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MECHANICS OF THE US WAVE
GENERATING US WAVES
US is a mechanical wave which passes through the tissues. In an US machine, the US wave is created at the transducer face. Most US machines contain piezoelectric crystals just beneath the face of the US transducer. These crystals have an intrinsic ability to generate US waves when activated by electrical current. The thickness of the crystal is the main determinant of the frequency of the US wave created. Thinner crystals create high frequency waves.
US waves are mechanical waves created by the electrical activation of piezoelectric crystals.
READING US WAVES
US waves pass through the tissues and depending on the characteristics of the tissues, some of the US wave is reflected back up to the transducer. These returning waves are echoes of the original wave and have the same characteristics of the original sound wave. It is important to realise that one US wave will create multiple echoes which return to the transducer at different times and amplitude depending on the depth and characteristics of the structure causing the reflection.
Piezoelectric crystals are able to create an electrical current when deformed by a pressure wave. Thus when the echo reaches the crystal at the transducer face, the crystal is deformed and creates an electric current which is read by the machine in shades of grey: creating the US image which we see on the screen.
Returning US waves are echoes
PULSE-ECHO TECHNIQUE
US imaging relies on pulses of US sent into the tissues by the transducer crystals followed by a pause during which the machine listens to the returning echoes. After a designated period of time, more US pulses are sent into the tissues. Thus a dynamic real-time image is created on the screen and the image is constantly updated with information returning from the next pulse.
The image on the screen is constantly refreshed by the information from returning echoes.
IMAGE ON THE SCREEN
The echoes returning from each pulse creates one vertical line of sight on the screen. Each returning echo creates one dot in that line of sight. Thus the arrangements of the crystals on the transducer face determine the shape of the image on the screen.
Linear transducer (vascular probe)
A linear transducer has crystals arranged parallel to each other. Thus each returning echo creates a straight vertical line. The final image on the screen is a rectangle.
Array transducer (echo probe)
In these transducers, the US waves originate from one position but with each pulse, the wave goes out at a slight angle to the previous one so that the US waves go out in lines which fan out from the transducer face. Thus the image on the screen has a wedge shape.
Curvilinear transducer (abdominal probe)
This one is a combination of the linear and the array transducers. The crystals are aligned parallel to each other but at the same time, with each the pulse, the US waves are sent out at slight angles to the previous. This creates a sector image on the screen.
US waves passing through tissues with activation of the crystals over time, showing the arrangement of the crystals for each type of transducer.
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