DOI:10.2214/AJR.07.2032
AJR 2007; 189:1435-1442
© American Roentgen Ray Society
Comparison of Contrast-Enhanced Sonography and MRI in Displaying Anatomic Features of Cystic Pancreatic Masses
Mirko D'Onofrio1,
Alec J. Megibow2,
Niccolò Faccioli1,
Roberto Malagò1,
Paola Capelli3,
Massimo Falconi4 and
Roberto Pozzi Mucelli1
1 Department of Radiology, University Hospital G. B. Rossi, Piazzale L. A. Scuro
10, University of Verona, Verona 37134, Italy.
2 Department of Radiology, New York University Medical Center, New York,
NY.
3 Department of Pathology, University Hospital G. B. Rossi, University of
Verona, Verona, Italy.
4 Department of Surgery, University Hospital G. B. Rossi, University of Verona,
Verona, Italy.
Received February 8, 2007;
accepted after revision June 24, 2007.
Address correspondence to M. D'Onofrio
(mirko.donofrio{at}univr.it).
Abstract
OBJECTIVE. The purpose of this study was to compare the accuracy
rates of unenhanced sonography, contrast-enhanced sonography, and MRI in
displaying the anatomic features of cystic pancreatic masses larger than 1.5
mm in diameter.
MATERIALS AND METHODS. Unenhanced and contrast-enhanced sonographic
and MRI examinations of 33 patients who underwent resection of a cystic
pancreatic mass were retrospectively reviewed. Two radiologists blinded to the
final histologic diagnosis reviewed the images, specifically assessing the
presence of intralesional mural nodules and septa. Sensitivity, specificity,
positive and negative predictive values, and accuracy were calculated on the
basis of correlation with surgical findings. Results of unenhanced sonography,
contrast-enhanced sonography, and MRI were compared by McNemar test.
Correlation of unenhanced and contrast-enhanced sonographic versus pathologic
results was established with Spearman's test. Interobserver variability was
determined.
RESULTS. Contrast-enhanced sonography correctly depicted
intralesional septa in 14 of 15 lesions (sensitivity, 93.3%; specificity,
88.8%; positive predictive value, 87.5%; negative predictive value, 94.1%;
accuracy, 90.9%) and nodules in six of eight lesions (sensitivity, 75%;
specificity, 96%; positive predictive value, 85.7%; negative predictive value,
92.3%; accuracy, 90.9%). MRI correctly depicted intralesional septa in 14 of
15 lesions (sensitivity, 93.3%; specificity, 61.1%; positive predictive value,
66.6%; negative predictive value, 91.6%; accuracy, 75.7%) and nodules in seven
of eight lesions (sensitivity, 87.5%; specificity, 80%; positive predictive
value, 58.3%; negative predictive value, 95.2%; accuracy, 81.8%). The
difference between the diagnostic accuracy of contrast-enhanced sonography and
that of MRI was not significant (p = 0.05, McNemar test) in the
identification of septa and nodules. The correlation between contrast-enhanced
sonographic findings and pathologic results (Rs = 0.93; p
< 0.001) was significantly better than that between sonographic and
pathologic results (Rs = 0.52; p < 0.0001). Interobserver
agreement had a kappa value of 0.86–0.94.
CONCLUSION. Contrast-enhanced sonography compares favorably with MRI
in displaying the anatomic features of cystic pancreatic masses seen on
transabdominal sonography.
Keywords: contrast-enhanced sonography • intraductal papillary mucinous neoplasm • MRI • pancreas • pancreatic cystic tumor • pancreatic pseudocyst • sonography
Introduction
Cystic pancreatic lesions are increasingly being found on imaging studies.
In a retrospective review [1],
cystic pancreatic lesions were found in 20% of patients undergoing abdominal
MRI examinations. Most cystic pancreatic lesions detected in clinical practice
are pseudocysts. Cystic pancreatic tumors represent 10–15% of cystic
lesions of the pancreas
[2–4]
and have variable histologic characteristics, malignant potential, and
biologic behavior. It is not surprising that there is wide variance in
agreement on clinical management
[5,
6]. It is critical that initial
imaging studies lead to definitive and reliable differentiation of benign from
malignant lesions [7].
