Medical Physics Ultrasound

1. Medical PhysicsUltrasound Option 9.6.1 2006

2. Syllabus - Contextual Outline Contextual Outline The use of other advances in technology, developed from our understanding of the electromagnetic spectrum, and based on sound physical principles, has allowed medical technologists more sophisticated tools to analyse and interpret bodily process for diagnostic purposes. Diagnostic imaging expands the knowledge of practitioners and the practice of medicine. It usually uses non-invasive methods for identifying and monitoring diseases or injuries via the generation of images representing internal anatomical structures and organs of the body. Technologies, such as ultrasound, compute axial tomography, positron emission tomography and magnetic resonance imaging, can often provide clear diagnostic pictures without surgery. A magnetic resonance image (MRI) scan of the spine, for example, provides a view of the discs in the back, as well as the nerves and other soft tissues. The practitioner can look at the MRI films and determine whether there is a pinched nerve, a degenerative disc or a tumour. The greatest advantage of these techniques are their ability to allow the practitioner to see inside the body without the need for surgery. This module increases students’ understanding of the history of physics and the implications of physics for society and the environment.

3. Syllabus 9.6.1 The properties of ultrasound waves can be used as diagnostic tools

4. Syllabus 9.6.2 The physical properties of electromagnetic radiation can be used as diagnostic tools

5. Syllabus 9.6.3 Radioactivity can be used as a diagnostic tool

6. Syllabus 9.6.4 The magnetic field produced by nuclear particles can be used as a diagnostic tool

7. Ultrasound X-rays Medical Physics MRI Nuclear - PET Endoscopy

8. Brainstorm ULTRASOUND Individual 1 minute Group 2 minutes

9. Ultrasonography • Ultrasonography is the process of obtaining medical images using high frequency sound waves. • The person who carries out the procedure is usually a medical technologist - a sonographer.

10. Syllabus 9.6.1 The properties of ultrasound waves can be used as diagnostic tools

11. About Ultrasound The properties* of ultrasound waves make them useful medical diagnostic tools • Pass through soft tissues • Reflect from tissue boundaries • Short wavelength => resolution Sonography uses reflected sound to “look” inside the body.

12. Ultrasound Imaging - Basic Principle High-frequency sound waves are passed into the body. The waves are reflected at boundaries between different tissues and organs in the body. Using known information about the speed of sound in the tissues, and the measured time for the echo to be received, the distance from the transmitter to organ can be calculated, and used to create an image.

13. Ultrasound Imaging - Basic Principle The principle of ultrasound is similar to SONAR and RADAR. Some animals use sound waves to produce a mental image of their surroundings to navigate and to locate prey. e.g. bats, some birds, dolphins. Advantages of ultrasound Ultrasound is non-invasive. Ultrasound is non-ionising. It is therefore very safe.

14. Australian Ultrasound

15. Reason for using short wavelengths • An image of an object cannot be produced if the object is smaller than a few wavelengths of the wave being used to examine it because there is little reflection of the wave • Electron microscopes can produce images of much smaller objects than a light microscope because the wavelength of electrons is much less that that of light • Some bats use ultrasound to navigate and to locate their prey - the high frequencies allowing them to locate small insects in flight organ

16. reflected wave incident wave Reason for using short wavelengths • Objects that are larger than a few wavelengths produce strong reflection of the waves

17. What is ultrasound? • Ultrasound is any sound having a frequency greater that the upper limit of human hearing • The human hearing range covers frequencies from 20 hertz to 20 kilohertz • Ultrasound used in medical imaging typically has frequencies from 2 MHz to 10 MHz • Ultrasound travels about 1500 m s–1 in soft tissues • The sound waves produced have wavelengths of about 1 mm Ultrasound machine 1970 • identify the differences between ultrasound and sound in normal hearing range

18. Frequency and wavelength of ultrasound waves • Ultrasound travels at 340 m s-1 in air and 1585 m s-1 in muscle. Calculate the wavelength in air and in muscle tissue of ultrasound having a frequency of 2 MHz Answer In air  = v/f = 340 / 2 x 106 = 1.7 x 10-4 m  = 0.17 mm In muscle  = v/f = 1585 / 2 x 106 = 7.9 x 10-4 m  = 0.79 mm • identify the differences between ultrasound and sound in normal hearing range

