Introduction
Chronic liver diseases can cause liver fibrosis, ultimately progressing to cirrhosis if left untreated. Individuals with liver cirrhosis are at risk of developing liver failure, portal hypertension (elevated blood pressure in liver veins), varices (enlarged veins), and hepatocellular carcinoma (liver cancer). However, liver fibrosis is known to be reversible in its early stages and requires an adequate diagnosis.
What Is Liver Fibrosis?
Liver fibrosis develops when infection or long-term injury causes an excessive build-up of scar tissue in the liver. The scar tissue formed is irreversible. Therefore, it reduces liver function and disrupts the regeneration potential of organs.
How to Diagnose Liver Fibrosis?
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Percutaneous Biopsy - It was a standard diagnostic test for liver fibrosis assessment. However, the procedure's limitations, such as invasive technique, expensiveness, poor patient tolerance, variation caused by the observer, biopsy sampling error, and risk of complications, have reduced its clinical application.
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Serum Markers - The technique is noninvasive but can exhibit variations in diagnostic potential.
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Transient Elastography (Fibroscan) - It is an ultrasound-based technique to analyze liver stiffness. The procedure's outcome strongly correlates with the severity of liver fibrosis.
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MRI-Based Imaging - Various MRI-based imaging techniques are used for liver fibrosis assessment. The methods used are diffusion-weighted imaging (water molecules are used to generate contrast MRI images), perfusion MRI (analyzing blood flow in tissues), MR spectroscopy (measures biochemical changes in tissues), and MRE.
What Is Magnetic Resonance Elastography (MRE)?
The imaging technology combines magnetic resonance imaging (MRI) with low-frequency vibrations to produce a visual image (elastogram). The most common use of magnetic resonance elastography is to identify liver stiffness. The imaging is useful for mapping the stiffness in larger areas and depths of the liver. The stiffness in chronic liver disease results from fibrosis (scarring of tissue) and inflammation (redness and swelling). In addition, the MRE of the liver helps assess liver fat content, focal disease (disease location), and complications seen in chronic liver disease. Imaging is also useful for analyzing the brain, thyroid, lungs, heart, skeletal muscle, and breast.
What Is the Principle of MRE Imaging of Liver Fibrosis?
The MRE imaging technique uses waves in the range of 20-200 Hz (hertz) for diagnosing the mechanical properties of liver tissue. An acoustic driver system for movement within tissues forms low-frequency waves. The waves generated move faster in stiffer tissue than in normal tissue. The continuous application of waves causes the formation of wavelengths depending on the speed of wave movement.
An increase in tissue stiffness can lead to longer wavelength formation. The propagating pattern of waves in the liver is imaged. The images are processed with specialized software to form cross-sectional images of liver tissue stiffness. The areas with larger wavelengths appear stiffer than those with shorter wavelengths.
Therefore, imaging with MRE includes:
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Mechanical wave formation in tissues.
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Imaging of the wave with MRI.
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Elastogram formation with image processing.
What Is the Procedure for Performing MRE Elastography of the Liver?
The hepatic MRE is performed during the patient's fasting phase. If the imaging is performed after food consumption, the blood flow to portal vessels can cause an increase in liver stiffness. In some patients, intravenous contrast agents like gadolinium are used because liver stiffness values do not alter.
The imaging procedure can be combined with standard MRI by including a few pieces of hardware to generate waves and software for image processing. The hardware for MRE consists of an active acoustic driver (located outside the magnet) and a passive driver (a disc-shaped, nonmetallic plastic tube) placed anterior to the chest over the region of the liver. A strap around the body fixes the passive driver. A frequency of 60 Hz is passed through a passive driver for liver fibrosis assessment. The patient can tolerate the procedure and mechanical wave.
What Are the Advantages of Using MRE in Liver Fibrosis Assessment?
Diagnosing the extent of fibrosis in suspected liver conditions is necessary. It is vital to diagnose fibrosis early to prevent the disease's progression to cirrhosis. Examination with MRE can measure disease severity, plan treatment, and evaluate response to treatment.
The advantages of using this imaging technique are
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It is a safe and noninvasive procedure.
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The entire region of the liver can be analyzed with a single scan.
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Fibrosis of the liver is detected earlier compared to other imaging techniques.
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Accurate imaging results are obtained in obese patients.
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The imaging technique can detect other abdominal complications like ascites (fluid accumulation in the abdomen).
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Imaging can be used effectively in liver fibrosis diagnosis even when bowel interposition (replacement of abnormal tissue with a part of the colon) between the liver and abdominal wall is present.
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The imaging technique depends less on the operator. Hence, the chances for error are minimal.
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The image acquisition time is minimal. Therefore, it can be combined with standard MRI imaging.
What Are the Risk Factors Associated With MRE Imaging?
Since MRE imaging includes the MRI, the presence of metal can become a safety issue for patients.
Therefore, the items to be avoided during MRE imaging are
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Metallic prosthesis used for joints or other body parts.
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Artificial heart valves (replacement of non-functional valve with an artificial one).
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Implantable heart defibrillators (battery-operated devices to stop irregular heartbeats).
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Heart pacemaker (device to control irregular heart rhythm).
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Cochlear implants (devices to improve hearing).
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Metal clips or fragments.
What Are the Limitations of MRE Use in Liver Fibrosis?
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The limitation of MRE evaluation of liver fibrosis is an iron overload in the liver. It results in weak signal intensity in the liver at a very low level in the MRE-based echo gradient. Thus leading to diagnostic errors.
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The technique requires patient cooperation, as breath-holding is necessary.
What Is the Role of MRE in Liver Fibrosis Assessment?
The stiffness of the normal liver is within 3 kPa (kilopascals). This imaging technique demonstrates high sensitivity and specificity in diagnosing individuals with liver fibrosis above 3 kPa. Furthermore, it can differentiate between different fibrosis stages and classify them. In addition, this technique can determine various stages of fibrosis and help with their grading.
The severity of liver stiffness increases with the histological grade of the liver biopsy. There is variation in the degree, extent, distribution, and pattern of fibrosis in chronic liver diseases caused by different etiologies. However, in a few inflammatory conditions of the liver, MRE examination shows fibrosis without any histological evidence. This may cause errors in diagnosis. An additional role for MRE imaging includes biopsy needle guidance into the liver parenchyma.
Conclusion
Recently, MRE has been used as a noninvasive alternative to biopsy. The imaging technique is essential for treatment planning and evaluating the response to treatment. The procedure is safe, less expensive, and reliable for liver fibrosis assessment.
