Magnetic resonance elastography: Non-invasive mapping of tissue elasticity
Introduction
There is strong precedent in clinical medicine for the concept that tissue viscoelastic properties, assessed by palpation, are markedly affected by a variety of disease processes. Student physicians learn that the presence of a hard mass in the thyroid, breast or prostate is suspicious for malignancy. Indeed, many tumors of these structures are still first detected by touch. It is not uncommon for surgeons at the time of laparotomy to palpate tumors that were undetected in preoperative imaging by CT, MRI or ultrasound. None of these modalities provide the information about the elastic properties of tissue elicited by palpation. The elastic moduli of various human soft tissues are known to vary over a wide range (more than four orders of magnitude). In contrast, most of the physical properties depicted by conventional medical imaging modalities are distributed over a much smaller numerical range.
Over the last decade, the recognition of the potential diagnostic value of characterizing mechanical properties has led a number of investigators to seek methods for imaging tissue elasticity. In materials science, the classic approach for measuring the elastic modulus of a sample is to apply a known stress and to measure the resulting strain. Refinements of this method involve the use of multiple measurements with varying stress, and/or the application of dynamic rather than static stress. Most of the proposed methods for elasticity imaging follow a similar approach. A stress is applied to tissue and the resulting strain distribution is observed or measured using a conventional imaging technique such as ultrasonography. The mode of stress application can be static, quasi-static, or dynamic. A recent review of such work is given by Gao et al. (1996).
Magnetic resonance elastography (MRE) is a recently developed technique that can directly visualize and quantitatively measure propagating acoustic strain waves in tissue-like materials subjected to harmonic mechanical excitation (Muthupillai et al., 1995, Muthupillai et al., 1996a). Shear waves at frequencies in the 10–1000 Hz range are used as a probe because they are much less attenuated than at higher frequencies, their wavelength in tissue-like materials is in the useful range of millimeters to tens of millimeters, and because shear modulus varies widely in bodily tissues. A phase-contrast MRI technique is used to spatially map and measure the shear wave displacement patterns. From this data, local quantitative values of shear modulus can be calculated and images (elastograms) that depict tissue elasticity or stiffness can be generated. In this paper we briefly summarize other techniques for imaging elasticity and describe the principles of MRE. We then consider the equations of harmonic motion in soft tissue and describe various approaches for reconstructing elastograms from MRE data and the assumptions inherent in each. These algorithms are then tested on synthetic and physical phantom data sets of known stiffness and issues such as noise sensitivity and resolution are discussed. Finally, a summary of recent ex vivo and in vivo results is presented.
Section snippets
Elastic properties of soft tissue
It is ironic that while the elastic properties of structural materials have been extensively characterized by engineers and physicists for more than a century, these properties are virtually unknown for biological soft tissues. The scarcity of such data in the literature most likely stems from the technical difficulty of measuring the elastic properties of semisolid biologic tissues using conventional laboratory methods such as mechanical load-cell testing devices, which rely on well-defined
Elasticity imaging techniques
Much of the pioneering work in elasticity imaging has been accomplished using ultrasound and either a quasi-static stress model (Ophir et al., 1991, O’Donnell et al., 1994, Cespedes et al., 1993, Garra et al., 1997) or a dynamic stress model (Gao et al., 1995, Huang and Roach, 1991, Lee et al., 1991, Lerner et al., 1990, Parker et al., 1990, Parker and Lerner, 1992, Rubens et al., 1995).
The quasi-static stress method employs an ultrasound transducer to apply a small axial compression to tissue.
