ULTRASOUND ARTEFACTS

Artefacts occur when any of four assumptions are broken:

- The speed of sound in tissue is constant

- Attenuation is constant

- The beam travels in a straight line

- Echoes arise from the centre of the beam

Speed of sound artefacts

Ultrasound machines are calibrated to assume a particular speed of sound in tissue. Unless a specific pre-set is selected, this is usually 1540 m/s. Most tissues within the body are within 5% of this value, but there are some exceptions. Fat has a significantly slower speed of sound, and bone far higher. This is due to the fact that speed of sound is related to both the density and stiffness of a material 

This can cause problem when performing measurements, if the measurement point crosses two or more different media each with a different speed of sound. It can be particularly problematic where boundaries are uneven (such as an uneven fat layer). Whilst tissue harmonic imaging can reduce the appearance of artefactual noise and reverberations, it cannot do anything about speed of sound artefacts.

A mismatch between the assumed and actual speed of sound can also cause phase aberration in scanners using electronic beam forming techniques. Electronic beam formers implement time delays, calibrated to the assumed speed of sound, in order to set focal points. Elements furthest away from the desired focal depth are excited before those closest to the focal zone, to ensure that all of the wave fronts arrive at the focal point together and add constructively.

When the speed of sound in the medium is different to the speed of sound assumed by the ultrasound scanner, the focal zone is misplaced.

Reverberation Artefacts

Reverberation artefacts are common beneath prosthetic valves in the heart, or gas in the bowel, as shown here by Sara Lamont on the Vet Image Solutions Facebook group.

An interface is highly reflective when there is a strong acoustic mismatch between two boundaries. The amplitude reflection coefficient is defined by the ratio between the reflected and transmitted amplitudes, i.e.:

Since acoustic impedance (z) is defined by , i.e. by the medium’s density and stiffness, it is not difficult to see why gas or bone would have a very different acoustic impedance to surrounding soft tissue.

Mirror image artefacts

Mirror image artefacts can occur when the ultrasound pulse hits a large, specular reflector and a significant amount of ultrasound energy is reflected at an angle away from the transducer. If this then encounters a scatterer and is reflected back along the path from which it originated, the specular reflector functions as a mirror, directing the echo back to the transducer face. This means that the transducer will place the specular reflector twice – once according to the time taken to receive the first echo, and once according to the time taken to receive the second, resulting in two images of the same reflector. Such an artefact can be eliminated by changing the angle of incidence, or decreasing the depth so as to not allow sufficient time for the second echo to return to the transducer.

Attenuation artefact

Attenuation occurs through absorption, reflection and scattering, and divergence. Certain aspects of attenuation are predictable. The rate of beam divergence in the far field is determined by the aperture of the source and the ultrasound wavelengths present. Attenuation also increases linearly with frequency (the attenuation coefficient in dB/cm/MHz), as it is a combination of the absorption coefficient and the scattering coefficient, both of which are themselves frequency dependent.

In the case of absorption, particle displacement at high frequencies cannot keep up with the pressure fluctuations of the passing wave and more energy is lost from the wave as heat. There is also an element of loss due to increased friction. In th case of scattering, scattering power (Raleigh scattering) is defined as being proportional to the sixth power of the diameter of the scatterer, and the forth power of the ultrasound frequency.

All of these known factors mean that attenuation can be assumed in soft tissues to follow a logarithmic decay curve, and the ultrasound scanner assumes a constant rate attenuation. When passing through an anechoic region, however, ultrasound is attenuated far less than expected. This results in the system’s TGC (time-gain control) over-compensating, and regions distal to the echo-free region are therefore shown as bright white or ‘over gained,' as seen in the below image from Vicky Cole.

The opposite effect can also occur distal to highly attenuating structures, which is the reason for the presence of a manually adjustable TGC scale. Distal to very strongly reflective materials, however (such as bone or gas, due to the high impedance mismatch between them and surrounding tissue), a complete acoustic shadow is likely to occur, beyond which no structures can be seen.

Refraction Artefact

Refraction artefacts arise from a changing in direction of the beam when there is a change in the speed of sound across an interface (recall that ). At a curved interface, this can sometimes cause an 'edge shadowing' artefact, when the beam is refracted away from the 'straight ahead' direction, thus creating an area where no echoes return. Many scanners now employ compound imaging, which offers one solution to this problem.

Beam width artefacts

The ultrasound beam has a width in both the scan plane and the elevation plane. The beam width in the elevation plane cannot be electronically focused unless using a 1D probe, and for this reason, its focus is usually at a fixed length set by an acoustic lens. The beam width in the scan plane depends upon the lateral resolution of the scanner and the position of the electronic focus.

The ultrasound scanner assumes that any echo along a scan line has originated from the centre of the beam. When scanning at depth, divergence can result in a considerable widening of the beam. A structure insonated at the edge of the beam will thus be incorrect

Sidelobes & Grating lobes