Resident Physics Lectures 01: Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology

Ultrasound Transducer 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 Microphone receives echoes Speaker transmits sound pulses

Piezoelectric Principle Voltage generated when certain materials are deformed by pressure Reverse also true! Some materials change dimensions when voltage applied dimensional change causes pressure change when voltage polarity reversed, so is dimensional change V

US Transducer Operation alternating voltage (AC) applied to piezoelectric element Causes alternating dimensional changes alternating pressure changes pressure propagates as sound wave

I’m a scanner, Jim, not a magician. Ultrasound Basics What does your scanner know about the sound echoes it hears? I’m a scanner, Jim, not a magician. Sound Acme Ultra- Sound Co. Echo

What does your scanner know about echoed sound? How loud is the echo? inferred from intensity of electrical pulse from transducer

What does your scanner know about echoed sound? What was the time delay between sound broadcast and the echo?

What else does your scanner know about sound echoes? Direction sound was emitted

What else does your scanner know about echoed sound? The sound’s pitch or frequency

What Does Your Scanner Assume about Echoes (or how the scanner can lie to you) Sound travels at 1540 m/s everywhere in body average speed of sound in soft tissue Sound travels in straight lines in direction transmitted Sound attenuated equally by everything in body (0.5 dB/cm/MHz, soft tissue average)

Luckily These Are Close Enough to Truth To Give Us Images Sound travels at 1540 m/s everywhere in body average speed of sound in soft tissue Sound travels in straight lines in direction transmitted Sound attenuated equally by everything in body (0.5 dB/cm/MHz, soft tissue average)

Ultrasound Display B-scan (“Brightness” Mode) Image series of gray shade dots For each dot, scanner must calculate position Gray shade

Images from Echos Let's see how this works!

? Dot Placement on Image Dot position ideally indicates source of echo scanner has no way of knowing exact location Infers location from echo ?

? Dot Placement on Image Scanner aims sound when transmitting echo assumed to originate from direction of scanner’s sound transmission ain’t necessarily so ?

? Positioning Dot Dot positioned along assumed line Position on assumed line calculated based upon speed of sound time delay between sound transmission & echo ?

Distance of Echo from Transducer Time delay accurately measured by scanner distance = time delay X speed of sound distance

What is the Speed of Sound? distance = time delay X speed of sound What is the Speed of Sound? scanner assumes speed of sound is that of soft tissue 1.54 mm/msec 1540 m/sec 13 usec required for echo object 1 cm from transducer (2 cm round trip) 13 msec 1 cm Handy rule of thumb

So the scanner assumes the wrong speed? Sometimes Luckily, the speed of sound is almost the same for most body parts soft tissue ==> 1.54 mm / msec fat ==> 1.44 mm / msec brain ==> 1.51 mm / msec liver, kidney ==> 1.56 mm / msec muscle ==> 1.57 mm / msec ?

? ? Gray Shade of Echo Ultrasound is gray shade modality Gray shade should indicate echogeneity of object ? ?

How does scanner know what gray shade to assign an echo? Based upon intensity (volume, loudness) of echo ? ?

Gray Shade Loud echo = bright dot Soft echo = dim dot Loud Soft

Complication Deep echoes are softer (lower volume) than surface echoes. Loud Soft ?

Gray Shade of Echo Correction needed to compensate for sound attenuation with distance Otherwise dots close to transducer would be brighter

I'd like to see an example! Depth Correction                          I'd like to see an example!

Echo’s Gray Shade Gray Shade determined by Measured echo strength accurate Calculated attenuation Who am I? Charles Lane

Attenuation Correction scanner assumes entire body has attenuation of soft tissue actual attenuation varies widely in body Fat 0.6 Brain 0.6 Liver 0.5 Kidney 0.9 Muscle 1.0 Heart 1.1 Tissue Attenuation Coefficient (dB / cm / MHz)

Ultrasound Display One sound pulse produces Multiple pulses one image scan line one series of gray shade dots in a line Multiple pulses two dimensional image obtained by moving direction in which sound transmitted

Moving the Sound Beam electronically phased or pulsed transducer arrays Focus Arrows indicate timing variations. Activating top & bottom elements earlier than center ones focuses beam.

Scan Patterns Linear sector beam translated produces rectangular image moved sideways produces rectangular image sector beam pivoted produces pie-shaped image

Th’ Th’ Th’ Th’ That’s All Folks