DOI:10.2214/AJR.04.1254
AJR 2005; 185:1015-1023
© American Roentgen Ray Society
Hepatic Attenuation Differences Associated with Obstruction of the Portal or Hepatic Veins in Patients with Hepatic Abscess
Kyoung Ho Lee1,
Joon Koo Han2,
Jun Yong Jeong2,
Young Jun Kim2,
Hak Jong Lee1,
Seong Ho Park3 and
Byung Ihn Choi2
1 Department of Radiology, Seoul National University Bundang Hospital, Seoul
National University College of Medicine, Institute of Radiation Medicine,
Seoul National University Medical Research Center, 300 Gumi-dong, Bundang-gu,
Seongnam-si, Gyeonggi-do 463-707, Korea.
2 Department of Radiology and the Institute of Radiation Medicine, Seoul
National University College of Medicine, Clinical Research Institute, 28
Yongon-dong, Chongno-gu, Seoul, 110-744, Korea.
3 Department of Radiology, University of Ulsan College of Medicine 388-1,
Poongnap-dong, Songpa-ku, Seoul 138-736, Korea.
Received November 22, 2002;
accepted after revision November 10, 2004.
Address correspondence to J. K. Han
(hanjk{at}radcom.snu.ac.kr).
Abstract
OBJECTIVE. The purpose of our study was to determine the nature of
the association between the attenuation difference of the hepatic parenchyma
surrounding an abscess and obstruction of the regional portal vein or of the
hepatic vein.
MATERIALS AND METHODS. Helical CT scans of 60 patients with hepatic
abscess were analyzed for the presence of complete or partial obstruction of
the portal or hepatic veins and for attenuation differences in the surrounding
parenchyma. Clinical (age, sex, underlying disease, and microorganism) and CT
(obstruction of the portal or hepatic vein and number, location, and size of
abscesses) findings were analyzed statistically for possible associations with
each of regional parenchymal hyper- and hypoattenuation by using the
chi-square test and multivariate logistic regression analysis.
RESULTS. Regional parenchymal hyperattenuation was identified in 40
patients (67%). More patients with portal vein obstruction showed regional
parenchymal hyperattenuation than patients without portal vein obstruction
(22/27 patients vs 18/33, p = 0.028), and more patients with hepatic
vein obstruction showed regional parenchymal hypoattenuation than those
without hepatic vein obstruction (11/21 vs 3/39, p = 0.0003).
Multivariate logistic regression analysis showed that portal venous
obstruction was the only statistically significant predictor of regional
parenchymal hyperattenuation (p = 0.032; odds ratio, 3.7) and that
parenchymal hypoattenuation was associated with hepatic venous obstruction
(p = 0.001; odds ratio, 44.9).
CONCLUSION. Parenchymal hypo- and hyperattenuation are frequently
observed in the hepatic region surrounding an abscess on dynamic CT. Moreover,
these parenchymal attenuation differences are associated with regional portal
or hepatic vein obstruction.
Introduction
Since Freeny [1] reported
regional hyperenhancement and an arterioportal shunt of the hepatic parenchyma
surrounding an abscess on angiography, several investigators have described
similar findings on dynamic CT
[2,
3] and MRI
[4]. In our experience,
regional parenchymal attenuation differences are frequently observed on the
dynamic CT images of patients with a hepatic abscess, which may be
hyperattenuated or hypoattenuated. As postulated in previous studies, this
parenchymal attenuation difference may be associated with portal venous
compromise
[2-4]
or possibly with hepatic venous compromise because abnormal dynamic
enhancement patterns in the pathologically normal hepatic parenchyma around
various hepatic lesions can be explained by hemodynamic alterations—that
is, a flow compromise in the portal or hepatic veins
[5,
6]. Although it has been
postulated that local hepatic inflammation, such as that due to an abscess
[5,
6] or cholangitis
[7], may alter regional
hemodynamics in the liver, the actual mechanism of hemodynamic alteration and
its appearance in dynamic imaging have not been as intensively investigated as
in neoplastic diseases such as hepatocellular carcinoma.
