; detector configura-
tion, 64 × 0.625 mm. Weight-based automated tube
current modulation technique with a tube current
range of 75–500 mA was used for scans recon-
structed with both techniques. However, the noise
index was increased by a factor of 1.3 for MDCT
examinations in the ASIR group to decrease the CT
dose (noise index is inversely related to radiation
dose). This noise index selection was based on our
experience with more than 2000 MDCT examina-
tions with this technique in routine abdom inal CT
studies and input from radiologists in the depar t-
ment regarding acceptable image quality and noise
on images.
Noise index is a term specific to the GE Health-
care CT systems and is used with automated tube
current modulation [21, 22]. Noise index repre-
sents the SD in the central region of the image
when a uniform (20 cm water) phantom is scanned
and reconst ructed with the standa rd reconstr uc-
tion algorith m [21]. Noise index is a measure of
image noise on CT sca ns and is inversely related
to tube cur rent, image qua lity, and radiation dose
[14–16, 21]. Hence a higher noise index implies
more noise on the CT i mages (i.e., poor image
quality) but at a lower tube current that results in
reduced radiation exposure.
For the MDCT studies reconst ructed with FBP
(n = 27), the noise indexes selected were 10 for
patients weighing less than 60 kg, 12.5 for those
weighing 61–90 kg, and 15 for those weighing more
than 91 kg (5-mm slice thickness). Therefore, the
noise indexes for ASIR were 13, 16, and 19 (5-mm
thickness). (For 0.625-mm settings, the approxi-
mate corresponding noise index levels would be 28
for < 60 kg, 32 for 61–90 kg, and 35 for > 91 kg for
standard-dose FBP scans and 36 for < 60 kg, 40 for
61–90 kg, and 45 for > 91 kg for low-dose ASIR.)
The ASIR percentage applied for image recon-
struction for MDCT scans was 30 % to strike an op-
timal ba lance of image quality and noise. This de-
termination was based on ou r initial efforts with
this technique for optimizing MDCT protocols for
various indications in the abdomen [19]. For image
reconstr uction, 30% ASIR selection yields blend-
ed image data sets in which 70% of image data are
reconstr ucted with FBP and 30% with ASIR with
30% reduction in image noise. For all patients,
immediately after completion of the diagnostic
MDCT exam ination, a technologist reconstructed
the images in the coronal and sagittal planes on the
scanner console using 2.5- to 3-m m thickness. All
image data sets were transm itted to a PACS for di-
agnostic interpretation.
Image Analysis
Review of the 75 MDCT studies (48 ASIR, 27
FBP) was perfor med at a PACS workstation ( PACS,
version 4, AGFA). The scans were random ized and
interpreted independently by two radiologists with
7 and 16 years of experience i n interpreting abdom-
inal CT images and blinded to the technical param-
eters of the sca ns, the clinical data, a nd the labora-
tory results. A predesigned template was used for

AJR :196, June 20 11 W745
MDCT and CT Enterography of Crohn Disease
image analysis, which included assessment of im-
age quality and image noise and determination of
Crohn disease findings (mural findings, extramural
features, and complications).
The image quality assessment included evalu-
ation of both ax ial images and coronal reforma-
tions in concordance with the European guidelines
on quality criteria for abdom inal CT exam inations
[8]. Dur ing the subjective image quality analysis,
the readers graded image quality on a 5-point scale
(1, poor image quality, not diagnostically accept-
able for interpretation; 2, suboptimal image qual-
ity, worse than routine dose; 3, acceptable image
quality, diagnostic interpret ation possible; 4, good
image quality; and 5, excellent image quality). The
readers evaluated image qual ity on the basis of
overall appea rance and on feat ures specific to the
evaluation of Crohn disease, including bowel wall
conspicuity, sharpness of bowel wall margin, clar-
ity of small-bowel mesentery, and sharpness of
mesenteric vessels. A n MDCT study with a score
of 3 or greater was considered of acceptable image
quality for rendering an interpretation.
Each reader assessed the MDCT scans for evi-
dence of Crohn disease, including mural thicken-
ing (defined as wall thickness > 3 mm), perienter-
ic inflammatory changes (mesenteric fat stranding
and prominent vasa recta), stricture, abscess, fistula,
small-bowel obstr uction, free fluid and loculated flu-
id collections, and lymphadenopathy. On the CT en-
terographic studies, the bowel wall was further eval-
uated for identification of mural hyperenhancement
(defined as increased attenuation of the bowel wall
compared with that of norma l-appearing bowel) and
mural stratification (defined as recognition of two or
three layers within the bowel wall). The diagnosis
of active disease was based on identification of the
following MDCT features: mura l hyperenha nce-
ment, mural stratification, mural thickening, perien-
teric in flammatory changes su ch as fat stranding and
vascular cha nges, and the presence of complications
such as fistula and abscess [3, 23–25].
