Physics, instrumentation and basic technique.
How it Works
The physics and the technology involved in ultrasound imaging has a profound effect on how structures appear. The dynamic nature of ultrasound scanning makes understanding of the processes involved essential.
Longitudinal ultrasound scan of biceps tendon within a distended sheath
Wayne Hedrick 2004 Ultrasound
Physics and Instrumentation Mosby;
For indepth review of the physics
Thomas Szaob, 2004 Diagnostic Ultrasound Imaging: Inside Out Academic Press
The probe contains a large number of transmitters set in a line along its length. Typically up to five of these firing simultaneously generate a short pulse of ultrasound that travels in a narrow column away from the probe. The transmitters then act as receivers and record the intensity of the reflected sound.
Figure 1 Time take for the transmitted pulse to be reflected back is used to calculated the distance of the reflecting boundary from the probe
The path that a single pulse passes along is described as the beam. The width of the beam determines the lateral resolution. The length of the pulse determines the axial resolution. Shorter pulses can be achieved using higher frequency, so the highest frequency practicable is generally used.
Different Types of Reflection
Two distinct patterns of reflection give rise to the echoes that make up an ultrasound image – specular reflection and scattering.
Specular reflection is responsible for the bright appearance of fibrous structures such as tendons and of boundaries between different tissues. It occurs when the sound wave meets a distinct surface (significantly larger than the wavelength of the ultrasound).
The process that occurs is similar to when light passes from air to water on the surface of a lake. Some of the light travels in to the water, while some is reflected back.
The amount of sound that is reflected at the boundary between two different tissues, such as fat and muscle, depends on how marked the difference is in their acoustic properties. Acoustic impedance, which is the measure of this, varies with the density and compressibility of the tissue.
Scattering gives rise to the characteristic texture (echo texture) of the image seen within soft tissue.
This occurs at the small (relative to the wave length of the ultrasound) subtle boundaries that exist within tissue. At these, small amounts of energy are absorbed and retransmitted in all directions as if from a point source, in a manor that loosely resembles a pebble dropped into a pond.Figure 2: Specular reflection and Scattering
Features of an Ultrasound Image
Recognising structures on ultrasound takes practice and a good knowledge of the anatomy is a big help. What follows is a brief description of some of the features that make up the image.
In almost all applications the top of the screen represents the probe and as you look further down the screen you are seeing progressively deeper tissues.
The depth to which you can see is normally shown on a scale running along side the image. This can be adjusted using the Depth adjustor, which will be a prominent control on any scanner. The maximum depth is dependent on the frequency, with lower frequencies penetrating much further, but at the cost of reduced resolution. Shoulder scanning is usually performed with high frequency probes using frequencies of around 10 MHz, which see reasonably well to a depth of 4cm or more.
Skin appears smooth and bright (echogenic, hyperechoic, highly reflective).
Fat can be bright or dark (hypoechoic), but subcutaneous fat is typically dark.
Muscle is also dark, when viewed in cross section. In long section sound is reflected back by the muscle fibres and the internal structure of the muscle can be easily seen.
Fluid, be it blood, effusion or cyst is generally black (anechoic), though thicker fluids such as puss can be bright or dark.
Tendons are typically bright, but this varies with their orientation relative to the probe.
Nerves are not normally seen when scanning the shoulder, but their appearance is similar to that of tendons.
Bone appears as a particularly bright line bright line due to the dramatic difference in acoustic impedance between bone and soft tissue. High frequency ultrasound does not penetrate bone effectively and therefore the screen is generally black deep to the bone.
Enhancement and Attenuation
One drawback of ultrasound is that each layer of tissue that is passed through reflects and absorbs the pulse to some extent, reducing the strength of the signal that reaches deeper tissue.
As less sound returns from deeper tissues than more superficial ones, the image is processed to compensate for this by applying a standard correction in proportion to the depth.
Some structures however allow sound to pass through them more easily than others. The most dramatic example is watery fluid, such as in an effusion, or in a cyst. These are described as being translucent. Because only a minimal amount of energy is absorbed by the fluid, the region that lies behind will receive more sound than the processor expects for that depth. This area will therefore appear uniformly brighter. This effect is called Enhancement.
