DOI:10.2214/AJR.06.1161
AJR 2007; 188:W522-W533
© American Roentgen Ray Society
B-Flow Imaging of the Hepatic Vasculature: Correlation with Color Doppler Sonography
Ronald H. Wachsberg1
1 Department of Radiology, University Hospital, New Jersey Medical School, 150
Bergen St., C-320, Newark, NJ 07103.
Received August 30, 2006;
accepted after revision November 28, 2006.
Address correspondence to R. H. Wachsberg
(wachsbrh{at}umdnj.edu).
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Abstract
OBJECTIVE. B-flow imaging is a non-Doppler technology that provides
real-time imaging of blood flow during gray-scale sonography. The utility of
B-flow imaging is reflected in numerous publications that describe normal and
pathologic findings in the carotid arteries and hemodialysis fistulas.
However, there is a paucity of articles describing B-flow imaging of the
abdominopelvic viscera. The purpose of this article is to illustrate a
spectrum of findings encountered during noninvasive flow evaluation of the
hepatic vasculature, correlating the Doppler sonography and B-flow imaging
findings.
CONCLUSION. Color and spectral Doppler sonography are invaluable for
noninvasive evaluation of the hepatic vasculature. However, a number of
pitfalls and artifacts have been described that can cause important pathologic
findings to be overlooked or can suggest incorrect diagnoses. In our
experience, B-flow imaging often correctly displays normal and pathological
vascular structures for which Doppler sonographic findings have been
misleading or erroneous.
Keywords: abdominal imaging • B-flow imaging • color Doppler sonography • Doppler sonography • liver
Introduction
When color Doppler sonography was originally introduced, sonologists hoped
that the new technology would provide reliable noninvasive mapping of blood
flow. Although color Doppler sonography remains an essential technology for
noninvasive vascular imaging, it is prone to various artifacts and
limitations, resulting in some instances in which true flow is not detected
and other instances where Doppler sonography falsely depicts blood flow
[1].
B-Flow Imaging
A technique for displaying flow information called B-flow imaging was
introduced several years ago
[2]. This is a non-Doppler
technology that directly displays flowing intravascular echoes during
real-time gray-scale sonography. Flow information is derived by digitally
encoding the outgoing ultrasound beam, then decoding and filtering the
returning beam so as to amplify echoes generated by the particulate
constituents of flowing blood.
The real-time B-flow imaging appearance of blood flow consists of mobile
intravascular echoes that simulate a conventional contrast angiogram, similar
to the appearance seen during infusion of a sonographic IV contrast agent. The
images are particularly impressive when viewed during real-time sonography or
on recorded movie clips. The technique is relatively simple to learn and
operate, with fewer parameters to manipulate than color Doppler sonography.
B-flow imaging was first introduced on linear transducers for vascular imaging
and subsequently became available on curved transducers suitable for general
abdominopelvic imaging as well.
Recent investigations have documented the value of B-flow imaging for
evaluation of flow in superficial vascular structures, in particular carotid
arteries and hemodialysis fistulas
[3,
4]. However, the literature
reveals scant interest in exploring abdominopelvic applications of B-flow
imaging. A MEDLINE search of the English-language literature using the term
"B-flow imaging" conducted on August 28, 2006, identified one
preliminary report on abdominal applications of B-flow imaging published in
2003 and two investigations of B-flow imaging for evaluating fetal
cardiovascular anomalies
[5–7].
In our experience, B-flow imaging is a powerful technique for noninvasive flow
evaluation of the liver. This article illustrates various advantages of B-flow
imaging as a complementary technique to color Doppler sonography of the
hepatic vasculature. All sonographic images were obtained using the 3.5 C
(2–4 MHz) curved abdominal transducer associated with the LOGIQ 700 and
LOGIQ 9 systems (GE Healthcare).
Comparison of B-Flow Imaging and Doppler Sonography
Doppler sonography uses a high-pass filter to suppress low-amplitude
frequency shifts caused by physiologic movement of soft-tissue structures.
Unfortunately, this filter also obliterates Doppler shifts produced by slowly
flowing blood and may cause a false diagnosis of vascular occlusion
[8]. This pitfall does not
apply to B-flow imaging, which excellently depicts slow blood flow (Figs.
1A,
1B and
2A,
2B,
2C with supplemental movie
clips). Doppler sonography is also prone to artifactual depiction of flow
signals within nonvascular hypoechoic structures, whereas B-flow imaging is
not plagued by such factitious flow (Fig.
3A,
3B).
