INIMAGING INSIGHT
MR Elastography of liver - Arobust emerging technique Dr Parul S Garde, Consultant Radiologist, Global Hospitals, Mumbai talks about new techniques revolutionising the diagnosis and management of chronic liver disease
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ast few decades have witnessed a major technological revolution in diagnostic as well as therapeutic aspects of medical science. As a practising radiologist in a tertiary care super speciality organ transplant hospital, I take this opportunity to write about one such revolutionary technique which has truly made its mark in the diagnosis and management of chronic liver disease MR elastography.
Techniques This MRI technique quantitatively evaluates tissue stiffness by propagating low frequency mechanical waves through the liver. It is performed using an MRI safe passive driver that is applied to the right upper abdomen and lower chest overlying the right lobe of the liver while the patient is being scanned in the MRI scanner. The MRE sequence is carried out with four short breathholds and completed within one to two minutes, without the need for intravenous contrast injection. An active driver generates low frequency mechanical waves (typically at 60 Hz) which are conducted to the passive driver through a long plastic tube. The passive driver vibrates and produces shear waves that are propagated across the liver. The wavelength of the propagating shear wave is directly proportional to the stiffness of the liver, that is, the stiffer the liver, the longer the wavelength. By applying an inversion al-
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EXPRESS HEALTHCARE
February 2017
gorithm to the raw data, elastograms or stiffness maps that depict tissue stiffness are generated. Elastograms may be displayed in a gray scale or with a colour scale. The result is obtained by placing the region of interest on the processed images. It is measured in Kilo Pascals.
Applications Staging of chronic liver disease : Regardless of the etiology, chronic liver disease can progress from inflammation to fibrosis (reversible in early stages) and finally cirrhosis (irreversible). Liver stiffness measured with MRE increases with increasing stage of fibrosis. The increase in stiffness increases significantly with advanced fibrosis and cirrhosis. MR elastography can non invasively detect various stages of fibrosis and thus obviate the need for liver biopsy, which has been the gold standard for detection, till the past decade. Patients who are treated in early stages of fibrosis stand a better chance of survival compared to those untreated, particularly due to availability of effective antiviral treatment for chronic viral hepatitis.
Quantification of fat content of liver Non alcoholic fatty liver disease is widely accepted as a cardiovascular and insulin resistance risk factor. Fatty liver is diagnosed when the intracytoplasmic fat deposition is found in greater than 5 per cent of hepatocytes. Till pres-
MRI based methods of hepatic fat quantification have proved to be able to reflect much smaller changes in the degree of steatosis ent times, histological fat analysis is a semi-quantitative method at best, with high rates of misdiagnosis due to sampling error. Currently, there is no cutoff or limit where liver fat content is considered too much or harmful. However, it is foreseeable that as we understand this disease spectrum as there might arise a need to know the exact amount of fat in the liver for risk stratification or re-
sponse assessment. Hence, there is a need for more precise quantitative methods of evaluating hepatic steatosis. MRI based methods of hepatic fat quantification have proved to be able to reflect much smaller changes in the degree of steatosis. Besides, the results thus obtained can be correlated with the patients’ body weights and serum alanine aminotransferase and aspartate amino-
transferase levels. This makes MRI ideal as an imaging biomarker for assessing response to treatment of fatty liver. In fact, recent papers have primarily used MRI as a means to quantify hepatic fat when studying the effects of prognostication and treatment of hepatic steatosis in relation to NASH and impaired glucose tolerance. There are two primary methods of evaluation for estimation of hepatic fat fraction on MRI. The first is an imaging-based, Chemical Shift Imaging (CSI) method which takes advantage of the fact that protons (hydrogen atoms) in fat and water molecules are quite different in magnetic resonance properties and that the signal intensity of the liver at different time-points of image data acquisition (echo time) varies, depending on the concentration of water and fat. The second method is MR spectroscopy (MRS), a purely quantitative method that measures the concentration of water and fat metabolites based on their resonant frequencies. Potentially, by combining the fat quantification information with MRE findings, one may be able to diagnosis simple hepatic steatosis from NASH and NASH with fibrosis.
Quantification of iron Content : Iron overload cannot be detected reliably on any other imaging technique except MRI. Iron is a paramagnetic substance and causes rapid decay of MRI signal and gener-