Helical CT is currently used as one of the primary diagnostic tests in
patients with a suspected hepatic abscess. However, most CT studies of hepatic
abscesses were published before the introduction of helical CT. Therefore, we
believed that an investigation of the hemodynamic alterations associated with
hepatic abscesses and of their presentations on dynamic CT was warranted. This
study was undertaken to describe regional attenuation differences of the
hepatic parenchyma surrounding hepatic abscesses and to determine their
possible associations with obstruction of the regional portal or hepatic
veins.
Materials and Methods
Patients
Seventy-eight patients underwent percutaneous drainage of a hepatic abscess
at our institution between January 1997 and June 2002. Of these 78 patients,
18 were excluded for the following reasons: no contrast-enhanced helical CT
scan was obtained before the percutaneous drainage procedure (n =
10), the abscess was considered to be a complication of transcatheter arterial
chemoembolization or of local ablation therapy for a hepatic tumor (n
= 5; these five patients were excluded because the regional liver
hemodynamics may have been altered by treatment), or the diagnosis of hepatic
abscess was incorrect and the hepatic mass was in fact a necrotic malignant
tumor (n = 3). Therefore, this study included 60 patients with
hepatic abscess who underwent contrast-enhanced helical CT and then
percutaneous drainage. The demographic details of these patients are shown in
Table 1.
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TABLE 1: Bivariate Relationships Between Regional Parenchymal Attenuation
Differences (Hyper- or Hypoattenuation) and Selected Variables (n =
60)
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Pyogenic abscess was confirmed by positive cultures from percutaneous
aspirates in 51 of the 60 patients (Table
1). Forty-four of these 51 patients had monomicrobial abscesses
and seven had polymicrobial abscesses. In three patients, amebic abscess was
diagnosed by positive serologic testing; these patients responded to
percutaneous drainage and amebicidal therapy. In the remaining six patients,
whose cultures were negative for microbial growth, the diagnosis of hepatic
abscess was based on clinical and CT findings. All six of these patients
responded to percutaneous drainage and antimicrobial therapy.
Of the 60 patients analyzed, 12 were documented to have diabetes mellitus.
Other factors predisposing to hepatic abscess were documented in 28 patients,
which included four of the 12 diabetes mellitus patients; these included a
history of bilioenteric anastomosis for biliary or pancreatic malignancy
(n = 12), hepatobiliary lithiasis (n = 8), a history of
other malignant tumor (n = 6) including gastrointestinal tract cancer
(n = 4), and abdominal infectious disease (n = 2, acute
cholecystitis and colonic diverticulitis in each patient). Of the remaining 32
patients, eight had diabetes mellitus and 24 patients had no documented
predisposing factor.
CT Acquisition
The CT examinations were performed using different scanners (Somatom Plus S
and Somatom Plus 4, Siemens Medical Solutions; HiSpeed Advantage, GE
Healthcare; MX 8000, Marconi Medical Systems), and CT techniques also varied
because of the retrospective nature of the study. In general, examinations
were performed using a spiral technique with 5- to 10-mm collimation and 5- to
10-mm reconstruction intervals. The X-ray tube voltage was 120-140 kV, and the
current varied between 150 and 220 mA. During the study period, our standard
protocol for dynamic CT consisted of a total volume of 100-150 mL of nonionic
IV contrast material (300-370 mg I/mL) administered by power injection at a
rate of 3 mL/sec, with a scanning delay of 30-40 sec for the hepatic artery
phase (HAP) and 60-80 sec for the portal venous phase (PVP). Of the 60
patients analyzed, 52 underwent dual-phase (HAP and PVP) CT, and the remaining
eight underwent single-phase (PVP) CT.
CT Analysis
Four abdominal radiologists participated in the analysis. Two reviewers
analyzed the presence of portal or hepatic vein obstruction, and the other two
analyzed the presence of regional parenchymal hyper- or hypoattenuation. All
four were blinded to the study concept when they analyzed CT images, and the
two pairs of radiologists were blinded to each other's results. All CT images
were displayed using a PACS (Marotech). Window and level setting manipulation
was permitted during the analysis.
