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Basic Physics of Ultrasound

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1 Basic Physics of Ultrasound

2 WHAT IS ULTRASOUND? Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound waves and their echoes. Known as a ‘pulse echo technique’ The technique is similar to the echolocation used by bats, whales and dolphins, as well as SONAR used by submarines etc.

3 In ultrasound, the following events happen:
The ultrasound machine transmits high-frequency (1 to 12 megahertz) sound pulses into the body using a probe. The sound waves travel into the body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone). 3. Some of the sound waves reflect back to the probe, while some travel on further until they reach another boundary and then reflect back to the probe . 4. The reflected waves are detected by the probe and relayed to the machine.

4 The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (1540 m/s) and the time of the each echo's return (usually on the order of millionths of a second). 6. The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image.

5 So…. All the energy comes from the transducer
All we “see” are reflections and scatter.

6 SOUND Sound waves consist of mechanical vibrations containing condensations (compressions) & rarefactions (decompressions)that are transmitted through a medium. Sound is mechanical. Sound is not electromagnetic. Matter must be present for sound to travel

7 Compression wave

8 CATEGORIES OF SOUND Infrasound (subsonic) below 20Hz
Audible sound 20-20,000Hz Ultrasound above 20,000Hz Nondiagnostic medical applications <1MHz Medical diagnostic ultrasound >1MHz

9 THE TRANSDUCER Piezo-electric crystal
Converts electric signals to mechanical & vice versa Transmits pulses of sound into tissue and listens for echos Most of the time is spent listening for echoes Piezo electric effect is a property of some crytalline materials eg quartz or PZT (lead zirconate titanate) When compressed or expanded these materials show a voltage between the compressed or expanded surfaces (due to the molecular alignment in the crystal lattice) Changing polarity is a means of electromechanical conversion

10 Transducer Power on Power off Transducer receiving echoes 10-6sec 10-3sec

11 Transducer components
Beam profile Profile of ultrasound pulse

Echoes occur when pulses of U/S hit reflectors A stream of echoes from each pulse return to transducer Deeper echoes from deeper tissues arrive later Stronger echoes arrive from stronger reflectors Each transducer has many elements each making pulses ( )

13 Perfect Reflection Transducer Object
Distance (d) is proportional to time (t) If you know the velocity (c) then the distance is d=1/2 (cxt) C=fxl, so we can work out frequency and wavelength too…

The image is a 2D map of reflections displayed as a grey scale B mode = brightness modulation “Real time” is lots of B mode images run together


16 SOUND WAVES Gathered into a narrow beam Reflected Refracted Scattered
Absorbed Undergo interference

17 Components Of B mode – Real time machine TIME IS OF THE ESSENCE

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