Diagnostic ultrasound ( PDFDrive )

CHAPTER 4 The Liver 105
HEPATIC MASSES
Focal liver masses include a variety of malignant and benign
neoplasms, as well as masses with developmental, inlammatory,
and traumatic causes. In cross-sectional imaging, two basic issues
relate to a focal liver lesion: characterization of a known liver
lesion (what is it?) and detection (is it there?). he answer to
either question requires a focused examination, oten adjusted
according to the clinical situation.
Liver Mass Characterization
Characterization of a liver mass on conventional sonography is
based on the appearance of the mass on gray-scale imaging and
vascular information derived from spectral, color, and power
Doppler sonography. With excellent spatial and contrast resolution,
the gray-scale morphology of a mass allows for the differentiation
of cystic and solid masses, and characteristic
appearances may suggest the correct diagnosis without further
evaluation. Oten, however, deinitive diagnosis is not based on
gray-scale information alone, but on vascular information
obtained on conventional Doppler ultrasound examination.
However, conventional Doppler oten fails in the evaluation of
a focal liver mass, particularly in a large patient or on a small
or deep liver lesion, or on a mass with inherent weak Doppler
signals. Motion artifact is also highly problematic for abdominal
Doppler ultrasound studies, and a let lobe liver mass close to
the pulsation of the cardiac apex, for example, can limit assessment
by conventional Doppler. For these reasons, conventional
ultrasound is not regarded highly for characterization of
focal liver masses, and a mass detected on ultrasound is
historically evaluated further with CECT or MRI for deinitive
characterization.
Role of Microbubble Contrast Agents
Worldwide, noninvasive diagnosis of focal liver masses is achieved
with CECT and MRI based on recognized enhancement patterns
in the arterial and portal venous phases. hese noninvasive
methods of characterization have become so accurate that
excisional and percutaneous biopsy for diagnosis of liver masses
is now rarely performed. In recent years, however, CEUS has
joined the ranks of CT and MRI in providing similar diagnostic
information as well as information unique to CEUS. 126 Injection
of a microbubble contrast agent to enhance the Doppler signal
from blood and imaging with a specialized imaging technique
such as pulse inversion sonography allow for preferential detection
of the signal from the contrast agent while suppressing the signal
from background tissue.
Ultrasound contrast agents currently in use are secondgeneration
agents comprising tiny bubbles of a perluorocarbon
gas contained within a stabilizing shell. Microbubble contrast
agents are blood pool agents that do not difuse through the
vascular endothelium. his is of potential importance when
imaging the liver because comparable contrast agents for CT
and MRI may difuse into the interstitium of a tumor. Our
personal experience with perluorocarbon microbubble agents
is largely based on the use of Deinity (Lantheus Medical Imaging,
Billerica, MA) and brief exposure to Optison (GE Healthcare,
Milwaukee, WI). 127,128 We routinely perform CEUS for characterization
of incidentally detected liver masses, those found on
surveillance scans of patients at risk for HCC, and any focal
mass referred by our clinicians found on outside imaging or
indeterminate on CT and MRI. 129
Microbubble contrast agents for ultrasound are unique in
that they interact with the imaging process. 127 he major determinant
of this interaction is the peak negative pressure of the
transmitted ultrasound pulse, relected by the mechanical index
(MI). he bubbles show stable, nonlinear oscillation when exposed
to an ultrasound ield with a low MI, with the production of
harmonics of the transmitted frequency, including the frequency
double that of the sound emitted by the transducer, the second
harmonic. When the MI is raised suiciently, the bubbles undergo
irreversible disruption, with the production of a brief but bright,
high-intensity ultrasound signal (see Chapter 3).
Liver lesion characterization with microbubble contrast agents
is based on lesional vascularity and lesional enhancement in the
arterial phase (10-40 sec), portal venous phase (40-90 sec), and
late phase (up to 5 minutes). Lesional vascularity assessment
depends on continuous imaging of the agents while they are
within the vascular pool. We document the presence, number,
distribution, and morphology of any lesional vessels. A low MI
is essential because it will preserve the contrast agent population
without destruction of the bubbles in the imaging ield, allowing
for prolonged periods of real-time observation. he morphology
of the lesional vessels is discriminatory and facilitates the diagnosis
of liver lesions (Fig. 4.41).
Lesional enhancement is best determined by comparing the
echogenicity of the lesion to the echogenicity of the liver at a
similar depth on the same frame and requires knowledge of liver
blood low. he liver has a dual blood supply from the hepatic
artery and portal vein. he liver derives a larger proportion of
its blood from the portal vein, whereas most liver tumors derive
their blood supply from the hepatic artery. At the initiation of
the injection, the low-MI technique will cause the entire ield
of view (FOV) to appear virtually black, regardless of the baseline
appearance of the liver and the lesion in question. In fact, a
known mass may be invisible at this point. As the microbubbles
arrive in the FOV, the discrete vessels in the liver and then those
within a liver lesion will be visualized, followed by increasing
generalized enhancement as the microvascular volume of liver
and lesion ills with the contrast agent. he liver parenchyma
will appear more echogenic in the arterial phase than at baseline,
and even more enhanced in the portal venous phase, as a relection
of its blood low. Vascularity and enhancement patterns of a
liver lesion, by comparison, will therefore relect the actual blood
low and hemodynamics of the lesion in question, such that a
hyperarterialized mass will appear more enhanced against a less
enhanced liver on an arterial phase sequence. Conversely, a
hypoperfused lesion will appear as a dark or hypoechoic region
within the enhanced liver on an arterial phase sequence.
Currently, evaluation of lesional enhancement is usually
performed with the low-MI technique just described. However,
details of vessel morphology and lesional enhancement are even
more sensitively assessed using a bubble-tracking technique called
maximum-intensity projection (MIP) imaging. 130 In this