Transabdominal contrast-enhanced sonography is performed with a purely
blood-pool contrast agent (microbubbles) that allows visualization of flow in
small vessels during a real-time examination
[8,
9]. The U.S. Food and Drug
Administration has not yet approved the technique for noncardiac use. Use of a
sonographic contrast medium consisting of sulfur hexafluoride–filled
microbubbles with a phospholipid peripheral shell, a mean diameter of 2.5
µm, and harmonic responses at low acoustic pressure (mechanical index, <
0.2) (SonoVue, Bracco) has been widely reported in hepatic imaging
[10–12].
Reports of use of this agent in pancreatic imaging are increasing
[8,
13–18].
However, the number of reports of studies dedicated to contrast-enhanced
sonography of pancreatic cystic lesions is small
[19,
20]. The purpose of our study
was to compare, using surgical pathologic findings as the reference standard,
the accuracy of contrast-enhanced sonography with that of MRI in the diagnosis
of cystic pancreatic lesions larger than 1.5 mm in diameter.
Materials and Methods
Patients
From the surgical pathology database at our hospital, we obtained the names
of all patients undergoing resection of a single cystic pancreatic mass from
January 2004 to January 2006. In 95 patients, a cystic mass had been detected
on transabdominal sonography and was studied with both contrast-enhanced
sonography and MRI before resection. Our institutional protocol calls for both
of these examinations when a cystic pancreatic mass is detected. Institutional
review board approval was obtained for this retrospective analysis. If the
mass is cystic, MRI often is recommended. Comparison of contrast-enhanced
sonography and MRI in the display of the anatomic features of cystic
pancreatic masses in the resected lesions therefore was possible.
The final number of patients included was 33 (15 men, 18 women; mean age,
50.9 years; range, 20–76 years)
(Table 1). The patients were
deidentified after being selected for the study. The 33 patients had 13
mucinous cystadenomas, four endocrine cystic tumors, four intraductal
papillary mucinous neoplasms, three mucinous cystadenocarcinomas, three
pseudopapillary tumors, three serous cystadenomas, and three pseudocysts.
Transabdominal sonography had been performed because of abdominal pain in 21
patients (nine with mucinous cystadenomas, two with endocrine tumors, two with
intraductal papillary mucinous neoplasms, two with mucinous
cystadenocarcinomas, two with pseudopapillary tumors, one with serous
cystadenoma, and three with pseudocysts). In the other 12 patients (four with
mucinous cystadenomas, two with endocrine tumors, two with intraductal
papillary mucinous neoplasms, one with mucinous cystadenocarcinoma, one with
pseudopapillary tumor, and two with serous cystadenomas), the cystic masses
were incidental findings on transabdominal sonography performed for other
indications (seven cases of follow-up of chronic hepatitis, three of
hypertension, one of Crohn's disease, and one of renal lithiasis). All
patients underwent both contrast-enhanced sonography and MRI. All unenhanced
sonographic, contrast-enhanced sonographic, and MRI examinations were
performed within 1 week before surgery.
Contrast-Enhanced Sonographic Technique
All patients were asked to ingest nothing by mouth in the 6 hours before
the sonographic examination. All contrast-enhanced sonographic examinations
were performed by radiologists with more than 5 years of experience and with
expertise in pancreatic imaging. A sonographic system (Sequoia 512 Acuson,
Siemens Medical Solutions) with contrast-specific sonographic imaging modes
and low acoustic pressure (2–4 MHz coherent contrast imaging or cadence
contrast pulse sequencing; mechanical index, 0.2; 12–13 frames/s) was
used. A 2.4-mL bolus of a second-generation contrast medium (SonoVue, Bracco)
was injected IV and immediately followed by a 5-mL bolus of saline solution.
Insonation of the pancreatic lesion was continuous with dynamic observation of
the passage from the unenhanced phase to the contrast-enhanced phases. The
arterial phase (
15–20 seconds after injection) was defined as
maximal hyperechogenicity within the aorta or other large peripancreatic
splanchnic arteries. The venous phase ( 30–45 seconds after
injection) was defined as the time at which the splenomesentericoportal tree
became hyperechoic. The maximum examination time was 5 minutes.
