Diagnostic ultrasound ( PDFDrive )

18 PART I Physics
A - Normal liver
v = 1.29 ± -.10 m/s
B - Cirrhotic liver
v = 4.41 ± -.17 m/s
FIG. 1.24 Shear Wave Elastograms of (A) Normal and (B) Cirrhotic Liver. Shear wave velocities measured in liver tissue samples by shear
wave elastography indicates a velocity of 1.29 ± 0.10 m/sec in the normal liver compared to a velocity of 4.41 ± 0.17 m/sec in the cirrhotic liver.
Increased shear wave velocity is associated with increased tissue stiffness due to hepatic ibrosis. (Courtesy of P. O’Kane, Thomas Jefferson
University.)
FIG. 1.25 Axial Resolution. Axial resolution is the
resolution along (A) the beam axis and is determined
by (B) the pulse length. The pulse length is the product
of the wavelength (which decreases with increasing
frequency) and the number of waves (usually two to
three). Because the pulse length determines axial resolution,
higher transducer frequencies provide higher image
resolution. In (B) for example, a transducer operating
at 5 MHz produces sound with a wavelength of 0.31 mm.
If each pulse consists of three cycles of sound, the
pulse length is slightly less than 1 mm, and objects A
and B, which are 0.5 mm apart, cannot be resolved as
separate structures. If the transducer frequency is
increased to 15 MHz, the pulse length is less than
0.3 mm, permitting A and B to be identiied as separate
structures.
A
B