Attenuation or Shadowing is the reverse effect, where some tissues absorb relatively more of the sound. The area of the image deep to this will appear darker. In the extreme almost no sound is transmitted, leaving a dark shadow behind the structure.
This effect is used as a diagnostic tool in identifying calculi, which if they are larger than the beam width cast a strong acoustic shadow.
This is the effect that makes a tendon appear bright when it runs at 90 Degrees to the ultrasound beam, but dark when the angle is changed.
The reason for this is that at particularly smooth boundaries, the angle of reflection and incidence are the same, just as they are with a conventional mirror. Thus the probe will only receive the reflected sound if the beam strikes the surface at a right angle
section of biceps
tendon in bicipital groove. On the left tendon appears bright,
This is how quickly the image is updated, and depends on how long the image takes to acquire. If it is too slow, the image becomes jerky and uncomfortable to view, and image quality can be lost due to movement. Too slow a frame rate is not normally a problem in scanning the shoulder. The most straight forward factors which slow down the frame rate are the number of focus levels used and the depth of the image.
Ultrasound is a very practical and personal skill, which take a considerable amount of practice to perfect. The following is a brief description of basic orientation of some of the controls that can be adjusted to improve the image.
Orientating the Probe
Probably the most important skill that must be learnt is orientation between hand and eye. This is made simpler if the probe and screen are in sync, i.e. if you move the probe to your left, the anatomy displayed on the screen appears to pass from left to right.
Because for the most common examinations (abdominal and gynae) patients in a radiology department are typically scanned on their backs, facing the sonographer, the convention is that the left side of the screen is right for the patient.
If the positioning of the subject is different then the orientation of the probe may change, so that when the probe is parallel to the display screen the left side of the probe corresponds to the left side of the screen.
It is also convention that when scanning in the sagittal or coronal planes, the left of the screen is superior and the right is inferior.
Positioning the Patient
There is no one best position for the opperator or the patient, but if either you or your subject is uncomfortable, you will struggle.
Adjusting the Controls
How much you adjust the machine controls and settings as you scan is very much a matter of taste. Some familiarity and understanding of the controls is essential especially if you are not the only one who uses your scanner.
When you press the freeze button the image displayed at that moment is captured on the screen so that measurements can be taken and a print can be made if required. Most modern machines will also have a Cine Loop function that allows you to scroll back through the preceding several seconds of the scan, frame by frame.
Increasing the depth allows deeper structures to be viewed, but reduces the scale and also slows down the frame rate, as each line of the image takes slightly longer to acquire.
The overall brightness of the image can be adjusted. Either too bright or too dark and it is difficult to see subtle differences in texture. This is the most important adjustment to become accustomed to making.
TGC (Time Gain Compensation)
Gain can also be adjusted selectively at different depths. This can be a simply near or far field adjustment on some portables, up to 10 separate depth adjustments on platform based machines. This is used to compensate for strong attenuation or enhancement by superficial tissue. It is more useful when examining an area. When examining relatively small specific structures, as is the case in the shoulder, adjustment of the overall gain is usually sufficient.
The pulse of ultrasound can be manipulated to be at its narrowest at a particular depth. This means that image quality including lateral resolution is maximised at that level. This can be manually adjusted so that a particular area can be examined in more detail. More than one focus level can be selected, though this can significantly slow down the frame rate.
This takes a portion of the screen and magnifies it. This can be done while scanning or once the image has been frozen. For superficial structures it is normally easier to magnify by just to reducing the depth of the image. For deep structures it is necessary to use the zoom; however orientation is more difficult if zoom is used while scanning.
Cursers are available on all modern machines and are calibrated so that reasonably accurate measurements can be made.
Taking Pictures and Labelling
It is foolhardy to form an opinion based solely on a still ultrasound image and so any conclusions should be drawn while actually scanning the patient. Images are of value as aid memoirs and for demonstration and discussion. To this end it can be useful to label them, if only with left or right.
The ultrasound image is produced assuming that the returning echoes faithfully represent the underlying tissue. Certain conditions can cause significant differences to occur. These are called artefacts.