MRI Technique
MRI examinations were performed on a 1.5-T system (Magnetom Symphony,
Siemens Medical Solutions) with a phased-array surface coil to obtain the best
signal-to-noise ratio at a given slice thickness ( 4 mm with a 0.1-mm
gap). All patients were asked to fast for a minimum of 4 hours. Twenty minutes
before the examination, the patients were given 150 mL of an oral
superparamagnetic contrast medium (ferumoxsil, Lumirem, Guerbet) to suppress
the high signal intensity from fluid in the stomach and duodenum on
T2-weighted sequences. T1- and T2-weighted and MR cholangiopancreatographic
sequences were used to evaluate the lesion. T1-weighted gradient-recalled echo
(TR/TE, 107/4.8) and turbo spin-echo (4,950/102) breath-hold sequences were
performed with and without fat suppression. MR cholangiopancreatography was
performed with a HASTE multislice sequence and a T2-weighted RARE thick-slab
sequence with a 40-mm thickness. The HASTE sequence (infinity/60; field of
view, 350 x 300 mm; matrix size, 240 x 256) was performed with
thin slices (4 mm with no gap) in the axial, coronal, paracoronal oblique, and
sagittal planes. To optimize visualization of the pancreatic ductal tree, RARE
thick-slab images (TE, 1,100 milliseconds) were obtained along an off-axis
oblique orientation paralleling the main pancreatic duct and in the axial
plane. Contrast-enhanced MRI was performed after administration of a 15-mL IV
bolus of gadoterate dimeglumine (Dotarem, Guerbet). A fat-suppressed 3D
volume-interpolated breath-hold sequence (4.5/1.7; flip angle, 10°) with
less than 2-mm voxel size was used. Unenhanced images were obtained in the
pancreatic (35 seconds after contrast injection) and venous (70 seconds after
contrast injection) phases. The maximum examination time was 35 minutes.
Pathologic Analysis
The resected specimens were examined by one pathologist with more than 20
years of experience in pancreatic pathology. After surgical resection, all
lesions were cut along the largest diameter. After removal of the cyst fluid,
the intracystic spaces were inspected for the presence of septa and nodules.
The cystic pancreatic lesions were classified in four categories depending on
the absence or presence of intralesional septa and nodules: 0, no
intralesional septa or nodules; 1, intralesional septa; 2, intralesional
nodules; 3, intralesional septa and nodules.
Image Analysis
Two radiologists blinded to the final histologic diagnosis retrospectively
reviewed sonograms, contrast-enhanced sonograms, and MR images specifically
assessing the presence of intralesional mural nodules and septa. In case of
disagreement, the final decision resulted from consensus. The cystic
pancreatic lesions were classified depending on the absence or presence of
intralesional septa and nodules: 0, no intralesional septa or nodules; 1,
intralesional septa; 2, intralesional nodules; 3, intralesional septa and
nodules.
Data Analysis
Sensitivity, specificity, positive predictive value (PPV), negative
predictive value (NPV), and diagnostic accuracy were calculated with respect
to the pathologic findings. A p value of 0.05 was considered the
limit for a statistically significant difference. Contrast-enhanced sonography
and MRI results were compared by McNemar test. The correlation of unenhanced
and contrast-enhanced sonographic findings with pathologic findings was
assessed as a correlation coefficient derived with Spearman's test
(Rs). Interobserver variability with kappa agreement was determined
for the contrast-enhanced examinations.
Results
The body mass index of the patients in this study was sufficiently low
(range, 18–23) that none of the studies was excluded because of poor
pancreatic visualization. At pathologic examination, the final diagnosis of
the 33 resected cystic pancreatic lesions was benign in six cases and
premalignant or malignant in 27 cases
(Table 2). The cystic
pancreatic lesions found benign at pathologic examination included three
serous cystadenomas (mean diameter, 47.3 ± 29 [SD] mm; range,
17–75 mm) and three pseudocysts (mean diameter, 38.3 ± 7.6 mm;
range, 30–45 mm). None of the pseudocysts in this series was entirely
extrapancreatic. The cystic pancreatic lesions found premalignant or malignant
at pathologic examination included 13 mucinous cystadenomas (mean diameter,
64.8 ± 29.8 mm; range, 30–130 mm), four cystic endocrine tumors
(mean diameter, 42.5 ± 38.8 mm; range, 15–100 mm), four
intraductal papillary mucinous neoplasms (mean diameter, 40 ± 16.3 mm;
range, 20–60 mm), three mucinous cystadenocarcinomas (mean diameter, 50
± 5 mm; range, 45–55 mm), three pseudopapillary tumors (mean
diameter, 31.6 ± 24.6 mm; range, 15–60 mm). Thirteen lesions were
located in the pancreatic head, 12 in the pancreatic body, and eight in the
pancreatic tail (Table 2).