19. Ultrasound propagation and properties • Velocity of sound in most soft tissues is about 1500 m/s • This is faster than the speed of sound in air (~340 ms-1) • Velocity of sound in bone is >> than in soft tissue • Velocity of sound = frequency x wavelength • Ultrasound (medical) has frequencies > 2 MHz • This is much higher than normal audible sounds (maximum 20 kHz) • Wavelength of ultrasound is therefore < 1.5 mm • The shorter the ultrasound wavelength, the better the resolution, however tissue penetration is poorer for shorter wavelengths • identify the differences between ultrasound and sound in normal hearing range

20. Contrasting audible sound waves and ultrasound waves Compared to sounds detectable by the human ear ultrasound . . . has a frequency that is higher has a wavelength that is shorter A significant difference between sound in air and ultrasound in human tissue is . . . the speed at which the waves travel. In air v ~ 340 m s-1 In human tissue v ~ 1500 m s-1 • identify the differences between ultrasound and sound in normal hearing range

21. 3-D ultrasound 1981 Frequency and wavelength of ultrasound waves • Waves can be used to produce an image of objects with a minimum diameter about equal to the wavelength of the wave. • Use of high frequencies, and hence short wavelengths produces an image with good resolution - that is, images of small objects can be produced • A 10 MHz wave can produce clear images of objects similar in size to the wavelength of the wave in tissue. If v = 1585 m s-1 this is . . . = v/f = 1585 / 10 x 106 = 1.585 x 10-4 m = 0.16 mm millimetres across • identify the differences between ultrasound and sound in normal hearing range

22. Normal iris Iris with tumour Ultrasound Images • gather secondary information to observe at least two ultrasound images of body organs

23. The Piezoelectric Effect Ultrasound is produced by a rapidly vibrating crystal transducer* (*a transducer converts energy from one form to another e.g. a loudspeaker) The piezoelectric effect The piezoelectric effect is the conversion of electrical to mechanical energy or mechanical to electrical energy by certain types of crystals. • describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal

24. + + + + – – – – The Piezoelectric Effect Ultrasound is produced by a rapidly vibrating crystal transducer*(*converts electrical energy to sound energy) When a voltage is applied across opposite faces of certain crystals, the distances between atoms in crystal lattice changes slightly deforming the crystal. crystal crystal crystal An alternating voltage causes the crystal to vibrate at the frequency of the applied voltage, producing sound in the surrounding medium. crystal • describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal

25. The Piezoelectric Effect Watch alarms, telephones and other electronic buzzers use the piezoelectric effect to make sound. [Demonstration - piezoelectric buzzer] If the frequency is greater than 20 kHz, ultrasound is produced. Quart watches use a rapidly vibrating crystal to keep time accurately. • describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal

26. The Piezoelectric Effect Piezoelectric materials can transform pressure changes into voltages - the reverse of the principle behind the production of ultrasound. This property allows the same material to be used as a detector of ultrasound, to convert pressure changes caused by the reflected wave into voltages that can be processed and analysed electronically. • describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal

27. The Piezoelectric Effect - Summary Ultrasound is produced by a piezoelectric material Producing ultrasound A piezoelectric crystal converts variations in electrical voltage to mechanical vibrations - producing ultrasound Detecting ultrasound The same transducer converts the reflected vibrations of the ultrasound into electrical signals for computer processing • describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal

28. Tutorial Questions Describe the production of ultrasound used for medical imaging. Answer Ultrasound is produced using a piezoelectric crystal transducer, which converts high frequency alternating potential differences into mechanical vibrations of the crystal at a corresponding frequency. These vibrations are used to create pressure variations that propagate through the surrounding medium. These pressure variations, if the frequency exceeds 20 kHz, are called ultrasound.

29. Tutorial Questions Describe the piezoelectric effect. Answer The piezoelectric effect occurs when a voltage is applied across opposite faces of certain crystals, causing the the crystal lattice to change size slightly. The effect is reversible, with pressure variations that deform the crystal slightly resulting in the production of a voltage across opposite faces.

30. Tutorial Question How is the piezoelectric effect used to detect ultrasound? Answer Ultrasound returning to the transducer deform the piezoelectric crystal in the transducer slightly, producing an alternating voltage across opposite faces. This is called the piezoelectric effect. The voltage variations correspond to the varying intensity of the ultrasound returning to the crystal.

