Abstract
Ultrasound image acquisition and interpretation are highly reliant on a reasonable depth of understanding of ‘how ultrasound works’. Where this is lacking, there is considerable scope for confusion. In a point-of-care setting, where novice practitioners may have limited direct supervision, this may lead to significant errors in diagnosis and subsequent patient management. This paper provides a brief introduction to the underlying physical principles of ultrasound and discussion of how these principles can be used to explain typical image appearances.
Keywords
Most point-of-care users begin their scanning experience with a reasonably good understanding of the basics of how ultrasound works. The aim of this brief physics overview is to explore in a bit more depth some of the physical principles of how sound behaves, to look at some of the compromises inherent in ultrasound image production and to consider how these principles may explain typical image appearances. The importance of equipment set-up in producing a diagnostic image will be covered in a subsequent paper.
What is ultrasound?
Sound of any description is simply the transfer of mechanical energy from a vibrating source through a medium. Ultrasound is defined as sound of a frequency above the human audible range, i.e. above 20 kHz. For most medical applications frequencies in the region of 1–15 MHz (1–15 million cycles per second) are required.
Piezoelectric materials within the face of the transducer contract or expand when a voltage is applied across them. A thin layer of a synthetic piezoelectric material (such as lead zirconate titanate – PZT) can be constructed to vibrate at a resonant frequency within the required range. This acts as a source of ultrasound. As the pressure wave of a returning ‘echo’ hits the transducer surface, a voltage is registered. The magnitude of this voltage is related directly to the amount of energy carried by the returning echo and will determine the brightness level stored from this location and displayed on the monitor. (It is worth noting that this construction of sensitive crystal elements does not respond favourably if the transducer is dropped.)
How are ultrasound images produced?
Diagnostic ultrasound utilizes the pulse-echo principle to construct a two-dimensional image of anatomical structures. This is essentially the same principle bats use to catch insects through echo-location. A pulse of sound leaving the transducer will travel into the patient until it encounters an ‘acoustic’ interface. At such an interface, a proportion of the sound energy is reflected back to the transducer and this return echo is detected. If the speed of sound is known and the time taken for the echo to return is measured, the depth of the reflecting interface can be calculated (Figure 1).
Time taken for echo to return and the speed of sound in soft tissue can be used to calculate the depth of the reflecting interface
Constructing the image
Each pulse of sound transmitted into the patient generates a stream of returning echoes from multiple reflecting and scattering targets at various depths within the tissue. The mechanical energy carried by each echo is converted into electrical energy by the piezoelectric crystals within the transducer. In simple terms, these values are then stored within the ultrasound machine's computer memory as a single ‘scan line’ of information; the voltage values are used to determine the brightness levels allocated to points in a vertical line on the image to represent the interfaces at corresponding depths in the patient. By firing pulses of sound in sequence from multiple adjacent crystals across the face of the transducer, numerous contiguous scan lines can be generated and a single brightness mode (B-mode) ‘frame’ of information produced to represent a two-dimensional anatomical cross-section.
If performed fast enough, rapid update of frames can create a ‘realtime’ image. Frame rate, however, is limited by several factors. In most systems, the equipment must wait for echoes to return from the maximum depth of interest along each scan line before the next pulse is sent out. The time taken to produce each frame depends on the depth of interest and the total number of scan lines in each frame. Frame rate is therefore limited by the width of the field of view (how many scan lines are required across the image), the line density (the number of lines per mm width in the image) and the depth of view selected (the deepest location from which echoes are displayed). Prioritizing between depth, field of view, spatial resolution and frame rate is one of the key compromises that the user needs to consider when setting equipment controls. (Imaging half way through the couch offers little useful diagnostic information and can seriously compromise image quality.)
Further compromise is found in the way that echo signal values are translated into a greyscale image. Due to the finite memory capacity of the computer, and the limited range of brightness levels available on the display monitor, it is impossible to either store or display the whole range of signal values that are generated within the patient. Significant ‘compression’ of the returning signals is necessary before they are displayed on the viewing monitor. Application-specific pre-sets are used to achieve this. For most point-of-care examinations, if the correct pre-set is selected, these processing curves rarely need to be altered. (Application-specific pre-sets will be discussed next in this series of papers.)
It is worth noting that the display monitor and room lighting also affect image quality. This can be a real challenge, particularly when a small flat screen monitor is viewed in high ambient lighting conditions.
