AN OVERVIEW OF ELASTOGRAPHY - AN EMERGING BRANCH OF MEDICAL IMAGING

Abstract

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 and, 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.

Figure 1

Propagation of shear wave induced by the radiation force impulse in homogeneous phantom (top row) and phantom with two inclusions (bottom row) recorded by MRI.

Figure 2

One of the first elastographic images published in 1988: a stress pattern recorded on the surface of compressed breast phantom (rubber prosthesis filled with silicone rubber gel) containing two lumps (nylon balls of diameters 25 mm and 6 mm). Reproduced with permission from [54].

Figure 3

Typical distribution of the axial (i.e., shear) displacement induced by sinusoidally modulated radiation force of focused ultrasound. Reproduced with permission from [38].

Figure 4

Generation and propagation of the shear wave shown at different times after transmission of the acoustic pulse. Reproduced with permission from [38].

Figure 5

Displacement at the focal point in response to ultrasound radiation force impulse for different (a) shear elasticity μ (b) shear viscosity D (Adapted from [82].)

Figure 6

Block diagram of elasticity measurement and imaging and different methods included within this imaging modality. The techniques are categorized by their excitation method, mechanical or ultrasound radiation force. Also, a classification is made between point measurement methods and imaging methods. Ultrasonic elasticity imaging methods are expanded to illustrate the broad range of approaches. A similar expansion of the MR techniques, as well as other approaches such as optical or X-ray methods, is excluded for brevity. [Acronyms: SWEI: Shear wave elasticity imaging; SSI: Supersonic shear imaging; SDUV: Shearwave dispersion ultrasound vibrometry; ARFI: Acoustic radiation force impulse imaging]

Figure 7

(a) MRE wave image from prostate phantom. Note the long wavelength on the left side of the phantom with respect to the wavelength on the right. (b) Elastogram showing lesion in red. (c) Profile along yellow line in (b). The scale is shear modulus in kPa. [© 2003 IEEE. Adapted with permission from [185]]

Figure 8

Sonogram (left side image) and elastogram (right side image) of an invasive ductal carcinoma showing a stiff lesion (dark) on the elastogram that is somewhat larger than the hypoechoic lesion seen on the sonogram (white arrows).

Figure 9

Sonogram (left image), 1% compression elastogram (middle) and 4% compression elastogram (right) of a fibroadenoma (arrows on sonogram). Note that the fibroadenoma is not really visible on the 1% compression elastogram but is visible as an area without decorrelation noise on the 4% compression elastogram. The 1% elastogram has less decorrelation noise than the 4% elastogram but a lesion may be more visible on the “noisier” elastogram.

Figure 10

On-screen quality indicators. Left image shows a numerical quality indicator (circled) at the bottom of the image. The closer the number to 100 the better. The right hand image shows a pie chart quality indicator--more pie segments in green means better quality.

Figure 11

Cyst appearance. Cyst may display as a dark area with a central brighter area (4a). Another common pattern is layered blue, green and red colors with in the lesion as shown in the color overlay image of figure 4b. (Images courtesy of Siemens and from Chiorean et al, Med Ultrasonography 2008;10(2): 73–82.

Figure 12

Drawing of the Fibroscan transducer. The piston like transducer rapidly indents the skin producing a compressional wave and shear waves. Ultrasound is emitted from the transducer to track tissue displacement caused by the shear wave to estimate shear wave speed. (Drawing from the Echosens web site)

Figure 13

Prostate B-mode sonogram (upper image) and corresponding elastogram (lower image) showing a small dark area (circled) corresponding to a malignancy. This focus is not visible on the sonogram. Images courtesy of Kaisar Alam, Riverside Research Institute.

Figure 14

Prostate sonogram (left image) showing somewhat increased echogenicity due to bubble formation during HIFU. Elastogram on right shows the ablated area as a clear dark (stiff) region. Images courtesy of Remi Souchon.

Figure 15

Strain ratio computation. Sonogram of breast lesion on left with color elastogram on right. The ratio of strain within the lesion (strain 2) and adjacent to the lesion (strain 1) is computed as 6.68

Figure 16

Normalized strain in venous thrombosis. The strain in the vein with chronic thrombosis (right hand image) is much lower than the strain in the more acute thrombosis (left hand image). The echogenicity of the thrombi are nearly identical in grayscale intensity. Images from Rubin JM, et al. J Ultrasound Med 2003;22:443–448.