The BTEC students need to know about the production of ultrasound and the basic principles of pulse echo technique, reflection and refraction, interaction with tissue, scattering and absorption; intensity measurement in decibels; specific acoustic impedance; sonar principle and ultrasonic scanning e.g. A-scan, B-scan and M-scan; Doppler effect; measurement of blood flow using Doppler ultrasound.
Ultrasound is an oscillating sound pressure wave with a frequency greater than the upper limit of the human hearing range. Ultrasound is only separated from ‘normal’ (audible) sound by the fact that humans cannot hear it. There are no other differences in physical properties. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz (frequency refers to the number of cycles of compressions and rarefactions in a sound wave per second).
Ultrasound is used in many different fields. Ultrasonic devices are used to detect objects and measure distances. Ultrasonic imaging (sonography) is used in both veterinary medicine and human medicine. In the non-destructive testing of products and structures, ultrasound is used to detect invisible flaws. Industrially, ultrasound is used for cleaning and for mixing, and to accelerate chemical processes. Animals such as bats and porpoises use ultrasound for locating prey and obstacles.
Ultrasonics is the application of ultrasound. Ultrasound can be used for medical imaging, detection, measurement and cleaning. At higher power levels, ultrasonics is useful for changing the chemical properties of substances.
Ultrasound imaging systems uses piezoelectric transducers as source and detector.
Piezoelectric crystals e.g. quartz vibrate in response to an alternating voltage, and when placed against a patient’s skin and driven at high frequencies produce ultrasound pulses that travel through the body. If the frequency of the voltage is equal to the natural frequency of the crystal it produces very large vibrations. This occurrence is resonance.
In physics, resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others. Frequencies at which the response amplitude is a relative maximum are known as the system’s resonant frequencies, or resonance frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy.
Normally piezoelectric crystals are used to produce potential difference when squeezed.
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.
As they travel outwards and encounter different layers within the body the ultrasound waves are reflected back towards the source.
The returning signal drives the crystals in reverse and produces an electronic signal that is processed to construct the image. Compared to MRI, ultrasound has the advantages of low cost and portability.
The image above shows the difference in how audible sound and ultrasound are produced.
Ultrasound scanner at Northwick Park Hospital
Diagnostic sonography (ultrasonography) is an ultrasound-based diagnostic imaging technique used for visualising internal body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions. The practice of examining pregnant women using ultrasound is called obstetric sonography, and is widely used.
Ultrasound images (sonograms) are made by sending a pulse of ultrasound into tissue using an ultrasound transducer (probe). The sound reflects (echoes) from parts of the tissue; these echoes are recorded and displayed as an image to the operator.
If the probe is placed straight on to the skin, almost all the energy will be reflected. So the probe has to have a coupling medium between it and the skin. This is a gel or an oil. If there is gas anywhere, it can cause big problems for imaging. The above image shows the ultrasound passing through the tissue and being reflected at various boundaries.
When the waves reach a boundary, a small amount, about 1 % gets reflected. However if the difference in acoustic impedance between two tissues is large, a high proportion of the ultrasound waves are reflected. For example the boundary between lung tissue and the air in the lungs leads to a 99.9% reflection, making it impossible to view structure behind the lungs.
The resolution of the beam means the smallest distance that can be discriminated in the image. The higher the frequency, the better the resolution. However as the beam passes through, the sound waves get scattered and absorbed by the molecules. This attenuation is more marked with higher frequency. Therefore a compromise has to be made. The optimum frequency for imaging the brain and abdomen is roughly 1 – 3 MHz.
Axial resolution is the resolution in the direction of the beam. It can be improved by making the pulses short. While the transducer is producing pulses, it cannot detect echoes. Therefore it makes sense to make the pulses as short as possible.
Lateral resolution is determined by the beam width. If two structures are within one beam, they cannot be discriminated.
Resolution is limited by diffraction effects. Just like in light, objects less than 1 wavelength apart cannot be resolved, the same is true of sound. 1 MHz waves can discriminate structures that are 1.5 mm apart.
