BME Seminar Series: Dr. Marvin M. Doyley, Ph.D.

A Model-Based Approach to Quasi-Static, Harmonic, and Transient Elastography

Department of Electrical and Computer Engineering, Department of Biomedical Engineering, Hajim School of Engineering and Applied Sciences, University of Rochester

Abstract

Elastography is emerging as an imaging modality that can detect and characterize atherosclerotic plaques; guide minimally invasive therapeutic techniques; and improve the differential diagnosis of breast and prostate cancers. Although current approaches to elastography vary considerably, the general principles of elastography can be summarized as follows:

1. Perturb the tissue using a quasi-static, harmonic, or transient mechanical source
2. Measure the internal tissue displacements using a suitable ultrasound, magnetic resonance, or optical displacement estimation method
3. Infer the mechanical properties from the measured mechanical response, using either a simplified or continuum mechanical model.

Several groups have developed approaches for obtaining approximate estimates of shear modulus with all three approaches to elastography, and despite their limited accuracy these techniques are fast and robust — traits that make them clinically appealing. For example, Ophir et al.1 used a simple mechanical model to compute approximate modulus elastograms, but stress concentration and target hardening artifacts compromised the diagnostic quality of the ensuing elastograms. Similarly, Parker et al.2 computed local estimates of shear modulus from local estimates for wavelength, but the weakness of this approach is that it is difficult to obtain accurate estimates of wavelength in complex organs such as the breast and brain. To overcome these issues, several groups, including our own, have proposed inversion schemes for computing the mechanical properties within soft tissues within the framework of solving an inverse problem. In this presentation, we will:

• Review a selection of the inversion schemes for all three areas — quasi-static, harmonic, and transient elastography
• Describe practical techniques for transforming the ill-posed problem to a well-posed one
• Describe better test procedures for evaluating the performance of modulus elastograms
• Report the results of pre-clinical studies conducted with simulated and physical phantoms, and volunteers
• Discuss new opportunities for model-based elastography.

References

• Ophir J, Cespedes I, Ponnekanti H, Yazdi Y and Li X 1991 Elastography: a quantitative method for imaging the elasticity of biological tissues Ultrason Imaging 13 111-34.
• Parker K J, Huang S R, Musulin R A and Lerner R M 1990 Tissue-response to mechanical vibrations for sonoelasticity imaging Ultrasound in Med.& Biol. 16 241-6.