AJR 2004; 183:1602-1604
© American Roentgen Ray Society
Volumetric Contrast Imaging in Bile Duct Sonography: Technology and Early Clinical Experience
Se Hyung Kim1,
Jeong Min Lee1,
Joon Koo Han1,
Helmut Brandl2 and
Byung Ihn Choi1
1 Department of Radiology and the Institute of Radiation Medicine, Seoul
National University Hospital, 28 Yongon-dong, Seoul 110-744, South
Korea.
2 GE Kretz Ultrasound, Tiefenbach 15, Zipf 4871, Austria.
Received August 20, 2003;
accepted after revision March 17, 2004.
Address correspondence to B. I. Choi
(choibi{at}radcom.snu.ac.kr).
Introduction
Sonography is the first-line technique for assessing biliary diseases.
Tissue harmonic imaging techniques have improved the depiction of the biliary
tree and intraductal abnormalities by decreasing reverberation or side lobe
artifact and increasing contrast resolution compared with conventional
sonographic techniques
[1–4].
However, tissue harmonic imaging has several limitations. First, the scattered
harmonic echoes are weak compared with the echoes of conventional sonography,
so electronic noise may be an issue. This problem could be magnified when
biliary sonography is performed in a patient with a large body habitus.
Additionally, separating harmonic echoes from conventional echoes and
minimizing noise in harmonic imaging require reductions in transmitted and
received bandwidths, which increase axial blurring on harmonic images
[5].
Volumetric contrast imaging is a "multisection" sonographic
technique that uses a volume probe to acquire a section of tissue continuously
and rapidly. (The word "contrast" in this context does not mean
the use of IV contrast material.) A volume probe is a linear-array transducer
that supports a stepping motor for computer-controlled section acquisition
during the scanning process. Images are obtained by swiveling the transducer
along a horizontal axis while adding or subtracting a predetermined degree
from a center position. The basic principle for volumetric contrast imaging is
a real-time volume acquisition with a small elevation angle in which the
combined surface and transparent maximum gradient-rendering processes are
performed (Fig. 1). The result
of volumetric contrast imaging is a thin volume-rendered image.
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Fig. 1. —Diagram of thin-volume image obtained using volumetric
contrast imaging with harmonic technique. Volume contains render box with
large surface. However, thickness of render box is kept small and can be
chosen by examiner—3, 5, 10, or 15 mm. Direction of rendering process
(arrow) correlates with plane (viewing plane [direction]) of
conventional 2D image.
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Volumetric contrast imaging provides more information from multiple
sections and theoretically might have an advantage by allowing greater
contrast as a result of an improved signal-to-noise ratio. Furthermore,
volumetric contrast imaging can be used in combination with harmonic imaging
and can boost the advantages of harmonic imaging and overcome the shortcomings
of that technique. The aim of our study was to introduce the new technology of
volumetric contrast imaging and to present the preliminary results comparing
volumetric contrast imaging combined with harmonic technique with tissue
harmonic imaging alone for evaluation of bile duct diseases.
Materials and Methods
A prospective study comparing volumetric contrast imaging with harmonic
technique and tissue harmonic imaging was approved by the ethics committee of
our hospital and was conducted with 20 consecutive patients who presented with
obstructive jaundice and therefore were believed to have bile duct disease.
Four of these patients were excluded from our study because they lacked
histopathologic results or confirmatory imaging. In addition, one patient who
had been diagnosed with fulminant hepatitis was also excluded. Thus, 15
patients were enrolled in our study. Written informed consent was obtained
from each patient. The patient population consisted of 11 men and four women
(mean age, 59 years; age range, 28–74 years). Final diagnosis included
choledocholithiasis in eight patients, malignant biliary obstruction in six
patients (Klatskin's tumor in three patients and common duct cancer, ampulla
of Vater cancer, and pancreatic head cancer in one patient each), and a
choledochal cyst in one patient. All cancers and the choledochal cyst were
confirmed histopathologically (six at surgery and one at biopsy). The
diagnosis of stones was verified on endoscopic retrograde cholangiography in
six cases and at surgery in two cases.
All images were obtained with a 2-5–MHz volume probe (Voluson 730
Expert, GE Kretz Ultrasound) and a 3-mm slice thickness. Tissue harmonic
images and volumetric contrast images were obtained simultaneously for a
side-by-side comparison. Volumetric contrast imaging techniques can be applied
with or without tissue harmonic imaging. In this study, volumetric contrast
imaging was obtained with the tissue harmonic technique. The best
representative images of the bile ducts in each patient were chosen.
Sonography and selection of image pairs were performed by one abdominal
radiologist (with 5 years' experience). The resulting 30 images were masked
using an overlay, obscuring identifying information and imaging parameters.
The images were randomly arranged as single images so that the volumetric
contrast image and tissue harmonic image from a single case were not together.
Individual images were then reviewed in consensus by two abdominal
radiologists (one with 13 and the other with 22 years' experience) who were
blinded to which imaging technique was used to obtain the image and the final
diagnosis. Four parameters— the sharpness of the bile duct wall,
internal artifacts, lesion conspicuity, and acoustic shadows from
stones—were graded on a 4-point scale (grade 1, poor, to grade 4,
excellent). Statistical analysis for the results of the data grading was
performed using Wilcoxon's signed rank test and Fisher's exact test.
