Ultrasound. Introduction. History (Hendee and Ritenour, 2002). In 1880, French physicists Pierre and Jacques Curie discovered the piezoelectric effect. “Piezo” is Greek for pressure. Piezoelectricity refers to the generation of an electrical response to an applied pressure .
“Piezo” is Greek for pressure. Piezoelectricity refers to the generation of
an electrical response to an applied pressure.
His specific application was the use of ultrasound to detect submarines
during Word War I. This technique, sound navigation and ranging
(SONAR), finally become practical during World War II.
“ultrasound at 50 dB was used” is nonsensical.
“the returning echo was 50 dB below the transmitted signal” is informative.
İn time t is given by
An ultrasound is a pressure wave. Power P carried by an ultrasonic wave is
Thus the instantaneous intensity can be expressed as
Maximum ultrasound intensities (mW/cm2) recommended by the US Food and Drug
Administration for various diagnostic applications.
the velocity of ultrasound wave is low.
the velocity of ultrasound wave is high.
As an ultrasound beam penetrates a medium, energy is removed from the beam by
As with x-rays, the term “attenuation” refers to any mechanism that removes energy
from the ultrasound beam.
Ultrasound is “absorbed” by the medium if part of the beam’s energy is converted
into other forms of energy, such as an increase in the random motion of molecules.
If the obstacle’s size is large compared with the wavelength of sound then part of the
beam may be “reflected” and the remainder “transmitted” through the obstacle as a
beam of lower intensity.
If the size of the obstacle is comparable to or smaler than the wavelength of the
ultraound, the obstacle will “scatter” energy in various directions.
Note, overall attenuation coefficient β, not only absorption or only (back)scattering
That is, ultrasound attenuation and backscatter measurements can be used (among many other things) to assess extent of tissue death in myocardial infarction
An alternative definition: Acoustic impedance = pressure/particle velocity
Compare electrical circuit analogue : impedance = voltage/current
Notice how similar these values are to each other and to that for water,
and how different they are from these.
Z2, u2Reflection and Refraction
Reflection Coefficient, R; fraction
of pressure transmitted =
Transmission Coefficient, T
1) Cos k2=1 l2=n2 (where n is an integer)
Transmission through such a layer is independent of the layer material.
2) Sin k2=1 l2=(2n-1)2/4
If Z2=(Z1Z3)1/2 then =1. This situation has considerable practical value in maximizing coupling between transducer materials and liquid media.
Example: a 1.5 mm thick quartz disk (c =5740 m/sec in quartz) has a
resonance frequency of v=5740/2 (0.0015)=1.91 MHz.
A (fres) = 0 dB
- 3 dB
weak resonance (broad curve)
Backing of transducer with an absorbing material(tungsten powder embedded in epoxy resin) reduces reflections from back, causes damping at resonance frequency
The characteristics of a 5MHz transducer for pulsed
IrTransducer – Tissue Mismatch
where is the Fraunhofer divergence angle in degrees. The region beyond the Fresnel zone is termed the Fraunhofer (or far) zone .
Example: What is the length of the Fresnel zone for a 10-mm diameter, 2MHz
unfocused ultrasound transducer?
= 1540 m/sec / 2x106/sec =0.77 mm.
Z0 = (5mm)2/0.77 mm = 32.5 mm
where the negative sign indicates that the frequency of the detectedultrasound is lower than that emitted by the source.
v: blood flow velocityc: speed of sound: angle between direction of blood flow and US beam
1) the transmitter transfer function relating the electrical signal to the resulting pressure wave, T1(f);
2) the attenuation exp[-w(f)lw1], and phase change exp [-jw(f)lw1], caused by the near side of the tissue,
3) the transmittence of the front surface of the tissue or the percentage of energy in the water that is coupled into the tissue, 1.
5) the transmittence of the rear surface of the tissue, 1.
6) the attenuation exp[-w(f)lw2], and phase change exp [-jw(f)lw2], caused by the near side of the tissue,
7) the receiver transfer function relating the pressure to the resulting electrical signal, T2(f);
Attenuation in water is negligible, i.e, w(f) 0
The corresponding signal can be obtained by taking the Inverse Fourier Transform:
Attenuated water path signal
(It is a hypothetical signal that would be received if it underwent
the same loss as the actual signal going through.)