Transducers. Ultrasound is produced and detected with a transducer, composed of one or more ceramic elements with electromechanical (piezoelectric) properties.
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Resonance transducers for pulse echo ultrasound imaging are manufactured to operate in a “resonance” mode, whereby a voItage (commonly 150 V) of very short duration (a voltage spike of 1 msec) is applied, causing the piezoelectric material to initially contract, and subsequently vibrate at a natural resonance frequency.
This frequency is selected by the “thickness cut,” due to the preferential emission of ultrasound waves whose wavelength is twice the thickness of the piezoelectric material.
Blood velocity measurements by Doppler instrumentation require a relatively narrow-band transducer response in order to preserve velocity information encoded by changes in the echo frequency relative to the incident frequency.
The optimal matching layer thickness is equal to ¼l =¼ x 0.4 mm = 0. 1 mm.
In addition to the matching layers, acoustic coupling gel (with acoustic impedance similar to soft tissue) is used between the transducer and the skin of the patient to eliminate air pockets that could attenuate and reflect the ultrasound beam.
lower frequency ultrasound is transmitted into the patient, and the higher frequency harmonics (e.g., two times the transmitted center frequency) created from the interaction with contrast agents and tissues, are received as echoes.
Shorter pulses, producing better axial resolution, can be achieved with greater damping of the transducer element (to reduce the pulse duration and number of cycles) or with higher frequency (to reduce wavelength).
Increasing the number of focal zones improves overall lateral resolution, but the amount of time required to produce an image increases and reduces the frame rate and/or number of scan lines per image.
The elevational or slice-thickness dimension of the ultrasound beam is perpendicular to the image plane.
Slice thickness plays a significant part in image resolution, particularly with respect to volume averaging of acoustic details in the regions dose to the transducer and in the far field beyond the focal zone.
The beam former is responsible for generating the electronic delays for individual transducer elements in an array to achieve transmit and receive focusing and, in phased arrays, beam steering.
Most modern, high-end ultrasound equipment incorporates a digital beam former and digital electronics for both transmit and receive functions.
A digital beam former controls application-specific integrated circuits (ASICs) that provide transmit/receive switches, digital-to-analog and analog-to-digital converters, and preamplification and time gain compensation circuitry for each of the transducer elements in the array.
The pulser (also known as the transmitter) provides the electrical voltage for exciting the piezoelectric transducer elcnwnts, and controls the output transmit power by adjustment of the applied voltage.
In digital beam-former systems, a digital-to analog-converter determines the amplitude of the voltage. An increase in transmit amplitude creates higher intensity sound and improves echo detection from weaker reflectors.
A direct consequence is higher signal-to-noise ratio in the images, but also higher power deposition to the patient. User controls of the output power are labeled “output,” “power,” “dB,” or “transmit” by the manufacturer. In some systems, a low power setting for obstetric imaging is available to reduce power deposition to the fetus. A method for indicating output power in terms of a thermal index (TI) and mechanical index (MI) is usually provided (see section 16.1 1).