TTE->Physics and Basic Principles->Interaction of Ultrasound Waves with Tissue
Objectives
Introduction
The perfect echocardiographic machine would produce an infinitesimally 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
Animation 2.1 Reflection Scattering
Animation 2.2 ScatteringScattering 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.
If a large number of boundaries that are smaller than the wavelength occurs (such as cells in the myocardium) occurs, the ultrasound beam scatters also. The scattering objects can be referred to as Rayleigh scaterers. The local scattering of the ultrasound beam causes areas that can be tracked throughout the cardiac cycle. The local scattering areas that can be tracked are called speckles.
Speckle tracking allows the recording of the speckle movement and is angle independent.
Refraction
Animation 2.3 RefractionRefraction 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
Animation 2.4 AttenuationAttenuation 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. Ultrasonic gel helps reduce the air-tissue interface. Air has a low level of accoustic impedence and results in over 99% of reflection of the ultrasound signal. Therefore, the use of ultrasonic gel in transthoracic echocardiography is required to obtain acceptable images.
