Mirror artefacts appear when a strong reflector angled to the probe causes structures to be mirrored on the other side of the strong reflector.
This effect is normally only produced by the diaphragm where for example anatomical structures in the liver are falsely mirrored in the lung.
It is possible to avoid this artefact by choosing a different examination position parallel to the strong reflector.
Many acoustic artefacts seen during an ultrasound examination can be directly attributed to the incorrect use of the ultrasonic hardware.
The most common mistakes are overgain and undergain artefacts.
Too low gain settings may result in the apparent absence of an existing structure (i.e.
A ghost image is an example of a more extreme refraction, where a structure is represented twice or more side by side.
This usually occurs deep to rectus abdominal muscles, which because of its shape acts as a lens and can produce the apparent duplication of the aorta.
Inappropriate focus points may result in beam width artefacts where echoes generated by the object located in the peripheral field are displayed as overlapping the object of interest.
The importance of adjusting the focal zone to the depth of the target structure is shown in the illustration.
You are now familiar with the ultrasound system, the probe and how to optimize the ultrasound image:
? Preparation of the ultrasound (US) system
? Selection of the appropriate ultrasound probe
? Appropriate placement of the US system, the patient and yourself
? Probe orientation, grip and movement
? Anatomy planes
? Acoustic coupling with ultrasound gel
? How the ultrasound beam is equivalent to a tissue slice
? How to optimise the image quality (depth, gain and focus)
? Imaging modes (B-mode, M-Mode, Colour Doppler and Power Doppler)
? Freeze, save, measure
? Image recognition
When performing ultrasonography, image artefacts are commonly encountered and may be confusing for the physician. Some artefacts can be avoided using correct scanning technique. Other artefacts are generated by the physical limitations of the modality.
Ultrasound artefacts arise secondary to errors inherent to the ultrasound beam characteristics, the presence of multiple echo paths, velocity errors, and attenuation errors.
Shadowing, refraction, reverberation, comet tail, ring-down and mirror image are some routinely encountered artefacts in clinical ultrasonography practice.
The learning objective of this module is to recognize, interpret and remedy potentially correctable US artefacts because it is important for image quality improvement and optimal patient care.
When the ultrasound pulse meets a border between two different tissues with a large difference in acoustic impedance, the pulse is almost entirely reflected by the tissue border.
That is why the surface of bone looks hyperechoic (white) with an anechoic (black) acoustic shadow extending from the bony surface to the bottom of the image on the screen.
On a frozen image it is possible to activate the “caliper” button in order to make measurements.
By moving a finger tip on the touch pad, the primary measurement point can be chosen. It is fixed by pushing the “Select” button. Then, by moving the finger tip on the touch pad, the second measurement point can be chosen and fixed by pushing the “Select” button. A connecting line and the distance between the two points are then displayed on the monitor.
Press the B-mode (2D button) to return to default 2D greyscale imaging.
Ultrasound cannot penetrate the airfilled lung parenchyma, because of the large difference in acoustic impedance between soft tissues and air.
The apparent lung parenchyma looks hypoechoic (greyish) but it is a reverberation artefact (see the next module about artefacts).
When an image is frozen it can be saved by pushing the “Save” button.
Pushing the “Clip” button saves a video clip. Usually without prior freezing.
Many ultrasound systems only allow saving images or video clips, if the recording has been named prior to saving.
The images and clips are saved in the archive on the hard disk of the ultrasound system. It can usually be retrieved and exported to USB or other external devices.