When the return pulses are received, the transducer turns them into electrical signals to be stored usually as digital data for analysis by a computer.
Many different types of images can be formed using ultrasound. The most well-known type is a B-mode image, which displays the acoustic impedance of a two-dimensional cross-section of tissue. Other types of image can display blood flow, motion of tissue over time, the location of blood, the presence of specific molecules, the stiffness of tissue, or the anatomy of a three-dimensional region.
Compared to other prominent methods of medical imaging, ultrasonography has several advantages. It provides images in real-time (rather than after an acquisition or processing delay), it is portable and can be brought to a sick patient’s bedside, it is substantially lower in cost, and it does not use harmful ionizing radiation. Drawbacks of ultrasonography include various limits on its field of view including difficulty imaging structures behind bone and air, and its relative dependence on a skilled operator.
The sonar or pulse-echo technique – where sonar stands for sound navigation ranging is used in medical imaging. In sonar, a transmitter sends out a sound pulse and a detector receives its reflection, or echo, a short time later. This time interval is carefully measured, and from it the distance to the reflecting object can be determined since the speed of sound in the medium is known. Sonar generally makes use of ultrasound frequencies.
A high-frequency sound pulse is directed into the body, and its reflections from boundaries or interfaces between organs and other structures and lesions in the body are then detected. By using this technique, tumours and other abnormal growths, or pockets of fluid, can be distinguished, the action of heart valves and the development of a foetus can be examined, and information about various organs of the body, such as the brain, heart, liver and kidneys, can be obtained.
One reason for using ultrasound waves, other than the fact that they are inaudible, is that for shorter wavelengths there is less diffraction, so the beam spreads less and smaller objects can be detected. For an obstacle intercepts and reflects a portion of a wave significantly only if the wavelength is less than the size of the object. Indeed, the smallest-sized objects that can be detected are on the order of the wavelength of the wave used. With the higher frequencies of ultrasound, the wavelength is smaller, so smaller objects can be detected.
(The wavelength is the distance travelled by sound in one cycle, or the distance between two identical points in the wave cycle i.e. the distance from a point of peak compression to the next point of peak compression. It is inversely proportional to the frequency. Wavelength is one of the main factors affecting axial resolution of an ultrasound image. The smaller it is the higher the resolution, unfortunately with less penetration. Therefore, higher frequency probes (5 to 10 MHz) provide better resolution but can be applied only for superficial structures and in children. Lower frequency probes (2 to 5MHz) provide better penetration albeit lower resolution and can be used to image deeper structures.)
The interaction of ultrasound waves with organs and tissues encountered along the ultrasound beam can be described in terms of attenuation, absorption, reflection, scattering, refraction and diffraction.
A reflection of the beam is called an echo and the production and detection of echoes forms the basis of ultrasound. A reflection occurs at the boundary between two materials provided that a certain property of the materials is different. This property is known as the acoustic impedance and is the product of the density and propagation speed. If two materials have the same acoustic impedance, their boundary will not produce an echo. If the difference in acoustic impedance is small, a weak echo will be produced, and most of the ultrasound will carry on through the second medium. If the difference in acoustic impedance is large, a strong echo will be produced. If the difference in acoustic impedance is very large, all the ultrasound will be totally reflected. Typically in soft tissues, the amplitude of an echo produced at a boundary is only a small percentage of the incident amplitudes, whereas areas containing bone or air can produce such large echoes that not enough ultrasound remains to image beyond the tissue interface.
At a tissue–air interface, 99% of the beam is reflected, so none is available for further imaging. Transducers, therefore, must be directly coupled to the patient’s skin without an air gap. Coupling is accomplished by use of gel between the transducer and the patient.