Subjective image noise was evaluated with the
following 5-point scale: 1, too little noise; 2, less
than usua l noise; 3, usual noise; 4, excessive noise
not affecting diagnostic interpretation; 5, exces-
sive noise rendering diagnostic inter pretation not
possible. A single reader performed the objective
evaluation of image noise by region-of-interest
analysis of the MDCT images. Objective image
noise (SD of mean CT number) was measured by
placement of a 70- to 110-mm2 ci rcular region of
interest in t he subcutaneous fat, fluid within t he
urina ry bladder (ga llbladder in patients in whom
urina ry bladder was collapsed), paraspinal mus-
cles, and right lobe of the liver. The lateral widt hs
in centimeters were recorded in all patients as the
maximum skin-to-skin transverse diameter of the
abdomen at the level of the upper pole of the right
kidney. The transverse dimension also was used to
compare habitus in the two groups because body
weight is not often a tr ue reflection of habitus,
which has image-quality implications.
Reference Standard
The final confirmat ion of disease status, activity,
and complications was done either singly or with a
combination of clinical evaluation (n = 75), ileoco-
lonoscopy (n = 46), surgery (n = 19), and follow-up
imaging (n = 8). An independent observer record-
ed the data from the medical records of the patient
population in the electronic medical database, and
this comprehensive evaluation acted as reference
standard for confirmation of the findings record-
ed on the MDCT studies. The details analyzed in
the clinical evaluation (n = 75) included history,
physical examination findings, details of hospital-
ization, notes from discharge summary, and labo-
ratory findings. The gastroenterologist’s report and
the histopathologic findings af ter biopsy were used
for confirmation of the diagnosis, disease activity,
and extent of involvement in patients who under-
went ileocolonoscopy (n = 46) within 3 months of
MDCT–CT enterography. In patients who under-
went surgical treatment (n = 19), the operative re-
ports and results of histopathologic evaluation of
the resected specimen were used for confirmation.
Radiation Dose
The radiation doses for all MDCT studies were
recorded as CT dose index (CTDI) and dose-length
product (DLP). The radiation dose measurements
for the MDCT–CT enterographic studies recon-
structed with ASIR and FBP were compared.
Statistical Analysis
Statistical analysis was performed with SAS
software (system release 8.2, SAS Institute) and
Excel 2003 (Microsoft). The subjective image
quality score, subjective image noise, and objec-
tive image noise measurements were estimated for
the MDCT scans reconstructed with adaptive sta-
tistical iterative technique and FBP technique and
compared by Wilcoxon signed rank and paired Stu-
dent t tests; p < 0.05 was considered to suggest a
statistically significant difference. The estimated
radiation doses for the ASIR and FBP scans were
compared by use of paired Student t test. Sensitiv-
ity, specificity, positive predictive value, negative
predictive value, and accuracy in the detection of
active Crohn disease with standard-dose CT en-
terography with FBP and low-dose CT enterog-
raphy with ASIR were calculated by comparison
with the reference standard. Agreement between
the two readers was determined with Cohen’s kap-
pa test. Interobserver agreement was considered
poor (κ ≤ 0.19), fair (κ = 0.2–0.39), moderate (κ =
0.4–0.59), substantial (κ = 0.6–0.79), or almost p er-
fect (κ = 0.8–1.00). The concordance between the
two readers for assessment of various CT entero-
graphic findings of Crohn disease was assessed by
estimation of interobserver agreement.
Results
Study Subjects
The mean weight of the patients in the co-
hort was 61.4 kg (range, 26.4–117.7 kg). Most
of the subjects in the study cohort weighed
less than 90 kg (29 patients, < 60 kg; 16 pa-
tients, 61–90 kg); only three patients weighed
more than 91 kg. The mean age, age range, sex
distribution, weight, and transverse axial di-
mensions in both patient groups (ASIR, FBP)
were comparable. The mean and median pa-
tient ages were 33.3 and 26 years (range, 17–
83 years) in the ASIR group and 31.8 and 27
years (range, 17–63 years) in the FBP group
(p = 0.7). There were 20 men and 28 wom-
en in the ASIR group and 13 men and 14
women in the FBP group. The mean and me-
dian weights were 60 and 59 kg (range, 26 –
118 kg) in the ASIR group and 64 and 58.6
kg (range, 29–117 kg) in the FBP group (p =
0.6). The mean and median transverse dimen-
sions (30.3 ± 3.6 [SD] and 30 cm) in the ASIR
group and FBP (29.9 ± 3.7 and 28.9 cm) were
not significantly different (p = 0.3).