CT images were analyzed independently by two radiologists who determined
the number of abscesses and whether the regional portal or hepatic vein was
obstructed around the abscess. Clustered lesions were counted as a solitary
lesion. The regional portal vein or the hepatic vein was considered to be
obstructed in the presence of at least one of the following CT findings: (a)
thrombosis was identified as an intravascular tubular low-attenuation lesion
during the PVP, (b) the portal vein or the hepatic vein was replaced or
displaced by the abscess and not identified at all while vessels in the other
hepatic regions were depicted by contrast-enhancement, or (c) the portal vein
or hepatic vein in contact with the abscess was stenotic as compared with
vessels in other hepatic regions. Because two or more of findings (a), (b),
and (c) could be observed at different segments of the same vessel, these
findings were analyzed separately. Findings (a) or (b) were considered to
indicate a complete obstruction; and finding (c), without findings (a) or (b),
was considered to indicate a partial obstruction. A consensus evaluation by
the two reviewers was obtained for images that caused reviewer disagreements.
One of the two reviewers determined abscess size by measuring the largest
transverse diameter on CT and classified the location of the abscess as left
liver, right anterior section, or right posterior section. A lesion in the
caudate lobe was classified as one in the right posterior section.
In addition to and independently from the analyses just described, all CT
images were also analyzed for the presence of each of parenchymal hyper- and
hypoattenuation area in the hepatic region surrounding the abscess, as
compared with the attenuation of other hepatic regions. Because results were
analyzed statistically on a per-patient basis, for patients with multiple
abscesses, regional parenchymal hyper- and hypoattenuation were determined to
be present if observed in a hepatic region adjacent to the single largest
abscess. If both regional parenchymal hyper- and hypoattenuation were present
in the same patient, this patient was allocated to both the hyper- and
hypoattenuation categories. This analysis was performed independently by the
two other radiologists. If the two reviewers disagreed, they together
performed region-of-interest (circular, 1 cm in diameter) measurements on the
mean attenuation number (in Hounsfield units) in hepatic regions devoid of
large vessels. The threshold attenuation difference was 10 H
[8,
9] in these patients. One of
the two reviewers recorded the temporal phases (HAP, PVP, or both phases) of
the CT scan during which regional parenchymal hyper- or hypoattenuation
appeared.
Statistical Analysis
Clinical and CT findings (the 10 variables listed in
Table 1) were statistically
analyzed to identify possible associations with each type of regional
parenchymal attenuation difference (hyper- or hypoattenuation) observed on CT.
In each patient with multiple abscesses, only the largest abscess was included
in the statistical analysis because it was impossible to determine which of
the multiple abscesses in any given hepatic region was responsible for a
regional parenchymal attenuation difference. In terms of portal or hepatic
vein obstruction, reviewers' responses were collapsed into a binary answer
(i.e., obstructed or not obstructed) regardless of whether the obstruction was
complete or partial. A regional parenchymal attenuation difference was
determined to be present if this finding was observed during at least one of
the two temporal phases (HAP and PVP) of the CT scanning. Bivariate
associations between variables were analyzed using the chi-square test. To
identify independent variables associated with regional parenchymal
hyperattenuation (or hypoattenuation) after controlling for all other factors,
the 10 listed variables were subjected to stepwise multivariate logistic
regression analysis. The presence of regional parenchymal hyperattenuation (or
hypoattenuation) was treated as the outcome variable. Relationship strengths
were ranked using odds ratios.
Kappa statistics were used to evaluate interobserver agreement with respect
to binary responses concerning the presence of portal or hepatic vein
obstruction and of regional parenchymal hyper- or hypoattenuation. Strengths
of agreements were interpreted according to the guidelines suggested by Landis
and Koch [10] as follows:
almost perfect, 
= 0.81-1.00; substantial, = 0.61-0.80;
moderate, = 0.41-0.60; fair, = 0.21-0.40; and slight,
= < 0.20.
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Fig. 1A —Transverse dynamic CT images in 47-year-old man with hepatic
abscess. CT scan obtained during hepatic artery phase shows wedge-shaped
regional hyperattenuation (e) in hepatic parenchyma surrounding abscess
(a).
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Fig. 1B —Transverse dynamic CT images in 47-year-old man with hepatic
abscess. This regional parenchymal hyperattenuation returned to normal during
portal venous phase. Note that posterior branch of right portal vein is
thrombosed (arrow).
