Open Journal of Radiology, 2013, 3, 112-116
http://dx.doi.org/10.4236/ojrad.2013.33018 Published Online September 2013 (http://www.scirp.org/journal/ojrad)
Portal Venous-Phase CT of the Liver in Patients without
Chronic Liver Damage: Does Portal-Inflow Tracking
Improve Enhancement and Image Quality?
Masayuki Kanematsu*, Haruo Watanabe, Hiroshi Kondo, Satoshi Goshima,
Hiroshi Kawada, Yoshifumi Noda
Department of Radiology, Gifu University Hospital, Gifu, Japan
Received June 7, 2013; revised July 7, 2013; accepted July 15, 2013
Copyright © 2013 Masayuki Kanematsu et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Purpose: This study was undertaken to determine if portal-inflow bolus tracking outperforms aortic bolus tracking with
respect to the image quality of contrast-enhanced portal venous-phase CT of the liver in patients without chronic liver
damage. Materials and Methods: Contrast-enhanced CT of the liver was performed in 132 consecutive patients with-
out chronic liver damage. Patients were prospectively assigned to three protocols: Protocol A—a portal venous-phase
scan delay of 6 seconds after superior mesenteric venous (SMV) enhancement increased by 70 HU or 14 seconds after
SMV enhancement was visually confirmed, and Protocols B and C—40 and 50 seconds, respectively, after abdominal
aortic enhancement increased by 100 HU. Enhancement (HU) of abdominal aorta, portal trunk, and liver parenchyma
and diagnostic acceptability were assessed. Results: HU of aorta was higher for protocol A than for protocols B and C
(P < 0.05), whereas HU of portal trunk was higher for protocol B than for protocols A and C (P < 0.05). HUs of liver
were similar in three protocols. No difference was found between diagnostic acceptabilities of three protocols. Conclu-
sion: Portal-inflow bolus tracking did not outperform aortic tracking in terms of optimization of portal venous-phase
CT in patients without chronic liver damage.
Keywords: CT; Liver; Contrast Enhancement; Bolus Tracking
Usefulness of contrast-enhanced CT for the diagnosis of
hepatic diseases is widely recognized and the technique
is employed at many centers. Furthermore, temporally
resolved multi-phasic CT scanning after the intravenous
bolus injection of contrast material is a crucial for the de-
tection and characterization of focal hepatic lesions [1-5].
In particular, the acquisition of optimal portal venous-
phase images is essential for the diagnosis of hyper- or
hypovascular liver metastases [3-5].
Several researchers have described methods of opti-
mizing scan delays after contrast injection, in particular,
for the hepatic arterial-dominant phase imaging, with the
use of bolus-tracking , test-bolus imaging , and
fixed injection duration  techniques. Bolus-tracking is
widely employed in many centers to optimize scan pro-
tocols of contrast-enhanced CT of the liver, including the
acquisition of hepatic arterial-dominant phase images
and subsequently of portal venous-phase images.
Portal venous-phase images play an important role in
the detection of malignant hepatic tumors by maximizing
the tumor-to-liver contrast, and in the characterization of
focal hepatic lesions by demonstrating washout of ma-
lignant hepatic tumors  or the peripheral paddling of
cavernous hemangiomas [2,10]. Nowadays, because the
total scan time for the entire liver has reduced to as little
as 2 seconds, we wondered whether a dedicated tech-
nique could allow scanning of the whole liver and cap-
ture the most intense enhancement of liver parenchyma
during the portal venous phase. The purpose of this study
was to determine if portal-inflow tracking outperforms
the widely employed aortic bolus tracking for the opti-
mization of contrast-enhanced portal venous-phase CT of
the liver in patients without chronic liver damage. A lit-
erature search failed to unearth any reports on this topic.
opyright © 2013 SciRes. OJRad
M. KANEMATSU ET AL. 113
2. Materials and Methods
This HIPAA-compliant study had institutional review
board approval and all patients provided written in-
formed consent. During a recent 5-month period, 176
consecutive patients with a known malignancy and with-
out known chronic liver disease due to viral, alcoholic,
autoimmune, or cryptogenic hepatitis underwent ab-
dominopelvic contrast-enhanced CT for a preoperative
work-up or a post-therapeutic survey. Of these, 44 pa-
tients were excluded because of; prior abdominal surgery
that could significantly alter the portal venous blood flow
dynamics (n = 38), diffuse fatty liver (n = 2), and nu-
merous liver metastases (n = 4). The remaining 132 pa-
tients (88 men and 44 women; age range, 19 - 91 years;
mean age, 62.1 years) constituted the study population.
