Principles of physics in Ultrasound
Principles of physics in Ultrasound
Physics has become an important part of medicine allowing specialist doctors and radiographers to rapidly access a patient's condition and to help in long-term diagnosis.
This enables doctor's to treat patients before their condition deteriorates.
This procedure would not be possible without the use of X-rays, CAT scans, MRI scans, ultrasound and endoscopes, which allow doctors to see inside the body with little or no surgery.
Without such equipment doctors would be forced to use invasive techniques, which could cause patients more harm as it increases the risk of infection.
A sound or ultrasound wave consists of a mechanical disturbance of a medium (gas, liquid or solid) which passes through the medium at a fixed speed.
Sound waves consist of a disturbance of air molecules, the vibrations which pass from molecule to molecules from the speaker to the ear of the listener.
The rate at which particles in the medium vibrate in the disturbance is the frequency or pitch of the sound measured in hertz (cycles/sound).
As the pitch increases there comes a frequency at about 20kHz when the sound is no longer audible and above the frequency disturbance, this is know as ultrasound.
The first major breakthrough in the evolution of high frequency echo-sounding techniques came when the piezo-electric effect in certain crystals was discovered by Pierre and Jacques Curie in Paris in 1880.
The turn of the century saw the invention of the Diode (component that restricts the direction of movement, allows an electric current to flow in one direction) and the Triode (type of vacuum tube), allowing powerful electronic amplification necessary for developments in ultrasonic instruments.
The early work in the 20th century used ultrasound as a therapy tool and it was not until the 1940's that research began into its use as a diagnostic tool.
The use of ultrasound in medicine began during and shortly after the 2nd World War in various centres around the world. (NSC, UK national screening committee)
The work of Dr.Karl Theodore Dussik in Austria in 1942 on transmission ultrasound investigation of the brain provided the first published work on medical ultrasonic's.
Although other workers in the USA, Japan and Europe have also been cited as pioneers, the work of Professor Ian Donald and his colleagues in Glasgow, in the mid 1950s, did much to facilitate the development of practical technology and applications.
This lead to the wider use of ultrasound in medical practice in the subsequent decades.
Rapid technological advances in electronics and piezoelectric materials provided further improvements from energy to greyscale images, and from still images to real-time moving images. The technical advances at this time led to a rapid growth in the applications to which ultrasound could be put.
The development of Doppler ultrasound had been progressing alongside the imaging technology, but the fusing of the two technologies in Duplex scanning and the subsequent development of colour Doppler imaging provided even more scope for investigating the circulation and blood supply to organs, tumours etc.
The advent of the microchip in the seventies and the processing of power have allowed faster and more powerful systems incorporating digital beam forming, more enhancement of the signal and new ways of interpreting and displaying data , such as power Doppler and 3d imaging.
Ultrasound refers to sound ...
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The development of Doppler ultrasound had been progressing alongside the imaging technology, but the fusing of the two technologies in Duplex scanning and the subsequent development of colour Doppler imaging provided even more scope for investigating the circulation and blood supply to organs, tumours etc.
The advent of the microchip in the seventies and the processing of power have allowed faster and more powerful systems incorporating digital beam forming, more enhancement of the signal and new ways of interpreting and displaying data , such as power Doppler and 3d imaging.
Ultrasound refers to sound waves of a high frequency, above 20kHz.
Audible sound can spread through a room, ultrasound is inaudible to humans, and has a shorter wavelength, this can be formed into a beam.
The normal range of hearing is from 20 to 20 000 Hz (hertz) any frequencies above this range are called ultrasound.
Typical frequencies used in medicine are in the megahertz range.
Ultrasound undergoes reflection and refraction at the interface between two different media, it is the reflections and echoes from different tissues that produce ultrasound images. (see appendix)
This use of echoes is the basis of sonar (sound navigation and ranging).
The pulse of sound that is used should be short, and high frequencies are usually used, as they travel further without being absorbed.
Ultrasound, nearly always in the form of pulses, is generated by a piezoelectric device. The same device can act as both the source and the detector of echoes reflected from different structures in the body since the signals are separated in time.
Useful information is provided by the delay time of echoes from different depths, the reflection of ultrasound at the interfaces between different tissues, the attenuation of the ultrasound, and the shift of frequency resulting from reflections from moving objects.
Ultrasonic waves are produced by a transducer (operating as a transmitter), this is a device, that changes energy from one form to another. (see appendix)
Ultrasound is produced by the piezoelectric effect, this effect occurs in certain crystals, when they are subjected to high electric filed of several volts per millimetre, which then causes the crystal to distort.
If a high frequency electrical signal is applied to a crystal a certain amount of energy will be converted into ultrasound.
The most commonly used transducer material in ultrasonic diagnosis is he synthetic ceramic, lead ziroconate titanate.
The two flat faces are made electrically conducting with a very thin coat of silver.
The goal is to produce a single focused arc-shaped sound wave from the sum of all the individual pulses emitted by the transducer.
To make sure the sound is transmitted efficiently into the body the transducer is coated with rubber and a special gel.
The sound wave, which is able to penetrate bodily fluids, but not solids, bounces off the solid object and returns to the Transducer, this return is an echo.
The return of the sound wave to the transducer results in the same process that it took to send the sound wave, just in reverse.
The return sound wave vibrates the Transducer and turns that vibration into an electrical pulse that is sent through the probe and into sonographer's computer where it can be interpreted and transformed into a digital image.