Magnetic resonance elastography (MRE)
MRE uses propagating mechanical waves rather than static stress as a probe. This provides major advantages in calculating elasticity because it does not require estimation of the regional static stress distribution. Shear waves at frequencies in the 50–1000 Hz range are suitable as a probe because they are much less attenuated than at higher frequencies, their wavelength in tissue-like materials is in the useful range of millimeters to tens of millimeters, and because shear modulus varies so
Data processing
A variety of approaches can be used to invert the displacement data to recover mechanical properties. These are characterized below by the assumptions or simplifications made in their derivations. It is possible to deduce quantitatively accurate values of properties such as shear modulus in favorable situations. In general, however, despite the richness of the data set and the variety of processing techniques, it remains a challenge to extract accurate results at high resolution in complex,
Results on test data
To illustrate the different noise sensitivity, resolution and accuracy of the various processing techniques, we present results on synthetic and physical phantoms of known parameters.
Experimental results
We briefly summarize some recent results, both ex vivo and in vivo, on both human and animal tissues. All the elastograms shown below are based on LFE processing.
Conclusion
MRE shows great potential for the non-invasive in vivo determination of mechanical properties in a variety of tissues. The detection of propagating acoustic waves has been demonstrated in vivo in breast, brain and muscle and ex vivo in numerous animal and human tissues. Reconstruction algorithms have been tested and characterized and, although far from perfect, yield quantitative measures of elasticity that clearly demarcate differences between tissue types and identify tumors as areas of
Acknowledgements
This research is supported by NIH grant CA75552.
References (62)
- et al.
Preliminary results for shear wave speed of sound and attenuation coefficients from excised specimens of human breast tissue
Ultrason. Imag.
(1990) - et al.
Elastography: elasticity imaging using ultrasound with application to muscle and breast in vivo
Ultrason. Imag.
(1993) - et al.
A viso-elastic study of scalp, brain and dura
J. Biomech.
(1970) - et al.
Imaging of the elastic properties of tissue – a review
Ultrasound Med. Biol.
(1996) - et al.
Sonoelasticity images derived from ultrasound signals in mechanically vibrated tissues
Ultrasound Med. Biol.
(1990) - et al.
Elastography: a quantitative method for imaging the elasticity of biological tissues
Ultrason. Imag.
(1991) - et al.
Tissue response to mechanical vibrations for sonoelasticity imaging
Ultrasound Med. Biol.
(1990) - et al.
Axial stress distributions between coaxial compressors in elastography: an analytical model
Ultrasound Med. Biol.
(1992) - et al.
Ultrasonic imaging of the stress distribution in elastic media due to an external compressor
Ultrasound Med. Biol.
(1994) - et al.
Fundamental mechanical limitations on the visualization of elasticity contrast in elastography
Ultrasound Med. Biol.
(1995)
MR imaging of motion with spatial modulation of magnetization
Radiology
Comparison of phase-difference and complex-difference processing in phase-contrast MR angiography
JMRI
Magnetic resonance imaging of shear wave propagation in excised tissue
JMRI
A new ultrasound tissue-equivalent material
Radiology
A solution to diffraction biases in sonoelasticity: the acoustic impulse technique
J. Acoust. Soc. Am.
Functional MR elastography of human skeletal muscle
Inverse approach to elasticity reconstruction in shear wave imaging
Granular convection observed by magnetic resonance imaging
Science
Magnetic-resonance imaging techniques for detection of elasticity variation
Med. Phys.
Sonoelasticity imaging: theory and experimental verification
J. Acoust. Soc. Am.
Elastography of breast lesions: initial clinical results
Radiology
Two-Dimensional Phase Unwrapping: Theory, Algorithms and Software
Comprehensive compilation of empirical ultrasonic properties of mammalian tissues
J. Acoust. Soc. Am.
Viscoelastic property of human brain — for the analysis of impact injury
No To Shinkei
Sonoelasticity imaging: results in in vitro tissue specimens
Radiology
Tissue elasticity reconstruction using linear perturbation method
IEEE Trans. Med. Imag.
Local multiscale frequency and bandwidth estimation
A pulsed Doppler ultrasonic system for making noninvasive measurements of the mechanical properties of soft tissue
J. Rehabil. Res. Dev.
Elastic moduli of breast and prostate tissues under compression
Ultrason. Imag.
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