Ultrasound images of various soft tissue organs- including a description of artefacts

Søren Torp-Pedersen1 and Jens E. Wilhjelm2

1Head of Rheumatological US, The Parker Institute, Frederiksberg Hosp., University of Copenhagen,

Nordre Fasanvej 57, DK-2000 Frederiksberg, DK2DTU Elektro, Ørsteds plads, building 348

Technical University of Denmark, 2800 Kgs. Lyngby(Ver. 1 26/11/09) © 2001-2009 by S. Torp-Pedersen & J. E. Wilhjelm

1 IntroductionThis document provides a number of examples of ultrasound images of various soft tissue organs.

The images are commented in the main text, but the letters specifying different organs are not placed on the images. This is up to the reader to put these on. All aspects discussed in these notes are based on clinical images, unless otherwise noted.

2 Clinical imagesAll images were recorded with a Logiq E9 ultrasound system (General Electric, ) 15MHz except the

abdominal images, which were recorded with a transducer with lower frequency. The scanner applied spatial compounding, speckle reduction imaging and harmonic imaging.

Figure 1 Thyroid scanned transversally (Danish: Thyreoidea transverselt).

1/11

Figure 1 shows a transverse scan of the right thyroid lobe. The thyroid (T) is seen as a light gray ho-mogeneous substance. Cc =common carotid artery, Tr =trachea (contains air artefact), M = muscles. S = shadow from tendon. Figure 2 shows a longitudinal scan of the left common carotid artery (Cc). M = muscles.

Figure 3 shows a longitudinal scan of the quadriceps tendon (T). P = patella, Sc = subcutaneous fat, J = joint cavity, F = femoral bone.

Figure 2 Arteria Carotis communis scanned longitudinally. The head is at the left on the image (Dan-ish: Arteria carotis communis longitudinelt).

Figure 3 Knee scanned suprapaterrally longitudinally. (Danish: Knæ suprapatellart longitudinelt)

2/11

Figure 4 shows a longitudinal frontal scan of the medial joint line in the right knee. M = meniscus, F = femur, T = tibia, MCL = medial collateral ligament, Sc = subcutaneous fat.

Figure 5 shows the right heel pad, longitudinal scan with light pressure. C = calcaneus. The calcaneus does not appear very clearly in this image. How can that be? Please make a drawing to try to answer this question.

Figure 4 Knee medial joint line in the right knee (Danish: Knæ mediale ledlinje frontalt).

Figure 5 Heel pad scanned longitudinally with light pressure (Danish: Hælpude longitudinelt let tryk).

3/11

Figure 6 shows the right heel pad, longitudinal scan harder pressure. C = calcaneus. The calcaneus is now easier to see..

Figure 7 shows a longitudinal scan of the left Achilles tendon (T). C = calcaneus, B = bursa.

Figure 6 (Heel pad scanned longitudinally with larger pressure than in previous figure (Danish: Hælpude longitudinelt med hårdere tryk end forrige figur).

Figure 7 Achilles tendon insertion scanned longitudinally (Danish: Achillessene insertion longi-tudinelt).

4/11

Figure 8 shows a longitudinal scan of the left Achilles tendon (T). M = muscle, Sc = subcutaneous fat.

Figure 9 shows a longitudinal scan of the flexor muscles of the forearm. M = muscle, V = vessel.

Figure 8 Achilles tendon scanned longitudinally. (Danish: Achillessene longitudinelt).

Figure 9 Flexor muscle scanned longitudinally. (Danish: Underarm flexormuskel longitudinelt)

5/11

Figure 10 shows the Achilles tendon (T) in transverse scan. F = fat.

Figure 11 shows a transverse scan of the flexor muscles of the forearm – orthogonal insonation. M = muscle, B = bone, Sc = subcutaneous fat.

Figure 10 Achilles tendon scanned transversally. (Danish: Achillessene transverselt).

Figure 11 Muscles in lower arm. (Danish: Underarm flexormuskel transverselt).

6/11

Figure 12 shows a longitudinal scan of the flexor muscles of the forearm with oblique insonation on the long axis of the muslces. The muscles (except one) appear more hypoechoic.

Figure 13 shows a transverse scan of the right liver lobe. The diaphragm is seen as a white line in the bottom of the image.

Figure 12 Muscles in lower arm. (Danish: Underarm flexormuskel transverselt skrå).

Figure 13 Liver. (Danish: Lever i epigastriet transverselt).

7/11

Figure 14 shows a longitudinal scan of the left liver lobe (L). V = vena cava.

Figure 15 shows a longitudinal scan of the left kidney (K). S = kidney sinus, B = bowel.

Figure 14 Liver scanned longitudinally. (Danish: Lever i epigastriet longitudinelt).

Figure 15 Kidney (left) scanned longitudinally. (Danish: Venstre nyre longitudinelt).

8/11

Figure 16 shows a longitudinal scan of the right kidney (K) seen through the liver (L).

Figure 17 shows a transverse scan of the right kidney (K) seen through the liver (L).

Figure 16 Kidney (right) scanned longitudinally (Danish: Højre nyre longitudinelt).

Figure 17 Kidney (right) scanned transversally (Danish: Højre nyre transversalt).

9/11

3 ArtefactsThe ultrasound artefacts are numerous and may be understood with basic understanding of the phys-

ical principles of US. Many of the artefacts may be controlled to some extent. It is therefore advisable to “read” and understand the ultrasound part of the image and not just the anatomical image.

