Introduction to Acoustics

852 Part F Biological and Medical Acoustics
Part F 21.3
Table 21.3 Ultrasound wavelength versus tissue thickness. To resolve the thickness of these structures, the ultrasound
frequency must be much higher than listed in this table. Much lower-frequency ultrasound passes through with little
interaction. For particles or cells, lower-frequency ultrasound is subject to Rayleigh scattering
Tissue Property Dimension (mm) Ultrasound
Speed (mm/µs) Frequency (MHz)
Trachea Lumen diameter 25 0.32 0.013
Aorta Lumen diameter 30 1.57 0.063
Fat layer Subcutaneous 20 1.45 0.080
Myocardium Thickness 10 1.6 0.16
Skull Thickness 10 4 0.4
Brain Cortex 3 1.5 0.5
Artery Wall 0.8 1.6 2
Trachea Wall 0.6 1.75 2.9
Bone Cortex 1 4 4
Eye Cornea 0.6 1.64 2.73
Skin Dermis 0.3 1.73 5.76
Fat Cell 0.1 1.45 14.5
Brain Cell, large 0.1 1.5 15
Skin Epidermis 0.05 1.73 35
Erythrocyte Cell 0.007 1.57 225
Bacteria Cell, large 0.005 1.5 300
Brain Cell, small 0.004 1.5 375
Notes:
1. Trachea lumen: for phonation below 13 kHz, the trachea diameter is smaller than the wavelength so that the trachea acts like
a waveguide, bounded by cartilage with acoustic velocity and impedance higher than the luminal air.
2. Bone cortex and thin skull have thicknesses near the wavelength of transcranial ultrasound and a much higher ultrasound
impedance, so the skull can act as an interference filter reflecting some wavelengths and passing others.
3. As the upper limit of most medical ultrasound systems is 15 MHz, small animal research instruments image at frequencies as high
as 70 MHz, high enough to resolve the epidermis and large brain cells, but not high enough to image most cells.
number varies depending on the numeric aperture of
the imaging system and on the acoustic contrast with
the surrounding material. Small round structures such
as tendons can be imaged using tricks, such as watching
for motion, but most objects successfully imaged
by ultrasound are larger. It is important to remember
that it is possible to see an object that you cannot
resolve. For instance, you are able see stars or
lights at night, even though you cannot resolve closely
spaced pairs of stars. When the signal from a target
is strong, the target can be detected and located, even
though a pair of targets cannot be resolved as two. If
Doppler demodulation is used, rather than amplitude
demodulation (used for B-mode), small vessels with
Fig. 21.10 Near-field versus far-field difference in ultrasound
image. This is an image taken with a diagnostic
scanner using a 5 MHz ultrasound transducer (wavelength
=0.3 mm) with a concavity radius of 16 mm and a transducer
diameter of 6.4 mm. The computed transition zone
is d 2 /4λ = 34 mm. The focus must be less than the 34 mm
distance to the transition zone to be effective. With a fixedfocus
fixed-aperture ultrasound transducer, the transition
from the near field to the far field is marked by lateral
spreading of the speckle in the far field. There is no effect
on the range (depth) speckle dimension