The primary malignancies in these 132 patients were;
rectal (n = 35), gastric (n = 29), colon (n = 28), breast (n
= 10), uterine cervical (n = 5), endometrial (n =3), eso-
phageal (n =3), ovarian (n = 3), prostate (n = 2), pancre-
atic (n = 2), renal cell (n = 1), bile duct (n = 1), appen-
diceal (n = 1), pulmonary (n = 1), urinary bladder (n = 1)
or testicular (n = 1) carcinoma, or gastrointestinal stromal
tumor of the stomach (n = 2), malignant lymphoma of the
cervical lymph nodes (n = 2), uterine leiomyosarcoma (n
= 1), and Paget's disease of the scrotum (n = 1).
2.2. Contrast Material Injection and Scan
A 16-detector CT scanner (Lightspeed 16; GE Healthcare,
Milwaukee, WI) with a fixed tube voltage of 120 kVp
and an automatic tube current modulation program (3D
mA Modulation; GE Healthcare) was used. Other CT
parameters were as follows: collimation, 1.25 mm; de-
tector configuration, 16 detectors with a 1.25-mm section
thickness (16 × 1.25 mm); table feed, 27.5 mm per rota-
tion; pitch, 1.37; craniocaudal scan range, 45 - 50 cm;
32-cm field of view; 0.5-second gantry rotation time; and
scan acquisition time, 8.8 - 9.7 seconds. All transverse
CT images were reconstructed at section thickness of 5
mm by using a standard reconstruction algorithm.
After the acquisition of unenhanced images, a bo-
lus-tracking program (Smart Prep; GE Medical Systems)
was used to determine the optimal time to initiate diag-
nostic portal venous-phase scanning following the ad-
ministration of contrast medium. All patients were ad-
ministered non-ionic iodinated contrast material contain-
ing 300 mg of iodine per milliliter warmed to body tem-
perature at a dose of 2 mL per kilograms injected intra-
venously over 30 seconds using a commercially available
power injector through a 21-gauge plastic catheter, which
was typically placed in either an antecubital vein or a
radial vein. The fractional dose was 20 mg iodine/kg/sec
in all patients.
Patients were prospectively randomized to three pro-
tocol groups, using a random-number table, as follows:
Protocol A—the portal venous-phase scan was started 6
seconds after enhancement of the superior mesenteric
vein (SMV) trunk increased by 70 HU or 14 seconds
after the SMV enhancement was visually confirmed, and
Protocols B and C—40 and 50 seconds, respectively,
after the abdominal aortic enhancement increased by 100
HU. For protocol A, CT number measurement in the
SMV occasionally failed due to respiratory motion. Hence,
we determined the empiric scan delay by visually con-
firming SMV enhancement prior to initiating the portal
venous-phase scan. The default empiric scan delay was
determined to be 14 seconds, which was obtained by
adding 8 seconds to the 6 seconds, because the mean
time in seconds from the initiation of contrast inflow in
the SMV to a CT number increase by 70 HU was deter-
mined to be 8.1 ± 3.7 (mean ± 1SD) seconds during our
preliminary evaluation of 40 patients (unpublished our
The region of interest cursor for bolus tracking was
placed in the SMV trunk or in the aorta just above the
diaphragmatic dome. Real-time low-dose (120 kVp, 50
mA) serial monitoring scans were started 30 (protocol A)
or 10 (protocols B and C) seconds after initiating the
contrast injection. The time in seconds from initiation of
contrast injection to diagnostic scan initiation was re-
corded in all patients.
2.3. Quantitative Image Analysis
A radiologist (H. W.) with 5 years of post-training ex-
perience at interpreting body CT images measured mean
CT numbers and determined standard deviations in the
abdominal aorta, portal vein trunk, and liver on a com-
mercially available DICOM viewer. Measurements were
performed on unenhanced and portal venous-phase axial
images. CT numbers of livers were measured in the left
lateral, right anterior, and right posterior segments devoid
of blood vessels, bile ducts, focal hepatic lesions, calci-
fications, and artifacts, and the numbers obtained were
averaged. Quantitative degrees of contrast enhancement
are expressed as CT numbers increases from unenhanced
to contrast-enhanced axial images (ΔHU).
2.4. Qualitative Image Analysis
Two radiologists (H. Kondo and S. G.) with 13 and 10
years of post-training experience of interpreting body CT
images, respectively, who were unaware of patient clini-
cal information and CT imaging parameters prospec-
tively and independently reviewed CT images. Each ra-
diologist first graded images alone, and subsequently,
consensus grades were reached by discussion.