Results
For the four sonographic parameters of biliary evaluation that we analyzed,
volumetric contrast imaging with harmonic technique was judged to be
significantly superior to tissue harmonic imaging alone (p <
0.05). The evaluation of unpaired data showed 27 grade-4 scores for the
volumetric contrast imaging compared with only three grade-4 scores for tissue
harmonic imaging (p = 0.001, Fisher's exact test).
With respect to the sharpness of the ductal wall, most images obtained with
volumetric contrast imaging combined with harmonic technique were rated as
grade 3 (n = 6) or 4 (n = 8), whereas images obtained with
tissue harmonic imaging alone were rated as grade 2 (n = 7) or 3
(n = 6). Significant improvement in the visualization of the ductal
wall was seen on the volumetric contrast images compared with the tissue
harmonic images (p = 0.003). In terms of internal artifacts, images
obtained with volumetric contrast imaging combined with harmonic technique
showed fewer artifacts than those obtained with tissue harmonic imaging alone
(p = 0.005). Of 10 patients in whom stones (n = 8), an
intraductal tumor (n = 1), or a periductal tumor (n = 1) was
seen, nine had volumetric contrast images that showed lesion conspicuity
better than tissue harmonic images (p = 0.003). Acoustic shadowing
from stones was seen more clearly in six of eight cases on images obtained
with volumetric contrast imaging combined with harmonic technique than on
those obtained with tissue harmonic imaging alone (p = 0.014).
Representative examples are shown in Figures
2A,
2B and
3A,
3B.
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Fig. 2A. —55-year-old man with periductal type of hilar
cholangiocarcinoma. Transverse images obtained with tissue harmonic imaging
(A) and volumetric contrast imaging with harmonic technique (B)
(3-mm slice thickness) show diffuse wall thickening (arrow) of hilar
duct. Volumetric contrast imaging with harmonic technique provides greater
conspicuity of thickened wall and clearer visualization of anterior wall of
normal duct (arrowhead) than tissue harmonic imaging.
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Fig. 2B. —55-year-old man with periductal type of hilar
cholangiocarcinoma. Transverse images obtained with tissue harmonic imaging
(A) and volumetric contrast imaging with harmonic technique (B)
(3-mm slice thickness) show diffuse wall thickening (arrow) of hilar
duct. Volumetric contrast imaging with harmonic technique provides greater
conspicuity of thickened wall and clearer visualization of anterior wall of
normal duct (arrowhead) than tissue harmonic imaging.
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Fig. 3B. —61-year-old man with stones in distal common bile duct. Image
(3-mm slice thickness) acquired with volumetric contrast imaging combined with
harmonic technique also shows stones (solid arrows) in common bile
duct. Sharpness of visualization of bile duct wall (arrowheads) and
lesion conspicuity are better than in tissue harmonic image (A). In
volumetric contrast image, internal artifacts are more suppressed and longer
length of common bile duct is shown with darker lumen (open arrows)
than seen in A. Acoustic shadow (curved arrow) has sharper
wall and is darker and better defined than shown in A.
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Discussion
In our study, volumetric contrast imaging provided better contrast
resolution and suppressed unwanted artifacts more effectively than tissue
harmonic imaging. The main aspect of volumetric contrast imaging is that a
thin volume contains approximately 10–25 bidirectional predicted slices
(B-slices), depending on the thickness setting, rather than a single B-slice.
The information is volume-rendered, and the user can adjust the thickness. The
direction of the rendering process in the render box correlates with the
direction on conventional 2D imaging. The default volume-rendering setting for
volumetric contrast imaging is a mixture of 70% surface texture rendering and
30% transparent maximum gradient. This mixture improves contrast and
signal-to-noise ratio. Furthermore, the rendering process has a high sample
rate, meaning the density of the rendering analysis is higher than the density
of pixels in a single B-plane. Therefore, the gaps of the speckle pattern are
filled with information from an adjacent slice, causing the speckle pattern
that is seen on any conventional B-mode image to be suppressed on volumetric
contrast imaging. In addition, a smooth image is produced with no loss of
detailed information because volume contrast imaging is not just a 2D filter.
These effects of speckle suppression and contrast enhancement could be
maximized when applied in the critical structure such as a small duct in which
a subtle artifact can cause a misinterpretation [Kim SH, unpublished data
2004].
Our preliminary results showed that volumetric contrast imaging provides
better depiction of the ductal wall and better suppression of internal
artifacts. Furthermore, volumetric contrast imaging improves both the
visualization of the actual intraductal lesion and the acoustic shadow behind
a stone. None of the information provided with tissue harmonic imaging is lost
with volumetric contrast imaging. Although better results were consistently
obtained using volumetric contrast imaging, our experience suggests that
volumetric contrast imaging was most advantageous for evaluating common bile
duct lesions, in which the role of sonography is limited by artifacts or
adjacent gas-filled structures (Fig.
3A,
3B).
Our study has some limitations. First, the number of cases was small, which
prevents us from generalizing on the basis of these preliminary results.
Further studies with more cases are warranted to investigate the feasibility
of volumetric contrast imaging in various clinical applications. Other
limitations in our study method include the fact that we used only a single
still image per case rather than multiple matched images or cine clips.
The objective of our study was to evaluate the feasibility of using
volumetric contrast imaging with harmonic technique, compared with tissue
harmonic imaging alone, to assess biliary diseases, not to evaluate its
diagnostic performance for revealing biliary lesions. Although our results
seem to be excellent, further prospective studies with more cases are needed
to assess the diagnostic performance and to confirm the feasibility of
volumetric contrast imaging.
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Introduction
Materials and Methods