Among the 75 MDCT studies included in
the final analysis, CT findings of Crohn disease
and its complications were seen on 63 scans
(84%); 12 scans (16%) showed no abnormal-
ities (Table 1). Mural thickening was present
on 49 scans (65.3%) and mural hyperenhance-
ment on 46 scans (61.3%) (Fig. 1). The extra-
mural findings and complications included 10
cases of abscess (13.3%), six of stricture (8%),
five of fistula (6.7%; two enteroenteric fistu-
las and three enterovesical fistulas), and seven
cases of partial small-bowel obstruction (9.3%)
(Figs. 2 and 3). Mesenteric lymph node en-
largement was present in 43 patients (57.3%).
Subjective Image Quality
All low-dose MDCT examinations per-
formed with ASIR were found diagnostical-
ly acceptable for interpretation in comparison
with standard-dose MDCT FBP scans (Table
2). The image quality scores for the MDCT
images (both axial and coronal reformations)
reconstructed with ASIR technique were sig-
nificantly higher than those for the FBP im-
ages. The mean image quality scores were 4.2
versus 3.87 (p = 0.007) for the axial images
and 4.6 versus 4.2 (p = 0.008) for the coronal

W74 6 AJR :196, June 20 11
Kambadakone et al.
images. Bowel wall conspicuity, sharpness
of bowel wall margin, clarity of small-bowel
mesentery, and mesenteric vessels were better
appreciated on MDCT images reconstructed
adaptive statistical iteration than those with
FBP. The improved performance of ASIR
was seen across all weight categories (Table
2). There was moderate interobserver agree-
ment on quality of the axial images (κ = 0.56 –
0.64) and substantial agreement on the coro-
nal images (κ = 0.67–0.78).
Image Noise
The subjective image noise of the ASIR
MDCT images was comparable to that of the
FBP images (mean image noise score, 1.43 vs
1.58 ; p = 0.2) (Table 3). There was moderate
interobserver agreement in assessment of im-
age noise (κ = 0.4–0.56). The objective image
noise determined as the SD of the mean CT
number was significantly lower for the ASIR
images than for the FBP images (p < 0.0 01)
for the four tissues (fat, fluid, muscle, and liv-
er) (Table 3).
CT Enterography With Neutral Oral
Contrast Medium
Sixteen patients (nine men, seven women;
mean age, 33.4 years; range, 17–57 years) un-
derwent both low-dose CT enterography re-
constructed with ASIR and standard-dose CT
enterography with FBP reconstruction (Ta-
bles 4–6). The image quality score was high-
er for axial data sets reconstructed with ASIR
than those reconstructed with FBP (mean im-
age quality score, 4.3 vs 3.85; p = 0.03), but
TABLE 1: MDCT Findings in Patients With Crohn Disease
Finding No. Filtered Back Projection (n = 2 7) Adaptive Statistical Iterative Reconstruction (n = 48)
Abnormality 63 23 (8 5.2) 40 (8 3.3)
Active disease 42 14 ( 51.8) 28 (58. 3)
Mural thickening 49 17 (62.9) 32 (66.7)
Mural hyperenhancement 46 14 ( 51.8) 32 (6 6.7 )
Perienteric changes 42 14 ( 51.8) 28 (58. 3)
Abscess 10 5 (18.5) 5 (10.4)
Stricture 6 1 (3.7 ) 5 (10.4)
Fistula 5 2 (7.4) 3 (6.25 )
Small-bowel obstruction 7 2 (7.4 ) 5 (10 .4)
Lymphadenopathy 43 14 (51.8) 2 9 (60.4)
Note —Values in par entheses are percent ages.
A
C
Fig. 1— 22-year-old man (b ody weight , 56 kg) wi th
multiple exacerbations of Crohn disease.
A and B, Ax ial (A) and coronal (B) contras t-enhanced
CT images obt ained during i nitial eval uation show
wall thickening and enhance ment wit h increased sur-
rounding perienteric vascularit y in descending colon
(arrows ). Biopsy confirmed Crohn disease. Radiation
dose de tails were vo lume CT dose i ndex, 14. 6 mGy;
dose-length produ ct, 773.3 mGy · cm; ef fective dose,
11.6 mSv. R emission occurred af ter institu tion of
steroid therapy, but patient presented 1 year later
wit h relapse.