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Fig. 1B —47-year-old woman with cirrhosis. On B-flow image, portal
vein lumen is filled with mobile echoes (arrowheads), best
appreciated on movie clip (Fig. S1C). Portal vein was patent at liver
transplantation 1 week later. Slow blood flow and large beam–vessel
angle made Doppler sonography detection of slow portal flow difficult, whereas
B-flow imaging performs well at large beam–vessel angles.
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Fig. 2B —44-year-old woman with poor graft function 2 days after liver
transplantation. Color Doppler sonogram optimized for slow-flow detection
shows questionable flow straddling margin of this collection
(arrow).
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Fig. 2C —44-year-old woman with poor graft function 2 days after liver
transplantation. B-flow image shows "puff of smoke" appearance
(arrowheads) indicating flowing blood within collection. This
appearance can be better appreciated on movie clip (Fig. S2D). Active venous
bleeding into intrahepatic pseudoaneurysm was identified and repaired at
laparotomy.
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Fig. 3A —37-year-old woman with Budd-Chiari syndrome who recently
underwent transjugular intrahepatic portosystemic shunt (TIPS) procedure.
Spectral Doppler sonogram reveals what appears to be flow signal within
hypoechoic structure adjacent to TIPS, suspicious for iatrogenic
pseudoaneurysm.
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Fig. 3B —37-year-old woman with Budd-Chiari syndrome who recently
underwent transjugular intrahepatic portosystemic shunt (TIPS) procedure.
B-flow image reveals flow within TIPS and inferior vena cava (IVC) but not
inside lesion. This finding resolved on follow-up, supporting diagnosis of
iatrogenic hematoma.
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The usual practice for optimizing color Doppler sonography is to increase
the color gain setting as high as possible until noise develops; then lower
the gain slightly to eliminate the noise
[8]. However, this practice can
exaggerate the spatial location of true flow signal, a phenomenon called
"oversaturation," resulting in flow signals that are not confined
to the patent lumen [9]. This
pitfall can cause thrombus to be overlooked (Fig.
4A,
4B,
4C) or improperly
characterized (Fig. 5A,
5B,
5C) and can prompt a false
diagnosis of vascular disease (Figs.
6A,
6B,
6C and
7A,
7B,
7C). Because of
oversaturation, the spatial distribution of color signals can substantially
exceed the true dimensions of a vascular structure, whereas the high spatial
resolution of B-flow imaging enables excellent display of even complex
vasculature (Figs. 8A,
8B and
9A,
9B). Soft-tissue vibration
associated with an arteriovenous fistula, known as "perivascular color
bruit," is a phenomenon that can significantly exaggerate the apparent
dimensions of a vascular fistula on Doppler sonography, whereas B-flow imaging
correctly displays the true dimensions (Fig.
10A,
10B,
10C).
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Fig. 4C —38-year-old woman with cirrhosis. Contrast-enhanced CT scan
shows nonocclusive thrombus (asterisk), confirming B-flow imaging
findings. This case illustrates how high color gain setting can cause
oversaturation and potentially obscure intravascular thrombus.
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Fig. 5A —45-year-old man with cirrhosis. All images are sagittal
sections of left lobe obtained near midline. Color Doppler sonogram obtained
with color gain increased until just below development of color noise reveals
what appears to be normal patency of left hepatic vein.
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Fig. 5B —45-year-old man with cirrhosis. All images are sagittal
sections of left lobe obtained near midline. B-flow image reveals curvilinear
intravascular channels within left hepatic vein lumen, consistent with
neovascularization within tumor thrombus rather than patent lumen.
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Fig. 5C —45-year-old man with cirrhosis. All images are sagittal
sections of left lobe obtained near midline. Color Doppler sonogram, repeated
with gain significantly lower than in A and other parameters unchanged,
now also reveals intrathrombus neovascularity. CT scan confirmed
hepatocellular carcinoma invading left hepatic vein. This case shows that
widespread practice of maximizing color gain until noise develops can lead to
color oversaturation and obscuration of nonocclusive thrombus.
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Fig. 6A —37-year-old male liver transplant recipient. Color Doppler
sonogram of liver hilum depicts relatively slow flow in portal vein (PV) and
aliased flow in hepatic artery (star). Note regions of focal widening
of arterial flow signal in porta hepatis (arrowheads), suggesting
looped vessel or pseudoaneurysm.
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Fig. 7A —33-year-old male liver transplant recipient. Color Doppler
sonogram of porta hepatis depicts pulsatile vascular structure containing
high-amplitude blue and red flow signals (arrow), suspicious for
pseudoaneurysm.
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Fig. 7C —33-year-old male liver transplant recipient. Color Doppler
sonogram obtained in different liver transplant recipient reveals hepatic
artery pseudoaneurysm (arrow), which was confirmed on arteriography.
Note similarity to looped artery in A.
