Ultrasound Elastography and MR Elastography for Assessing Liver Fibrosis: Part 1, Principles and Techniques

Abstract

OBJECTIVE The purpose of this article is to provide an overview of ultrasound and MR elastography, including a glossary of relevant terminology, a classification of elastography techniques, and a discussion of their respective strengths and limitations. CONCLUSION Elastography is an emerging technique for the noninvasive assessment of mechanical tissue properties. These techniques report metrics related to tissue stiffness, such as shear-wave speed, magnitude of the complex shear modulus, and the Young modulus.

Nonalcoholic Fatty Liver Disease

“…The measurement involves the assessment of the propagation of shear waves within the liver, from which the hepatic stiffness is inferred. Depending on the device, results can be reported as shear wave speed in m/s or as one of several elastic moduli (e.g., Young modulus, complex shear modulus) in kPa 157 . These various elastography-derived parameters have emerged as the leading imaging biomarkers of hepatic 'fibrosis'.…”

“…Elastography-enabled scanners use ultrasound to track shear waves generated transiently in situ within the liver by an ultrasound push pulse (also known as an acoustic radiation force impulse); measurement locations are recorded on an anatomical image 157,159,160 (Figure 5b). Because each manufacturer uses proprietary hardware and software, measurements made with different devices and even with different transducers (probes) on the same device do not agree 160,161 .…”

Elastography in Chronic Liver Disease: Modalities, Techniques, Limitations, and Future Directions

“…Shear-wave velocity stiffness values obtained with different techniques and manufacturers are not directly comparable, because shear-wave speed depends on tissue stiffness as well as the applied frequency of the shear wave; with all other things being equal, shear-wave speed and inferred stiffness are greater if the shear waves are applied at higher frequency (54). In addition, the technique is operator dependent, and simultaneous measurements in the same patient may vary, depending on the operator's expertise.…”

“…It is important to recognize that MR elastographic stiffness values increase with the frequency of applied mechanical waves; thus, stiffness values obtained with MR elastography at 60 Hz cannot be directly compared with values obtained with MR elastography performed at other frequencies. In addition, the methods and assumptions used in stiffness calculations differ between MR elastography and US-based methods, and so the stiffness values from each modality are not directly comparable (54).…”

Editorial: A New Spin on Magnetic Resonance Elastography

“…Magnetic resonance elastography (MRE) is another non-invasive technique that has been developed and appears to be the most accurate modality to assess hepatic fi brosis (13)(14)(15) as well as being utilized in newer epidemiological studies ( 16 ). Two-dimensional gradient-recalled echo (2D-GRE) has been well studied and is available in many specialized centers (17)(18)(19). Although MRE is not limited by obesity and ascites, it has a failure rate of 1-2% due to low signal-to-noise due to iron overload based on previous studies (16)(17)(18)(19)(20).…”