C and D, Repea ted axial (C) an d coronal (D) contrast-
enhance d CT images reco nstructed with adapt ive
statistical iterative reconstruction show image qual-
it y and depiction of ent eric and ext raenteric changes
(arrows ) comparable to A and B but with 53 % reduc-
tion in radiat ion dose (volume CT do se index, 7.8 m Gy;
dose-length produ ct, 36 6.7 mGy · cm; e ffecti ve dose,
5.5 m Sv).
B
D

AJR :196, June 20 11 W747
MDCT and CT Enterography of Crohn Disease
the scores were comparable for coronal im-
ages reconstructed with the two techniques
(mean image quality score, 4.55 vs 4.35; p =
0.2). Low-dose CT enterography with ASIR
had better subjective image quality than stan-
dard-dose CT enterography with FBP (mean
image noise score, 1.3 vs 1.8; p = 0.003) and
had a lower objective image noise assessment
for fat, fluid, muscle, and liver (p = 0.01).
Active Crohn Disease
Among the 16 patients who underwent both
standard-dose CT enterography with FBP
and low-dose CT enterography with ASIR,
10 (62.5%) had evidence of active disease on
FBP scans and 12 (75%) on ASIR scans. The
readers had equivalent sensitivity, specificity,
negative predictive value, and accuracy in the
detection of active Crohn disease at ASIR CT
enterographic examinations (90%, 83%, 83%,
and 87.5%) and FBP CT enterographic exami-
nations (87%, 75%, 67%, and 84%) (Table 4).
Readers had similar concordance for the rec-
ognition of mural thickening, mural hyperen-
hancement, mural stratification, and perien-
teric fat stranding with ASIR CT enterography
and FBP CT enterography (κ = 0.7–0.83 vs κ =
0.4– 0.83) (Table 5).
Radiation Dose
The estimated radiation dose for the stan-
dard-dose MDCT scans reconstructed with
FBP was a volume CTDI (CTDIvol) of 12 ±
5.5 mGy and a DLP of 577 ± 297 mGy · cm.
An average dose reduction of 34.5% (range,
6–57%) was found for ASIR MDCT (CTDIvol,
7.7 ± 2.1 mGy; DLP, 380.3 ± 150 mGy · cm;
p < 0.001) (Table 7). The radiation dose re-
duction was more substantial for the patients
weighing less than 90 kg (mean reduction,
36%) than for those weighing more than 91 kg
(mean reduction, 26%). This difference was
attributed to the fact that in examinations of
patients weighing more than 90 kg, the maxi-
mum tube current of 500 mA (upper limit for
this protocol) was reached for the noise index
selection for standard-dose MDCT scans with
FBP (auto tube current range, 75–500 mA).
Compared with standard-dose CT enterogra-
phy with FBP reconstruction, low-dose CT
A
Fig. 2 —24-y ear-old man (bod y weight, 5 1 kg) with
known Crohn disease, spiking fever, and abdominal
pain.
A, Axial contrast-enhanced MD CT image wi th 30%
ASIR shows peripherally enhancing abscess (arrow )
wit h air pockets i n right lowe r quadrant ex tending
into right psoas muscle.
B, Coronal refor mated MDC T image wit h 30% ASI R
show s mural thickening and en hancement (thin ar-
row ) in ter minal ileum wi th hyper emia appearin g as
engorged perienteric vascularit y and comb sign (thick
arrow ). Rad iation dos e details wer e volume CT dose
index , 6.8 mGy ; dose-le ngth produ ct, 32 3.3 mGy • cm;
eff ective do se, 4.8 4 mSv.
B
TABLE 2: Results of Image Quality Assessment of MDCT Scans Reconstructed With Two Techniques
Weight Group
Filtered Back Projection (n = 27) Adaptive Statistical Iterative Reconstruction (n = 48)
pReader 1 Reader 2 Reader 1 Reader 2
Ax ial images 0.007
All weights 3.88 3.86 4.1 4.26
< 60 kg 3.9 3.8 4.0 4.22
61– 90 kg 3.8 3.9 4.1 4 .1
> 91 kg 44.5 4.4 4.6
Weighed k0.56 0.64
Coronal images 0.008
All weights 4.2 4.2 4.5 4.6
< 60 kg 4.1 4.2 4.6 4.5
61– 90 kg 4.1 4.4 4.6 4.5
> 91 kg 4.5 4.6 4.5 4.6
Weighed k0.78 0.67
Note —Image qu ality was assesse d on the follo wing 5-p oint scale: 1, po or image quali ty, not diagn osticall y acceptable f or interpr etatio n; 2, subop timal image qua lity,
wors e than rout ine dose; 3, acce ptable image q uality, diagnost ic interp retation possi ble; 4, good i mage qualit y; 5, excell ent image quali ty.