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Fig. 2A —Transverse dynamic CT images in 76-year-old man with hepatic
abscess. CT scan obtained during hepatic artery phase shows regional
hyperattenuation (e) in hepatic parenchyma surrounding abscess (a). Arrowhead
indicates hepatic artery at segment II.
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For all statistical tests, a p value of less than 0.05 was
considered to indicate a statistically significant difference. All statistical
analyses were performed by a qualified statistician using SPSS software,
version 10.0 (SPSS).
Results
CT Findings
CT revealed 115 intrahepatic abscesses in 60 patients. Forty-eight patients
had a solitary abscess and 12 had multiple abscesses (two in three patients;
three in three patients, four in two patients, and more than four in four
patients). The sizes and locations of the abscesses included in the
statistical analysis are summarized in
Table 1.
The regional portal vein was considered to be completely (n = 20)
or partially (n = 7) obstructed in 27 patients (45%). In these 27
patients, portal venous thrombosis was observed in 17, the regional portal
vein was not identified in five, and stenosis of the portal vein was observed
in 18. Stenotic portal veins were surrounded by a thin low-attenuation cuff,
although this finding was indistinguishable from dilated bile ducts in six
patients (Figs. 1A,
1B,
2A,
2B, and
2C). The regional hepatic vein
was considered to be completely (n = 17) or partially (n =
4) obstructed in 21 patients (35%). In these patients, hepatic venous
thrombosis was observed in 12 patients, and the regional hepatic vein was not
identified in eight. Stenosis of the hepatic vein was observed in five
patients; however, the perivascular low-attenuation cuff was not observed
around the stenotic hepatic vein (Figs.
3A,
3B, and
3C). In 11 patients, both
portal venous and hepatic venous obstructions were observed.
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Fig. 2B —Transverse dynamic CT images in 76-year-old man with hepatic
abscess. This regional parenchymal hyperattenuation returned to normal during
portal venous phase. Note stenosis (open arrows, B) and
thrombosis (curved arrow, C) of left portal vein in contact
with abscess (a in C). Note also periportal low-attenuation cuff, which
extends to other hepatic regions. Arrowhead in C indicates hepatic
artery at segment II, white arrows indicate wall or septation of abscess.
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Fig. 2C —Transverse dynamic CT images in 76-year-old man with hepatic
abscess. This regional parenchymal hyperattenuation returned to normal during
portal venous phase. Note stenosis (open arrows, B) and
thrombosis (curved arrow, C) of left portal vein in contact
with abscess (a in C). Note also periportal low-attenuation cuff, which
extends to other hepatic regions. Arrowhead in C indicates hepatic
artery at segment II, white arrows indicate wall or septation of abscess.
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Fig. 3A —Transverse dynamic CT images in 55-year-old man with hepatic
abscess. CT scans obtained during hepatic arterial (A) and portal
venous (B and C) phases, show thrombosis of right hepatic vein
(open arrows, B and C) adjacent to hepatic abscess (a
in A and B) and normal opacification of middle hepatic vein
(curved arrow, B). Note regional hypoattenuation (o in
B and C), and smaller hyperattenuated area (e in A) in
hepatic parenchyma surrounding abscess. Vertex of wedge-shaped hypoattenuating
area points to inferior vena cava (I in B and C). Note dilated
intrahepatic bile ducts in left liver (white arrows, B and
C).
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Fig. 3B —Transverse dynamic CT images in 55-year-old man with hepatic
abscess. CT scans obtained during hepatic arterial (A) and portal
venous (B and C) phases, show thrombosis of right hepatic vein
(open arrows, B and C) adjacent to hepatic abscess (a
in A and B) and normal opacification of middle hepatic vein
(curved arrow, B). Note regional hypoattenuation (o in
B and C), and smaller hyperattenuated area (e in A) in
hepatic parenchyma surrounding abscess. Vertex of wedge-shaped hypoattenuating
area points to inferior vena cava (I in B and C). Note dilated
intrahepatic bile ducts in left liver (white arrows, B and
C).