Making sense of ultrasound images
During an ultrasound examination, most of the diagnostic conclusions about both normal and abnormal appearances are based on pattern recognition. This includes a number of key observations:
The spatial definition of tissue boundaries; Relative tissue reflectivity; Echo texture; The effect of tissue on the through transmission of sound.
These appearances are determined by the assumptions that the equipment is making when the image is constructed, the settings selected by the operator and by the way in which sound waves interact with tissue. Some of these key interactions are outline below.
What happens to a pulse of sound as it travels through a patient?
In many ways, sound and light behave in a similar fashion. Concepts such as reflection, scattering and refraction are common to both. An appreciation of this behaviour will help trainees gain a better understanding of why structures appear as they do in an ultrasound image. This is particularly important when learning to distinguish between artefact, normal and pathological appearances.
Acoustic impedance
Acoustic impedance (Z) is a measure of the response that particles within a specific tissue make as sound waves pass through it. Changes in pressure associated with transmission of the sound wave give rise to particle displacement. The acceleration and degree of this displacement depends on the impedance of the tissue; impedance is determined by tissue density and compressibility. Tissues that are dense and have low compressibility (e.g. bone) have high impedance and tissues that are less dense and more compressible (e.g. fat) have low impedance. At an interface where there is a change in impedance (an acoustic boundary), sound energy will be reflected.
Reflection
Reflection of the ultrasound pulse occurs when there is a change in acoustic impedance. A proportion of the sound energy is reflected and the remaining sound energy is transmitted beyond the boundary.
If the boundary is smooth and large compared with the wavelength of the sound,
As with light, where the sound pulse hits a boundary at an angle other than 90° it will be reflected at an equal and opposite angle, where the angle of incidence equals the angle of reflection. This equates to the basic ‘law of reflection’ that we observe when light hits a highly reflective surface. If the angle at which the sound hits a boundary is sufficiently steep, the reflected echo may not be detected by the transducer. In practice, boundaries will be detected most clearly, if insonated at 90°.
This angle dependence of specular reflection can be seen clearly when imaging the abdominal aorta. The vessel walls are better visualized if the transducer is tilted towards the patient's feet and the aorta is viewed at 90° (Figure 2).
The divergent field of view produced by a curved array transducer demonstrates clearly the effect of angle of insonation on the visualization of vessel walls. Beam to vessel angle varies across the image. A pulse of sound hitting the wall at 90° (yellow arrow) will be reflected back to the transducer. A pulse hitting the wall at an angle less than 90° will be reflected at an equal and opposite angle
Scattering and echo texture
Acoustic impedance changes occur at large-scale boundaries, but are also present throughout soft tissue structures. Small-scale localized changes in acoustic property act as tiny reflecting targets that scatter the sound in many directions. This is what produces the characteristic echo texture (graininess) associated with solid structures on ultrasound and the relative echogenicity (brightness) of adjacent organs.
Attenuation
As sound travels through tissues, it loses energy. A number of interactions contribute to this process of attenuation including reflection, scattering and absorption. This results in the pulse becoming progressively lower in intensity (and therefore producing weaker echoes) the deeper it travels into the patient.
If the impedance difference is high enough, for example at a soft tissue/air interface, total attenuation occurs – all of the sound energy will be reflected and none transmitted to deeper structures.
Scattering contributes to beam attenuation and is highly frequency dependent. This results in increased attenuation, and thus reduced penetration of the sound beam to deeper structures, when higher transmit frequencies are selected.
Absorption
Absorption is the process by which the mechanical energy carried by the pulse is converted into heat within the tissues. The exact mechanisms for energy loss are not fully understood, but absorption is the most significant form of attenuation in soft tissue.
As sound travels though the patient, there is the potential for tissue damage, either through heating or mechanical effects. Anyone using ultrasound within their clinical practice should be familiar with current safety statements. The British Medical Ultrasound Society Safety Statements and Guidelines (BMUS 2010)
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can be found in full at
Beam penetration
The frequency-dependent nature of both absorption and scattering results in limited ability to penetrate to deeper tissues when using high-frequency transducers. Unfortunately, high-frequency pulses produce better image resolution. Therefore, there is always a compromise between image quality and penetration. In practice, use the highest frequency that allows adequate penetration to the depth of interest.
Ultrasound artefacts – avoiding the obvious pitfalls
In any diagnostic application of ultrasound, the recognition of what is ‘real’ in the image and what is ‘artefact’ is an important part of the image interpretation process. Even in the hands of a relative ‘expert’ ultrasound appearances can be misleading and may result in significant patient mismanagement.
