Medical Physics Ultrasound

Medical Physics Ultrasound
Option 9.6.1 2006

Syllabus - 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.

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

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

Syllabus 9.6.3 Radioactivity can be used as a diagnostic tool

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

Ultrasound X-rays Medical Physics MRI Nuclear - PET Endoscopy

Brainstorm ULTRASOUND Individual 1 minute Group 2 minutes

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.

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

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.

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.

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.

Australian Ultrasound

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

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

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

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

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

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

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  = x 10-4 m  = 0.16 mm millimetres across 1981 3-D ultrasound identify the differences between ultrasound and sound in normal hearing range

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

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. Reference describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal

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. Reference crystal describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal

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. Notes describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal

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. Notes describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal

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 Notes describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal

Describe the production of ultrasound used for medical imaging.
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.

Describe the piezoelectric effect.
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.

How is the piezoelectric effect used to detect ultrasound?
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.

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.

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

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

Acoustic Impedance Source: Butler Physics 2 Use the information in these tables to calculate the acoustic impedance of water and blood and compare these to bone. Conclusion . . . 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 define acoustic impedance … and identify that different materials have different acoustic impedances

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 = 399 R 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

Acoustic impedance of non-biological materials
For interest only! Reference: File Wave Reflection 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

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

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

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

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

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

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 . . .

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

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

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

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

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

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 = x 103 x 1560 = x 106 R Proportion of reflected ultrasound Ir/Io = ( )2/( )2 Ir/Io = 6.35 x 10-3 solve problems and analyse information using [the above equations]

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

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

A-scans are now obsolete A-scans were useful in measuring the thickness of tissues such as the cornea of the eye. Improvements in technology have made A-scans obsolete. 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

Reflection of Ultrasound and B-scan Use
The B-scan mode was developed to show directly on a display the distances of each tissue boundary from the surface of the body. B-scan results could be combined to produce a 2-D section (see right) 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

The B-scan mode enables a true 2-D image to be produced. It is therefore useful because it enables the size and extent (as well as thickness) of a particular organ or tumour to be determined. Question A simple B-scan was produced using a single ultrasound beam that was moved across a patient’s abdomen from the patient’s right to left just below the navel. Describe* the B-scan image produced from the accumulated data. Answer The image is 2-D. It is a transverse slice from right to left with the section showing parts of body structures from the front to the back. Parts of the body closer to the navel or closer to the feet than the scan path are not visible in the image Answer: The image is 2-D. It is a transverse slice from right to left with the section showing parts of body structures from the front to the back. Parts of the body closer to the navel or closer to the feet than the scan path are not visible in the image. * “Provide characteristics and features” 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

A convex array scanner produces a sector shaped beam The image produced is a two-dimensional slice This is a common type of scan used in obstetrics Advantage This array permits a large imaging area through a small window 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

Reflection of Ultrasound and Sector scan Use
Animation A convex array transducer Below - typical sector scan 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

Ultrasound and Phased Arrays
A linear array produces parallel wavefronts from a line of transducers The resulting image is a sectional slice parallel to the transducer array ADVANTAGE: This type of scan results in accurate linear distances being displayed i.e. correct proportions are maintained 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

A steerable beam is created by some modern ultrasound scanners Successive transducers produce circular wavefronts with a slight delay between each wave Interference between the waves results in a strong linear wavefronts The direction of propagation is controlled by changing the time delay between transducers The advantage of this is that the beam does not have to be steered manually by the operator - the process is automatic 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

Animation: Phased array used to create a steerable beam 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

The Doppler Effect The Doppler effect refers to the property of waves that results in a change in frequency of the wave when the source and the observer are moving relative to each other. The Doppler effect can be heard when moving vehicles are producing a constant pitch sound as they pass by. describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood moving through the heart

The Doppler Effect

The Doppler Effect and Sound Waves
The Doppler effect can be the result of… movement of the source relative to the observer movement of the observer relative to the source movement of both objects at different velocities in a common reference frame In medical ultrasound imaging, the relative movement is due to the movement of a tissue inside the body from which the sound is reflecting, relative to the ultrasound head. e.g. Blood flow, heart beat, breathing describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood moving through the heart

A first hand investigation to demonstrate the Doppler effect piezoelectric buzzer oscilloscope (use computer software - Audacity*) * Audacity is downloadable freeware and can be used as an oscilloscope for investigation of sounds describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood moving through the heart

The Doppler effect results in an increase in the frequency of a sound wave when the source is moving relatively towards the observer, compared to when there is no relative motion. It results in a decrease in the frequency of a sound wave when the source is moving relatively away from the observer, compared to when there is no relative motion. describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood moving through the heart

The Doppler Effect The Doppler effect is used in medical imaging to produce a Doppler ultrasound image showing whether or not movement, such as blood flow or heartbeat is normal. Doppler ultrasound images are normally colour coded, with different colours representing different velocities relative to the ultrasound head. describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood moving through the heart

The Doppler Effect A Doppler ultrasound image uses colour coding to show different rates of movement of the tissues being imaged. describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood moving through the heart

The Doppler Effect Colour Doppler image showing leakage of blood through a hole in the septum separating the left and right ventricles describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood moving through the heart

The Doppler Effect describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood moving through the heart

The Doppler Effect Guess what this baby is doing!
describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood moving through the heart

Doppler ultrasound - heart blood flow
Red indicates blood flow towards the US detector and blue indicates blood flow away from the US detector [Coincidentally the opposite of red/blue shift in astronomy] identify data sources and gather information to observe the flow of blood through the heart from a Doppler ultrasound video image