Speckle is the fine-dotted background of the image i.e., the mottled appearance where the intensity change from white to black. It is created by the huge amount of subresolveable echoes due to scatter-ing and reflection and is an interference-pattern. It is a function of the distribution of the point-scat-terers and reflectors and of the wavelength. It will therefore look different from tissue to tissue, from frequency to frequency and from angle to angle. It provides us with tissue contrast. Some machines have a speckle reduction function, e.g., by applying so-called spatial compounding, where images are recorded from different angles and added.

Reverberation describes artefacts created by the sound pulse being reflected multiple times between the transducer and an interface (simple reverberation) and/or between multiple interfaces (multiple layers of the abdominal wall, complex reverberation). Simple reverberation is easily understood when the same echo is repeated equidistantly down through the image. Complex reverberation results in a “tail” of weak echoes being emitted from, for instance, the abdominal wall some time after the pulse has travelled through. Complex reverberation explains why the transducer-near-end of cysts has ech-oes. Harmonic imaging removes reverberation artefacts.

Colour/power-Doppler is also subject to reverberation artefacts and it is advisable always to let the colour box go to the transducer sole in order to know of any reverberation sources.

Comet tail When metal in soft tissue is insonated at 90°, a comet tail (white band) is seen. Typically, this is seen with needles. It is a reverberation artefact and is not seen behind all metal. It is also seen behind gall stones (in a shorter version).

Sidelobes The transducer crystals also emit sound at some angle to the intended direction. This ex-plains why very good reflectors may generate echoes even when they are not in the path of the pulse. The abdomen is filled with very good reflectors (gas-filled bowel loops) and they may generate ech-oes inside the urinary bladder or amniotic cavity (they are best seen when the background is black). The side lobe issue plays a role both on send and receive: If the crystal emits a sidelobe at a given angle it is also more sensitive on receive to an echo coming from that angle. The angle is frequency dependent.

This is why imaging based on harmonics is effective in reducing sidelobe artefacts. The transmit and receive sidelobes are at different angles (because send and receive frequencies are different).

Mirror. Good reflectors may generate mirror images. Best mirrors are smooth surfaces with high acoustic impedance difference: gas filled bowel loops, pleura, smooth bone surface (extremity). The mirror is easily identified and understood when the mirror image and “original” image both are present in the image. The mirror may, however, be at an angle to the image plane and the “original” may be placed in front of or behind the image plane.

Colour/power Doppler is also mirrored.

Shadow A shadow is a defect (black region) behind a structure that attenuates the pulse - absolutely (stone) or relatively (dense connective tissue). Or a shadow generated by a smooth and rounded sur-

10/11

face when the sound hits like a tangent (vessels, subcutaneous fat, muscles, benign tumours). Some shadows are actually mirror artefacts (both clean and dirty shadows behind bowel loops).

Enhancement When a structure attenuates the sound less than neighboring regions, the pulse will re-tain more energy and so will the echoes travelling back through this structure. The result is that the structure has a band of higher amplitude echoes behind it (a band of more white gray scale compared to neighboring regions. It is probably a function of water content and not pathognomic of cyst!). Some liver metastasis, most breast fibroadenomas and areas of inflammation have enhancement.

Anisotrophy The human body is isotropic to CT and MR imaging meaning that a specific point in the body looks the same when the body is rotated in the scanner. This does not apply to US which displays anisotrophy. An interface appear whiter when insonated at 90 degrees, than at any other angle! Mus-cles and tendons display anisotrophy.

Interface thickness In reality, an interface has no thickness. However, the interface appears to have a certain thickness, which is a function of pulse length, reflectivity, gain settings etc. The transducer-near-side of an interface is trustworthy, not the posterior side. The interface thickness also accounts for number of layers being overestimated if the interfaces are regarded as layers.

Sound speed The ultrasound scanner assumes that the pulse travels with 1540 m/s and uses this pa-rameter to construct the depth dimension of the image. The sound speed is, however, different from tissue to tissue (which is one of the reasons that echoes are generated). The error in distance may be as high as 10 % in in vivo human imaging. Examples of errors are needles looking bent and pleura being elevated behind cartilaginous ribs.

Refraction Ultrasound respects sound wave theory and refraction occurs when the pulse traverses an interface at some angle (non-oblique incidence). It may become pronounced in the image when the pulse travels through a trapezoid shape (with two successive additive refractions). The liver and spleen may duplicate the upper renal pole with resulting pseudotumor.

3.1 Specific Doppler artefactsThe artefacts above also apply to the behaviour of the Doppler pulse, e.g. mirror, attenuation, rever-

beration. The following artefacts are inherent Doppler-artefacts:

Blooming Blooming is when the colour information bleeds outside the vessel. Intrasynovial vessels are often below the resolution of gray-scale imaging and blooming is of course present when we see these vessels. Blooming is Doppler gain dependent.

Aliasing This occurs when the PRF is too low (the Doppler shift is more than half of the PRF – the Nyquist limit). The speed and direction of the blood is misinterpreted and a wrong colour is displayed. This is only a problem if direction and speed are important parameters.

Twinkling Hard surfaces (bone, calcification) may display a rapid twinkling between red and blue, although no movement occurs.

Generally, the way to distinguish between true and false flow is to use the spectral Doppler. In case of mirroring and reverberation, however, the spectral Doppler is of no help since it is true flow shown in a wrong place.

11/11