W74 8 AJR :196, June 20 11
Kambadakone et al.
enterography with ASIR had an average ra-
diation dose reduction of 29.5% (CTDIvol,
10.9 ± 3.8 Gy vs 7.7 ± 1.7 Gy; DLP, 580 ± 248
mGy · cm vs 408 ± 139 mGy · cm; p = 0.03).
Discussion
Compared with the standard-dose MDCT–
CT enterographic scans reconstructed with
FBP, low-dose MDCT–CT enterographic
images reconstructed with ASIR technique
had optimal image quality for diagnosing the
manifestations of Crohn disease. Crohn dis-
ease has a complex clinical course that poses
several challenges in management [26, 27].
The availability of newer therapeutic op-
tions, including immunosuppressive drugs
and novel biologic agents, has allowed phy-
sicians to pursue more aggressive strategies
in the treatment of patients with Crohn dis-
ease [26, 27]. However, appropriate patient
selection and stratification based on disease
severity and risk are key to instituting indi-
vidualized treatment with these novel but
expensive therapies [5, 26–28]. Therefore,
there is a growing impetus to develop sero-
logic, genetic, and imaging biomarkers to
facilitate treatment decisions and monitor
response [27]. Manifestations of Crohn dis-
ease on contrast-enhanced MDCT images
have already had positive correlation with es-
tablished serologic biomarkers of inflamma-
tion, thereby providing accurate information
on disease activity and response [5, 26–28].
The increasing use of MDCT in the eval-
uation of Crohn disease has raised concerns
about the long-term adverse effects of exces-
sive radiation exposure [11–13, 29, 30]. Be-
cause of their young age at disease onset and
because the relapsing and recurrent nature of
the disease necessitates multiple imaging in-
vestigations, patients with Crohn disease are
more susceptible than the general popula-
tion to the detrimental effects of ionizing ra-
diation [11, 13, 30]. Heightened awareness of
cancer risk due to ionizing radiation exposure
TABLE 3: Results of Subjective and Objective Image Noise Assessment of MDCT Scans Reconstructed With
Two Techniques
Weight Group
Filtered Back Projection (n = 27) Adaptive Statistical Iterative Reconstruction (n = 48)
pReader 1 Reader 2 Reader 1 Reader 2
Subjective image noisea0.2
All w t 1.7 1.5 1.5 1.4
< 60 kg 1.8 1.6 1. 6 1.3
61– 90 kg 1.6 1.3 1. 5 1.3
> 91 kg 1.8 1.2 1.3 1.2
Weighed k0.56 0.4
Objective image noiseb
Fat < 0.0001
All weights 15.7 ± 4 .8 12 ± 2.8
< 60 kg 15.7 ± 5.1 11.7 ± 2.6
61– 90 kg 15.9 ± 4 .9 12.7 ± 3 .2
> 91 kg 14.6 ± 3 .7 11 ± 2
Muscle < 0.0001
All weights 17 ± 5.4 12.6 ± 2.9
< 60 kg 16 ± 4.6 12.3 ± 2.7
61– 90 kg 19 ± 7.4 13. 2 ± 3.1
> 91 kg 19.1 ± 2.4 11.2 ± 3 .5
Fluid < 0.0001
All weights 15.2 ± 3 .4 12.1 ± 3. 4
< 60 kg 14.7 ± 3 .1 11.9 ± 3. 9
61– 90 kg 15.6 ± 3 .6 12.5 ± 2.7
> 91 kg 19.5 ± 2 .1 12.9 ± 4 .1
Liver < 0.0001
All weights 16.6 ± 4.1 14.1 ± 3.7
< 60 kg 15.7 ± 3.7 14 ± 4.4
61– 90 kg 17.7 ± 4. 6 15.2 ± 3 .8
> 91 kg 16 ± 4.2 14.9 ± 6.2
aAsse ssed on the f ollowing 5 -point sca le: 1, too lit tle noise ; 2, less than usual noi se; 3, usual noise; 4, ex cessive noi se not aff ecting diag nostic in terpret ation; 5, e xcessive
noise rendering diagnostic interpretation not possible.
bValues re present t he image noise es timate d with regi on of inter est analys is of SD of mean C T number (HU) i n subcutane ous fat, flu id, muscle, and right lobe of liv er.
