Physics
Home > Physics > Options > Medical physics > Medical Physics: 1. Ultrasound
9.6 Option – Medical Physics: 1. Ultrasound
Extract from Physics Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit 30 June 09]
Prior Learning: Preliminary module 8.2
You should have a clear understanding of the nature of sound, including transmission and reflection of sound in solids, liquids and gases. You should have a clear understanding of the terms amplitude, frequency, wavelength, velocity and phase as they apply to sound waves.
Background: Diagnosis means identifying a disease or disorder from its symptoms. A diagnostic tool is a device or a procedure that assists in diagnosis. Today, medical imaging technologies exploit a variety of physical phenomena to investigate the body without resorting to surgery. One such technology is based on the properties of ultrasound waves, which make them very useful as a diagnostic tool.
gather secondary information to observe at least two ultrasound images of body organs
- Gather secondary information by
looking for ultrasound images in popular science and
medical books. An Internet search will also return a range
of images. Check that the images are of body organs.
Recognising an ultrasound image takes practice so a
labelled image would be best. You will make better
observations if you can find an image with a scale. Observe
the shades of grey (greyscale) or any colour added to the
image. Look for the sharpness of the image (its clarity and
resolution).
- Try to identify parts of the organ. Look for any relationships between the colour, greyscale and type of tissue. Identify any parts of the image which are very dark (no echo), very white (good echo), or are intensified, in shadow or repeat (possibly artefacts).
identify the differences between ultrasound and sound in normal hearing range
| Characteristic | Sound in normal
hearing range |
Ultrasound |
|---|---|---|
| Frequency | 20 Hz – 20 kHz | Over 20 kHz
For medical imaging, 1 MHz – 15 MHz |
| Audibility | Audible to humans | Inaudible to humans |
| Wavelength | Longer wavelength | Shorter wavelength |
| Scattering | More easily scattered | Less easily scattered by body tissue |
describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal
- Certain crystalline materials, with fixed ions in the
crystalline lattice, exhibit an effect in which mechanical
stress applied to the crystal produces a potential
difference between opposite faces. This effect is called
the piezoelectric effect.
- Conversely, a potential difference applied to opposite
faces of the crystal, say by two electrodes on either side
of a flat slab of piezoelectric crystal, causes mechanical
deformation of the crystal. If an alternating potential
difference is applied, the crystal vibrates, that is,
becomes alternately thinner then fatter between the
electrodes, at the same the frequency as the applied
potential difference.
- If the vibrating crystal is in contact with air, it will produce a sound wave of the same frequency as the alternating potential difference. The frequency of the applied alternating potential difference is chosen to give the desired frequency of ultrasound. Careful sizing of the crystal material, to match its natural resonant vibration to this frequency, makes for the most efficient transfer of electrical energy to ultrasound energy.
define
acoustic impedance: 
and identify
that different materials have different acoustic
impedances
- Acoustic impedance, Z, is the
opposition of a medium to the passage of sound waves. A
substance with a high acoustic impedance hinders the
movement of sound energy more than a substance with a low
acoustic impedance.
- Impedance is proportional to both the density of the
medium and the velocity of the sound within it. The units
for acoustic impedance can be found by multiplying the
units for density by the units for velocity. Hence,
acoustic impedance is measured in
kgm-2s-1.
- The body consists of a range of materials, such as air in the lungs, gas in the bowel, water, blood, muscle, fat and bone. Each body component has a characteristic impedance that depends upon the nature of the matter in it. Gases have very low density, therefore very low acoustic impedance. The impedance of a particular tissue will vary within a range around a typical value.
| Substance | Characteristic acoustic impedance |
|---|---|
| Air | 429 kgm-2s-1 |
| Water | 1.43x106 kgm-2s-1 |
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
Solve problems
- You are required to perform numerical calculations to determine acoustic impedance. To do this you will need to know the velocity of the sound in the material and the material’s density. You will need to examine the problem for information that either gives you these values directly or will allow you to determine these values. You may need to revise density and velocity. Check the units of the given data. Density should be in kgm-3. Velocity should be in ms-1.
Analyse information
- You must be able to examine information in order to extract useful data. Density and velocity data may be given directly or found in tables, on diagrams or in graphs. To find the required data you may need to make inferences.
Example of inferring data. You are given a problem which includes a graph of the velocity of sound against the temperature of air, and a table of air density and air temperature values. To calculate the acoustic impedance you should choose a density value from the table. Read its corresponding temperature and then use the graph to find the velocity at that temperature.
Calculate the acoustic impedance of a range of materials including bone, muscle, soft tissue, fat, blood and air.
You will need to use the formula Z="ρv" and the data as determined from the above.
Sample calculation
The density of blood is 1060 kgm-3and its ultrasound velocity is 1570 ms-1
Acoustic impedance, Z="ρv" = 1060 x 1570 = 1.59 x 106 kgm-2s-1
Test yourself
Fat has an ultrasound velocity of 1450 ms-1 and a density of 952 kgm-3. Find its acoustic impedance
Explain the types of tissues that ultrasound can be used to examine.
- Ultrasound will move through a medium until it
encounters a boundary. When this happens some ultrasound
will be reflected from the boundary and some will cross the
boundary. The ratio of these two amounts depends upon the
difference between their acoustic impedance. A big
difference will mean that very little of the sound will
cross the boundary. This means that the boundary will give
a strong echo. Ultrasound echoes are the basis of examining
tissues. Bone has the highest acoustic impedance in the
body and so most sound is reflected from the surface of the
bone. This means that the sound cannot penetrate the bone.
Tissues enclosed by bone, (eg in the skull) hidden by bone
or within bone cannot be examined.
- For a similar reason air interfaces have almost
complete reflection. This means that tissues with enclosed
air, such as the lungs, are difficult to image.
- On the other hand adjacent tissues with similar acoustic impedances will allow some ultrasound to be transmitted and some reflected. This allows both an echo and further penetration of the ultrasound. Each boundary will give an echo a short time later than the preceding one. This allows us to examine the tissue the sound has just passed through. Thus soft or watery tissues such as muscle, fat and blood can be examined.
describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound
Prior learning: Module 8.2 and 8.4 should be revised to check your understanding of reflection and refraction.
- This description may be supported with a diagram. A
short burst (pulse) of ultrasound is produced by a
piezoelectric transducer. This pulse will travel through a
medium until it reaches the boundary with another medium.
- Some of the pulse will be reflected and will return to
the transducer. The distance from the transducer to the
boundary (ie the depth of the boundary) can be found by
recording the time between the pulse and its echo.
- Some of the pulse will cross the boundary (ie be
refracted) into the second medium. This refracted pulse
will continue into the second medium until it reaches
another boundary, where some of it will be reflected to
return to the transducer and some will be refracted a
second time. In this way a series of echoes having
different time lags from the initial burst will be
recorded. Each represents a boundary at a different
distance (depth) from the transducer.
- The amount of ultrasound reflected compared to the amount refracted at a boundary will depend upon the different acoustic impedances of each medium. A large difference in impedance means that there is more reflection and less refraction at a boundary.
solve
problems and analyse
information using: and 
- To solve problems using these formulae
you will need to extract the information from the problem
and recognise the quantities you are asked to find and
those you are given. You may need to rearrange the
equations prior to substituting the data. Check that the
units for the quantities are correct.
Note that the intensity reflection coefficient is a dimensionless quantity.
- You can analyse information by taking data given in the problem and finding acoustic impedance and/or the intensity reflection coefficient. Using these calculated or given values you can make predictions about the boundary between two materials. For example, You could predict a strong echo if the intensity reflection coefficient was large. You should have a knowledge of the range of values that this coefficient may take.
identify that the greater the difference in acoustic impedance between two materials, the greater is the reflected proportion of the incident pulse
- Different substances have different acoustic impedance.
When an ultrasound pulse crosses a boundary between two
different materials, some of the incident pulse is
reflected back into the first material, giving an echo that
can be recorded, while the remainder is refracted into the
second material.
- The amount of the incident pulse that is reflected depends on the difference in acoustic impedance between the materials. A large difference in acoustic impedance will mean that more of the wave is reflected and less transmitted by the boundary.
describe the situations in which A scans, B scans and sector scans would be used and the reasons for the use of each
A range of communication strategies would enable you to fully address these descriptions, including paragraphs, bullet points, diagrams, a table or a combination of these. Choose a strategy that best fits your description.
Example of a paragraph
The A scan consists of a series of amplitude peaks on a cathode ray oscilloscope (CRO) trace, each peak corresponding to an echo from a boundary of a certain depth. The CRO trace is actually a time scale but knowing the pulse velocity allows us to determine distance, that is, the depth of the boundary that returned an echo. A scans are used in situations where only distance measurements are required. Two such situations are measurements in the eye and foetal skull size. The latter can be used to estimate the developmental stage of the foetus. A scans require less complex equipment than other ultrasound techniques.
Example of bullet points
B scans
- B (brightness) scans show the echo as a brightness signal on the CRO.
- A static B scan consists of a series of bright dots.
- Each dot on a static B scan corresponds to an echo from a boundary of a certain depth.
- Static B scans are not very useful on their own
- B scans form the basis of sector scans.
Example of a table
| Scan type | Description | Example of use | Reason for use |
|---|---|---|---|
| Sector scan | Successive B scans are made as the transducer probe is rocked sideways on the patient. Each static B scan is added to form a fan-shaped (sector) brightness image. This is a cross-sectional image. | imaging of the infant brain through the fontanel | shows a two dimensional image only needs a small entry ‘window’. |
identify data sources and gather information to observe the flow of blood through the heart from a Doppler ultrasound video image
- Determine the type of data that needs to be collected
and explain the qualitative or quantitative analysis that
will be required for this data to be useful.
- Amongst a range of sources of videos, an Internet
search may locate mpeg videos of Doppler ultrasound. Other
sources may include a medical imaging facility or hospital.
- Check that your video footage has had false colour applied to it, as this will reveal the most information from a Doppler ultrasound. A ‘colour overlay’ may reveal the direction, velocity or volume of blood flow. The colours may reveal problems with the blood flow such as damage to heart valves. Video footage may include the Doppler shifts as audible 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 is the apparent
change in wavelength of a wave when it is produced by a
source moving relative to a stationary observer. When a
source of sound waves moves towards an observer, the waves
are ‘bunched up’; their wavelength is shorter
and the frequency heard by the observer is higher. The
converse is true when the source is moving away from the
observer. The Doppler effect also happens if the source is
stationary and the observer is moving.
- If a pulse of ultrasound reflects off a stationary
boundary it will return with the same frequency and
wavelength as was emitted. If the boundary is moving away
from the transducer there will be a Doppler shift effect.
The waves will undergo a Doppler shift on their outward and
reflected journeys, producing a double Doppler shift.
- Ultrasound used in blood flow measurement is typically
in the range 5 to 15 MHz. The ‘moving boundary’
comprises the surfaces of multiple red blood cells, as an
individual red blood cell is too small to be a boundary on
its own. The Doppler shift is typically a change in
frequency of up to 3 kHz. This value is positive when blood
flows towards the probe, and negative when the blood flows
away.
- Sophisticated computer software can assign colours to an ultrasound scan on the basis of its Doppler shift. In this way flow velocity can be seen. A colour change can indicate increased velocity, indicating a narrowed artery. Colour intensity can indicate flow volume. Mixed colours can indicate flow turbulence due to a partial blockage. The wrong colour can indicate a leaking heart valve. Doppler shifts can be in the audible range and so can be heard. An experienced operator can make a diagnosis using this sound.
outline some cardiac problems that can be detected through the use of the Doppler effect
- You answer should connect a use of the Doppler effect
to a cardiac problem. Cardiac problems that can be detected
by the Doppler effect are those that produce a difference
in the flow of the blood through the heart:-
- The size of the Doppler frequency shift is an
indication of the velocity of the blood. Velocity will
increase where the blood vessel is narrowed.
The direction of the Doppler shift will tell us if the blood is flowing in a reverse direction. This can tell us if the valves are functioning correctly.
- A range of Doppler shifts indicates that the flow is turbulent. This can tell us that the blood is encountering a blood vessel blockage or surface unevenness due to deposits.
identify data sources, gather, process and analyse information to describe how ultrasound is used to measure bone density
- You will need to have a clear idea of the sorts of data
you need. Bone density and issues around bone density
should be clearly understood. Your data
sources will need to provide this. You need to
have a good understanding of ultrasound as well. Data
sources could include medical texts or public health
publications.
- Gather information by checking your
data sources for an appropriate level of depth. Note that a
range of techniques are used to measure bone density so be
careful to focus on the ultrasound method. An Internet
search will provide a variety of data sources.
- Bone densitometry is a common and widely used procedure
in the diagnosis of osteoporosis. Process
information by checking that any data you find
from mass media sources is accurate. Also check any
websites for accuracy. Do they have a scientific basis? Are
they someone else’s synthesis of data? Are they
biased or are commercial in nature? Comparing with
scientific literature allows you to assess the accuracy of
your data.
- Ultrasound and bone density measurements can be
understood in terms of a series of causes and effects. Look
for these as part of your analysis. Be
sure that you don’t omit important cause-effect
relationships simply because these appear to be obvious to
you. A good description will include all of them.
- To describe how ultrasound is used to measure bone density. A pulse of ultrasound is produced by a piezoelectric transducer in a probe. This pulse contains a known amount of energy. The pulse is directed through a patients’ heel bone. The bone absorbs some of the energy. A comparison is made of the energy lost by the pulse with the energy of the initial pulse. The amount absorbed is related to the quality of the bone.
define the
ratio of reflected to initial intensity as:
- You are asked to state the meaning and so should be
able to say in words that this ratio gives an indication of
how well a pulse of ultrasound will cross the boundary
between two different materials. You should be able to
clearly indicate that ‘intensity’ is a measure
of the energy in the pulse of ultrasound and that it
depends upon the amplitude of the waves in a pulse of a
certain frequency. You should also be able to indicate that
acoustic impedance is a function of the density of a
material and the velocity of the waves in that material.
- You are also asked to identify the essential qualities of this relationship. Make sure you understand the various subscripts and symbols. Note that the right hand side involves the square of an addition and the square of a subtraction. Ie the quantity [Z2-Z1]2 is NOT the same as Z22 – Z12. Also note that the relationship is a dimensionless ratio that will be between 1 and 0.
|