At gross pathologic examination, a thick wall was present in 26 lesions (13
mucinous cystadenomas, four cystic endocrine tumors, three mucinous
cystadenocarcinomas, three pseudopapillary tumors, two intraductal papillary
mucinous neoplasms, and one pseudocyst) and a thin wall in seven lesions
(three serous cystadenomas, two intraductal papillary mucinous neoplasms, and
two pseudocysts). A thick wall and intralesional septa were found in 13
lesions (10 mucinous cystadenomas, two cystic endocrine tumors, and one
mucinous cystadenocarcinoma). A thick wall and intralesional nodules were
found in eight lesions (three mucinous cystadenomas, two intraductal papillary
mucinous neoplasms, two mucinous cystadenocarcinomas, and one pseudopapillary
tumor). A thin wall and intralesional septa were found in two lesions (two
serous cystadenomas). No lesions were found to have a thin wall and parietal
nodules at pathologic examination.
Unenhanced sonography correctly depicted intralesional septa in 13 of 15
lesions (13 true-positive results, 12 true-negative results, six
false-positive results, and two false-negative results) and nodules in five of
eight lesions (Tables 3 and
4). Contrast-enhanced
sonography correctly depicted intralesional septa (Figs.
1A,
1B,
1C,
1D,
2A,
2B, and
2C) in 14 of 15 lesions (14
true-positive results, 16 true-negative results, two false-positive results,
and one false-negative result) and nodules (Figs.
3A,
3B,
3C, and
3D) in six of eight lesions
(six true-positive results, 24 true-negative results, one false-positive
result, and two false-negative results). MRI correctly depicted intralesional
septa (Figs. 1A,
1B,
1C,
1D,
2A,
2B, and
2C) in 14 of 15 lesions (14
true-positive results, 11 true-negative results, seven false-positive results,
and one false-negative result) and nodules (Figs.
3A,
3B,
3C, and
3D) in seven of eight lesions
(seven true-positive results, 20 true-negative results, five false-positive
results, and one false-negative result).
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TABLE 3: Final Diagnosis and Number of True-Positive and True-Negative Results of
Resected Cystic Pancreatic Lesions (n = 33)
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TABLE 4: Imaging Results in Identification of Intralesional Septa and Nodules in
Resected Cystic Pancreatic Lesions (n = 33)
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Fig. 2A —47-year-old woman with mucinous cystadenoma. Transabdominal
sonogram reveals 5-cm cystic mass in pancreatic body. Visible inside lesion
are thin septa (arrow) along anterior wall and small nodule
(arrowhead) along posterior wall.
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Fig. 2B —47-year-old woman with mucinous cystadenoma.
Contrast-enhanced sonogram reveals vascularization of entire thick-walled
mural structure with vascularized thin intralesional septa (arrow).
Nodule is not evident.
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Fig. 3A —68-year-old man with branch duct intraductal papillary
mucinous neoplasm. Transabdominal sonogram reveals 2-cm cystic lesion in
pancreatic tail. Some internal structure is visible but poorly defined
(arrow).
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Fig. 3C —68-year-old man with branch duct intraductal papillary
mucinous neoplasm. T2-weighted turbo spin-echo MR image (TR/TE, 4,950/102)
confirms presence of mass. Low signal intensity within mass results from
nodule (arrow) as predicted with contrast-enhanced sonography.
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Fig. 3D —68-year-old man with branch duct intraductal papillary
mucinous neoplasm. Photograph of operative specimen shows whitish nodule
(arrow) projecting into mucinous cystic lesion. Final pathologic
diagnosis was branch duct intraductal papillary mucinous neoplasm with
invasive carcinoma.
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The difference in diagnostic accuracy between unenhanced sonography,
contrast-enhanced sonography, and MRI was not significant (p = 0.05,
McNemar test) in identification of intralesional septa and nodules. In
comparison with the gross pathologic findings, however, contrast-enhanced
sonography (Rs = 0.93; p < 0.001) was superior to
unenhanced sonography (Rs = 0.52; p < 0.0001).
Contrast-enhanced sonography had fewer false-positive (Figs.
