Evaluation of Ultrasound Elastography Quantification Accuracy
FibroScanner (EchoSens, France)
The aim of this study was to investigate the use of poly-vinyl alcohol cryogel (PVA-C) as a tissue-mimicking material (TMM) for liver elastography by manipulating its mechanical and acoustic properties to achieve liver stiffness values spanning the range from healthy liver tissue to severe fibrosis, and to develop a range of test phantoms based on this TMM to quantify the measurement accuracy of an ultrasound elastography system, the Fibroscan® system.
Chronic liver disease is extremely common worldwide, with estimated figures of 350 and 180 million people chronically infected with the hepatitis B and hepatitis C viruses, respectively. These conditions, together with alcoholic liver disease and non-alcoholic fatty liver disease, among others, may lead to the development of extensive fibrosis or cirrhosis of the liver, with the latter associated with an increased risk of morbidity and mortality. Large needle biopsy (LNB) followed by histopathological analysis is regarded as the gold standard method for assessing the health of the liver, however this invasive procedure is painful and has a non-zero mortality rate. Elastography is a relatively new approach to the diagnosis of liver disease, and can provide a measure of the Young's modulus is a non-invasive manner.
The mechanical properties of tissue correlate with its pathological condition and hence the study of properties such as the Young's modulus can provide a method for differentiating between different pathological states in tissue.
PVA-c is a versatile material which has found use as a TMM for ultrasound and MRI phantom devices. The stiffness of PVA-C can be manipulated by processing the material through a number of freeze/thaw cycles, varying the PVA concentration and also by controlling the rate of thawing
Fibroscan system (EchoSens, France), probe, and typical measured 'elastogram'
Liver biopsy versus FibroScan measurement
Mechanical testing of PVA-c samples
Effect of over-lying fat layer on the phantom's surface. Left: no fat layer; Right: fat layer causes an error in the measures slope, which translates into an error in the measures stiffness.
Increasing fat layer thickness lead to increasing errors in the measures stiffness values, compared to a gold standard mechanical measurement of the samples.
Shear Wave Elastography
Shear Wave Elastography: quantitative map of the stiffness of a calibrated phantom, measured using an Aixplorer scanner (Supersonic Imagine, France)
Calibration phantoms produced for testing shear wave elastography systems
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