The computer must determine three things from each electrical impulse received, which wire did the impulse come from.
There are multiple receiving wires on a transducer.
Also how strong the impulse was, and lastly, how long did it take the impulse to be received from when it was sent.
Once the computer determines these three things, it can locate which portion of the monitor to light up.
Speed of Ultrasound
The speed with which the oscillations can be passed from one atom to its neighbours depends on the stiffness of the medium (i.e. the magnitude of the force between atoms).
The stronger the force of attraction the greater the acceleration of an atom when influenced by an oscillating neighbour will be, the greater the mass the lower the acceleration of a disturbed atom will be.
The basic ultrasound system works on a pulse echo technique using the equation:
Distance = Speed x Time
If the speed of sound in the different substances between the transducer and object is known and the time interval between the pulse and reflected signal can be measured, then the distance travelled by the pulse can be calculated.
The velocity of ultrasound depends on the material through which it travels, the greater the density, the lower the velocity.
Electrical energy is used to drive the transducer and is converted into mechanical energy.
This mechanical energy, when absorbed into the tissue can generate heat.
There are three types of scanner used in ultrasound, the A scan, the B scan and the Doppler scan. These are used depending on the organ being scanned.
A scan (amplitude-scan)
A pulse generator is connected to the ultrasound transducer and the time base of an oscilloscope.
At the start of each sweep the pulse generator will send a pulse to the oscilloscope and at the same time trigger the transducer to send an ultrasound pulse into the tissue.
When the ultrasound pulse hits a boundary between two tissue types some of the signal is then reflected back to the receiver where it is amplified and shown as a second pulse on the oscilloscope screen.
An A scan is a sequence of individual echoes due to reflections along one direction only.
B scan (brightness)
To obtain an image, the B scan uses sensors attached to the probe which can define position and orientation of the organ in a two dimensional plane.
The ultrasound beam is swept across the plane and the image is built up from a collection of already-existing images, from the A scans.
As this takes several seconds, any movements within the organ will degrade the quality of the image.
This is overcome using real time scanners of which there are two types phased array scanners and sector scanners.
The phased array scanner has small transducers, which are triggered individually very close together, with a small phase difference between each one.
This creates a composite ultrasound scan, which does not need the sensors to define the orientation of the beam.
Sector scanners use one or more transducers, which are scanned mechanically across an arc of 60 degrees.
Phased array and sector scanners are hand held and scan fast enough for the images to be viewed as a film on a television screen.
Doppler ultrasonography
Ultrasound can also be used in a completely different way to measure movement.
When an object which reflects, or emits, waves is moving with constant velocity, a stationary onlooker will find that the frequency of the ways received is different depending on whether the object is moving towards or away from the onlooker.
This is the Doppler effect.
Doppler ultrasound devices measure the frequency changes in the ultrasound signal reflected from a moving object, the frequency change being proportional to the velocity of the object along the axis of the beam.
In a continuous Doppler system, a narrow beam of waves between 2 and 10 MHz is transmitted from one transducer while a second transducer acts as a receiver.
Mixing the transmitted and received signals generates the Doppler signals.
A pulsed Doppler system gives range resolution that is defining a small volume at a given depth from which signals can be analysed.
This can be done by transmitting pulses of ultrasound and opening a receiver gate for a short period between pulses.
The delay in the gate determines the maximum distance between the transducer and the reflecting surface.
In the past 2 years, 3D ultrasound machines have been developed; they work on the same principle as ordinary ultrasound.
The difference is that they take a number of scans by moving the probe over the body, or rotating a probe in the body, these scans are then combined by a computer to form a 3D image.
The 3D scan is best used for early detection of tumours, to asses the development of a child, and to visualise the blood flow in various organs or a foetus.
Ultrasound is a safe and painless method for examining the internal organs that avoids the use of X-rays. Instead, high-frequency sound waves are used and the echoes that result as the sound waves reflect off the soft tissue structures in the body form an image.
Ultrasound images, displayed on a video monitor, can be used during pregnancy to measure the size of a developing foetus and to detect abnormalities. Ultrasound can also be used to look at other parts of the body to, for example, determine whether a lump is solid or fluid-filled.
Ultrasound can be used for many things including finding the thickness of the eye lens, viewing the foetus during pregnancy and blood flow.
Ultrasound can be used to monitor growth of a baby during pregnancy and check there are no abnormalities.
Also in diagnose abnormalities of the liver, gallbladder, pancreas, thyroid gland, lymph nodes, ovaries, breasts, bladder, prostate, scrotum and kidney, diagnose abnormalities of blood vessels such as aneurysms
It can help to look for blockages of blood flow in blood vessels, such as a deep vein thrombosis, abnormalities of the heart valves or other heart structures (this type of examination is called echocardiography)
Without the use of physics the medical equipment, such as X-rays, ultrasounds and endoscopes, would not exist.
This would cause diagnosis of patients to be a long and complex procedure.
This equipment has revolutionised medical practise and will continue doing so for years to come.
References
http://www.nelh.nhs.uk/screening/fasp/history.htm.
Physics for medical imaging, RF Farr and PJ Allisy Roberts.
Basic physics for medical imaging, Edwin GA Aird.
Physics and Instrumentation of Diagnostic Medical Ultrasound, Peter Fish.
Biophysical science 1
Bianca Rackham
