ISUOG Physical Principles of Ultrasound including Safety
Goals To understand Physics of ultrasound How is a b-mode image is generated The effects of ultrasound on human tissue To know Settings of your machine Safety issues & artefacts
Sound Sound is a vibration Gas, liquid or solid matter Speed of sound depends on phase and temperature Air 343 m/s Water 1482 m/s Steel 5960 m/s Biological tissue 1540 m/s
wavelength Physics of sound Fibration sinusoid Frequency = number of sinusoids / second Hertz louder softer Human ear 20 20.000 Hz (500 8000) Ultrasound = > 20.000 Hz Medical US ~ 1 20 MHz lower highe r
From US to image D =½T x v
From US to image
Crystal sending 1 ~ 20 MHz 100 V A-mode
Crystal receiving 1 mv
From US to image Distance of reflector is calculated (time window send-receive) D =½T x v B-mode imaging
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From US to image Large angle / width takes time! Large number of sectors takes time!
From US to image Depth takes time!
From US to image TIME Distance of reflector is calculated (time window send-receive) M-Mode (Motion)
Resolution Lateral resolution Axial resolution Temporal resolution
Understand your machine
First step: Transducer! Different transducers -> different properties Curved array Linear array Small cavity probes Handle w ith care
Frequency and shape determine transducer (± 3D) High frequency: High resolution Less penetration Low frequency: Less resolution More penetration 10-20 cm = 3.5 mhz 5-10 cm = 5 mhz 2-5 cm = 7.5 mhz 1-4 cm = 10 mhz B O T H 2 1 W E E K S!
Multiple frequency transducers Combines several frequencies Thus best of both Penetration and resolution!
Before we start: Safety issues Biological effects of US: Increased movement of molecules results in warmth Gas bubbles can collapse -> results in shock wave
Before we start: Safety issues Thermal Index (< 6.0) Current energy / energy needed to warm 1 C Mechanical Index (cavitation) PNP / Fc ALARA principle -> as low as reasonably achievable
Before we start: safety issues R e a d s a f e t y s t a t e m e n t s I S U O G / A I U M / B M U S
Let s get started.. POWER Output energy Turn down in slim patients!
How to optimize your image DEPTH Determines waiting time
How to optimize your image Use your screen! Better resolution better image
How to optimize your image FOCUS Compare to a lens Extremely important in US guided needling Often forgotten!
How to optimize your image FRAME RATE = number images / second fast moving structures depth and width CD and post processing
How to optimize your image GAIN Compare to amplifier Artifacts more prominent as well Obese women: don t turn up the volume ;-)
How to optimize your image
How to optimize your image DYNAMIC RANGE Presets!
Images different dynamic ranges
How to optimize your image.. or scanning time CINELOOP Scroll in the images Measurements Moving elements
Technological image enhancement SPECKLE REDUCTION IMAGING (SRI) Random granular texture that obscures anatomy SRI = filter that detects and deletes patterns of speckles
Technological image enhancement TISSUE HARMONIC IMAGING 2f, 3f, 4f develop : laws of physics Probe is open to receive harmonics as well! Less penetration Decreased axial resolution
Technological image enhancement COMPOUND IMAGING Insonation angle changes Less shadowing More contrast, less speckles
Artifacts In US: A depiction which does not match the actual anatomy
Artifacts DROP-OUT / SHADOWING Hypo-echogenic area behind dense reflector The strongest in US beam angle U S E AN O T H E R I N S O N AT I O N AN G L E
Artifacts AMPLIFICATION Better image behind lucent area U S E A N O T H E R I N S O N AT I O N A N G L E
Artifacts REVERBERATION Results from back-and-forth travelling of US between reflecting surfaces Extra depiction at deeper distance D =½T x v
Artifacts REVERBERATION Second reflecting surface is the probe U S E A N O T H E R I N S O N AT I O N A N G L E D E C R E A S E P O W E R U S E T H I S A N D C O M P O U N D I M A G I N G
Artifacts SIDE LOBE ARTIFACT Results from a reflector outside the US beam Depicts reflector in the US beam
Key points 1. Realise how your machine works 2. Turn down frequency in obese women (NOT: GAIN) 3. Recognize artifacts (and know them!) 4. If in doubt: Scan from another angle
Conclusions Ultrasound is not difficult to understand If you study the technique, it will help you Scan according to safety regulations (ALARA)
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