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Fig. 8A —68-year-old woman with cirrhosis and apparently spontaneous
fistulous connection between right portal and right hepatic veins. Sagittal
color Doppler sonogram of inferior right hepatic lobe shows region of color
aliasing (asterisk) supplied by portal vein (red signal) and
drained by hepatic vein (blue signal). Arrows indicate direction of
flow. Note that vessels within fistula are not individually discernible.
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Fig. 8B —68-year-old woman with cirrhosis and apparently spontaneous
fistulous connection between right portal and right hepatic veins. B-flow
image depicts abnormal blood vessels in exquisite detail.
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Fig. 10A —42-year-old woman with arterioportal fistula. LHA = left
hepatic artery, LPV = left portal vein. Color Doppler sonogram reveals large
"perivascular color bruit" (arrow) generated by fistulous
connection between LHA and LPV.
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Fig. 10B —42-year-old woman with arterioportal fistula. LHA = left
hepatic artery, LPV = left portal vein. B-flow image reveals that fistula
(arrow) is much smaller than suggested on color Doppler sonogram
(A).
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Fig. 10C —42-year-old woman with arterioportal fistula. LHA = left
hepatic artery, LPV = left portal vein. Contrast-enhanced CT scan indicates
that true size of fistula (arrow) is similar to B-flow imaging
findings.
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Vascular stenosis is typically diagnosed when Doppler sonography reveals a
localized flow jet. However, turbulence and other factors can also cause
localized acceleration of flow unassociated with anatomic narrowing. B-flow
imaging is very helpful at distinguishing between a false-positive diagnosis
of vascular stenosis and a true-positive case (Figs.
11A,
11B,
11C and
12A,
12B,
12C).
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Fig. 11A —32-year-old male liver transplant recipient with intractable
ascites. Color Doppler sonogram reveals color aliasing in hilar portal vein,
with threefold focal velocity acceleration (from 45 to 149 cm/s).
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Fig. 11C —32-year-old male liver transplant recipient with intractable
ascites. Transhepatic portogram confirms absence of significant luminal
narrowing or pressure gradient across portal anastomosis (arrow).
False-positive Doppler sonography diagnosis was presumably caused by
anastomotic turbulence.
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Fig. 12A —53-year-old man with hilar cholangiocarcinoma. Color Doppler
sonogram of porta hepatis reveals apparent stenosis of hilar portal vein
(arrowhead) with color aliasing. Spectral Doppler sonogram (not
shown) revealed angle-corrected velocity measuring 163 cm/s flow jet at this
level.
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Noninvasive evaluation of transjugular intrahepatic portosystemic shunts
(TIPS) function with Doppler sonography is complex and fraught with pitfalls
that can potentially yield misleading findings
[10]. In our experience,
B-flow imaging is very helpful in supplementing the Doppler sonography TIPS
evaluation (Fig. 13A,
13B,
13C).
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Fig. 13A —36-year-old man with cirrhosis and hemorrhage from esophageal
varices treated with transjugular intrahepatic portosystemic shunt (TIPS).
Oblique transverse color Doppler sonogram reveals focal flow jet
(arrowhead) within TIPS. Angle-corrected velocity at this location
revealed localized 60 cm/s acceleration of flow velocity, suspicious for
stenosis.
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Fig. 13B —36-year-old man with cirrhosis and hemorrhage from esophageal
varices treated with transjugular intrahepatic portosystemic shunt (TIPS).
B-flow image in same plane as A reveals normal luminal diameter without
evidence of stenosis (arrowhead).
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Fig. 13C —36-year-old man with cirrhosis and hemorrhage from esophageal
varices treated with transjugular intrahepatic portosystemic shunt (TIPS).
Coronal curved multiplanar reformat of contrast-enhanced CT scan reveals
nonstenotic TIPS lumen, confirming B-flow imaging findings.
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The applications illustrated in this article should not be misinterpreted
as suggesting that color Doppler sonography has been or will be eclipsed by
B-flow imaging. B-flow imaging does not provide information regarding flow
velocity and directionality, and its current iteration has other limitations
that require improvement. We anticipate that color Doppler sonography will
continue to be the primary technique for noninvasive flow mapping, with B-flow
imaging as a complementary technique for use in situations where color Doppler
sonography findings are ambiguous or otherwise uncertain.
Conclusion
B-flow imaging is an exciting, relatively new non-Doppler technology for
noninvasive flow imaging. Our experience exploring hepatic B-flow imaging
indicates that this currently underused technology has the potential to
substantially improve noninvasive blood flow evaluation. As the imaging
community becomes increasingly aware of abdominopelvic applications of B-flow
imaging, it is hoped that manufacturers will advance the capabilities of this
technology.
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Abstract
Introduction
