IMAGE DATA ACQUISITION l Understanding ultrasonic image formation

IMAGE DATA ACQUISITION l 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.

l 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.

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

Beam Formers l 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.

l A digital beam former controls applicationspecific integrated circuits (ASICs) that provide transmit/receive switches, digital-toanalog and analog-to-digital converters, and preamplification and time gain compensation circuitry for each of the transducer elements in the array.

l 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 l 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 analogconverter determines the amplitude of the voltage. An increase in transmit amplitude creates higher intensity sound and improves echo detection from weaker reflectors.

l A direct consequence is higher signal-tonoise ratio in the images, but also higher power deposition to the patient. • User controls of the output power are labeled “output, ” “power, ” “d. B, ” or “transmit” by the manufacturer. In some systems, a low power setting for obstetric imaging is available to reduce power deposition to the fetus.

l A method for indicating output power in terms of a thermal index (TI) and mechanical index (MI) is usually provided.

Transmit/Receive Switch l 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 m. 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 msec (1 msec).

Pulse Echo Operation l 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.

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