31. Tutorial Question Compare the properties of medical ultrasound with sound in the normal hearing range. (10 lines - 4 marks) Answer The sounds are similar because they are both longitudinal waves requiring a medium through which to propagate. Both types of waves can be reflected from a boundary between two media having different acoustic impedances. Ultrasound has frequencies extending up from the upper limit of human hearing, which has a range from 20 Hz to 20 kHz. Medical ultrasound frequencies fall in the range 2 MHz to 10 MHz and therefore have frequencies much greater than those that humans can hear. Ultrasound has a much shorter wavelength, of the order of a millimetre, than the sounds that humans can hear. Both have the same speed in the same medium.Medical ultrasound has a velocity of approximately 1500 m s-1 in soft human tissues whereas sound in air has velocity of about 340 m s-1.

32. Acoustic Impedance • Acoustic impedance is the product of density and acoustic velocity* The logical units for acoustic impedance are kg m–3 x m s–1 or kg m–2 s–1 This unit is given the special name - a rayl Z = acoustic impedance (rayls) r = density (kg m–3) v = acoustic velocity (m s–1) *speed of sound in the medium • define acoustic impedance … and identify that different materials have different acoustic impedances

33. Acoustic Impedance Bone has a density of 2 x 103 kg m–3. The speed of sound in bone is 4080 m s–1. Calculate the acoustic impedance of bone. Answer Z = v Z = 2 x 103 kg m–3 x  4080 m s–1 = 8.16 x 10 6 rayls  • define acoustic impedance … and identify that different materials have different acoustic impedances

34. Acoustic Impedance Source: Butler Physics 2 Answers Water Z = 1.00 x 103 x 1540 = 1.54 x 106 R Blood Z = 1.05 x 103 x 1570 = 1.65 x 106 R Bone Z = 2.0 x 103 x 4080 = 8.16 x 106 R Bone has an acoustic impedance approximately 5 times that of blood and water Use the information in these tables to calculate the acoustic impedance of water and blood and compare these to bone. Conclusion . . . • define acoustic impedance … and identify that different materials have different acoustic impedances

35. Calculating Acoustic Impedance Answers Bone Z = 1.9 x 103 x 4080 = 7.8 x 106 R Soft tissue Z 1.06 x 103 x 1540 = 1.63 x 106 R Fat Z = 9.52 x 102 x 1450 = 1.38 x 106 R Blood Z = 1025 x 1570 = 1.61 x 106 R Air Z = 1.21 x 330 = 399R Compare the acoustic impedances of bone, soft tissue, fat, blood and air Blood and soft tissues have approximately the same acoustic impedance. Fat has the smallest acoustic impedance of these tissues and bone has the greatest acoustic impedance. The acoustic impedance of less than 0.1% that of the human tissues. • solve problems and analyse information to calculate the acoustic impedance of a range of materials, including bone, muscle, soft tissue, fat, blood and air and explain the types of tissues that ultrasound can be used to examine

36. Acoustic impedance of non-biological materials For interest only! Reference: File Wave Reflection http://freespace.virgin.net/mark.davidson3/reflection/reflection.html • solve problems and analyse information to calculate the acoustic impedance of a range of materials, including bone, muscle, soft tissue, fat, blood and air and explain the types of tissues that ultrasound can be used to examine

37. Acoustic Impedance and Reflection fat muscle Represented as Io It = Io – Ir Ir Consider two different tissues - such as fat and muscle. A boundary or interface exists between the two tissues. Sound travelling and meeting the interface will be partly reflected and partly transmitted. • describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound

38. Acoustic Impedance and Reflection • If two tissues have the same acoustic impedance, no reflection of ultrasound takes place at a boundary between them • The greater the difference in acoustic impedance between two tissues at a boundary, the greater the reflection • Identify the two tissues in this table, a boundary between which would produce the greatest reflection and the least reflection Answer Greatest reflection: fat/skull bone Least reflection: blood/kidney OR kidney/soft tissue • describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound

39. Acoustic Impedance and Reflection • The ultrasound machine measures the time for the incident wave to reach the boundary and return to the detector • Since the time and speed are known, the distance (d) can be calculated • 2d = v x t incident reflected d v transmitted • describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound

40. Acoustic Impedance and Reflection • Images are clearer if there is a strong reflection (a large difference in acoustic impedance at the reflecting boundary) • Ideally the ultrasound should strike tissue boundaries normal to the surface so that it reflects directly back to the transducer incident incident reflected reflected transmitted transmitted Reflected ray does not strike detector • describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound

41. Acoustic Impedance and Refraction • Ultrasound meeting a tissue boundary at an angle other than 90° are refracted on crossing the boundary • This complicates the processes of detection and analysis • Ultrasound waves reflected perpendicular to the boundary are simpler to analyse Analysis of refracted waves is more complex • describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound

42. Reflection - Quantitative Definition The ratio of the reflected intensity of ultrasound at a tissue boundary to the original intensity of the ultrasound at the boundary is equal to the ratio of the square of the acoustic impedance difference to the square of the sum of the acoustic impedances Write this definition in symbolic form if the two tissues have acoustic impedances Z1 and Z2 and the reflected intensities is Ir and the incident intensity is Io • define the ratio of reflected to initial intensity as . . .

43. Acoustic Impedance and Reflection Io Ir It = Io – Ir • identify that the greater the difference in acoustic impedance between two materials, the greater is the reflected proportion of the incident pulse

44. Acoustic Impedance and Reflection Io Ir It = Io – Ir Answer muscle-fat boundary Ir/Io = 0.01 Muscle/bone Ir/Io = 0.41 Compare the proportion of the ultrasound signal reflected at a muscle/fat boundary with the proportion reflected at a muscle/bone boundary. What is illustrated by these calculations? • identify that the greater the difference in acoustic impedance between two materials, the greater is the reflected proportion of the incident pulse

45. Calculating Acoustic Impedance Explain the types of tissues that ultrasound can be used to examine. Discuss in class and make appropriate notes! • solve problems and analyse information to calculate the acoustic impedance of a range of materials, including bone, muscle, soft tissue, fat, blood and air and explain the types of tissues that ultrasound can be used to examine

46. Acoustic Impedance and Reflection • Air between the ultrasound scanner head and the body, causes most of the sound energy to be reflected from the skin surface due to the poor impedance match. • A gel with approximately the same acoustic impedance as flesh is placed between the scanner head and the body. The gel • ensures most sound energy enters the body • makes it easier to move the ultrasound head over the body Ultrasound does not enter the body • identify that the greater the difference in acoustic impedance between two materials, the greater is the reflected proportion of the incident pulse

47. Acoustic Impedance and Reflection • Acoustic energy is reflected at interfaces between tissues with different acoustic impedances (Z) • Acoustic impedance = product of density and acoustic velocity (Z=v) • The unit of acoustic impedance is the rayl • The proportion of acoustic reflection increases as the difference in acoustic impedances increases • For soft tissue/air, soft tissue/bone and bone/air interfaces, almost total reflection occurs • identify that the greater the difference in acoustic impedance between two materials, the greater is the reflected proportion of the incident pulse

48. Problem Solving Determine the proportion of ultrasound reflected at a boundary between fat and kidney tissue. Answer Fat Z = 952 x 1450 = 1.38 x 106 R Kidney Z = 1.038 x 103 x 1560 = 1.619 x 106 R Proportion of reflected ultrasound Ir/Io = (1.619 - 1.38)2/(1.619 + 1.38)2 Ir/Io = 6.35 x 10-3 • solve problems and analyse information using [the above equations]

49. Bone Density Measurement Using Ultrasound • Why measure bone density? • Low bone density is associated with osteoporosis - risk of breaks • Two methods are currently used to measure bone density • X-rays (Called DXA or DEXA – “Dual x-ray absorption”) – Measures spine, hip or total body. • Ultrasound – measurements are taken at the heel - safer than DEXA - proportion of ultrasound transmitted through the heel is a measure of bone density • During an ultrasound exam, two soft rubber pads come in contact with either side of the heel. These pads send and receive high-frequency sound waves through the heel bone • Ultrasound is not as reliable as DEXA • identify data sources, gather, process and analyse information to describe how ultrasound is used to measure bone density

50. Reflection of Ultrasound and A-scan Use • The earliest ultrasound scans used a simple ray - effectively one-dimensional that entered the body and was reflected back. The intensity of the reflected ray was displayed on an intensity vs time graph. • This is called an A-scan. • Using the A-Scan mode, the distance to each boundary between different tissues could be calculated from the known speed of sound in the tissues. • describe the situations in which A scans, B scans, and phase and sector scans would be used and the reasons for the use of each