Ultrasonic Methods for Imaging the Elastic Properties of Tissue

Imaging the elastic properties of soft tissue using ultrasonic methods has been the focus of many international research efforts. The primary goal of these labors has been development of novel` imaging methodologies and techniques for estimating the elastic properties of soft tissue to differentiate normal from pathological states. In this paper, we focus on ultrasound-based tissue elasticity estimation strategies and imaging methods. More specifically, we will introduce and discuss the techniques known as compression elastography and vibration sonoelastography. Introduction For more than a decade, imaging the elastic properties of soft biological tissues using ultrasonic methods has become the focal point of many research efforts. The goal of these initiatives revolves around mapping some tissue mechanical property in an anatomically meaningful manner in order to provide useful clinical information. Since changes in tissue stiffness may be indicative of an abnormal pathological process, imaging parameters related to tissue elasticity may provide a suitable gateway for differentiating normal from abnormal tissue types. It has been shown experimentally that there is in fact a difference in stiffness between abnormal and normal tissues types owing to the pathological state. As Krouskop et al. reported from their work with breast tissue samples, ductal carcinomas exhibited a stiffness contrast of approximately 23 dB whereas invasive and infiltrating carcinomas exhibited a 28 dB contrast relative to the softer subcutaneous adipose tissue [1]. Additionally, it was shown that cancers of the prostate exhibited a stiffness contrast in excess of 10 dB when compared to the normal prostate tissue [1]. This observation confirmed an earlier report by Parker et al. [2] albeit using a very limited set of tissue samples. Palpation is a routine physical examination where the clinician qualitatively assesses low-frequency tissue stiffness. During palpation, lumps that are discrete and differ from the surrounding tissue are identified for further diagnosis. Though these lumps may dislocate or feel fixed within the tissue, subtle finding are much more difficult to interpret. Specifically, in many tumor cases and despite the difference in stiffness, the small size of a pathological lesion and/or its nonsuperficial location impedes detection and evaluation by palpation. Additionally, lesions may or may not possess echogenic properties that make it detectable ultrasonically. For example, tumors of the prostate or the breast can be isoechoic or barely visible in standard ultrasound examinations, yet be much harder than the embedding tissue. It is hypothesized that imaging parameters related to tissue stiffness will provide novel information that is associated with tissue structure and/or pathology. Elasticity Imaging using Ultrasound Within the context of elasticity imaging, research devoted to ultrasonic-based methods have received a lot of attention. Some of these methods have focused on quantitatively measuring physical parameters (e.g. shear modulus reconstruction) whereas others strive to provide an image that qualitatively relates to the tissue stiffness distribution (e.g., strain estimation and low frequency vibrational response). The fundamental goal of these efforts has been the development of novel imaging methodologies and techniques for estimating the elastic properties of soft tissue in order to differentiate normal from pathological tissue structures. Based on results that can be found in the literature, these efforts are slowly coming to fruition. An important distinction of ultrasound-based elasticity imaging systems is that tissue echogenicity and stiffness, attributed to the Bulk and shear moduli distributions, respectively, are disjoint. Figure 1 illustrates the variation of the shear and bulk moduli for various materials and body tissues (adapted from [3]). Note that though the Bulk moduli exhibits limited dynamic range, the opposite is true for the shear moduli distribution, which spans several orders of magnitude for the various biological materials and tissues. From Fig. 1, it can be inferred that though there may exist a relatively limited spatial fluctuation in the underlying Bulk moduli (which determines ultrasonic wave propagation and scattering) for given tissue volume, the shear moduli (and thus the elastic properties) may exhibit considerably larger spatial fluctuations. It is this large dynamic range in the elastic properties that motivates clinicians to routinely use palpation for the physical inspection of tissues and drives research in the field of elasticity imaging. Several groups have contributed to the growing field of ultrasound-based elasticity imaging. While considerable variations exist in how the details are implemented, all methods share the following basic elements: (1) A mechanical force is applied to the tissue, (2) Local tissue motion is measured, (3) Some elasticity parameter is estimated. Since no known modality is capable of imaging the elastic properties of tissue directly, the application of a mechanical force in order to induce tissue motion is a prerequisite to all elasticity imaging techniques. Additionally, the mechanical sources can be grouped based on the spatial characteristics of the tissue excitation. External methods apply a compressive force on the skin in order to deform the underlying tissue structures. These external sources can be further classified by the temporal characteristics of the mechanical source: static (or quasistatic) methods and dynamic methods. In contrast, internal methods apply the mechanical force internally and directly on the tissue region of interest. These mechanical sources can either be biological in nature (such as cardiovascular pulsation) or manually induced using approaches such as the acoustic radiation force of a focused ultrasound beam. 10 10 10 10 10 10 10 10 10 Glandular breast tissue