IMAGE DATA ACQUISITION

Understanding ultrasonic image formation requires knowledge of ultrasound production, propagation, and interactions. • Images are created using a pulse echo

method of ultrasound production and detection.

• Each pulse transmits directionally into the patient, and then experiences partial reflections from tissue interfaces that create echoes, which return to the transducer.

Image formation using the pulse echo approach requires a number of hardware components: • the beam former,

• pulser,

• receiver,

• amplifier,

• scan converter/image memory, and

• display system.

Ultrasound equipment is rapidly evolving toward digital electronics and processing, and current state-of-the-art systems use various combin-ations of analog and digital electronics.

Beam Formers

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.

Major advantages of digital acquisition and processing include the flexibility to introduce new ultrasound capabilities by programmable software algorithms and to enhance control of the acoustic beam.

Pulser

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.

Transmit/Receive Switch

The transmit/receive switch, synchronized with the pulser, isolates the high voltage used for pulsing (~150 V) from the sensitive amplification stages during receive mode, with induced voltages ranging from ~1 V to ~2 V from the returning echoes. • After the ring-down time, when vibration of the

piezoelectric material has stopped, the transducer electronics are switched to sensing small voltages caused by the returning echoes, over a period up to about 1000 sec (1 msec).

Pulse Echo Operation

In the pulse echo mode of transducer operation, the ultrasound beam is intermittently transmitted, with a majority of the time occupied by listening for echoes. • The ultrasound pulse is created with a short

voltage waveform provided by the pulser of the ultrasound system.

This event is sometimes called the main bang. The generated pulse is typically two to three cycles long, dependent on the damping charac