2A,
2B,
2C,
4A, and
4B) and false-negative (Figs.
5A,
5B,
5C, and
5D) results. The errors on
contrast-enhanced sonography and MRI did not occur in the same lesions. On
contrast-enhanced sonography, a septum was not seen in a small lesion
(endocrine tumor) measuring 15 mm, and nodules were not seen in two lesions
(mucinous cystic tumor, adenocarcinoma; intraductal papillary mucinous
neoplasm) measuring 45 and 60 mm (Table
3). On MRI a septum was not seen in a small lesion (mucinous
cystic tumor, adenoma) measuring 33 mm, and a nodule was not seen in one
lesion (mucinous cystic tumor, adenoma) measuring 40 mm
(Table 3). Although small septa
were not seen in small lesions, the undetected nodules were present in larger
lesions. Interobserver agreement was good for both readers (overall, 
=
0.86–0.94). In particular, there was good agreement in the detection of
septa (contrast-enhanced sonography, = 0.94; MRI, = 0.86) and
nodules (contrast-enhanced sonography, = 0.91; MRI, = 0.93)
for both imaging methods.
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Fig. 4B —47-year-old woman with cystic endocrine tumor.
Contrast-enhanced sonogram obtained during dynamic examination shows no
enhancement of intralesional structures. Absence of intralesional septa and
nodules was confirmed at gross pathologic examination. Final pathologic
diagnosis was cystic endocrine tumor.
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Fig. 5B —55-year-old woman with mucinous cystadenoma.
Contrast-enhanced (B) and unenhanced (C) sonograms from same
frame of dynamic examination. Contrast-enhanced image (B) shows very
small septa (arrow), not visible in unenhanced image (C).
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Fig. 5C —55-year-old woman with mucinous cystadenoma.
Contrast-enhanced (B) and unenhanced (C) sonograms from same
frame of dynamic examination. Contrast-enhanced image (B) shows very
small septa (arrow), not visible in unenhanced image (C).
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Discussion
Contrast-enhanced sonography is performed with a blood-pool contrast agent
(microbubbles) that when injected IV allows real-time visualization of the
perfusion of an organ or a lesion. Contrast-enhanced sonography has been found
accurate in evaluation of the vascularization of solid pancreatic lesions
[8,
14], although the number of
reports of studies of contrast-enhanced sonography of pancreatic cystic
lesions is small [19,
20]. Oshikawa et al.
[15], who used an air-filled
microbubble contrast agent, concluded that dynamic sonography can assist in
the characterization of pancreatic tumors. Visualization of tumor vascularity
is the direct result of the use of a blood-pool contrast agent, dynamic
observation of the contrast-enhanced phases, and the high contrast enhancement
and spatial resolution of current sonographic harmonic imaging
[16]. During contrast-enhanced
sonography, microbubble-specific software on the sonographic console deletes
all background signal intensity so that the operator sees only the signal
intensity produced by the contrast agent passing under the sonographic probe
while the nonvascularized (unenhanced) tissue remains invisible
[13]. This property can be
readily exploited in evaluation of the wall and architecture of cystic
pancreatic lesions.
The viable vascularized portions of cystic pancreatic tumors become
progressively echogenic during contrast-enhanced sonography as the contrast
material passes into the capillary beds of the septa (Figs.
1A,
1B,
1C,
1D,
2A,
2B, and
2C) or nodules (Figs.
3A,
3B,
3C, and
3D) inside the cysts.
Conversely, intralesional blood clots and debris, easily detectable on
baseline sonograms, are completely invisible (Figs.
2A,
2B, and
2C) during contrast-enhanced
sonography [13,
19,
20]. For this reason,
contrast-enhanced sonography is reported to improve the characterization of
pseudocysts [13,
19,
20]. Moreover, owing to the
cancellation of the background tissue and of echogenic intracystic content
(i.e., mucinous content), the detection rate of septa and nodules on
contrast-enhanced sonography is improved
[21] compared with that on
transabdominal sonography. During unenhanced sonography, the viscosity of
mucin within a lesion results in increased echogenicity, which can obscure the
internal wall, leading to misdiagnosis
[13,
19–22].
Septa and nodules may be seen only on T2-weighted MR images, possibly
explaining the higher number of false-positive results on MRI than on
contrast-enhanced sonography in our study. The errors with contrast-enhanced
sonography and MRI did not occur in the same lesions. Small septa were not
seen in small lesions, and undetected nodules were present in larger
lesions.