The image below shows the production of an echo depending on relative acoustic impedances of the two media: From: Aldrich: Crit Care Med, Volume 35(5) Suppl.May 2007.S131-S137
The image below shows the percentage reflection of ultrasound at boundaries: From: Aldrich: Crit Care Med, Volume 35(5) Suppl.May 2007.S131-S137
When an ultrasonic pulse enters the body it is reflected from the boundary between different types of tissue. The ease with which an ultrasonic pulse can travel through a material depends on a property of the material called acoustic impedance (Z). This is defined as:
Acoustic impedance (Z) = density of material (ρ) x speed of sound in the material (v)
(The propagation velocity is the velocity v at which sound travels through a particular medium and is dependent on the compressibility and density of the medium. Usually, the harder the tissue, the greater is its value. The average velocity of sound in soft tissues such as the chest wall and heart is 1540 metres/second.)
The greater the difference between the acoustic impedances of the two materials at a boundary in the body the greater the amount of reflection – two materials with the same acoustic impedance would give no reflection (or refraction) while two with widely separated values would give much larger reflections.
The ratio of the reflected intensity (Ir) to the incident intensity (Io) is given by:
For example a boundary between fat and muscle would give 1% reflection while that between fat and air would give almost 100% reflection. Hence the need for a coupling gel between the transducer and the skin.
The specific acoustic impedance z is a ratio of acoustic pressure to specific flow, which is the same as flow per unit area, or flow velocity. In all cases, ‘acoustic’ refers to the oscillating component. With this proviso, we can say that acoustic impedance Z = pressure/flow and specific acoustic impedance z = pressure/velocity.
The change in the direction of a sound wave on being incident upon a tissue interface at an oblique angle is refraction and is determined by Snell’s law. Follow this link for an explanation of this law:
Tissue absorption of sound energy contributes most to the attenuation of an ultrasound wave in tissues.
Sound energy is attenuated or weakened as it passes through tissue because parts of it are reflected, scattered, absorbed, refracted or diffracted.
Not all echoes are reflected back to the probe. Some of it is scattered in all directions in a non-uniform manner. This is especially true for very small objects or rough surfaces. The part of the scattering that goes back to reach the transducer and generate images is called backscatter.
Decibels are one of the most confusing units of measurement that we use. It’s not like other measurements in that it stays the same every time. It’s not a set size or distance. The thing you must understand is that a Decibel is a relationship between two values of power or intensity (intensity is a measure of energy).
Generally the numbers that are being compared are of a vastly different magnitude and we need a way to compare them that is easily understood. This is where Decibels come in.
Decibels are a generally a ratio between units of power, intensity or amplitude.
Example 1: If Intensity or Amplitude of the wave has a factor of 4 = how many decibels?
Answer: 4 x or 2 x 2 = 6dB
The intensity, I, relative to a reference intensity Io, is defined as:
Relative intensity (dB) = 10log10 (I/Io)
Ultrasound scanning has different modes:
A mode (amplitude modulated display); A stands for Amplitude. Information of the reflected signal in a single ultrasound beam is continually displayed distance from the transducer and intensity are shown by position and amplitude in a line on an oscilloscope. They only give one-dimensional information and therefore are not useful for imaging. This mode is mainly of historical interest, may be rarely used in gynaecology or ophthalmology.
A-scans are used where the anatomy of a section is well known and a precise depth measurement is needed. One example is where the position of the midline of the brain is needed. Any delay could indicate the presence of a tumour or a fluid filled space.
B mode (Brightness modulated display); B stands for Brightness. In this case A-mode information from many beams, typically forming a sector in a plane of the body, is shown as pixel intensity on a monitor. B mode is often referred to as 2D, and is the most important modality for anatomic assessment and orientation in the body, also for localising and as a background for display of other information such as Doppler signals. It can be used to take an image of a cross-section through the body. The transducer is swept across the area and the time taken for pulses to return is used to determine distances, which are plotted as a series of dots on the image. B-Scans will give two-dimensional information about the cross-section.
The B-scan is the basis of two-dimensional scanning. The transducer is moved about to view the body from a variety of angles. The probe can be moved in a line (linear scan), or rotated from a particular position (sector scan).
The two movements (A and B) can be combined to give a compound scan. It requires considerable skill and a good knowledge of anatomy for the sonographer to get a decent image and to interpret it.
However it is a widely used technique for assessing the growth of the prenatal foetus. It can give early indications of any problems that may arise.