Values ar e mean ± SD of image n oise.

AJR :196, June 20 11 W749
MDCT and CT Enterography of Crohn Disease
for imaging has alarmed treating physicians
and radiologists [10, 11, 30–32]. Therefore,
MR enterography is fast gaining attention as
a suitable alternative to MDCT in the evalu-
ation of young patients with Crohn disease
[28, 33, 34]. There is increasing evidence
that with appropriate protocols and expertise,
high-quality MR enterography can be as ef-
fective as CT enterography and therefore may
be an appropriate imaging substitute for CT
enterography at centers experienced in body
MRI [28, 33, 34]. But MR enterography is yet
to gain widespread acceptance owing to pro-
tocol complexity, lack of availability, and lim-
ited expertise. Even at centers with experience
in MR enterography, initial evaluation is often
performed with MDCT, and follow-up is un-
dertaken with MR enterography [28, 33–35].
The beneficial role of MDCT in the man-
agement of Crohn disease is undeniable be-
cause according to the American College of
Radiology appropriateness criteria [24], both
routine MDCT and CT enterography are the
most appropriate imaging investigations in
Crohn disease. The need for adapting dose-
saving measures has been highlighted for
MDCT [24]. Proposed strategies for mini-
mizing radiation dose at MDCT include use of
low tube current, low tube voltage, automatic
tube current modulation, and noise reduction
filters [14, 16]. Using simulation techniques in
the abdomen, several authors have validat-
ed the accuracy of low tube current images
in the diagnosis of conditions such as uroli-
thiasis, appendicitis, and Crohn disease [15,
25, 36–38]. However, these authors also have
cautioned about image-quality limitations
due to excessive dose reduction. The con-
sequences are especially undesired in Crohn
disease because early detection, determina-
tion of disease activity on the basis of mural
hyperenhancement, and recognition of com-
plications such as small abscesses and fistulas
have ramifications in management. Therefore,
dose reduction strategies should strike an op-
timal balance between image quality and ra-
diation dose. Moreover, CT findings are often
interpreted by trainees and by treating physi-
cians of varying expertise, further emphasiz-
ing the need for optimal image quality. ASIR
improves image quality by eliminating im-
age noise from noisy low-dose CT images
[19, 20]. This technique has shown prom-
ise in several preliminary studies, allowing
acquisition of low-dose CT scans with im-
proved image quality [18–20].
The results of this study confirmed that
images obtained at low-dose MDCT–CT en-
terography and reconstructed with adaptive
statistical iteration were of optimal quality
for rendering diagnostic interpretations in
the care of patients with Crohn disease. In
addition, an average radiation dose reduction
of 34% was achieved for low-dose MDCT
with ASIR. All the CT enterographic imag-
es in the ASIR group allowed confident and
accurate determination of various mural and
extramural manifestations of Crohn disease
in patients in all weight categories.
Mural hyperenhancement is a recognized
feature of disease activity and severity [5, 8,
9, 23–25]. Both readers confidently identi-
fied the mural features, including mural hy-
perenhancement, on ASIR images. The image
quality of axial and coronal reconstructed im-
ages was consistently found a aesthetically es-
thetically pleasing. Although the image qual-
ity assessment was quite subjective, we found
TABLE 4: Comparison of Standard- and Low-Dose CT Enterography With Two Reconstruction Techniques in
Determination of Clinical Activity
Diagnostic At tribute
Filtered Back Projection (n = 16)aAdaptive Statistical Iterative Reconstruction (n = 16)
Reader 1 Reader 2 Reader 1 Reader 2
Sensitivity 83.3 91.6 90.0 90.0
Specificity 75.0 75.0 83.3 83.3
Positive predictive value 90.9 91.6 90.0 90.0
Negative predictive value 60.0 75.0 83.3 83.3
Accuracy 81.3 8 7.5 8 7.5 87. 5
Note —Values ar e percentage s. In 16 cases, s tandard- dose CT en terograph y with fil tered back pro jection was c ompared wi th low-do se CT ente rography wi th adapti ve
sta tistica l iterative recon struct ion to determine t he presence of ac tive Cro hn disease in accor dance with t he refere nce standar d.
aIn the ad aptive st atist ical itera tive recons truction group , active Cr ohn disease con firmed in accor dance with t he refere nce standar d was found in 12 of 1 6 patient s
(75 %); in the fil tered back pr ojection gr oup, activ e Crohn diseas e was found in 10 o f 16 patient s (62. 5%).
