BASIC ULTRASONIC PRINCIPLES

Sound?

Sound is a mechanical, longitudinal wave that travels in a straight line

Sound requires a medium through which to travel

Acoustic spectrum

Basic Ultrasound Physics

Amplitude

oscillations/sec = frequency - expressed in Hertz (Hz)

What is Ultrasound?

Ultrasound is a mechanical, longitudinal wave with a frequency exceeding the upper limit of human hearing, which is 20,000 Hz or 20 kHz.

ULTRASOUND – How is it produced?

Produced by passing an electrical current through a piezoelectrical crystal

Transducer Construction

Human Hair

Single Crystal

Microscopic view of scanhead

Piezoelectric material

• AC applied to a piezoelectric crystal causes it to expand and contract – generating ultrasound, and vice versa

• Naturally occurring - quartz

• Synthetic - Lead zirconate titanate (PZT)

Ultrasound Production

• Transducer contains piezoelectric elements/crystals which produce the ultrasound pulses (transmit 1% of the time)

• These elements convert electrical energy into a mechanical ultrasound wave

The Returning Echo

• Reflected echoes return to the scanhead where the piezoelectric elements convert the ultrasound wave back into an electrical signal

• The electrical signal is then processed by the ultrasound system

Piezoelectric Crystals

• The thickness of the crystal determines the frequency of the scanhead

Low Frequency3 MHz

High Frequency10 MHz

Frequency vs. Resolution

• The frequency also affects the QUALITY of the ultrasound image– The HIGHER the frequency, the

BETTER the resolution– The LOWER the frequency, the LESS

the resolution

Frequency vs. Resolution

• A 12 MHz transducer has very good resolution, but cannot penetrate very deep into the body

• A 3 MHz transducer can penetrate deep into the body, but the resolution is not as good as the 12 MHz

BASIC ULTRASONIC PRINCIPLES

• Sound generated above the human hearing range (typically 20 kHz) is called ultrasound.

• However, the frequency range normally employed in ultrasonic nondestructive testingand thickness gaging is 100 kHz to 50 MHz.

• This means it can be reflected off very small surfaces such as defects inside

Ultrasound Transducer

Speakertransmits sound pulses

Microphonereceives echoes

• Acts as both speaker & microphone– Emits very short sound pulse– Listens a very long time for returning echoes

• Can only do one at a time

BASIC CMUT STRUCTURE:

Basic CMUT structure.

Operating Principles of CMUTsWhen a DC voltage is applied between the metallized membrane and the substrate of a CMUT, the membrane is attracted toward the bulk by the electrostatic force, and induced stress within the membrane balances the attraction. Driving the membrane with an AC voltage superposed on the bias generates ultrasound. If the biased membrane is subjected to ultrasound, a current is produced due to the capacitance change under constant bias voltage. The amplitude of the current is a function of frequency, bias voltage, and device capacitance. The efficiency of CMUTs is determined by the

electromechanical transformer ratio, which can be expressed as the product of the device capacitance and the electric field strength across the gap. Planar fabrication enables building a thin membrane above a sub-micron sealed cavity, which is crucial to obtain high electric fields for improved transducer performance.

  Operating Principles of CMUTs

• When a DC voltage is applied between the metallized membrane and the substrate of a CMUT, the membrane is attracted toward the bulk by the electrostatic force, and induced stress within the membrane balances the attraction. Driving the membrane with an AC voltage superposed on the bias generates ultrasound. If the biased membrane is subjected to ultrasound, a current is produced due to the capacitance change under constant bias voltage.