Current Medical Imaging - Volume 7, Issue 4, 2011

From times immemorial, medical practitioners have used palpation for detection and characterization of pathologies. Recently, a new diagnostic imaging modality has emerged, called elasticity imaging (EI), which is an extension of the ancient art of palpation. Various imaging modalities may be used for EI, including ultrasound, MRI or even X-ray, to visualize the elasticity (stiffness) of soft tissue structures. Elasticity imaging is a cutting-edge diagnostic modality that can be adapted for a very wide range of medical applications. Because many diseases are accompanied by significant changes in tissue mechanical properties and various tumors, especially malignancies, have elastic properties that are often markedly different from surrounding tissues, EI could provide a significant adjunct to current diagnostic methods. Lesions in the breast, liver, thyroid, prostate, pancreas, and lymph nodes have been successfully imaged using EI. Elasticity imaging has the potential to differentiate benign and malignant lesions, to detect steatohepatitis in patients with fatty liver disease, to monitor ablation and other therapeutic lesion treatments. EI has demonstrated its effectiveness in assessing the progress of ablation therapy. In this two-volume special issue on hot topics, some of the world's leading research groups present reviews of many different approaches to EI. The first issue begins with an overview of the historical and theoretical basis of EI. A review of ultrasound methods for elasticity imaging using external vibrators is then presented. That article is followed by two articles where the ultrasound transducer is used as the source of deformation. The first volume of this two-issue series closes with three articles that involve acoustic radiation force techniques for EI. The second issue begins with three more articles that involve acoustic radiation force in EI. The final ultrasound-based EI approach described here uses physiological motion as the source of deformation to image the elastic properties of vasculature. Two papers then review the developments and status of elasticity imaging based on magnetic resonance imaging methods. The final paper of the second volume of this special issue reviews a method for elasticity imaging based on measurements of the surface stress distribution - a method that closely approximates manual palpation by humans. This is a rapidly developing field and, despite efforts to provide a comprehensive review, inevitably some new developments have been left out. We apologize in advance for this and suppose that there will be future reviews where recent meritorious work will be included. We take this opportunity to provide special thanks to the authors of these review articles. Each group has made important contributions to the field of elasticity imaging, has provided their expertise in reporting the current state of affairs and provided enthusiastic support for this project. We are deeply grateful.

From times immemorial manual palpation served as a source of information on the state of soft tissues and allowed detection of various diseases accompanied by changes in tissue elasticity. During the last two decades, the ancient art of palpation gained new life due to numerous emerging elasticity imaging (EI) methods. Areas of applications of EI in medical diagnostics and treatment monitoring are steadily expanding. Elasticity imaging methods are emerging as commercial applications, a true testament to the progress and importance of the field. In this paper we present a brief history and theoretical basis of EI, describe various techniques of EI, analyze their advantages and limitations, and overview main clinical applications. We present a classification of elasticity measurement and imaging techniques based on the methods used for generating a stress in the tissue (external mechanical force, internal ultrasound radiation force, or an internal endogenous force), and measurement of the tissue response. The measurement method can be performed using differing physical principles including magnetic resonance imaging (MRI), ultrasound imaging, X-ray imaging, optical and acoustic signals. Until recently, EI was largely a research method used by a few select institutions having the special equipment needed to perform the studies. Since 2005 however, increasing numbers of mainstream manufacturers have added EI to their ultrasound systems so that today the majority of manufacturers offer some sort of Elastography or tissue stiffness imaging on their clinical systems. Now it is safe to say that some sort of elasticity imaging may be performed on virtually all types of focal and diffuse disease. Most of the new applications are still in the early stages of research, but a few are becoming common applications in clinical practice.

The field of imaging the elastic properties of tissue has grown to an impressive variety of techniques and applications, some of which are currently in clinical trials. This review focuses on the specific role of vibration sonoelastography, beginning with the first images of tissue “hardness”, through the development of real time scanning techniques, 3D reconstructions, and applications to major disease categories, especially the detection of prostate cancer.

The specific purpose of this review is to describe the progress of our work on elastography at the University of Texas Medical School-Houston in the past decade (2000-2010), and to relate it to our earlier work on this topic in the preceding decade (1991-2000). This review is neither intended to cover all specific aspects of this fast growing field, nor to be an exhaustive review of the literature. Such information is available separately and in several literary reviews. The early work in our Laboratory was started [1] with the fundamental theoretical and experimental development of elastography and ended with demonstration of the feasibility of producing elastograms in a clinical setting [2]. During the following decade our work has branched out into three main directions. These were (1) a continued effort to demonstrate the ability of elastography to depict the elastic properties of tissues and to develop improved algorithms for attaining quality strain estimations; (2) the development and practical in vivo demonstration of Poisson's ratio elastography (poroelastography) for the study of poroelastic materials such as lymphedematous tissues; and (3) the development of axial-shear strain elastography (ASSE) for imaging the mechanical boundary conditions at tissue interfaces, and to demonstrate the utility of this modality in the differentiation between benign and malignant breast lesions in vivo. These three areas are the main topics that are covered in this review.