Doppler Ultrasound and Cardiac Problems
Doppler ultrasound can be used to detect Leakage of blood through heart walls - holes Backflow of blood through faulty valves Poor blood flow due to fat deposits in arteries Irregular flow due to heart malfunction outline some cardiac problems that can be detected through the use of the Doppler effect

Ultrasound Advantages and Disadvantages
Advantages of using ultrasound It is non-invasive – does not require surgical procedures Ill patients can be examined without sedation, and relatively quickly and conveniently Since sound is non-ionising it does not damage DNA, cells and tissues It is relatively cheap (compared with other scanning technologies) Disadvantages of using ultrasound The images obtained are highly dependent on the operator’s skill Images are not as easy to interpret as x-rays or MRI It is difficult to produced clear images with obese patients (due to sound absorption and reflection from fat) The presence of air and bone obscure objects behind them because both reflect ultrasound strongly at boundaries with other tissues

Ultrasound Advantages and Disadvantages
Ultrasound is used for medical imaging because Ultrasound is extremely safe and can be used for obstetrics and it can show tumours and some soft tissue injuries. Ultrasound provides a real-time image, and the sonographer can change the way the scan is done to show a desired part of the body most clearly Ultrasound technology is relatively cheap and widely available Ultrasound’s disadvantage is that the image does not show fine detail visible in an X-ray or MRI scan

Review and PFAs H1. evaluates how major advances in scientific understanding and technology have changed the direction or nature of scientific thinking

Review and PFAs H2. analyses the ways in which models, theories and laws in physics have been tested and validated

Review and PFAs H3. assesses the impact of particular advances in physics on the development of technologies

Review and PFAs H4. assesses the impacts of applications of physics on society and the environment The impact on society of the application of our knowledge of ultrasound has been very significant Improved diagnosis of pre-natal medical problems e.g. spina bifita, cleft palate, foetus developmental problems - benefits individuals and society by reducing treatment costs Safe, non-invasive imaging technology which is cost effective

Review and PFAs H5. identifies possible future directions of physics research

A word from the creator This PowerPoint presentation was prepared by
Greg Pitt of Hurlstone Agricultural High School Please feel free to use this material as you see fit, but if you use substantial parts of this presentation, leave this slide in the presentation. Share resources with your fellow teachers and students.

  Describe the production of ultrasound used for medical imaging.
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. 

 Describe the piezoelectric effect. Answer
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. 

 How is the piezoelectric effect used to detect ultrasound?
Tutorial Questions How is the piezoelectric effect used to detect ultrasound? Pressure variations produced by the ultrasound deform the piezoelectric crystal slightly, producing an alternating voltage across opposite faces. The voltage variations correspond to the varying intensity of the ultrasound returning to the crystal. 

Tutorial Questions Compare the properties of medical ultrasound with sound in the normal hearing range. The sounds are similar because they are both longitudinal waves. 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. 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. 

 Define acoustic impedance.
Tutorial Questions Define acoustic impedance. Acoustic impedance (Z) of a medium is the product of the density of the medium () and the speed of sound (v) in the medium. Hence… 

Do all materials have the same acoustic impedance?
Tutorial Questions Do all materials have the same acoustic impedance? Explain your answer. No. The acoustic impedance (Z) of a medium is the product of the density of the medium () and the speed of sound (v) in the medium. Two materials may have the same density and the speed of sound may different in them. Hence their acoustic impedances will differ.

Tutorial Questions Do materials in which the speed of sound is the same always have the same acoustic impedance? Explain your answer. No The acoustic impedance (Z) of a medium is the product of the density of the medium () and the speed of sound (v) in the medium hence two media through which the speed of sound is the same, but which have different densities, have different Z values. The speed of sound in fat and mercury is the same, however mercury is much more dense, and therefore has a higher acoustic impedance* * There is no need to remember such detail - This example is for the purpose of illustration only.

Tutorial Questions Explain why is ultrasound not very useful in the diagnosis of adult brain disorders. Bone has a significantly higher acoustic impedance than soft tissue and therefore any ultrasound entering the scalp will be reflected strongly from the scull, resulting in very little energy entering the brain. The homogeneous nature of tissue in the brain also results in very little reflection from different areas of the brain, so even if ultrasound did enter the brain, it would be difficult to produce any image of structures within the brain itself. Ultrasound is sometimes used to investigate foetal brains because the skull in early development is softer cartilage, rather than calcified bone, and therefore ultrasound can penetrate the foetal brain more readily.

Tutorial Questions Calculate the acoustic impedance of bone and blood
Z = 2 x 103 x 4080 = 8.16 x 106 R Blood Z = 1.05 x 103 x 1570 = 1.65 x 106 R muscle density = 1070 kg m–3 fat density = 1070 kg m–3

Tutorial Questions Predict whether any ultrasound energy striking an interface between water and aluminium would be reflected. Justify your prediction. Water Z = 1 x 103 x 1540 = 1.54 x 106 R Aluminium Z = 2.7 x 103 x 6400 = x 107 R Since the acoustic impedance of aluminium is much greater than that of water, most of the energy would be reflected at the boundary.

Tutorial Questions Calculate the density of fat and muscle given their acoustic impedances fat 1.38 x 106 R muscle 1.7 x 106 R fat 1.38 x 106 =  x 1585  = 870 kg m–3 muscle 1.7 x 106 =  x 1585  = 1070 kg m–3

………………………………………………………………………………….
Tutorial Questions Question Explain how medical ultrasound is produced. Identify the effect. ………………………………………………………………………………….

Do we have to memorise this?
NO!

Medical Physics Ultrasound

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