Triple Physics Topic 2 - Ultrasound

Ultrasound

Ultrasound is sound of any frequency above the range of human hearing, about 20 000 Hz.  Many animals can hear ultrasound.  Early remote controllers for TV sets used ultrasound, which meant that each time you changed channel, the cat would leap up the curtains.  Modern remote controllers use infra red.

Bats and dolphins are animals that use ultrasound as SONAR (Sound Navigation and Ranging) for both hunting and navigation.  It is believed that they can make a picture in their brains from the ultrasound echoes they pick up.

Source not known

Note the large ears that funnel the sound down to the animal.  The shape of the mouth and nose focus the sound waves.

Bats and dolphins use SONAR (Sound navigation and ranging), not RADAR (radio detection and ranging).  This is a common bear trap in exams.

 But calls have a frequency of about 18 000 Hz.  What is the wavelength of these calls?

Generation and Detection of Ultrasound

The ultrasound probe or transducer is used to generate and detect the ultrasound waves.  The most common method is by use of a piezoelectric transducer.  If you squeeze or stretch a crystal of quartz (easier than might be thought) a voltage is induced.  It can be high enough for a spark to jump.  Gas lighters use the effect.  Conversely, if you apply a voltage to a piezoelectric material, you can make it change shape.  If the voltage is alternating, the crystal will vibrate.

The material, usually lead zirconate titanate (PZT), an artificial ceramic, has a thickness of half a wavelength of the ultrasound wave.

The lens protects the PZT slice and acts to converge the beam slightly.  The beam consists of short pulses of frequencies of several megahertz.  The vibrations are damped by the backing block which is made of epoxy resin.  The whole is contained in a metal case which protects the probe mechanically and electrically.

Ultrasound in Medicine

Ultrasonic devices are used primarily for non-destructive investigation:

• In engineering to test that a structure doesn't have flaws in it.

• In medicine to diagnose a range of conditions.

The most obvious use in human and veterinary medicine is the use of the ultrasound test in pregnancy.  The equipment can nowadays reveal very detailed pictures:

The ultrasound equipment consists of:

• a transducer (probe), which converts electrical signals into ultrasound, and detects the reflected signals that come back.

• a signal generator that makes pulses, typically at a frequency of about 2 000 000 Hz (2 megahertz).

• a computer to convert the pattern of signals back into a meaningful picture.

The transducer is coupled to the skin using a jelly.  This is because ultrasound is reflected as soon as it hits a material-air boundary.  Therefore without a jelly the waves would reflect as soon as they were transmitted, and not show anything.

Imaging in the Body

When ultrasound passes into the body:

• it is a longitudinal wave travelling in a material

• it is reflected at boundaries between different tissues;

• it is absorbed by tissues.

The extent to which the ultrasound is absorbed or reflected gives information about the structures below.  The ultrasound passes through the tissue and is reflected at various boundaries as shown:

The sonographer (operator) moves the transducer about to get patterns of signals from a wide area.

The beam of ultrasound can be focused to improve the picture.  The higher the frequency, the better the resolution (how much detail can be picked up).

 Question 2 The speed of sound through the body is about 1400 m/s.  What is the wavelength of an ultrasound signal of frequency 2 000 000 Hz?

If you want to go into ultrasound in medicine in more detail, go to:

Ultrasound can also be used for treatment.  Ultrasound probes can be used to break up gallstones.  In dentistry, ultrasonic probes can clean plaque off your teeth.

Ultrasound is generally a very safe diagnostic technique:

• There are no known hazards with low frequency (low energy) beams.

• It is non-invasive.

• There is no discomfort apart from a cold probe!

• More effective than X-ray techniques in producing images of soft tissue;

• The equipment is relatively inexpensive, can be moved about very easily, and does not need a specialist room.

• There are no hazards for the operator.

However:

• The sonographer has to be skilled at operating the probe and its associated equipment to get a decent image.

• The image needs skilful interpretation.

• Attenuation can reduce the resolution of the image.

• Bone absorbs ultrasound so that brain images are hard to get;

• Gas-soft tissue interfaces reflect 99.9% of the incident energy.  Images of tissues on the far side of lungs are impossible to get.

High energy ultrasound waves can be used for therapeutic purposes.  Low intensity ultrasound can be used in healing wounds and relieving discomfort and pain in conditions like arthritis.  High intensity beams can shatter kidney stones.  Ultrasound treatment has to be done with care because:

• the temperature in the tissues can rise;

• the pressure changes can rupture cells;

• bone is a strong absorber of ultrasound.

While this would not cause many problems for an adult, it must be avoided where there is a growing foetus.

Photo by Joseph Caballero, Wikimedia Commons

Ultrasound in Industry

A common use for ultrasound is to check for flaws in castings.  Flaws are bubbles of gas that can occur while metal is cast.  As the metal solidifies, the bubbles get trapped.  They can seriously weaken the casting.  If they are bad enough, the casting may have to be scrapped.

The ultrasound probe send pulses through the casting.  Where the pulses meet a boundary, for example, if there are two layers of metal, they are partially reflected.  If there is a flaw, most of the waves get reflected.

 Question 3 What else happens to the sound waves as they pass the boundary?  Why?

Now look at what the CRO shows:

You can see that there is a transmitted pulse, which is quite big, and a smaller received echo, which is the reflected pulse.

 Question 4 Why is the reflected pulse smaller than the transmitted pulse?

 Question 5 Each square on the horizontal axis represents 1 microsecond (1 × 10-6 s).  How long does it take for the echo to arrive back at the probe after the pulse has left?

 Question 6 The speed of sound in the casting is 6000 m/s.  What is the depth of the flaw?

Ultrasound is often used for cleaning.  In water and other solvents small bubbles are formed by cavitation, as a result of the ultrasound.  These are very effective at removing dirt from components.  Ultrasonically cleaned components are very clean indeed.  Here is a picture of an ultrasonic component cleaner used to clean instruments:

Here is another ultrasound device used to clean wires.

 Question 7 Complete the interactive gap-fill exercise

 Question 8 Now try the Crossword that gets you to think about sound and ultrasound.

 Summary Ultrasound is any sound of frequency greater than 20 000 Hz Ultrasound is used in medical imaging Ultrasound can be used in industry for checking the quality of components. The ultrasound probe transmits pulses and receives the echoes. The distance to a boundary is half the distance worked out from the time between pulse and echo. Ultrasound can be used to clean components.

Now try the Topic Quiz

Home                    Unit Index