TABLE 5: Reader Concordance in Determination of Crohn Disease Findings on CT Enterographic Images
Reconstructed With Two Techniques
Finding
Filtered Back Projection (n = 16) Adaptive Statistical Iterative Reconstruction (n = 16)
No. Reader 1 Reader 2 kNo. Reader 1 Reader 2 k
Mural hyper enhancement 12 12/12 11/12 0.85 13 12/13 13/13 0.82
Mural thickening 13 12/13 12 /13 0.67 13 12/ 13 13/13 0.82
Mural stratification 11 9/11 10 /11 0.61 12 10/12 12 /12 0.71
Perienteric changes 10 8 /10 10/10 0.43 12 10/12 12/12 0.71
Abscess 44 /4 4 /4 1 3 3/3 3/3 1
Fistula 43/4 2 /4 0.63 0 — — —
Stricture 44/4 3 /4 0.82 43/4 4/4 0.82
Small-bowel obstruction 33/3 3/3 1 4 4/4 3/4 0.82
Lymph nodes 65/6 6/6 0.86 55/5 5/5 1
Note —dash ( —) means not ap plicable.

W750 AJR:196, June 2011
Kambadakone et al.
moderate to substantial agreement between
the readers. Although subjective image noise
was perceived as comparable between the
low-dose ASIR and standard-dose FBP im-
ages, objective image noise was consistently
lower on low-dose ASIR images. In a previous
study [15], we found that when simulated low-
dose MDCT scans were used, confident inter-
pretation of Crohn disease manifestations was
possible despite the presence of image noise.
This approach can be used if iterative recon-
struction techniques or other image quality
improvement methods are not available on the
scanner. With ASIR, image quality improve-
ments in low-dose MDCT examinations can
ensure consistent examination quality and af-
ford an opportunity to achieve even higher re-
duction in radiation dose without compromis-
ing diagnostic performance.
The modest radiation dose benefit (mean,
34.5%) facilitated by ASIR can translate into
substantial cumulative radiation dose bene-
fits for patients undergoing serial MDCT ex-
aminations. We observed that the radiation
dose reduction achieved was higher in patients
weighing less than 91 kg (< 200 lb), and these
observations are in agreement with those made
by Hara and colleagues in their study of rou-
tine abdominopelvic CT with ASIR [18–20].
Because of the debilitating nature of the dis-
ease, a considerable number of the pat ients with
Crohn disease have lower body weight; this
finding therefore has important ramifications.
Although the radiation dose reduction achieved
in this study was modest, the dose for the regu-
lar FBP-enabled MDCT scans obtained at our
institution is lower than that in the American
College of Radiology appropriateness criteria
radiation guidelines [24]. This translates into
more substantial overall radiation dose reduc-
tion, and improvement in image quality despite
the use of low-dose technique opens opportuni-
ties for further reduction of CT dose.
The rationale for using a 1.3 times high-
er noise index for the low-dose CT entero-
graphic studies and corresponding 30%
ASIR was based on vendor-defined protocol
suggestions and our own initial experience
with ASIR in abdominopelvic CT. In addi-
tion, before starting the study, we obtained
feedback from radiologists interpreting CT
enterographic images at our institution dur-
ing the phase of protocol optimization, and
ASIR of 30% was chosen. Previous standard-
dose FBP scans of the same patient served
as controls in evaluation of the low-dose CT
enterographic studies with ASIR. This study
design enabled us to observe the image qual-
ity and radiation dose considerations of the
two techniques in the same group of patients
undergoing scanning in a short time interval.
The results of this type of comparative anal-
ysis are of immense practical value, and the
radiation dose benefits observed are realistic
within the scope of further dose reduction.
