Liver Imaging
assessment of diffuse liver disease, in particular, liver fibrosis. Grading of liver fibrosis is essential for evaluation of chronic liver disease and impacts prognosis and treatment options.9
fibrogenesis suggest that clinically significant histological improvement can occur even in a cirrhotic liver.10
Virtual Touch Quantification and Non-alcoholic Fatty Liver Disease
Recent insights into liver Liver biopsy is still seen as the most
Results of Virtual Touch quantification in evaluating liver fibrosis have shown a stepwise increase in the median shear wave velocity with increasing histological severity of liver fibrosis.11
Measurement Accuracy and Reproducibility Studies measuring the reproducibility and accuracy of measurement techniques suggest that the superficial portions of liver are subject to potential interference from factors both intrinsic and extrinsic to the liver, such as manual compression.12
Virtual Touch quantification of
the liver is believed to be more reliable when it is applied to the deep portion of the right lobe compared with the left lobe.12
It is thought that
measurements made in the right lobe are more accurate for diagnosing liver fibrosis than those from the left lobe.13
Factors such as aortic
pulsation, respiratory fluctuations and presence of food in the stomach may be responsible for left lobe measurements being less accurate.12,13
However, there have been several studies comparing the utility and limitations of the FibroScan device with ARFI elastography and liver biopsy.10–12
Current Status of Liver Fibrosis Assessment Transient elastography (TE) using the FibroScan® (EchoSens, Paris) device is a widely used method for the assessment of hepatic fibrosis.14,15
The limitations of the FibroScan device include that it has a fixed depth of evaluation and a fixed ROI. This is believed to cause some difficulty in patients who have increased subcutaneous fat or those with ascites where the liver is at a greater depth from the skin surface, and patients with narrow intercostal spaces.10
Although
FibroScan is contraindicated in ascites, small volumes of ascites can be difficult to detect clinically. The measurement procedure is a blind one and, furthermore, the presence of hepatic steatosis can adversely affect FibroScan results.9
Virtual Touch quantification can be performed to a larger depth and is under direct sonographic visualization because it uses conventional B-mode imaging for ROI placement. In patients with narrow intercostal spaces in whom FibroScan evaluation can be difficult, ARFI imaging can be performed using a subcostal window under direct visualization. Ascites and large amounts of subcutaneous fat do not significantly affect Virtual Touch quantification unlike FibroScan.14
Furthermore,
placing the ROI over large vessels can be avoided with ARFI quantification imaging and increase accuracy. Excellent correlation between ARFI measurements and liver biopsy histological scores has been demonstrated in several series.14,15
50
effective method of evaluating liver fibrosis but is associated with significant risks such as bleeding and infection and is also costly to perform. In recent years, noninvasive methods have been developed, with the aim of reducing the number of liver biopsies in individual patients and providing a method for reliably assessing liver fibrosis. Several treatments are being studied for arresting or treating fibrosis and there is a need for an imaging modality that can monitor disease progression during the course of clinical treatment trials. ARFI data studies in vivo have shown accurate and repeatable liver stiffness measurements.11
Non-alcoholic fatty liver disease (NAFLD) traditionally has been a diagnosis of exclusion as a cause for chronic liver disease. In recent years, its incidence has increased, affecting between 20 and 30 % of the population of the US and other western countries.16
NAFLD is the hepatic
The risks of progressing to NAFLD-related cirrhosis and developing hepatocellular carcinoma and liver failure are now thought to be similar to chronic alcoholic liver disease and viral hepatitis.16–18 A recent study has also shown Virtual Touch quantification to be promising for assessing the presence or absence of fibrosis in patients with the metabolic syndrome and NAFLD.18
manifestation of the metabolic syndrome and is related to obesity and insulin resistance. It comprises a spectrum of disease, pathologically ranging from simple steatosis to non-alcoholic steatohepatitis (NASH) and fibrosis.17
In addition, a significant positive correlation was found by Yoneda et al. between ARFI elastography and severity of liver fibrosis in NAFLD.19 The stages of fibrosis in NAFLD (stage 0 to stage 4) were found to have specific median velocities that correlated well with transient liver elastography and serum fibrosis marker tests.
Focal Liver Lesions—Current Status Solid focal liver lesions are commonly found during routine abdominal ultrasound imaging. Conventional ultrasound is frequently the first imaging modality performed for clinical symptoms of right upper quadrant pain and deranged liver function tests owing to its low cost, wide availability and reliability. Contrast-enhanced ultrasound (CEUS) is widely used in Canada, Asia and Europe to characterize liver lesions owing to its realtime capability of assessing lesion enhancement.20,21
Its limitations include
the cost of the microbubble agent, requirement for a second operator to perform the contrast injection, necessity for intravenous access and contraindications in certain patient groups, including those with cardiac disease (particularly intracardiac shunts), pulmonary hypertension and pregnancy. However, CEUS has significant advantages because it can be used in patients with renal impairment and obviates the need for a computed tomography (CT) scan, thereby saving radiation exposure.
In assessing focal liver lesions with ARFI, generating contrast between a lesion and the surrounding tissue is the key to characterization. As a result of the simultaneous availability of conventional high-quality ultrasound imaging and ARFI elastography in the same machine, Virtual Touch tissue quantification can provide complementary noninvasive information on shear wave velocities within a focal lesion. Virtual Touch imaging generates high-contrast images between normal liver and focal lesions, improving visualization.
Several studies have evaluated ARFI elastography in patients with focal solid liver lesions.22–25
A recent study by Tian et al. in 116 patients
concluded that the median shear wave velocities in malignant lesions were 3.14 m/sec and 1.35 m/sec in benign liver lesions.22
With a cut-off
value of 2.22 m/sec the study concluded a sensitivity, specificity, and accuracy for malignancy of 89.7, 95, and 92.2 %, respectively. False-negative lesions below the cut off were all hepatocellular carcinomas that were found to be relatively less stiff. Another study by Davies et al. used a cut-off value of 2.5 m/sec to differentiate between
US RADIOLOGY
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