How Do Ultrasound Transducers Work?
Other People Are Reading
-
Transducers
-
A transducer is a device that converts one form of energy into another. The camera used for ultrasound imaging is a transducer. It converts voltage into vibrations and vice-versa. The vibrations are mechanical sound waves, while the voltage is electropotential energy. The transducer consists of several parts that are integral to producing the wave, transmitting it into the body and receiving echoes from body structures.
Crystals
-
Crystals are the source of transducers' mechanical waves. Voltage is applied to a crystal, which causes it to vibrate, a characteristic called the piezoelectric effect. The amount of voltage controls the frequency of the vibration, which, in turn, produces the desired frequency of the sound wave. Lead zirconate titanate (PZT) is a man-made material commonly used for transducer crystals.
-
Focus
-
The crystal is shaped like a circular lens. The sound beam initially projects from the crystal at the same diameter as the crystal and gradually decreases to half the diameter. This is the focus of the beam. After the focus, the beam gradually increases in diameter. Ultrasound transducers use many crystals to produce a two-dimensional image.
Settings
-
Specific structures are examined using ultrasound, so the natural focus of a beam is not sufficient for adequate imaging. The focus must be different for structures based on their distance from the transducer. Lenses, curved elements and mirrors may be used in transducers to improve focusing and cannot be changed. Electronic focusing is controlled by the sonographer by making adjustments to the settings on the machine. Changing the focus causes the transducer to apply voltage to different crystals at different times. This timing changes the focus of the beam.
Acoustic Impedance
-
Acoustic impedance is determined by the density of a material and the velocity of the sound wave, which is determined by the material through which it travels. If two materials have different acoustic impedances, sound will reflect off the structure producing a reading on the sonogram. The difference in acoustic impedances will determine how much sound is reflected, and how much will continue to transmit through the body. The acoustic impedance of the crystal and that of air are quite different, so there will be no ultrasound transmission beyond the surface of the transducer.
Matching Layers
-
To minimize acoustic impedance between the crystal and the body, several matching layers are placed between the crystal and the surface of the transducer. Several matching layers are used, starting with one with an acoustic impedance close to that of the crystal, and ending with a layer that has an acoustic impedance close to that of the skin. This decreases reflections and allows more sound to propagate into the body.
Gel
-
Ultrasound gel is applied to the skin to remove air between the transducer and the body. This eliminates the reflection that would be caused by the difference in the acoustic impedance of air. The ultrasound gel aids in the propagation of sound waves into the body.
Producing an Image
-
Ultrasound waves reflect off tissues. These reflections are called echoes, and they travel back through the ultrasound gel, the matching layer and the crystal. From the crystal, the ultrasound waves are converted from mechanical energy to electropotential energy, or voltage. This energy is sent to the rest of the ultrasound system for conversion into a digital image.
-
