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Physics of musculoskeletal ultrasound

How it Works

The ultrasound image is created by first transmitting sound waves into the body and then interpreting the intensity of the reflected echoes.  This is achieved using a hand held probe which contacts the body via a water based gel.  The data collected is then processed within the body of the scanner and displayed as a black and white image generally referred to as grey scale.

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 and physics of musculoskeletal ultrasound essential.

Image construction

The probe contains a large number of transmitters (crystals) 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. The process is repeated sequentially along the length of the probe.  The time taken for an echo to return is used determine the distance from the probe and is calculated assuming that sound has a constant speed (1540m/s). The strength of the echoes returning from any point is represented by the brightness of that point on the screen.

 

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Figure 1: Time taken 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

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.

Figure 3: Reflections on still water are an example of specular reflection.
Figure 2: Reflections on still water are an example of specular reflection.

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Figure 3: Specular and diffuse reflection

Scattering

Scattering occurs when an interface is equivalent to 1 wavelength in size, which leads to the reflected echoes being scattered in many directions. When the interface is smaller than the wavelength, then echoes scatter in all directions equally, these are known as Rayleigh scatterers. This can be seen in Figure 3, termed diffuse reflection.

Features of an Ultrasound Image

Recognising structures on ultrasound takes practice and a knowledge of the anatomy does help!. What follows is a brief description of some of the features that make up the image.

Presentation

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, starting with skin.

Typical Appearance of Normal tissue

Figure 4: A summary of the appearance of tissues on musculoskeletal ultrasound
Figure 4: A summary of the appearance of tissues on musculoskeletal ultrasound

 

Muscle is dark, when viewed in transverse view. In longitudinal view echoes are reflected back by the muscle fibres and the internal structure of the muscle can be easily seen.

Figure 5: Transverse view of muscle tissue
Figure 5: Transverse view of muscle tissue

 

Linear patterning of a longitudinal view of muscle fibres
Figure 6: Linear patterning of a longitudinal view of muscle fibres

 

 

Fluid, be it blood, effusion or a cyst is generally black (anechoic), though thicker fluids such as puss can be bright or dark.
Fluid demonstrated in a Bakers cyst
Figure 7: Fluid demonstrated in a Bakers cyst
Tendons are typically bright, but this varies with their orientation relative to the probe. You can see a longitudinal view here of the long head of biceps tendon, sitting superficial to a tendon sheath effusion and the bright white cortex of the humerus (Figure 8).

Longitudinal view of long head of biceps tendon
Figure 8: Longitudinal view of long head of biceps tendon

Nerves..The median nerve can be seen in Figure 9, positioned in the centre, at the top of the screen, with a darker appearance to the surrounding circular tendons. They have a classic 'Pepperpot' appearance.

Transverse view of median nerve, demonstrated as a honeycomb/pepperpot appearance
Transverse view of median nerve, demonstrated as a honeycomb/pepperpot appearance


Bone appears as a particularly 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.

Figure 10: Bright white cortex of the humeral head
Figure 10: Bright white cortex of the humeral head

 

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