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Fig. 3C —Transverse dynamic CT images in 55-year-old man with hepatic
abscess. CT scans obtained during hepatic arterial (A) and portal
venous (B and C) phases, show thrombosis of right hepatic vein
(open arrows, B and C) adjacent to hepatic abscess (a
in A and B) and normal opacification of middle hepatic vein
(curved arrow, B). Note regional hypoattenuation (o in
B and C), and smaller hyperattenuated area (e in A) in
hepatic parenchyma surrounding abscess. Vertex of wedge-shaped hypoattenuating
area points to inferior vena cava (I in B and C). Note dilated
intrahepatic bile ducts in left liver (white arrows, B and
C).
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Regional parenchymal hyperattenuation was identified in 40 (67%) of the 60
patients analyzed. In 52 patients who underwent dual-phase CT, this finding
was observed in 36 (69%) patients during HAP (n = 14), PVP (n
= 5), or both phases (n = 17). The observed regional parenchymal
hyperattenuation always involved the subcapsular region of the liver and
usually appeared as a wedge shape with straight margins (Figs.
1A,
1B,
2A,
2B, and
2C). Regional parenchymal
hypoattenuation was identified in 14 (23%) of the 60 patients analyzed during
HAP (n = 2), PVP (n = 7), or both phases (n = 5).
Regional parenchymal hypoattenuation areas were all located peripheral to the
abscess and had ill-defined margins (Figs.
3A,
3B, and
3C). In 10 patients, both
hyperattenuation and hypoattenuation of the regional parenchyma were observed
in different areas adjacent to the abscess.
Associations Between Regional Parenchymal Attenuation Differences and Selected Variables
Table 1 presents the
percentages of patients who showed regional hyper- or hypoattenuation in the
surrounding parenchyma by categories of various factors, with the results of
chi-square testing. Bivariate analysis using the chi-square test revealed that
more patients with portal vein obstruction had regional parenchymal
hyperattenuation than those without portal vein obstruction (22/27 patients vs
18/33, p = 0.028), and that more patients with hepatic vein
obstruction had regional parenchymal hypoattenuation than those without
hepatic vein obstruction (11/21 vs 3/39, p = 0.0003). However, no
other significant association was found between any combination of the 12
variables, including regional parenchymal hyper- or hypoattenuation, listed in
Table 1.
Stepwise multivariate logistic regression analysis revealed that of the 10
variables examined, the presence of portal venous obstruction was the only
significant predictor of regional parenchymal hyperattenuation (p =
0.032; odds ratio, 3.7; 95% confidence interval [CI], 1.1, 12.0). The presence
of hepatic venous obstruction was the most significant predictor of regional
parenchymal hypoattenuation (p = 0.001; odds ratio, 44.9; 95% CI,
4.6, 435.6). The likelihood of regional parenchymal hypoattenuation also
increased when the abscess was located in the right anterior section rather
than in the left liver (p = 0.030; odds ratio, 14.5; 95% CI, 1.3,
162.9). None of the other eight variables was found to be predictive of
regional parenchymal hypoattenuation.
Reviewer Disagreements
Reviewer disagreement concerning the number of abscesses (two adjacent
lesions vs a clustered solitary lesion) occurred in three instances. After a
consensus evaluation, these lesions were considered to be single abscesses in
each patient. Table 2 summarizes the kappa statistics for interobserver agreement with respect to
binary responses for the presence of obstruction of the portal or hepatic vein
and of regional parenchymal differences, and the results of consensus
evaluation. Interobserver agreement was almost perfect or substantial for all
CT findings analyzed except for two variables (nonidentification of the portal
vein and stenosis of the hepatic vein), for which interobserver agreement was
moderate.
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TABLE 2: Kappa Statistics for Interobserver Agreement and the Results of
Consensus Evaluations of CT Findings (n = 60)
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Discussion
Our results suggest that complete or partial obstruction of the regional
portal vein (45%) or of the hepatic vein (35%) is frequently observed around a
hepatic abscess, that regional parenchymal hypoattenuation (23%) or
hyperattenuation (67%) is also frequently observed around hepatic abscesses,
and that regional parenchymal hyper- or hypoattenuation is associated with
obstruction of the regional portal vein or the hepatic vein, respectively.
One clinical implication of our observations is that the CT findings of
intrahepatic vascular obstruction and associated hemodynamic alteration around
a hepatic focal lesion do not necessarily indicate malignancy. Because such
findings typically have been described to be helpful in the diagnosis of a
malignant tumor, especially hepatocellular carcinoma
[5,
11], they may lead to a
misdiagnosis [12] if other CT
findings of a hepatic abscess are atypical.
