Medical ultrasound: imaging of soft tissue strain and elasticity

Abstract

After X-radiography, ultrasound is now the most common of all the medical imaging technologies. For millennia, manual palpation has been used to assist in diagnosis, but it is subjective and restricted to larger and more superficial structures. Following an introduction to the subject of elasticity, the elasticity of biological soft tissues is discussed and published data are presented. The basic physical principles of pulse-echo and Doppler ultrasonic techniques are explained. The history of ultrasonic imaging of soft tissue strain and elasticity is summarized, together with a brief critique of previously published reviews. The relevant techniques-low-frequency vibration, step, freehand and physiological displacement, and radiation force (displacement, impulse, shear wave and acoustic emission)-are described. Tissue-mimicking materials are indispensible for the assessment of these techniques and their characteristics are reported. Emerging clinical applications in breast disease, cardiology, dermatology, gastroenterology, gynaecology, minimally invasive surgery, musculoskeletal studies, radiotherapy, tissue engineering, urology and vascular disease are critically discussed. It is concluded that ultrasonic imaging of soft tissue strain and elasticity is now sufficiently well developed to have clinical utility. The potential for further research is examined and it is anticipated that the technology will become a powerful mainstream investigative tool.

Figures

Figure 1.

Idealized stress–strain relationship for soft tissues. The inset (with stress on a magnified scale) shows that, when tissue is stressed, the strain initially increases rapidly (corresponding to the elimination of free fluid), after which the relationship is effectively linear over a small increase in stress. With further increase in stress, the tissue becomes strained decreasing rapidly (as it approaches the limit of its elasticity). The practical implication of this is that, in order to obtain reproducible and useful values of Young's modulus, the tissue needs to be slightly statically preloaded and the measurement needs to be made over a small increment in stress (i.e. in the linear region).

Figure 2.

Young's moduli of different types of breast tissues, measured with the same static preloading (5%) but at two loading frequencies (0.1 Hz, black bars; 4 Hz, white bars). Although the measured value of Young's modulus increases slightly with the loading frequency, the effect is not particularly marked. The practical implication of this is that, provided that the rate of change of the tissue displacement for the measurement is slow (i.e. quasi-static), static conditions can be assumed to apply. Data from Krouskop et al. [14]. DCIS, ductal carcinoma in situ.

Figure 3.

Young's moduli of different types of breast tissues, measured at the same (quasi-static) loading frequency (0.1 Hz) but at two levels of static preloading (5%, black bars; 20%, white bars). The measured value of Young's modulus increases markedly with the level of static preloading. The practical implication of this is that, in order to obtain reproducible measurements of Young's modulus, the level of static preloading (provided that it is sufficient for the free fluid to be eliminated) should be kept as small as practicable. Data from Krouskop et al. [14].

Figure 4.

Overview of approaches to elastography. The methods of excitation are classified broadly as being by direct mechanical action or by radiation force: the section number in the paper corresponding to each technique is shown. The method of detecting and processing the effect of the excitation is also shown for each technique, as is the characteristic (stiffness, strain or (Young's) elastic modulus), which is displayed in the corresponding image. ARFI, acoustic radiation force impulse.