A wide variety of cystic pancreatic masses encompass lesions with different
natures and biologic behaviors but often with similar morphologic features
[3,
23,
24]. Our understanding of
cystic pancreatic lesions has dramatically increased over the past 10 years
[7,
23]. Many authors have
reported that although the clinical, radiologic, and pathologic features of
cystic pancreatic lesions are now well known, accurate preoperative diagnosis
remains difficult [23,
24]. More frequent use of
radiologic examinations and advances in imaging techniques have led to
identification of a large number of cystic pancreatic lesions
[1,
7,
25]. Most of these lesions are
incidental findings and, especially when smaller than 2 cm in diameter, are
rarely malignant. Immediate surgical treatment therefore may not be necessary
unless the lesion is suspected of being a mucinous cystic tumor
[24].
There have been a few studies
[19,
20] of the use of
contrast-enhanced sonography in the evaluation of cystic pancreatic lesions;
the findings, however, have not been correlated with the anatomic appearance
of the specimens. Rickes and Wermke
[19] found 95% sensitivity and
92% specificity in the diagnosis of cystadenoma and 100% sensitivity and
specificity in the diagnosis of pseudocyst in a cohort of 31 patients with
cystic pancreatic masses. Itoh et al.
[22] found contrast-enhanced
sonography useful in the detection of vascularization of the internal features
of intraductal papillary mucinous neoplasm in 21 patients, and there was
excellent correlation with pathologic findings.
Our findings are in agreement with previous results in that
contrast-enhanced sonography proved accurate in the detection of septa and
nodules in cystic pancreatic tumors. Moreover, the presence of septa and
nodules was correctly excluded in all the pseudocysts surgically managed owing
to involvement of the main pancreatic duct. Differences in diagnostic accuracy
between contrast-enhanced sonography and MRI were not significant in the
identification of septa and nodules.
Data in the literature
[26–28]
suggest that follow-up should be provided to all patients with asymptomatic
cystic pancreatic lesions without criteria for suspicion of malignancy. For
the branch duct type of tumor, the recommendation of a consensus meeting
[27] on the management of
intraductal papillary mucinous neoplasms of the pancreas was "until
definitive studies are performed..., yearly follow-up if lesion is < 10 mm
in size, 6–12 monthly follow-up for lesions between 10 and 20 mm, and
3–6 monthly follow-up for lesions > 20 mm" (p. 28). Moreover,
patients with asymptomatic serous cystadenoma with a maximum diameter less
than 4 cm are candidates for nonoperative treatment with clinical and
radiologic follow-up [28].
Observation also is reported to be a safe management option for simple
pancreatic cysts 2 cm in diameter or smaller
[29]. On the basis of these
results, we believe that after the initial comprehensive imaging assessment of
a cystic pancreatic mass, sonography can be used as a follow-up technique for
lesions that do not necessitate surgery. When changes are detected at
sonographic surveillance, contrast-enhanced sonography can be used. This
practice decreases the frequency of CT and MRI examinations, limiting
radiation and expense.
The main limitation of this study relates to the retrospective evaluation.
Prospective analysis comparing contrast-enhanced sonography with MRI, CT, or
endoscopic sonography is necessary to assess the true specificity and
diagnostic accuracy. Beyond its ability to provide superior anatomic detail,
endoscopic sonography allows fine-needle aspiration
[30]. The second major
limitation of this study is related to the detectability of cystic masses on
transabdominal sonography. In our study, only cystic masses seen during
transabdominal sonography were included. The limitations of transabdominal
sonography are related to body habitus and the presence of bowel obscuring the
left upper quadrant when the lesions are located in the distal pancreatic body
and tail. Our findings, however, prove that in patients in whom the cystic
pancreatic mass is visible on transabdominal sonography, the results of
contrast-enhanced sonography and those of MRI in detecting intralesional septa
and nodules are very similar. Contrast-enhanced sonography can be considered a
complementary examination for the characterization of cystic pancreatic masses
seen on transabdominal sonography and can be included in the follow-up of
borderline lesions. In this subgroup of patients in whom a cystic mass can be
visualized, contrast-enhanced sonography may be a less expensive,
radiation-free, and effective imaging technique of lesion follow-up.
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Abstract
Introduction
