Principles of ultrasound elastography

Abstract

Tissue stiffness has long been known to be a biomarker of tissue pathology. Ultrasound elastography measures tissue mechanical properties by monitoring the response of tissue to acoustic energy. Different elastographic techniques have been applied to many different tissues and diseases. Depending on the pathology, patient-based factors, and ultrasound operator-based factors, these techniques vary in accuracy and reliability. In this review, we discuss the physical principles of ultrasound elastography, discuss differences between different ultrasound elastographic techniques, and review the advantages and disadvantages of these techniques in clinical practice.

Keywords: Elastography; Shear wave; Strain; Ultrasound.

Figure 1

Palpable breast mass from 24year old woman proven to be a benign fibroadenoma. A Conventional B mode image on the left, and a map of relative tissue stiffness in the same region of interest on the right. On the elastogram, bright areas depict tissue that is less stiff than tissue in the dark areas. Images were acquired using a L9 probe on a Siemens S2000 US system with manual strain (Courtesy of Dr. Richard Barr, MD, PhD).

Figure 2

Transient elastography acquisition on a phantom. a)Time-motion(TM) mode b)Amplitude(A) mode. TM and A modes are used to locate ideal liver part. c)Shear wave propagation image. y-axis is distance from skin, x-axis is time. Slope of the dashed line is shear wave speed(V_s)[23]. Tissue stiffness value is indicated in kPa. In the left panel, controlled attenuation parameter(CAP) value, which quantifies steatosis level is indicated in dB/m.

Figure 3

pSWE acquisition on a phantom. Green box is the focus of ARFI excitation. Shear wave speed value is indicated in left panel.

Figure 4

2D-SWE acquisition on a phantom. Blue box denotes elastographic field of view (FOV) and circle decodes region of interest. Tissue stiffness in kPa, is indicated at the bottom of the image. The color scale can be adjusted by the user. Blue areas are less stiff than red areas.

Figure 5

Liver elastography image examples; 1)2D-SWE acquisition of liver with Supersonic Aixplorer. Color coded elastogram with color scale on right top. SWS values are indicated below the scale. 2)pSWE acquisition of liver with Siemens ACUSON S3000. ARFI induced technique measures SWS in the center area. 3) Transient Elastography measurement example with FibroScan.

Figure 6

2D-SWE image of kidney. Mean stiffness is 10.4kPa for this patient, likely reflecting elevated renal stiffness due to CKD –related fibrosis [11].

Figure 7

Strain image of prostate with distribution of stress in color coded map(transrectal approach) Blue color represents hard tissue. Red color represents soft tissue. Real time display of compression is indicated at bottomside.

Figure 8

2D-SWE image of thyroid. Quantitative SWS value is indicated in right side of the image

Figure 9

Summary and classification of elastography techniques

Figure 10

Strong features of elastography techniques

Figure 11

Limitations of elastography techniques