Diagnostic ultrasound ( PDFDrive )

CHAPTER 4 The Liver 107
FIG. 4.42 Normal Liver Vasculature. Temporal maximum-intensity
projection image shows accumulated enhancement in 11 seconds after
contrast material arrives in liver. Depiction of vessel structure to ifth-order
branching is evident. Focal unenhanced region (arrow) is slowly perfusing
hemangioma. (With permission from Wilson S, Jang H, Kim T, et al.
Real-time temporal maximum-intensity-projection imaging of hepatic
lesions with contrast-enhanced sonography. AJR Am J Roentgenol.
2008;190[3]:691-695. 130 )
determines lesion conspicuity on a sonogram. hat is, a tiny
mass of only a few millimeters will be easily seen if it is increased
or decreased in echogenicity compared with the adjacent liver
parenchyma. Because many metastases are either hypoechoic or
hyperechoic relative to the liver, a careful examination should
allow for their detection. Nonetheless, many metastatic lesions
are of similar echogenicity to the background liver, making their
detection diicult or impossible, even if they are of a substantial
size. his occurs when the backscatter from the lesion is virtually
identical to the backscatter from the liver parenchyma.
To combat this inherent problem of lack of contrast between
many metastatic liver lesions and the background liver on
conventional sonography, contrast-enhanced liver ultrasound is
helpful (Fig. 4.45). CEUS increases the backscatter from the liver
compared with the liver lesions, thereby improving their detection.
his occurs rapidly following the arterial phase of enhancement
and generally lasts for several minutes beginning in the portal
venous and persisting for the late phase.
Of historic interest is the use of the irst-generation contrast
agent Levovist (Schering AG, Berlin). Ater clearance of the
contrast agent from the vascular pool, the microbubble persisted
in the liver, probably within the Kupfer cells on the basis of
phagocytosis. A high-MI sweep through the liver produced bright
enhancement in the distribution of the bubbles. herefore all
normal liver enhances. Liver metastases, lacking Kupfer cells,
do not enhance and therefore show as black or hypoechoic holes
within the enhanced parenchyma 135 (Fig. 4.45A and B). In a
multicenter study conducted in Europe and Canada, more and
smaller lesions were seen than on baseline scan. 136 Overall, lesion
detection was equivalent to that of CT and MRI. he decibel
diference between the lesions and the liver parenchyma is
increased many fold because of increased backscatter from
contrast agent within the normal liver tissue. Although many
results were compelling, these irst-generation contrast agents
are no longer marketed.
Today, current requirements for improved lesion detection
use a similar technique of CEUS with a perluorocarbon contrast
agent and low-MI scanning in both the arterial and the portal
venous phase. he use of a low-MI imaging technique for lesion
detection has advantages in terms of scanning because the
microbubble population is preserved and timing is not so critical.
Virtually all metastases will be unenhanced relative to the liver
in the portal and the late phase and the liver parenchyma will
remain optimally enhanced. herefore malignant lesions tend
to appear hypoechoic in the portal phase, allowing for improved
lesion detection (Fig. 4.45C and D). his observation, that
malignant lesions tend to be hypoechoic in the portal venous
phase of perluorocarbon liver enhancement, is helpful for both
lesion detection and lesion characterization. Enhancement of
benign lesions, FNH, and hemangioma generally equals or exceeds
liver enhancement in the portal venous phase.
Detection of hypervascular liver masses (e.g., HCC, metastases)
is also improved by scanning with perluorocarbon agents in
the arterial phase. hese lesions will generally show as hyperechoic
masses relative to the liver parenchyma in the arterial phase
because they are predominantly supplied by hepatic arterial low.
HEPATIC NEOPLASMS
Sonographic visualization of a focal liver mass may occur in a
variety of clinical scenarios, ranging from incidental detection
to identiication in a symptomatic patient or as part of a focused
search in a patient at risk for hepatic neoplasm. Hemangiomas,
FNH, and adenomas are the benign neoplasms typically encountered
in the liver, whereas HCC and metastases account for the
majority of malignant tumors.
he role of imaging in the evaluation of an identiied focal
liver mass is to determine which masses are potentially clinically
important, requiring conirmations of their diagnoses, and which
masses are likely to be insigniicant and benign, not requiring
further evaluation to conirm their nature. On a sonographic
study, there is considerable overlap in the appearances of focal
liver masses. Once a liver mass is seen, however, the excellent
contrast and spatial resolution of state-of-the-art ultrasound
equipment have provided guidelines for the initial management
of patients, which include recognition of the following
features:
• A hypoechoic halo identiied around an echogenic or isoechoic
liver mass is an ominous sonographic sign necessitating
deinitive diagnosis.
• A hypoechoic and solid liver mass is highly likely to be
signiicant and also requires deinitive diagnosis.