Copyright © 2013 SciRes. OJRad
M. KANEMATSU ET AL.
The radiologists independently graded portal venous-
phases CT images separately for diagnostic acceptability
using a five-point scale (grade 1 = unacceptable, grade 2
= suboptimal, grade 3 = acceptable, grade 4 = good,
grade 5 = excellent). A grade was awarded to each pa-
tient after reviewing all images. Grade 5 was given when
the image quality (soft tissue contrast, sharpness of tissue
interfaces, lesion conspicuity, and paucity of image deg-
radation caused by streaking noise or beam-hardening
artifacts) was deemed superb; grade 3 when image qual-
ity was fair and did not hamper image interpretation; and
grade 1 when image quality was considerably poor enough
to hampered image interpretation. Grades 4 and 2 were
defined as being intermediate between grades 5 and 3
and grades 1 and 3, respectively.
2.5. Statistical Analysis
Statistical analyses were performed using commercially
available software (SPSS, version 17; SPSS, Chicago,
Ill). For quantitative measurements, one-way analysis of
variances (ANOVA) was performed to compare back-
ground factors (patient age and body weight) and mean
ΔHUs of the abdominal aorta, portal trunk, and liver be-
tween the three protocols. When a statistically significant
intergroup difference was found by ANOVA, pair-wise
comparisons were performed using the Mann-Whitney
test with Bonferroni correction, and a stricter p value cri-
terion of <0.017 was considered significant. The Kruskal-
Wallis test was used to compare qualitative grades. When
a significant difference was found between the three
protocols, pairwise comparisons were performed using
the Mann-Whitney test with Bonferroni correction, and
again a stricter p value criterion <0.017 was considered
3.1. Patient Background Factors
The protocol A, B, and C groups consisted of 46, 43, and
43 patients, respectively. No significant difference was
found between any two groups in terms of age or body
weight (Tabl e 1). Medians times from initiations of con-
trast injections to initiations of diagnostic scans were 47,
56, and 67 seconds for the three protocols, respectively
3.2. Quantitative Image Analysis
Means values and the 1 SDs of the HUs of the abdomi-
nal aortas, portal veins, and livers for the three protocols
are summarized in Table 2, and medians and HU vari-
abilities are summarized in Figure 2. The mean HU of
the abdominal aorta was higher for protocol A than for
protocols B and C (P < 0.05), and the mean HU of the
Table 1. Patient age and body weight in the three protocols.
(n = 46)
(n = 43)
(n = 43)
Age (y) 61.3 ± 11.6 63.1 ± 10.7 61.9 ± 14.3
Body weight (kg) 60.1 ± 10.5 57.1 ± 9.8 55.7 ± 11.6
No significant difference in age (P = 0.77) and body weight (P = 0.15) was
found between any two protocols.
Table 2. Contrast enhancement of the abdominal aorta,
portal vein, and liver in the three protocols.
(n = 46)
(n = 43)
(n = 43)
Aorta 130.0 ± 17.6* 120.4 ± 18.6 116.6 ± 16.6
Portal vein 135.8 ± 14.7 144.7 ± 21.3** 128.7 ± 15.5
Liver 58.2 ± 9.2 56.2 ± 8.8 54.8 ± 9.3
Note: Numbers are mean ΔHU ± 1SD. *Value was significantly greater than
those in protocols B and C (P < 0.05). **Value was significantly greater than
those in protocols A and C (P < 0.05).
Figure 1. Box plot showing time in seconds from initiation
of intravenous contrast injection to initiation of portal ve-
nous diagnostic scan. Median values were 47, 56, and 67
seconds for protocols A, B, and C, re spe c tively. Boundary of
boxes closest to zero indicates 25th percentile, line within
boxes indicates median, and boundary of boxes farthest
from zero indicates 75th percentile. Error bars indicate
smallest and largest values within 1.5 box lengths of 25th
and 75th percentiles. Outliers are represented as individual
Figure 2. Box plot showing increased contrast enhancement
in HU from unenhanced to portal venous-phase images for
abdominal aorta, portal vein trunk, and liver. Note same
definitions of symbols as those in Figure 1.
Copyright © 2013 SciRes. OJRad
M. KANEMATSU ET AL. 115
portal trunk was higher for protocol B than for protocols
A and C (P < 0.05). On the other hand, the mean liver
HUs in the three groups were comparable.
3.3. Qualitative Image Analysis
The mean grades for diagnostic acceptability were 4.30 ±
0.70, 3.93 ± 0.73, and 4.19 ± 3.88 for protocols A, B, and
C, respectively (Figures 3 and 4). No difference was
found between the three protocols.