Attenuation, enhancement and anisotropy described above fall into this category, and can be helpful in scanning. Conversely anisotropy can make part of a tendon appear bright while an adjacent section that is not at right angles to the beam appears dark. This can give the impression that there is a tear present. Changing the angle of the probe dispels this illusion and you will quickly become accustomed to this effect.
There are a number of other common artefacts, several of which are listed below (this list is by no means comprehensive)
Mirror images – This is where a strong reflector at an angle to the probe causes structures that lie in front and to the side of it to appear as if they lie behind it, just as something viewed through a mirror appears to lie behind it. This effect is normally only achieved by the diaphragm.
artefact. Percistant haematoma anterior to the tibia
Reverberation – this causes evenly spaced lines at increasing depths and is caused by sound reflecting back and forth between the surface of the probe and a strong reflector close to the surface.
Comet tail – this is the same process as reverberation, but occurs within a very small structure, with smooth highly reflective borders, such as a metal fragment. Tiny bright reverberations are seen deep to the structure slowly diminishing is size as if it had a tail.
Refraction – Sound is refracted in the same way that light is as it passes from one medium to another. Thus the direction in which it travels changes when it passes through a boundary at an angle less than 90 degrees. This can lead to subtle miss placement of structures and some degradation of image quality when the angle of incidence is particularly acute.
Ghost images – This is a dramatic example of refraction, where a structure is represented twice or more side by side. This classically occurs deep to Rectus Abdominis which due to its shape acts as a lens and can lead to the apparent duplication of the aorta or early foetal sack.
Range distortion – Ultrasound travels at slightly different speeds through different tissues. A rough average of 1540m/s is used, but the velocity through fat (~1460m/s) and water (~1480m/s) is somewhat slower. Structures deep to a large fluid collection can therefore appear a little further away than they actually are.
Side Lobe Artefacts – the probe cannot produce a pulse that travels purely in one direction. Pulses also travel off at specific angles. These side lobes are relatively weak and so normally do little to degrade the image. Their effect is only normally seen faintly superimposed in fluid filled areas which are anechoic and so do not obscure the weak side lobe reflections. The exception is when a side lobe strikes a particularly strong reflector at 90 degrees. In this case the reflector can appear within the image.
Partial volume – the slice that makes up the ultrasound image is 3 dimensional, just as with MR. This typically means that fluid filled areas, where they are very small or adjacent to soft tissue will not appear anechoic (Black) as would be expected, but often contain low level echoes which can be mistaken for debris or even soft tissue.
These notes cover some of the basics of scanning, but serve only as an introduction.
Further study of the physics and technology will greatly improve your ability to use and interpret ultrasound scans. Formal training is the ideal, but as yet is very hard to come by in this country, though we are looking into ways we can address this.
There are a large number of textbooks covering the physics of ultrasound and most clinical texts will have a separate chapter covering the subject. These are worth taking time to study. Good luck!
For straightforward observational tasks such as evaluating the recruitment of muscles and biofeedback, the operator needs to have a basic understanding of the main controls, along with an overview of the way the image is acquired and the safety issues.
When ultrasound is used to make objective measurement or to monitor changes in structures, a greater understanding of the way in which the image is formed and displayed is needed if such observations are to be accurate and reproducible.
An indepth knowledge of how different conditions effect the ultrasound image and how and where artefact occur is required for many diagnostic applications and this level of knowledge is best acquired through a formal physic of ultrasound course of the type offered by colleges that provide sonographer training.
Low energy ultrasound waves can generally be treated as sinusoidal waves when considering their interaction with soft tissue. As the intensity of the ultrasound increases, the shape of the wave changes and it displays non linear properties which lead to increased absorption, streaming and cavitation.
The rate at which the ultrasound is absorbed as it passes through soft tissues is dependent on the frequency
The passage of ultrasound produces eddy effects known as streaming within the liquid elements of soft tissue. This effect is credited with many of the benefits ascribed to conventional therapeutic ultrasound.
A more dramatic mechanical effect is cavitation. This is where tiny collections of gas, known as microbubles oscilate in response to the ultrasound beam. This can cause them to enlarge and then spontaneously implode. The resultant release of energy has the potential to damage delicate soft tissue structures.