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ECHOES
DETERMINING DEPTH
The returning echoes are read by the machine, interpreted and presented as a grey scale image on the screen. The machine calculates the depth of the reflecting structure by the time the echo takes to return to the machine. To do this the machine assumes the average speed of US in tissues (1540m/s) and that the echo was formed by US waves from the previous pulse.
13 microseconds for an US wave to travel 1cm
You need to remember that the US wave and echo do a round trip, and so the time taken for the echo to return is a combination of the two. To determine the depth of an object, the distance travelled and the time of return needs to be halved.
depth = speed of US x time for echo to return / 2
DETERMINING BRIGHTNESS
The brightness of a structure on the screen is mainly due to reflection. The degree of reflection depends on the impedence of two adjacent tissues.
All tissues have an innate impedance. Impedence (z) is a measure of how easily US is able to pass through a tissue. If a tissue is very dense and/or has a high propagation speed for US, its impedance is increased. Tissues with high impedance have a high level of reflection of the US wave creating echoes of high intensity. These high intensity echoes are interpreted as bright white (hyperechoic) by the machine. The greatest reflection occurs at tissue interfaces where there is a lot of impedance mismatch.
Air and bone have the highest levels of reflection because they have a high degree of impedence mismatch with soft tissue.
Bone has a higher density and a faster propagation speed (high impedance) than soft tissue leading to impedance mismatch. Thus at the bone soft tissue interface there is a high level of reflection. Hence bone and calcium containing structures appear bright white on the screen. At the same time bone and calcium absorbs a lot of the US wave creating loss of returning echoes posterior to bone (shadowing).
Conversely fluid filled structures do not reflect much US, so these appear anechoic (or black).
Air has a lower density and a slower propagation speed (low impedance) to soft tissue creating impedance mismatch. Unlike bone, air does not absorb much of the wave. At an air soft tissue interface, the US wave bounces back and forth between the air and the US transducer face, causing a reverberation artefact.
The highest level of reflection occurs where there is a large impedance mismatch at a tissue interface
DIRECTION OF RETURNING ECHOES
The machine assumes that all echoes returning to a crystal are a product of the reflection (echoes) of the US wave created by that crystal. However, in order for this to occur, the US wave needs to be perpendicular to the structure. When the US wave is perpendicular to the structure, the angle of incidence is 0.
On the other hand, if the US wave is oblique to the structure, the angle of incidence will be greater than zero. This means that the returning echo will never reach the original crystal and the information will be lost or misinterpreted by the machine.
Perpendicular and oblique orientation of the US wave to the reflecting structure.
For optimal imagine ensure the US beam is perpendicular to the structure you are imaging
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