We report a summary of recent developments and current status of our team's efforts to image and quantify in vivo nonlinear strain and tissue mechanical properties. Our work is guided by a focus on applications to cancer diagnosis and treatment using clinical ultrasound imaging and quasi-static tissue deformations. We review our recent developments in displacement estimation from ultrasound image sequences. We discuss cross correlation approaches, regularized optimization approaches, guided search methods, multiscale methods, and hybrid methods. Current implementations can return results of high accuracy in both axial and lateral directions at several frames per second. We compare several strain estimators. Again we see a benefit from a regularized optimization approach. We then discuss both direct and iterative methods to reconstruct tissue mechanical property distributions from measured strain and displacement fields. We review the formulation, discretization, and algorithmic considerations that come into play when attempting to infer linear and nonlinear elastic properties from strain and displacement measurements. Finally we illustrate our progress with example applications in breast disease diagnosis and tumor ablation monitoring. Our current status shows that we have demonstrated quantitative determination of nonlinear parameters in phantoms and in vivo, in the context of 2D models and data. We look forward to incorporating 3D data from 2D transducer arrays to noninvasively create calibrated 3D quantitative maps of nonlinear elastic properties of breast tissues in vivo.

Acoustic radiation force based elasticity imaging methods are under investigation by many groups. These methods differ from traditional ultrasonic elasticity imaging methods in that they do not require compression of the transducer, and are thus expected to be less operator dependent. Methods have been developed that utilize impulsive (i.e. < 1 ms), harmonic (pulsed), and steady state radiation force excitations. The work discussed in this paper utilizes impulsive methods, for which two imaging approaches have been pursued: 1) monitoring the tissue response within the radiation force region of excitation (ROE) and generating images of relative differences in tissue stiffness (Acoustic Radiation Force Impulse (ARFI) imaging); and 2) monitoring the speed of shear wave propagation away from the ROE to quantify tissue stiffness (Shear Wave Elasticity Imaging (SWEI)). For these methods, a single ultrasound transducer on a commercial ultrasound system can be used to both generate acoustic radiation force in tissue, and to monitor the tissue displacement response. The response of tissue to this transient excitation is complicated and depends upon tissue geometry, radiation force field geometry, and tissue mechanical and acoustic properties. Higher shear wave speeds and smaller displacements are associated with stiffer tissues, and slower shear wave speeds and larger displacements occur with more compliant tissues. ARFI images have spatial resolution comparable to that of B-mode, often with greater contrast, providing matched, adjunctive information. SWEI images provide quantitative information about the tissue stiffness, typically with lower spatial resolution. A review these methods and examples of clinical applications are presented herein.

Interactions between waves can be turned into profit to break diffraction limits and invent new kinds of medical images. It consists in productively combining two very different waves -- one to provide contrast, another to provide spatial resolution -in order to build a new kind of image, Multiwave imaging provides a unique image of the most interesting contrast with the most interesting resolution. We will show in this paper how this general concept of multiwave imaging allow to perform high resolution and quantitative elasticity imaging of the human body. Here, the two waves are sonic shear waves and ultrasonic compressional waves. A comparison with static elastography (a single wave technique) will show the interest of multiwave imaging and various clinical examples will be presented to illustrate the efficiency of this approach.

In recent years, several new techniques based on the radiation force of ultrasound have been developed. Vibroacoustography is a speckle-free ultrasound based imaging modality that can visualize normal and abnormal soft tissue through mapping the acoustic response of the object to a harmonic radiation force induced by ultrasound. In vibroacoustography, the ultrasound energy is converted from high ultrasound frequencies to a low acoustic frequency (acoustic emission) that is often two orders of magnitude smaller than the ultrasound frequency. The acoustic emission is normally detected by a hydrophone. In medical imaging, vibro-acoustography has been tested on breast, prostate, arteries, liver, and thyroid. These studies have shown that vibro-acoustic data can be used for quantitative evaluation of elastic properties. This paper presents an overview of vibro-acoustography and its applications in the areas of biomedicine.