The Current Status of Non-Invasive Assessment of Liver Fibrosis: Real Time Tissue Elastography

Abstract

Liver biopsy has long been an important tool for assessing the degree of liver fibrosis. Currently, there are many reports and approaches to evaluate the staging of hepatic fibrosis by using noninvasive imaging methods. The main modalities are based on ultrasonography (US) and magnetic resonance imaging (MRI). Among these techniques, Transient Elastography (TE, Fibroscan) has taken a leading role in the noninvasive assessment of liver stiffness at current clinical medicine in Europe. The principal of TE is simple: TE measures the propagation speed of elastic shear wave within the liver parenchyma. The velocity of the wave propagation correlates directly to tissue stiffness; that is, the harder the tissue is, the faster the shear wave spread. In this chapter, we introduce a new US-based approach for non-invasive assessment of liver fibrosis using real-time tissue elastography (RTE) that can be performed simultaneously with conventional ultrasound probes during routine US examination. RTE is technically different from TE: echo signals representing tissue strain before and under mild compression are compared and analyzed. RTE was developed in Japan for the visual assessment of tissue elasticity integrated in a sonography machine. The technique is based on the Combined Autocorrelation Method that calculates the relative hardness of tissue rapidly from the degree of tissue distortion and displays this information as a color image [Shiina T, 2002]. The distortion of tissue is transferred to a color-coded image according to its magnitude and superimposed translucently on the conventional B-mode image. Such a simultaneous display of tissue elasticity and B-mode images enables us to evaluate the anatomical correspondence between these modalities. The RTE image is constructed by the amount of displacement of the reflected ultrasound echoes under compression. Although US-based elastography is unable to demonstrate physical elasticity directly, it shows the relative degree of tissue strain when subtle compression is applied. In hard tissue, the amount of displacement of the reflected ultrasound echoes is low, whereas, in soft tissue, the amount of displacement is high because soft tissue can be compressed more than hard tissue. This technology has already been proved to be diagnostically valuable in detecting space occupying lesions in the breast, prostate, and pancreas. In 2007, Friedrich-Rust applied this technique to measure liver stiffness and reported its usefulness for the detection of significant fibrosis (≧F2, area under the receiver operating characteristic curve; AUC 0.93) in combination with AST to platelet ratio index (APRI). In 2010, we reported that AUC for no significant