The Effect of Frequency on the Uniformity of Hepatic Shear Wave Illumination

Introduction: MR elastography (MRE) has recently emerged as a tool for the noninvasive assessment of liver fibrosis and a number of studies have indicated its potential role as an alternative to biopsy [1-5]. MRE uses phase-contrast MR techniques to image the propagation of shear waves through tissue in order to determine the mechanical properties of the tissue. One of the advantages that MRE has over liver biopsy is that biopsies only sample a very small percentage of the liver volume and thus are subject to sampling errors. MRE, on the other hand, has the capability to measure tissue stiffness throughout the liver and is less susceptible to local sampling errors. The ability to differentiate healthy liver tissue from fibrotic tissue varies with fibrosis stage and is easier at the later stages. However, being able to identify the early stages of fibrosis is important for providing an early diagnosis and beginning early treatment of the underlying disease. Unfortunately, a challenge with performing MRE on patients with normal liver stiffness and those with mild fibrosis is that the liver is soft and can quickly attenuate the MRE acoustical vibrations. This characteristic can limit the volume of liver tissue which receives adequate shear wave illumination, thus increasing the likelihood of sampling errors in this segment of the patient population. One approach to trying to increase the wave penetration in these patients is to use a lower mechanical vibration frequency. The hypothesis of this study was that decreasing the frequency of the vibrations used for MRE would significantly increase the volume of hepatic tissue illuminated by shear wave motion.