Interaction of Ultrasound Waves with Tissue

Introduction  

The perfect echocardiographic machine would produce an infinitely small ultrasound beam, an incredible high sweep speed, and a uniform energy throughout its beam length. Even with the perfect echocardiographic machine, we are still left with the ultrasound interaction with tissues. The interaction can cause measurement errors, artifacts, and poor picture quality. An understanding of the basic interactions of tissue with ultrasound provides the basis of avoiding errors and misdiagnosis.  

Tissue Interactions  

When ultrasound waves strike a medium, they cause expansion and compression of the medium. Ultrasound waves have four basic interactions with tissues. These interactions are: Reflection, Scattering, Refraction, Attenuation  

Reflection

Reflection occurs when the ultrasound wave is deflected towards the transducer.

The major factors affecting the amount of reflection are:

• Angle of incidence

• Acoustic impedance mismatch

• Width of the tissue boundary

• Angle of tissue boundary

Scattering

Scattering occurs when the width or lateral dimension of the tissue boundary is less than one wavelength.

If a large number of small tissue boundaries occurs, the scattering can radiate in all directions. The signal that reaches the transducer is a much weaker signal than the transmitted signal. Most scattering occurs with red blood cells, which have a width of 7-10 µm which is 20 times smaller than the ultrasound wavelength (0.2 to 1 mm). A filter can ignore small signals from red blood cells below a threshold value. Hematocrit has very little effect on the Doppler signal.

Refraction

Refraction occurs when the ultrasound signal is deflected from a straight path and the angle of deflection is away from the transducer. Ultrasound waves are only refracted at a different medium interface. Refraction can result in ultrasound double-image artifacts.  

Attenuation  

Attenuation is the result of an ultrasound wave losing energy. As the ultrasound wave travels through a medium, the medium absorbs some of the ultrasound wave energy.

During attenuation the ultrasound wave stays on the same path and is not deflected.  As it passes through tissues of different densities, the amplitude decreases.  If all of the ultrasound wave energy is absorbed then structures distal to the point of total attenuation will not be visualized and will appear to be "dropped".  This is called dropout.   

Conclusions   

In conclusion, ultrasound energy is lost by reflection, scattering, and attenuation. The loss in energy results in a "noisy" background. If the signal-to-noise ratio is good then a clear picture will be displayed. A poor signal-to-noise ratio results in a blurry picture. Attenuation is frequency dependent. Low frequencies have better penetration and are therefore not attenuated as much as higher frequencies.  

Transducer  

The mechanism that allows transmission and reception of ultrasonic waves is the piezoelectric crystal. The piezoelectric crystal can transmit ultrasound waves by being stimulated by electrical pulses. When the electrical pulse stops, the crystal continues to emit a signal for a short period while it stops vibrating. The period from the electrical pulse ending to the end of the vibration of the crystal is call the "ring down" period. Once the pulsed signal is reflected back to the piezoelectric crystal, the crystal then generates an electrical signal which can be processed and displayed. The piezoelectric crystal is housed in a transducer which also contains a damping material, an acoustic lens and an electrical cable.