In the context of clinical imaging, artefacts are appearances within the image that do not correspond directly to the reality of structures within the patient.
However, ultrasound artefacts are not always undesirable. Indeed some pathologies (for example the presence of pneumothorax) are recognized and characterized by the artefacts they produce. To avoid confusion, it is helpful to have a clear understanding of how ultrasound artefacts originate.
Some false assumptions
In ultrasound imaging, most artefacts arise because of one or more false assumptions that are basic design features of all ultrasound equipment. These assumptions include that:
The speed of sound is constant; Sound always travels in a straight line along the axis of the beam; The beam has negligible thickness; The rate of attenuation in tissue is constant.
There are a number of features of both ultrasound transducer design and sound propagation that violate these basic assumptions and result in image artefacts.
Some common artefacts are outlined.
Acoustic shadowing
Acoustic shadowing is probably the easiest artefact to identify and can be the most frustrating. Shadowing occurs when the proportion of sound energy transmitted beyond a specific target is too small to produce detectable echo signals. This occurs when the target attenuates a high proportion of the energy carried by the pulse through reflection or absorption. Shadowing from ribs and bowel gas can limit views of abdominal anatomy and a flexible scanning technique is needed to work around this. Novice ultrasound users often marvel at the apparent ease with which an experienced sonographer can produce an excellent view of a structure that seemed unobtainable. This is particularly common in abdominal examinations where the novice may persist in trying to scan through bowel or rib shadowing. The ‘expert’ simply moves the transducer searches for an alternative acoustic window that avoids shadowing artefacts and the structure of interest appears as if by magic.
However, shadowing is not always problematic and can be helpful in identifying pathology such as gallstones or renal calculi.
Increased through transmission
Fluid filled structures appear echo-free or ‘anechoic’ within the image; i.e. they are displayed as uniformly black. As the sound pulse travels through fluid it encounters no changes in acoustic impedance. Since the fluid contains no scattering targets, no reflected echoes are returned to the transducer and attenuation is minimal. Therefore, more energy penetrates to structures deep to the fluid and they, in turn, will appear brighter.
A fluid filled space can be identified with confidence from this anechoic appearance and the presence of increased through transmission. This is apparent when scanning through a full urinary bladder. Care must be taken to adjust time gain compensation settings to compensate for this increase through transmission or echoes from deep pelvic structures may be saturated. This is particularly important during FAST examinations looking for free fluid in the pelvis.
Mirror image artefact
A large soft tissue–air interface such as the lung/hemi-diaphragm may act as an acoustic mirror. At an air interface, impedance mismatch is so high that total reflection of sound occurs. When imaging the right lobe of liver and right hemi-diaphragm in a normal patient, a mirror image of the liver can be seen above the diaphragm (Figure 3).
At the lung interface, the air–soft tissue interface acts as an acoustic mirror, producing a ‘mirror image’ of the liver above the right hemi-diaphragm
A pulse of sound travelling through the liver bounces off the highly reflective lung interface and is directed back into the liver at an equal and opposite angle. Echoes from structures within the liver are subsequently reflected back to the diaphragm and on to the transducer. The equipment assumes a single, straight-line journey, and will therefore place these echoes above the diaphragm. The absence of this appearance can indicate the presence of pathology within the chest.
Refraction artefacts
Ultrasound equipment is programmed to assume that sound travels in straight lines and that received echoes have come from a source directly in line with the transmitted sound pulse. This is not always a correct assumption. Refraction occurs when the ultrasound beam crosses a boundary between tissues that have different speeds of sound propagation and the sound beam ‘bends’ from its original path.
If we observe a straw placed within a glass of water, it is easy to appreciate the effect of optical refraction. The impact of acoustic refraction is far less obvious, but can have a significant impact on image quality. It can also result in some interesting artefacts. An example of this can be seen when imaging the abdominal aorta in transverse section. When the anterior abdominal muscles are tensed, refraction of the beam can produce a ‘double image’ of the mid line vessels (Figure 4).
Refraction artefact producing apparent duplication of the aorta and superior mesenteric artery
In practice, if the ultrasound beam hits a boundary at 90° no refraction occurs, as there is no relative increase in the speed of either edge of the wave front. Refraction artefact can therefore be reduced considerably by moving the transducer and scanning from a variety of different angles.