Our results concerning the association between portal venous obstruction
and regional parenchymal hyperattenuation validate the hypothesis proposed by
several researchers
[1-4]
who attributed parenchymal hyperenhancement around a hepatic abscess to an
arterioportal shunt resulting from portal venous compromise.
In cases of diminished hepatic venous flow, the hemodynamics is complex and
can differ according to the site and the chronicity of the occlusion
[5]. When the hepatic vein is
acutely obstructed, which is likely in the case of a hepatic abscess, it is
generally believed that the portal vein becomes a draining vein, thus
resulting in a functional arterioportal shunt, which appears as parenchymal
hyperattenuation in the congested hepatic region during the HAP
[13,
14]. Although statistically
not significant, regional parenchymal hyperattenuation was observed in 16 of
21 patients with hepatic venous obstruction. Three of these patients had CT
findings that were relevant to this hypothesis. The findings of one of these
patients are illustrated in Figures
4A,
4B, and
4C. In the other two patients,
the vertex of the wedge-shaped hyperattenuating area pointed to the thrombosed
hepatic vein [5,
15]. However, in the remaining
13 patients, we could not recognize any distinguishing feature of regional
parenchymal hyperattenuation that might be specific to this patient
subgroup.
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Fig. 4A —Transverse dynamic CT images in 79-year-old man with hepatic
abscess. CT scan obtained during hepatic artery phase shows wedge-shaped
regional parenchymal hyperattenuation (e) in hepatic parenchyma surrounding an
abscess (a) that ruptured into peritoneal cavity. Left portal vein (l in
A) is highly opacified compared with right portal vein (r in B),
indicating presence of arterioportal shunt. Note periportal low-attenuation
cuff surrounding left portal vein and air within bile ducts (b).
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Fig. 4B —Transverse dynamic CT images in 79-year-old man with hepatic
abscess. CT scan obtained during hepatic artery phase shows wedge-shaped
regional parenchymal hyperattenuation (e) in hepatic parenchyma surrounding an
abscess (a) that ruptured into peritoneal cavity. Left portal vein (l in
A) is highly opacified compared with right portal vein (r in B),
indicating presence of arterioportal shunt. Note periportal low-attenuation
cuff surrounding left portal vein and air within bile ducts (b).
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Fig. 4C —Transverse dynamic CT images in 79-year-old man with hepatic
abscess. During portal venous phase, left hepatic vein was not identified
except for small central portion (solid arrow), whereas middle and
right hepatic veins were normally opacified (open arrows). a =
abscess, e = hyperattenuation.
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Our statistical analysis showed that hepatic venous obstruction is
associated with regional parenchymal hypoattenuation, but not with
hyperattenuation. This is not a surprising result when one considers the
complexities of hemodynamics in cases of hepatic venous compromise. An
experimental study [16]
recently revealed that acute occlusion of the hepatic vein can induce
hypoattenuation in the affected parenchyma, probably because an arterioportal
shunt occurs at the presinusoidal level (through the transplexal route), and
the reversed opacified portal flow escapes from (or does not enter) the
congested segment and is then dispersed into adjacent segments. The
parenchymal hyperenhancement that results from an arterioportal shunt, which
is an idea derived mainly from studies
[13,
14] using angiography or CT
during arteriography, might not be observed in dynamic CT after IV contrast
enhancement because reduced portal venous enhancement outweighs increased
arterial enhancement in a congested hepatic region. Although this complex
facet of hemodynamics that results in regional parenchymal hypoattenuation has
not been described in human patients with hepatic venous obstruction, regional
parenchymal hypoattenuation has been recently described in acutely contested
hepatic segments after living donor liver transplantation
[17]. This may be observed
only when the hepatic vein is acutely obstructed but collateral drainage has
not had time to develop, which is likely if the obstruction is caused by acute
inflammation such as is caused by a hepatic abscess.
Our multivariate statistical analysis showed an unexpected association
between abscess location and regional parenchymal hypoattenuation; however,
the implications of this association are not clear.
