Diagnostic ultrasound ( PDFDrive )

CHAPTER 1 Physics of Ultrasound 17
x
x’
A
y
y’
x = y
x’ < y’
precompression compression shear waves
B
FIG. 1.22 Elastography. (A) Strain elastography (SE), and (B) shear wave elastography (SWE). Strain elastograms are images of tissue
stiffness generated by analysis of speckle displacements before and after mechanical compression of tissue. The precompression frame is compared
to a frame obtained after compression. In this example, the lesion is compressed much less than the surrounding tissue, indicating relative stiffness.
SE is not quantitative and indicates only the relative hardness or softness of lesions compared to their surroundings. In SWE (B) high-intensity
compression pulses from the transducer are focused on an area of interest, resulting in the generation of low-frequency shear waves. Speckle
displacement resulting from shear (transverse) waves is tracked with multiple imaging frames in order to estimate shear wave velocity. Shear wave
velocity is directly related to Young modulus, permitting a quantitative estimate of tissue stiffness.
A
B
FIG. 1.23 Strain Elastograms. The upper frames (A) show in vivo images of swine liver containing a lesion produced by the injection of a
small volume of absolute ethanol. In the precompression image (left) the lesion located within the circle is invisible. The elastogram (right) clearly
delineates the lesion as an area of increased stiffness compared to the surrounding tissue. The lower frames (B) show a gray-scale image (left)
and strain elastogram (right) of a mixed solid and cystic thyroid nodule. In the elastogram the color map displays relative stiffness with softer areas
appearing as shades of red, orange, and yellow, and stiffer areas as dark blue. The nodule is heterogeneous with the relatively noncompressible
cystic portions differentiated from more compressible surrounding tissue. (Courtesy of P. O’Kane, Thomas Jefferson University.)