The median time from initiating contrast injection to ini-
tiating a diagnostic scan with SMV tracking (protocol A)
was 47 seconds, and this was rather shorter than 56 and
67 seconds required for aortic tracking (protocols A and
B, respectively). Furthermore, the variability of these
durations was greater with SMV tracking. The greater
variability observed when employing SMV tracking was
expected, because these durations were strongly affected
by circulation time variations of the portal venous system
Figure 3. Bar chart showing diagnostic acceptability grades.
No difference was found between three protocols.
Figure 4. Portal venous-phase CT image in 56-year-old
woman weighing 63 kilogram examined using SMV track-
ing (protocol A). Intensity of hepatic enhancement and im-
age quality are acceptable in this case.
and systemic circulation. Although we set the delay time
from bolus detection in the SMV to maximize hepatic
contrast enhancement based on the results of our pre-
liminary study, the actual time from initiation of contrast
injection to portal venous-phase diagnostic scan was
shortest for protocol A. However, had we even extended
the delay time with SMV tracking by for example nine
seconds, so that median of the duration was the same as
that of protocol B, liver enhancement might have been
similar to that observed in the present study.
Despite the difference of no less than 20 seconds in
median times from initiation of contrast injection to scan
start between SMV and aortic tracking methods, no in-
tergroup difference in hepatic enhancement was observed,
which suggested that durations of peak hepatic enhance-
ment in the portal venous phase was sustained fairly long.
Therefore, in contrast to scan timing optimization for the
hepatic arterial-dominant phase, the scan time window
for the portal venous phase was rather wide, and hence
strict scan timing optimization might not be necessary for
portal venous-phase imaging.
Livers were enhanced on average by 54 - 58 HU dur-
ing the portal venous phase for all three protocols and no
significant difference was observed. HU increases of
livers in the present study are consistent with those of a
previous study , in which it was shown that maxi-
mum hepatic enhancement in the portal venous phase
was linearly correlated with the iodine dosage (mg/kg of
body weight). In this previous study, an iodine dose of
520 mg/kg yielded a hepatic enhancement by 50 HU .
In the present study, the standard deviations of hepatic
enhancement for all three protocols lay in the range 8.8 -
9.3 HU, and no difference was found between the proto-
cols. Furthermore, qualitative image qualities (based on
diagnostic acceptability) were similar for the three pro-
tocols. These observations also concur well with those of
the previous study , in which maximum hepatic en-
hancement was found to be strongly correlated with io-
dine dosage (mg/kg) as long as portal venous-phase im-
ages were obtained during the fairly long, peak liver en-
When we undertook this study, we also sought to de-
termine whether portal venous tracking allows variabili-
ties in the timing of contrast material portal inflow to be
corrected and allows stably capture of maximum hepatic
enhancement using a rapid CT scanner that enables a
whole liver scan to be conducted within seconds. How-
ever, SMV tracking was not found to produce stronger
hepatic enhancement despite the somewhat more com-
plex procedure required as compared with aortic track-
In the present study, enhancement of the portal vein
was significantly greater for aortic tracking with a 40-
second scan delay (protocol B). One possible explanation
Copyright © 2013 SciRes. OJRad
M. KANEMATSU ET AL.
Copyright © 2013 SciRes. OJRad
for the significant difference in portal venous enhance-
ment observed despite no difference in hepatic enhance-
ment in the portal venous phase is that in the protocol B
the scan timing was most consistent with the timing of
the portal inflow of dense contrast material and the tim-
ing was somewhat premature for the hepatic parenchy-
mal imaging, although no differences in hepatic enhance-
ment were found.
This study has some limitations that warrant consid-
eration. First, this study was conducted on a limited co-
hort at a single institution. Second, the technical diffi-
culty of portal venous bolus tracking in the presence of
respiratory motion probably caused the wide variability
of scan timing observed when SMV tracking was used.
Third, we did not evaluate the diagnostic performance for
livers with diseases such as tumors, fatty liver, or cirrho-
sis. Furthermore, in patients with cirrhosis accompanied
by portal hypertension, time to the portal inflow of con-
trast material may well be variably delayed and portal
venous enhancement reduced, which may affect the fea-
sibility of portal venous bolus tracking.
Intense hepatic enhancement during the portal venous
phase was robustly achieved using aortic bolus tracking
in patients without chronic liver disease. Our results
showed that portal-inflow bolus tracking was ineffective
for increasing hepatic enhancement, reducing enhance-
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