Ultrasound is used in other investigations such as detections of cysts, abscesses, and tumours.
Real time B-scans use a linear array of up to 100 transducers to get a cross section of the body. Moving images are possible.
M-mode; M stands for motion. This approach is used for the analysis of moving organs. It is based on A-mode data from a single ultrasound beam that are represented as function of time. This does not require a sweep through many ultrasound beams which allows for high temporal resolution.
The Doppler Effect in ultrasound is used to measure blood flow and can therefor locate blockages.
The Doppler Effect is observed whenever there is relative motion between the source of waves and the observer. The Doppler Effect can be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding. It is important to note that the effect does not result because of an actual change in the frequency of the source.
The above left image shows that a stationary blood cell reflects the incoming wave with the same wavelength: there is no Doppler shift.
The above middle image shows that a blood cell moving away from the probe reflects the incoming wave with a longer wavelength.
The above right image shows that as the blood cell moves towards the probe it reflects the incoming wave with a shorter wavelength.
So the Doppler mode exploits the frequency shift due to relative motion between two objects. With this approach information regarding blood velocity and cardiac valves can be obtained.
It is interesting to realise that there are two Doppler shifts occurring. The first occurs between the transmitted wave and the blood cell (with the probe acting as a source). But then, the beam echoes off the blood cell, so the moving blood cell behaves as a source and there is a second Doppler shift of the echoed wave, between the blood cell and the detector. Hence, the Doppler shifted frequency is twice what you would initially expect.
This is a Doppler ultrasound probe. It is being used to examine blood flow in the radial artery, the same one that you would use to measure your pulse.
There is some gel on the skin to make sure the probe makes a good contact so the sound can pass easily into the body.
Doppler mode can be obtained by continuous (CW) or pulsed wave (PW); in addition, velocity data can be shown as overlaying colour on B-mode images (colour Doppler, power Doppler and Tissue Doppler).
In continuous wave (CW) Doppler an ultrasound beam is sent in a single direction, and Doppler shift in the reflected sound is displayed. Since the sound is sent continuously there is no way to determine the time between emitted and reflected sound, therefore the depth where reflection occurs is indeterminate, but this modality allows determination of much higher velocities at large depth than PW Doppler.
A Doppler ultrasound may help diagnose many conditions, including:
Poorly functioning valves in your leg veins, which can cause blood or other fluids to pool in your legs (venous insufficiency)
Heart valve defects and congenital heart disease
A blocked artery (arterial occlusion)
Decreased blood circulation into your legs (peripheral artery disease)
Bulging arteries (aneurysms)
Narrowing of an artery, such as in your neck (carotid artery stenosis)
It can estimate how fast blood flows by measuring the rate of change in its pitch (frequency). During a Doppler ultrasound, a technician trained in ultrasound imaging (sonographer) presses a small hand-held device (transducer), about the size of a bar of soap, against your skin over the area of your body being examined, moving from one area to another as necessary.
PW Doppler flow image of blood flow – Doppler images showing blood flow in the carotid artery. Both the image and the Doppler data of the healthy carotid artery are clean and smooth as the smooth walled vessels lead to laminar flow. The partially blocked carotid artery causes turbulent flow, as seen from the red and blue regions in the Doppler image (blood flow towards and away from the probe).
The image above shows the Doppler ultrasound signal measured from a healthy artery. When the heart beats it pushes blood at a velocity of over 100cms^-1 away from the probe.
The information can be colour coded and combined with conventional ultrasound images, which is particularly useful in diagnosing blockages in blood vessels.
Doppler imaging looks at a carotid artery and produces an image and trace of blood flow.
The image below left is a combined image showing a healthy artery with the blood all flowing smoothly and in the same direction. This indicates that the blood vessel is healthy.
The image above right shows the flow is not all in the same direction in the artery. Some blood is moving away from the probe and some blood towards it. This is turbulent flow, like rapids in a river, and is caused by a blockage in the blood vessel. This blockage is usually due to the build-up of fatty deposits.
Advantages and Disadvantages of Ultrasound Scanning:
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.
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.