Diagnostic Ultrasound - Abdomen and Pelvis

Approach to Hepatic Sonography
Diagnoses: Liver
Patient Preparation
Although no specific patient preparation is generally required
to visualize the liver, the other organs of the hepatobiliary
system, such as the gallbladder, are ordinarily examined as
part of a complete sonographic liver assessment. Therefore,
for a comprehensive hepatobiliary assessment, patients
should be instructed to fast for 6-8 hours prior to the
ultrasound examination. If dehydration is a concern, water
may be given, but other activities that increase stomach and
intestinal gas production should be limited.
Imaging Protocols
The liver is the largest organ in the abdomen and must be
scanned systematically and carefully to ensure a thorough
examination. The patient should be positioned supine initially,
but a variety of positions, including decubitus scanning, may
be needed depending on organ size and orientation and the
presence of overlying bowel gas. Imaging from the subcostal
approach with cranial angulation of the transducer in normal
and deep suspended respiration typically provides optimal
visualization. If the liver is high in position, or shrunken and
cirrhotic, an intercostal approach may be necessary.
The scan protocol ordinarily begins by visualizing the left lobe
in the mid sagittal plane beneath the sternum and xiphoid
process and proceeds with the sonographer scanning laterally
to visualize first the left lobe and then the right lobe. The
transverse plane is scanned next, followed by oblique planes
along the long and short axes of specific anatomic structures,
such as the gallbladder and common bile duct. The
sonographer must visualize the entire liver in real time from
the most lateral portions of the left lobe through the right
lobe, and then capture a series of standard sagittal and
parasagittal views for documentation purposes. Similarly, in
the transverse plane, the liver must be examined from the
dome to the inferior tip. Standard views to be documented
include the following:
• Longitudinal views: Lateral segments of left lobe, aorta,
inferior vena cava, caudate lobe and ligamentum
venosum, porta hepatis, gallbladder fossa, right lobe
segments, right lobe with longitudinal view of right
adrenal fossa and kidney
• Transverse views: Left lobe dome, left portal vein,
caudate lobe and ligamentum venosum, right lobe
dome, hepatic venous confluence and individual right,
middle, and left hepatic veins, gallbladder, and liver with
right kidney
• Oblique views: Common bile duct in long axis, main
portal vein in long axis, gallbladder in long axis, short axis,
and decubitus views
Transducer Selection and Technical Factors
The sonographer should select the highest frequency
transducer that provides sufficient penetration to visualize the
entire depth of the liver. As a visual check, in a normal liver the
diaphragm should be clearly visible on longitudinal and
transverse images of the right lobe. With modern ultrasound
technology, this usually implies broad-bandwidth 3-5 MHz
transducers capable of imaging in both harmonic and
fundamental modes and with multiple focal zones. Curved
linear transducers generally provide the best compromise
between good near-field imaging and wide field of view. For
images of the hepatic capsule, a high-frequency linear
transducer should be used.
Time-gain compensation and the overall receiver gain settings
should be set so that the liver has a homogeneous and
uniform echo texture from the near field to the far field.
Speckle reduction techniques, such as spatial and frequency
compounding, and adaptive filtering techniques can work
synergistically with harmonic imaging modes to reduce image
noise.
Color, spectral, and power Doppler modes should be part of
every dedicated assessment of the liver. Color Doppler mode
should be used to document patency and direction of flow in
the portal and hepatic veins as well as the hepatic artery. If
flow appears absent on color Doppler, power Doppler should
be used since it is less dependent on insonation angle. Power
Doppler is also useful for minimizing artifacts resulting from
background motion, and for distinguishing dilated bile ducts
from vascular structures within the liver.
The Doppler signatures of the hepatic vessels obtained in
spectral Doppler mode are often quite useful in the
identification and characterization of hepatic pathology. The
normal portal vein shows hepatopetal, minimally undulating
flow. By contrast, the normal hepatic veins have a triphasic
velocity waveform similar to the appearance of the central
venous pressure waveforms. The hepatic artery interrogated
in the hepatic hilum shows peak velocity ranging from 30-60
cm/sec, and a low-resistance waveform with continuous
antegrade flow during diastole.
In liver transplants, careful evaluation of the hepatic artery,
portal vein, and hepatic veins are important for detection of
potential complications including stenosis or thrombosis. The
hepatic duct should be carefully evaluated for developing
strictures at the anastomosis.
Equipment parameters in Doppler modes, such as the wall
filter and velocity scale (pulse repetition frequency), should be
set so as to correctly display the expected range of velocities
in the vessels being interrogated. For quantitative assessment
of flow velocity, angle correction should be performed in
spectral Doppler mode in order to calibrate the machine to
calculate an accurate velocity from the frequency shift
information that is detected at the transducer.
Anatomy-Based Imaging Issues
The normal liver should display relatively uniform,
intermediate-level echogenicity slightly higher than that of
the renal cortex. Its length should typically not extend below
the inferior pole of the right kidney unless a Reidellobe is
present. Portal triads ramifying within the liver typically display
echogenic walls, whereas the hepatic venous walls will not be
echogenic unless insonated at exactly ninety degrees.
Certain normal hepatic structures can simulate pathology. For
example, the fibrous ligamentum teres and ligamentum
venosum may cause acoustic shadowing and create the
appearance of mass lesions or abnormal echogenicity of the
caudate lobe. Overlying ribs may also cause shadowing,
particularly during intercostal scanning.
Selected References
1. Heller MT et al: The role of ultrasonography in the evaluation of diffuse liver
disease. Radiol Clin North Am. 52(6):1163-75, 2014
2. McNaughton DA et al: Doppler US of the liver made simple. Radiographics.
31(1):161-88, 2011
3. Kruskal JB et al: Optimizing Doppler and color flow US: application to
hepatic sonography. Radiographics. 24(3):657-75, 2004
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