Basic Physics of Ultrasound and Knobology

Transcription

1 WELCOME TO UTMB

2 Basic Physics of Ultrasound and Knobology By Daneshvari Solanki, FRCA Laura B. McDaniel Distinguished Professor Anesthesiology and Pain Medicine University of Texas Medical Branch Galveston, Texas

3 Physics Sound waves with frequencies greater than 20 khz are called ultrasound Ultrasound waves used in medical imaging have frequencies between 2 million to 20 million Hz (2-20MHz) These sound waves are mechanical waves that are created by back and forth displacement of a transducer

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5 Generation of Ultrasound Voltage is applied to a piezo electric crystal and it expands. When the voltage is removed it contracts. When this is done rapid succession it creates ultrasound This process of transmission and reception occurs 7000 times per second

6 When we use ultraound to locate something the sound waves bounce off the objects. There has to be a way to listen to those sounds Piezo electric crystal that sends out the ultrasound also has the capability of listening to those reflected sounds

7 Physics Ultrasound transducers send out sound waves and then listen for returning echoes Most transducers at this time send out waves only approximately 1% of the time

8 Ultrasound Probe Single piezo electric crystal transmits and receives ultrasounds Ultrasound probe has large number of such piezo electric crystals

9 Transmission of ultrasounds Air is the enemy of ultrasound. It reflects the sound waves away without penetration of the skin so you cannot get an image

10 Transmission of ultrasounds Ultrasound travels well in liquids. So we use thick liquid or jelly between the probe and the skin It keeps air bubbles out and transmits ultrasound through the skin

11 Physics Acoustic impedance is the measure of the opposition that a system presents to an acoustic flow when an acoustic pressure is applied to it. Acoustic impedance is the tendency to resist the passage of ultrasound. Acoustic impedance determines the amount of sound waves transmitted and reflected by the tissues Reflection occurs when the ultrasound beam hits two tissues (areas) having different acoustic impedance Large differences in impedances inhibit useful information

12 Acoustic Impedance Every tissue has a unique property called acoustic impedance It depends on the density of the tissues This then determines the speed of ultrasound in that tissue Acoustic impedance alters the course of the ultrasound waves

13 Acoustic Impedance Material Z (Impedance) Air 400 Fat 1,380,000 Water 1,430,000 Tissue Bone 1,700,00o 7,800,000 Acoustic impedance is the complex representation of acoustic resistance. Acoustic impedance (Z) = density of material (ρ) x speed of sound in the material (v) Z = pv Acoustic impedance determines the amount of sound waves transmitted and reflected by tissues

14 Acoustic Impedance values at the interface Interface Percent Reflection Fat /Muscle 1% Fat / Bone 50% Tissue/Air 100% Reflection occurs when the ultrasound beam hits two tissues (areas) having different acoustic impedances The greater the difference between the acoustic impedances of the two materials at a boundary in the body the greater the amount of reflection

15 What happens to ultrasound waves when they hit the tissues? They are 1. Attenuated 2. Refracted 3. Reflected

16 Attenuation Body absorbs ultrasound energy so some of the waves will not reflect back This is attenuation More the body tissues ultrasound has to cross more it will be attenuated

17 Refraction Ultrasound waves travel to the tissues with different acoustic impedance Some of the waves are REFRACTED (bent) Some of the waves are REFLECTED back to the probe Amount of reflection is difference in acoustic impedance So more the difference in Impedance more is the reflection These reflected sound waves produce the ultrasound image

18 Reflection Ultrasound travels through different tissues and some waves are refracted and others are reflected form each tissue This produces images of different tissues

19 Scattered Reflection Specular Reflection Irregular surfaces like nerves scatter ultrasound in all directions They only reflect a portion of the ultrasound A smooth object like a needle reflects all the ultrasound This is a mirror like reflection

20 Echogenicity It is the ability of the tissues to reflect or transmit ultrasound waves. A visible contrast is seen when the ultrasound travels through tissues with different echogenicity This creates images that are 1. Hyperechoic White image -Reflects most of the waves 2. Hypoechoic Gray image -Allows some reflects others 3. Anechoic Black image -Allows most waves through

21 Hypoech0ic and Hyperechoic images Hypoechoic Gray Hyperechoic Hyperechoic White Hypoechoic

22 Interscalene brachial plexus

23 Ultrasound of the lumbar spine

24 Anechoic Images Anechoic Anechoic Black

25 Transducers These are ultrasound probes

26 These transducers have different frequencies foot prints Ultrasound probes for regional anesthesia There are two transducers we use for regional anesthesia 1. Linear array 2. Curvilinear array

27 Foot print The footprint refers to the portion of the transducer that comes in contact with the patient This can be linear or curvilinear Curvilinear transducers come with different footprints for use for different purposes

28 Linear transducer gives a square image Linear Curvilinear Curvilinear transducer gives a curved

29 Frequency Ultrasound waves are created by vibration of piezo electric crystals. This creates high pressure and low pressure area that travel forwards Frequency is the number of high pressure and low pressure cycles per second

30 Wavelength Wavelength is the distance between identical points in adjacent cycls of a waveform Wavelength is the distance between identical points in adjacent cycles of a waveform

31 Frequency and Wavelength Wavelength is inversely proportional to frequency Higher frequency has shorter wavelength Lower frequency had longer wavelength This phenomenon affects RESOLUTION

32 Resolution What is resolution? It is the ability to see two things clearly as two things Good resolution allows us to do that Poor resolution makes the picture of the two things blurred and makes it look like one

33 Higher frequency and shorter wavelength produce good resolution Lower frequency and longer wavelength produce poor resolution

34 Depth Ultrasound has to penetrate sufficient depth of the tissues to produce an image Depth of penetration is dependent upon the frequency Higher frequency has shorter depth of penetration Lower frequency has higher depth of penetration

35 Transducers

36 Transducers Linear Transducer High frequency Gives a square image High resolution Good for viewing small areas Good for shallow depth Curvilinear Transducer Low frequency Gives a curved image Lower resolution Good for viewing large area Good for deeper structures

37 Summary

38 Controls you need to know Power button Battery Track ball or mouse pad Zoom Freeze

39 Ultrasound Machines

40 Ultrasound Machines

41 Track ball is like using your mouse on PC. It is used in conjunction with measuring, annotating, and moving Doppler box to the desired location Zoom allows magnification of the ultrasound picture Freeze allows you to freeze the image

42 Terminology you must know Frequency Gain Time gain compensation Depth Focus

43 Gain This function is very similar to a brightness control Echo signal is converted to electronic signal This signal needs to be amplified to produce an image Signal amplification is called gain Gain adjusts acoustic power of the signal

44 Time gain compensation T.G.C. TGC compensates for the effects of attenuation by progressively increasing amplification applied to signals with depth. Buttons for this are sliders and they are usually located on the side Each slider adjusts amplification of 2D mode signal at a specific image depth Sliders for TGC

45 Depth Changing the depth varies the field of view (F.O.V.) Excessively large field of view reduces spatial resolution

46 Focus The point at which the beam is at its narrowest is the focal point or focal zone It is the point of greatest intensity and best lateral resolution.

47 Focus The point at which the beam is at its narrowest It is the point of greatest intensity and best lateral resolution. Focal zone is off Focal zone is right

48 In Plane and Out of Plane Needle Placement

49 Needle visualization Out of plane In Plane

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51 References 1. asics/ 2. Ultrasound Podcast - Physics Basics YouTube 3.

52 QUESTIONS?