The perfect echocardiographic machine would produce an infinitesimally
small ultrasound beam, an incredible 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. A understanding
of the basic interactions of tissue with ultrasound provides the basis
of avoiding errors and misdiagnosis. Tissue interaction has also lead
to the development of new technologies, such as automatic border detection.
Tissue Interactions
Ultrasound waves, when they strike a medium, cause expansion and compression of the medium.
Ultrasound waves
interact with tissue in four basic manners. Those 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
The angle of incidence is the angle of the ultrasound beam and the tissue plane. For reflection, the angle of incidence is less than 90 degrees. Reflection occurs at tissue boundaries and tissue interfaces. Tissue boundaries that are perpendicular to the ultrasound wave's path act as excellent reflectors, whereas, tissue boundaries that are parallel to the ultrasound wave's path act as poor reflectors. When the ultrasound wave is not reflected, either due to a parallel tissue boundary or high acoustic impedance, the ultrasound signal is not recorded and the display shows a lack of signal or dropout of the ultrasound picture.
For example, in Figure 2.1 and Video 2.1, the myocardial walls that are perpendicular to the ultrasound beam are easily visualized. The myocardial walls that are perpendicular to the ultrasound beam act as excellent reflectors of the ultrasound signal. The walls parallel to the beam are not easily visualized. Walls parallel to the ultrasound signal result in poor signal reflection. In fact, the part of the parallel wall is not seen because the signal has been lost, which is commonly called dropout.
Besides angle of incidence, the tissue
boundary width impacts the amount of the reflected signal.
If the tissue boundary width is
less than the wavelength of the ultrasound wave, the ultrasound
wave will not be reflected. Tissue boundaries that are smooth and
have a width greater than the ultrasound wave act as a mirror or
a specular reflector which
results in a significant amount of reflection of the signal.
As the ultrasound wave travels thru one medium or tissue into another medium or tissue, a change in acoustic impedance occurs. The amount of change of acoustic impedance will determine the amount of reflection. Acoustic impedence is determined by the density of the tissue. Large changes in density between two tissues will result in a large changes of acoustic impedance. The change in impedance between two structures is call acoustic impedance mismatch. The difference in acoustic impedance between two tissues accounts for the amount of reflection that will occur at the tissue border. In animation 2.2, as the ultrasound wave passes thru each tissue boundary, it loses some energy or amplitude while some of the wave is reflected.
Animation 2.2 Reflection
thru multiple boundaries
Scattering
Animation 2.4 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 and is typically 100-1000 (40 - 60
dB) less 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
of different acoustic impedance. Refraction allows
enhanced image quality by using acoustic lenses.
Refraction can result in ultrasound double-image
artifacts.
Animation 2.5 Refraction
Attenuation
Attenuation
is the result of an ultrasound wave losing energy. As the
ultrasound wave travels thru a medium, the medium absorbs
some of the energy of the ultrasound wave. The amount of
energy absorption, or acoustic impedance (Z),
is determined by the product of the density
of the medium and the propagation
velocity of the ultrasound wave. The acoustic
impedance formula is shown below:
Figure 2.2 Acoustic Impedance Formula
Animation 2.6 Attenuation
During attenuation the
ultrasound wave stays on the same path and
is not deflected. As it passes thru
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.
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.
Below is a chart showing the frequency
of the probe and the depth of penetration for that frequency. Note
that as the frequency is increased penetration is decreased.
MHz
Depth (cm)
1
30
5
6
20
1.5
Table 2.1 Frequency & Depth
In conclusion, the factors that affect attenuation are:
