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Medical Physics: Ultrasounds

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Peter Robson

on 12 June 2012

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Transcript of Medical Physics: Ultrasounds

Medical Physics
Ultrasounds
The Properties of
ultrasound waves can
be used as
diagnostic
tools
1
Normal hearing range is between 20 - 20 000 Hz
Ultrasound is above 20 000 Hz
Dolphins and bats use ultrasound for navigation
Medical ultrasounds are produced using a piezoelectric crystal and range in frequency from ~1 MHz to 10 MHz
1 MHz are low frequency ultrasounds producing low resolution but have high penetration
10 MHz are high frequency ultrasounds producing high resolution but have a low penetration.
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Identify the differences between ultrasound and sound in normal hearing range
Describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal
Define acoustic impedance:



and identify that different materials have different acoustic impedances
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 and analyse information using:
Describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound
Define the ratio of reflected to initial intensity as:
Identify that the greater the difference in acoustic impedance between two materials, the greater is the reflected proportion of the incident pulse
Solve problems and analyse information using:
Describe situations in which A scans, B scans and sector scans would be used and the reasons for the use of each
Gather secondary information to observe at least two ultrasound images of body organs
Identify data sources, gather, process and analyse information to describe how ultrasound is used to measure bone density
Describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood moving through the heart
Outline some cardiac problems that can be detected through the use of the Doppler effect
Identify data sources and gather information to observe the flow of blood through the heart from a Doppler ultrasound video image
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Piezoelectric is a word derived from two words. Piezo meaning pressure and Electric meaning electricity. Therefore Piezoelectric means Pressure from Electricity
The Piezoelectric effect is the phenomenon where mechanical vibrations of a substance (leadzirconate titanate for example) are converted into electrical signals and vice versa
Ultrasound, just like any wave, will reflect off boundaries between different mediums.
Acoustic impedance refers to the ease with which a sound wave can pass through a certain material.
Different materials will have different acoustic impedances.
ACTIVITY
Look at the following table
(also found on page 345 of your textbook)
Use the data to calculate Z
Place all relevant data in a spreadsheet
Upload to the Acoustic Impedance Task
Ultrasound Transducer
Remember:
Acoustic impedance by itself does not determine the reflection or penetration of soundwaves.
It is the difference in the acoustic impedance encountered as the sound waves enter from one medium to another.
More precisely, the proportion of soundwaves that will be reflected back when they hit the boundary between any two media is the information that can be gathered to create the image.
The bigger the difference in Z values the larger the proportion of sound reflected back.
The smaller the difference in Z values the smaller the proportion of sound reflected back.
Be consistent in your use of Z and Z
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ACTIVITY
Access the spreadsheet you constructed for acoustic impedance
Use that data to find the ratio of reflected ultrasound between the following:
-Liver and Fat
-Soft Tissue and Muscle
-Brain and Bone
Upload the new spreadsheet to the Ratio of Reflected Ultrasound task
A-Scan (Amplitude Scan)
B-Scan (Brightness Scan)
Sector Scan
An A-scan is a range-measuring system that records the time for an ultrasonic pulse to travel to an interface in the body and be reflected back.
In an A-scan the ultrasound pulses are directed into the body in one line and the reflected signal is detected.
The intensity of the reflected beams is plotted on a graph as a function of time.
In this way the position of various features can be determined from the time lapse between sending the signal and receiving its echo and a knowledge of the speed of sound in the tissue.
The intensity of the reflected beam provides information about the type of material through which the ultrasound is travelling.
An A-scan provides one-dimensional information about the location of the reflecting boundaries.
Originally this type of scan was used to determine the midline position of the brain and detect any abnormalities there caused by tumours, because the midline would be displaced by a tumour.
A-scans are no longer used for this as more sophisticated methods of imaging the brain have been developed.
A-scans are still used in ophthalmology for the diagnosis of eye disease and for measurements of distances in the eye, where no image of the interior of the eye is needed
In a B-scan the intensities of the reflected ultrasound are represented as spots of varying brightness.
the brightest spot correspond to the most intense reflected ultrasound.
By moving the transducer probe, the body is viewed from a range of angles.
A series of spots are obtained, each series corresponding to a different line through the body.
These spots can give a 2-D picture of a cross-section through the body
Sector scans are scans of a fan-shaped section of the body.
They are made of a number of B-scans, which build up an image of the sector in the body through a series of dots of varying intensities.
When this type of scanning was first used, a single transducer was rocked back and forth manually so that the ultrasound pulses would sweep across a sector of the body.
This required skill and experience to achieve clear images and is now rarely used in hospital work.
However, its advantage is that it requires a small entry ‘window’ into the body and is still valuable in imaging through a small space, such as the space between the bones of an infant's skull to obtain an image of the infant's brain
Modern scanning techniques use an array of transducers very close together in the one probe head.
This enables very clear images to be produced and also allows the possibility of real-time scans (scans that are produced faster than 16 images per second and displayed on the monitor at that rate).
Real-time scans allow movement to be monitored and so are used to examine, for example, foetal movement or heart movement.
There may be as many as several hundred transducers in an array.
These transducers may be fired simultaneously in which case they produce a wavefront parallel to the transducers.
If the transducers are fired in close succession so that they are slightly out of phase with one another, they produce a wavefront that strikes the surface at an angle other than 90°.
By changing the time between firing of the transducers, the phase difference of the waves from the transducers and hence the direction of the ultrasound beam can be altered.
The beam can be swept from side to side, producing a scan over a wide arc.
Improvements in transducer array construction and electronic processing have led to improvements in image quality.
The firing is electronic and very accurate.
By using this phased array of transducers, a phase scan is produced.
Ultrasound scanning using phase scans is the most common scanning technique used today.
http://www.ultrasound-images.com/
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).
ACTIVITY
Visit the following website
Gather three different ultrasound images
Create a word document with those images
label the organ(s) and different parts of the organ(s)
Upload to the Ultrasound Images Task
Ultrasound and bone density

There are several techniques for measuring bone density. These vary in their usefulness for detecting the risk of osteoporosis, a disorder in which reduced bone density leads to brittle bones.

Normal X-rays are not suitable for the early detection of osteoporosis. This is because X-rays show changes due to loss of bone density only when approximately 30 per cent of bone has been lost. Diagnosis needs to be made earlier than this.

Ultrasound measurement of bone density is more effective and the technique is readily available, often through mobile units at pharmacies. The patient inserts a foot into a warm water bath and ultrasound waves are directed through the heel
The speed of the ultrasound through the bone and the ultrasound attenuation (degree of absorption) are measured. Normal bone has a higher speed of ultrasound and larger attenuation than osteoporotic bone. Speed and attenuation are combined to give an index from which an estimate of heel bone mineral density is reported.

It is not possible by this method to measure sites of fractures that occur in people with osteoporosis, such as at the hip or spine. Although it is a cost-effective method of screening for bone density, ultrasound on its own does not predict the probability of fractures and so is not recommended for the diagnosing of osteoporosis. If an abnormal result is obtained from the bone density analysis, a DEXA examination should be undertaken to test for osteoporosis.

DEXA (Dual Energy X-ray Absorptiometry) using X-rays is regarded as the most reliable method of measuring bone density and can detect small changes only 6–12 months after a previous measurement. The density of the lumbar spine and left hip are usually measured. The DEXA procedure is different from a normal X-ray because low-energy X-rays are used. (The X-ray dose is so low that the radiographer can remain in the room with the patient.) People who are shown to have low bone density through ultrasound measurement are referred for a DEXA scan, because DEXA measures bone density with high accuracy and precision. More commonly, a person will be sent directly for a DEXA scan and not for ultra-sound testing for bone density.
http://simple.wikipedia.org/wiki/Doppler_effect
Have you forgotten what the Doppler Effect is? Click Below
Doppler ultrasound in practice

In Doppler ultrasound, the change in frequency is measured and analysed to give information about rate of blood flow in the body, and particularly through the heart.

An ultrasound is directed into the body and some of this ultrasound is reflected off blood cells moving with the blood. Due to the movement of the blood cells, the reflected ultrasound that is received by the transducer will have changed in frequency compared with the incoming signal.

In fact, the Doppler effect has to be taken into account twice. To illustrate this, imagine the blood is moving towards the transducer. The blood cells will receive a signal at a higher frequency than that given out by the transducer. These blood cells then act as a source when they reflect the signal. They reflect the higher frequency wave and then move into the wave at the same time, resulting in a further increase in frequency. This higher frequency is received by the transducer.
For example, if the blood flow is 1 ms and 5 MHz ultrasound is used, the frequency change is approximately 3 kHz, which is in the audible range. Note that the frequency change is what is measured. An experienced practitioner can listen to the frequency change and make judgements about whether the flow is towards or away from the transducer and whether the blood flow rate is normal. Of course the signal can also be electronically analysed and displayed on a screen for examination.

The ultrasound reflected from internal tissues may pose a problem as these waves interfere with the echo ultrasound being analysed. Particular types of signals must be used to overcome this problem.
-1
http://hsconline.nsw.edu.au/mod/assignment/view.php?id=93266
http://hsconline.nsw.edu.au/mod/assignment/view.php?id=93267
Echocardiography
Echocardiography is the use of ultrasound to diagnose heart (cardiac) problems.
In its simplest form, a transducer placed on the chest wall emits a short pulse of ultrasound.
The reflection (echo) is detected from cardiac structures such as valves and heart muscle walls.
From this, the condition of the heart and its components can be determined.
This is the preferred method of detecting heart valve infection (endocarditis) and intracardiac tumors.
Doppler Echocardiography
The most common form of echocardiography is Doppler echocardiography.
In this method, ultrasound waves reflect off individual blood cells moving towards or away from the transducer.
If the blood flow is parallel to the ultrasound beam, the velocity of blood flow can be measured.
The greater the speed of flow, the more the frequency shifts.
To improve the information displayed with Doppler ultrasonography, the computer can add 'false colour'.
Commonly blood flow towards the transducer is coloured red and flow away is blue.
The speed can be indicated by variations in brigtness and colour.
Cardiac problems that can be detected by Doppler echocardiography include:
-Narrowing of the arteries
-Leaking valves
-Blood vessel blockages
Youtube has plenty of videos of Doppler ultrasounds

The following link is of a dog with mild aortic regurgitation. Notice how the colours are mixed
http://www.youtube.com/watch?v=po3lYcAjg9k
http://hsconline.nsw.edu.au/mod/assignment/view.php?id=93275
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