Ultrasound Imaging and Therapy

Aaron Fenster, James C. Lacefield, Aaron Fenster, James C. Lacefield

• Publisher: CRC Press
• Year: 2015
• ISBN:
  9781498759977
• Edition: 1
• Topic: Médecine
• Subtopic: Oncologie

Up-to-Date Details on Using Ultrasound Imaging to Help Diagnose Various DiseasesDue to improvements in image quality and the reduced cost of advanced features, ultrasound imaging is playing a greater role in the diagnosis and image-guided intervention of a wide range of diseases. Ultrasound Imaging and Therapy highlights the latest advances in usin

Section II

Diagnostic Ultrasound Imaging

Chapter 4

Ultrasound Elastography

Timothy J. Hall, Assad A. Oberai, Paul E. Barbone, and Matthew Bayer

4.1 Introduction

Ultrasound is a commonly used imaging modality that is still under active development with great potential for future breakthroughs although it has been used for decades. One such breakthrough is the recent commercialization of methods to estimate and image the (relative and absolute) elastic properties of tissues. Most leading clinical ultrasound system manufacturers offer some form of elasticity imaging software on at least one of their ultrasound systems. The most common elasticity imaging method is based on a surrogate of manual palpation.

There is a growing emphasis in medical imaging toward quantification. Ultrasound imaging systems are well suited toward those goals because a great deal of information about tissues and their microstructure can be extracted from ultrasound wave propagation and motion tracking phenomena. Estimating tissue viscoelasticity using ultrasound as a quantitative surrogate for palpation is one of those methods.

This chapter will review the methods used in quasi-static (palpation-type elastography). The primary considerations in data acquisition and analysis in commercial implementations will be discussed. Moreover, methods to extend palpation-type elastography from images of relative deformation (mechanical strain) to quantitative images of elastic modulus and even the elastic nonlinearity of tissue will also be presented. Section 4.4 will review promising clinical results obtained to date.

4.1.1 Brief Clinical Motivation

Manual palpation has been a common component of medical diagnosis for millennia. It is well understood that physiological and pathological changes alter the stiffness of tissues. Common examples are the breast self-examination (or clinical breast examination) and the digital rectal examination. Although palpation is commonly used, it is known to lose sensitivity for smaller and deeper isolated abnormalities. Palpation is also limited in its ability to estimate the size, depth, and relative stiffness of an inclusion or to monitor changes over time.

Given the long history of successful use of palpation, even with its limits, there was a strong motivation to develop a surrogate that could remove a great deal of the subjectivity, provide better spatial localization, provide spatial context of surrounding tissues, and improve estimates of tumor size and relative stiffness. The spatial and temporal sampling provided by clinical ultrasound systems, as well as their temporal stability, make them very well suited to this task. The first clinically viable real-time elasticity imaging system was reported in 2001 [1], and significant improvements have been made since then.

The typical commercial elasticity imaging system provides real-time elasticity imaging with either a side-by-side display of standard B-mode and strain images or a color overlay of elasticity image information registered on the B-mode image (or both options). Some metric of feedback to the user is also often provided so the user knows if the scanning methods are appropriate and/or if the data acquired are high quality.

4.1.2 Theory Supporting Quasi-Static Elastography

A basic assumption commonly used in palpation-type elastography is that the loading applied to deform the tissue is quasi-static, meaning that motion is slow enough that inertial effects—time required and inertial mass—are irrelevant. Following the presentation of Fung [2], a basic description of the...