Analysis of Dynamic Magnetic Resonance Elastography Data - Advance Image Processing in Magnetic Resonance Imaging - page 442

14.6.5 Muscle.....................................................................................470
14.6.6 Ultrasound Wave Field Visualization .....................................471
14.6.7 Characterization of Thermally Ablated
Tissue ......................................................................................472
14.7 Conclusion ..............................................................................................473
Acknowledgments .............................................................................................473
References .........................................................................................................474

14.1 INTRODUCTION

For centuries, palpation has been used as an efficient detection tool by physicians,
based on the fact that many diseases cause changes in the mechanical properties
of tissues. Student physicians learn that the presence of a hard mass in the thyroid,
breast, or prostate is suspicious for malignancy. Indeed, even today many tumors
of these structures are 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 provides informa-
tion about the elastic properties of tissues elicited by palpation. The elastic moduli
of various human soft tissues are known to vary over a wide range. The Young's
modulus of soft tissues can vary as much as four orders of magnitude [1,2] in
healthy and diseased tissues. The literature on mechanical properties of abnormal
tissues is limited, but it is known that the elastic modulus of the breast may differ
from surrounding tissues by a factor of 90 [3,4]. It is also known that the shear
modulus of many tissues can vary in response to changes in the physiologic state
[1,5]. The elasticity of muscle in the relaxed and contracted states can differ by
more than 100-fold [1]. In contrast, most of the other 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. For reviews of such work, see

Reference 6 and Reference 7 [6,7].

Magnetic resonance elastography (MRE) is a technique that can directly
image and quantitatively measure displacements due to propagating acoustic
strain waves in tissue-like materials subjected to harmonic mechanical excita-
tion [8,9]. A phase-contrast MRI technique is used to spatially map and measure
the shear-wave displacement patterns. From this data, local quantitative values
of the shear modulus can be calculated and images (elastograms) that depict
tissue elasticity or stiffness can be generated. In this chapter, we describe the
principles of MRE, consider the equations of harmonic motion in soft tissue,
and describe approaches for reconstructing elastograms from MRE data and
the assumptions inherent in each, and present a summary of some

ex vivo and

in vivo results.