In our patients, it was virtually impossible to determine whether a given
hepatic abscess was the result of pylephlebitis or vice versa. However, we
believe that portal or hepatic vein obstruction occurred as a result of
hepatic abscess development in most patients because none of our patients with
these venous obstructions had a documented abdominal infectious disease likely
to have caused pylephlebitis
[18]. Regional venous
thrombosis is a well-known complication of various local infectious diseases
throughout the body
[19-22].
Furthermore, it has been reported that thrombosis of the portal
[4,
23,
24] and hepatic
[23,
25] veins might be the result
of an intrahepatic inflammatory lesion. Hanazaki et al.
[24] proposed that a hepatic
abscess may lead to infectious damage of the portal vein, which might result
in thrombosis. This hypothesis was reinforced by Gabata et al.
[3], who reported marked
periportal inflammation and stenosis of the portal venules surrounding hepatic
abscesses. In our study, diffuse stenosis of the regional portal vein, with a
periportal low-attenuation cuff, was frequently (11/18 patients) found to be
accompanied by thrombosis at distal branches. We believe that these CT
findings may represent different stages or degrees of infectious
phlebitis.
In our study, obstruction of the portal or hepatic veins did not always
induce a parenchymal attenuation difference, and a parenchymal attenuation
difference was not always accompanied by portal or hepatic vein obstruction.
This can be explained by several factors, as follows: First, hemodynamic
alterations in a hepatic region can certainly occur without a large vascular
obstruction identifiable on CT. Smaller peripheral vessels might be obstructed
that are not visualized on CT. Second, in addition to vascular obstruction,
the compressive effect of an abscess on the regional parenchyma might have a
role in the determination of parenchymal attenuation differences
[26]. The degree of this
compressive effect may be correlated with abscess size; however, no
significant association was observed in our study between abscess size and
parenchymal attenuation differences. Finally, the parenchymal attenuation
differences observed in our study may not necessarily indicate hemodynamic
alterations in the normal hepatic parenchyma. Without histopathologic data, it
is unclear whether parenchymal attenuation differences reflect histopathologic
changes, such as edema or inflammation, in some patients.
The major limitation of our study is that the variables analyzed did not
include clinical and laboratory findings describing abscess inflammatory
activity, and therefore our statistical analysis might represent a compilation
of snapshots of abscesses at different stages of evolution and resolution.
Regional parenchymal hyperattenuation has been reported to disappear after
antibiotic therapy for hepatic abscess
[3]. It is unclear whether the
disappearance of a parenchymal attenuation difference is accompanied by a
reinstatement of blood flow in an obstructed vessel. We did not stratify
inflammatory activity because all of our patients required a percutaneous
drainage procedure, which entails high levels of inflammatory activity in most
patients. The second limitation of our study concerns the variability of the
CT protocols and iodine concentrations in contrast material. Because many
patients were scanned before the introduction of the bolus tracking technique,
the temporal phase of CT scans varied. These factors could not be controlled
for because of the retrospective nature of our study. Moreover, this
limitation precluded a consistent quantitative analysis of regional
attenuation values. Third, we pooled the results of parenchymal attenuation
differences between the HAP and the PVP for the statistical analysis for
simplification reasons and to compensate for inconsistencies in the temporal
phases of CT scanning. This limitation precluded a more systematic analysis by
substratifying data according to the temporal phases of the CT scanning. For
example, there may have been more uncounted regional parenchymal
hyperattenuation in the eight patients who underwent single-phase CT.
Therefore, a more elaborate study, such as a prospective study using a
consistent CT protocol, is needed to confirm our results. The fourth
limitation is that we did not consider other factors affecting hepatic
attenuation on dynamic CT. These factors include the presence of liver
cirrhosis and the patient's hemodynamic status, the latter of which is likely
to be unstable in such acutely ill patients.
In conclusion, obstruction of the portal or hepatic vein and parenchymal
hypo- and hyperattenuation are frequently observed in the hepatic region
surrounding an abscess on dynamic CT. Parenchymal hyper- or hypoattenuation of
the hepatic region surrounding an abscess was found to be associated with
obstruction of the regional portal vein or the hepatic vein, respectively,
although parenchymal hyperattenuation can be observed without visible portal
venous thrombosis.
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Abstract
Introduction
