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 SOUNDSound 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
8 CATEGORIES OF SOUND Infrasound (subsonic) below 20Hz Audible sound 20-20,000HzUltrasound above 20,000HzNondiagnostic medical applications <1MHzMedical diagnostic ultrasound >1MHz
9 THE TRANSDUCER Piezo-electric crystal Converts electric signals to mechanical & vice versaTransmits pulses of sound into tissue and listens for echosMost of the time is spent listening for echoesPiezo 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 onPower offTransducer receivingechoes10-6sec10-3sec
11 Transducer components Beam profileProfile of ultrasound pulse
12 ULTRASOUND PULSES MAKING THE IMAGE Echoes occur when pulses of U/S hit reflectorsA stream of echoes from each pulse return to transducerDeeper echoes from deeper tissues arrive laterStronger echoes arrive from stronger reflectorsEach 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 isd=1/2 (cxt)C=fxl, so we can work out frequency and wavelength too…
14 ULTRASOUND PULSES MAKING THE IMAGE The image is a 2D map of reflections displayed as a grey scaleB mode = brightness modulation“Real time” is lots of B mode images run together
15 SOUND WAVESWAVELENGTH IS VERY SMALLOBEY THE LAWS OF OPTICS
16 SOUND WAVES Gathered into a narrow beam Reflected Refracted Scattered AbsorbedUndergo interference
17 ComponentsOf B mode –Real time machineTIMEIS OF THE ESSENCE