Reverberation
Sometimes, particularly just below a strongly reflecting superficial interface that lies parallel to the transducer face, multiple horizontal linear echoes are displayed that do not represent real tissue boundaries. This reverberation artefact is due to multiple sound reflections between the interface and the transducer. Since the reverberation echo has taken twice as long to arrive, the scanner assumes that the reflecting interface is at twice the depth. As the reverberating pulse-echo travels increasing distances through tissue it undergoes corresponding attenuation. The reverberation lines therefore diminish in brightness with depth.
Comet tail artefact
These fleeting artefacts are seen when scanning over regions of bowel gas or at other small air interfaces. Comet tail artefacts are generated where reverberation occurs within a small but highly reflective object. This short path reverberation produces a series of closely spaced echoes giving a characteristic banded appearance. The appearance of comet tail artefacts at the pleural interface is helpful in the exclusion of pneumothorax in the normal patient (Figure 5).
Comet tail artefacts seen at normal pleural interface
Slice thickness artefact
Since ultrasound is viewed as a series of two-dimensional images, it is easy to think of them representing anatomy in two dimensions only. In reality, the ultrasound beam is three-dimensional and the tissues imaged are interrogated in slices of finite width. The operator has no control of this and simply needs to be aware of resultant image appearances.
The effect of slice thickness artefact can be appreciated when imaging vascular structures. The lumen of the vessel may appear to contain low level echoes that are in reality generated by adjacent soft tissue.
How does Doppler work?
The Doppler effect is named after Christian Andreas Doppler (1803–1853), the Austrian mathematician who first described it. This is the phenomenon that causes a stationary observer to hear a change in the pitch of a police siren or racing car as it speeds past them. The effect can be used in medical ultrasound to detect and evaluate blood flow.
When a pulse of sound is reflected from a target that is moving either towards or away from the transducer, the reflected echo will have a different frequency to that of the transmitted pulse. If the reflecting target is moving towards the transducer, the echo frequency will be higher than the transmit frequency; if the target is moving away the echo frequency will be lower. The Doppler signal can be displayed as either a colour-coded overlay on the B-mode image – colour flow mapping, or as a graphical representation of the velocity profile over time at a specific operator selected anatomical location – spectral Doppler.
Power Doppler displays the amplitude (strength) of the frequency shift signal, rather than the actual frequency shift itself. As such, velocity and directional information is lost, but the technique is more sensitive to weaker signals from smaller vessels than conventional colour flow Doppler.
In practice, Doppler is often regarded by point-of-care users as either a ‘dark art’ to be avoided or a ‘magic button’ that makes vessels appear, in full colour, out of a fog of grey. In fact, when using Doppler, a great deal of care does need to be taken in selecting appropriate equipment settings and a few basic limitations need to be remembered if confusion is to be avoided.
Angle matters
The Doppler shift is highly angle dependent. The greatest frequency shift occurs if the sound pulse encounters blood cells moving either directly towards or away from the transducer (i.e. looking along the length of the vessel lumen). However, when the sound pulse is travelling perpendicular to the vessel (i.e. the vessel is imaged at 90°) no frequency shift occurs and no flow will be detected.
Deep is difficult
One of the key limitations of Doppler imaging is that the signal is always weak compared with acoustic noise or ‘clutter’ within the image. In a standard B-mode image, if equipment settings are correct, the lumen of a vessel should appear black. This indicates that there are no strong reflectors within the vessel. Scatter from blood cells is too weak to be allocated a grey level, even if the vessel is superficial. Obtaining a Doppler signal from a deep vessel is particularly challenging and problems with penetration may lead to the false assumption that there is no flow.
In a point-of-care setting, Doppler is generally used simply to identify vascular structures rather than acquire detailed quantitative data. However, to avoid confusion, it is well worth investing some time exploring this useful technique in more detail. As a basic rule of thumb, if no flow is seen, do not assume that no flow is present or that the structure is non-vascular.
Thrush and Hartshorne 4 provide an excellent overview of how Doppler works.
Summary
Ultrasound is a seductive tool. It can be enormously useful and the literature is overflowing with evidence of its potential in point-of-care settings. Understandably, it is not uncommon for point-of-care users to be carried along on an initial wave of enthusiasm for how this technique can change their own practice. However, getting to grips with how ultrasound works and recognizing its limitations is essential if confusion is to be avoided.
Physics textbooks are rarely standard coffee table reading of choice, but those listed below are accessible and well worth the investment of time and a modest outlay. 2,3,4
The next in this series of papers will focus on equipment controls and the art of image optimization.