Several previous FBP CT studies used for
comparison were performed with positive
oral contrast medium because admitted pa-
tients, who are often sicker than outpatients,
better tolerate ingestion of positive oral contrast
medium or cannot conform to the desired time
TABLE 6: Comparison of Image Quality and Radiation Dose of Standard- and Low- Dose CT Enterography With Two
Reconstruction Techniques
Variable
Filtered Back Projection (n = 16) Adaptive Statistical Iterative Reconstruction (n = 16)
Reader 1 Reader 2 Reader 1 Reader 2
Image quality score
Axial 3.8 3.9 4.2 4.4
Coronal 4.4 4.3 4.5 4.6
Subjective noise 1.8 1.7 1.3 1.3
Objective noisea
Fat 13.8 ± 3.5 11.3 ± 2. 4
Fluid 16. 5 ± 3.2 12.4 ± 3.2
Muscle 14.5 ± 4 12.4 ± 3.3
Liver 15.8 ± 4. 2 14.5 ± 4.2
Radiation dosea
CT dose index (mG y) 10.9 ± 3. 8 7.7 ± 1.7
Dose-lengt h product (mGy · cm) 580 ± 24 8 40 8 ± 139
aMean ± SD.
A
Fig. 3 —37- year-old woman (body we ight, 58 .6 kg) wit h Crohn disea se at follow -up evaluat ion.
A, Axial contrast-enhanced C T image obtai ned after i leostomy shows wall thickening and enhancement wit h
surro unding fat s tranding in i leum at ost omy site (arro w) . Radiation d ose details w ere as follows: volume CT
dose in dex, 9.4 m Gy; dose- length pr oduct, 4 85 mGy · cm; ef fective dose, 7.2 mSv. Rem ission occurr ed after
immun osuppress ive treat ment. Eig ht months later, pat ient presented with relapse.
B, Repeat ed axial CT e nterographic image r econstru cted wit h adaptive s tatistical it erative r econstru ction
shows wall thi ckening and mural hyper enhancement o f ileal loop at ileost omy (arrow ), narr owing of jux tasto mal
ileum , and proximal dilate d loops of small b owel, indica ting bowel obstructi on. Average dose reduct ion was
37% w ith main tained image qualit y (volume CT dose inde x, 6.5 mG y; dose-l ength prod uct, 315 mG y · cm;
eff ective do se, 4.73 mS v).
B

AJR :196, June 20 11 W751
MDCT and CT Enterography of Crohn Disease
frame for consumption of neutral oral contrast
medium. These patients also have a relatively
higher incidence of complications, including
abscesses and fistulas. In our practice, posi-
tive oral contrast medium is often preferred.
We also offer positive oral contrast medium
to outpatients unable to tolerate large volumes
of neutral oral contrast medium or who have
had a prior undesirable experience with neu-
ral oral contrast medium. These protocol vari-
abilities with oral contrast medium reflect our
practice pattern and the complexity of evalu-
ation of Crohn disease. It is conceivable that
the presence of positive oral contrast medi-
um can affect a reader’s confidence in iden-
tification of subtle mural hyperenhancement.
In this study, all mural hyperenhancement was
confidently recognized on CT enterographic
images. Moreover, to minimize any effect of
bias due to use of oral contrast medium on
reader performance and perceived improve-
ments in image quality with ASIR, we per-
formed subgroup analysis of the data on 16
patients who underwent CT enterography in
which images were reconstr ucted with both
adaptive statistical iterative technique and
FBP. The results showed consistent improve-
ment in image quality with low-dose ASIR.
This pilot study had a few other limita-
tions. Although the study was performed
prospectively, only the diagnostic interpreta-
tions were rendered initially, and the detailed
analysis was retrospectively undertaken. We
also did not randomize patient assignment
to MDCT–CT enterography with ASIR or
FBP but used previous images reconstruct-
ed with FBP for comparison. In addition, the
ASIR application used in this study is ven-
dor specific, and the observations may not be
applicable to CT protocols from other ven-
dors. Nonetheless, our results confirm the
feasibility of low-dose MDCT in the evalu-
ation of Crohn disease. Likewise, we relied
on the readers’ subjective assessment of im-
age quality with the two reconstruction tech-
niques and their confidence in rendering di-
agnostic interpretations, and we did validate
each CT finding of Crohn disease to study
its accuracy. The value of CT enterography
for defining the relevant manifestations of
Crohn disease is well established, and its as-
sociation with high diagnostic confidence
supports the performance of MDCT.
Conclusion
Low-dose 64-MDCT images reconstruct-
ed with adaptive statistical iterative tech-
nique had optimal image quality and af-
forded high diagnostic confidence in the
evaluation of manifestations of Crohn dis-
ease compared with standard-dose CT im-
ages reconstructed with FBP. The improved
image quality achieved with the low-dose
scans opens oppor tunities to further dose
reduction in CT enterography and related
MDCT examinations of the abdomen in pa-
tients with Crohn disease.
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