Sound/Tissue Interaction

Tissue appearance on ultrasound is determined by their physical properties and its interaction with the sound beam.  The varying degrees of brightness (and darkness) seen on the BMode image are the result of reflection, refraction, and scattering.  This causes tissues to appear either as anechoic, hypoechoic, or hyperechoic depending on the differences in acoustic impedance at the borders of two tissues.  Additionally, tissues may appear as homogeneous or heterogeneous depending their make-up.

In this image of the distal thigh, adipose tissue (indicated by the yellow arrow) appears hypoechoic with streaks of  hyperechoic lines that are irregular in length and texture.  The adipose layer is most superficial; note the difference in its appearance compared to the adductor longus (AL) and vastis medialis (VM) muscles. The saphenous nerve (SN) is situated in the fascia between the two muscles.

Arteries, veins and cysts appear as anechoic structures on ultrasound because of Rayleigh scattering.  Arteries are round on a Bmode image, and pulsatile by nature.  Veins are not as round, and are usually easy to compress.  Both arteries and veins will appear as tube-like structures when the Bmode image is changed from a transverse to longitudinal view; cysts will still appear round.

The pectoris major (PM) muscle shown between the white arrows in this infraclavicular image to the left, is an excellent example of how muscle appears on ultrasound.  It is heterogeneous in nature because of the varying acoustic impedances between the cell structures, high water content of the cells and the intertwined fascia.

 

In this supraclavicular image, the lung is in the lower left hand corner.  It is characterized by a thin hyperechoic line created by the visceral and parietal pleura, with areas of reverberation (commonly referred to as “comet tails“) in the tissue.  These “comet tails” are created when the sound beam contacts small areas of air/fluid between the pleura, causing it to vibrate.

Bone, such as the second rib, seen here in a sagittal view, are large specular reflectors that produce a bright (hyperechoic) thin line.  Because the acoustic impedance of bone is much higher than that of tissue, the majority of the sound beam is reflected back to the transducer creating a “shadow” below it.  In this image, the pleura can be seen both cephalad and caudad to the rib in the intercostal spaces, but not below the rib itself.

Nerves and tendons both appear as hyperechoic structures in the periphery.  The median nerve (white arrow) can be distinguished from the tendon (yellow arrow) by following its course proximally from the wrist to the elbow.  Nerves will maintain their appearance along the entire length of the forearm, while tendons will become flat, and eventually disappear as it attaches to muscle.

Nerves can appear either hypoechoic or hyperechoic depending on the location in the body.  In the image below left, yellow arrows point to hypoechoic cervical nerve roots 5-7 in the brachial plexus.   By comparison, the tibial and peroneal nerves imaged in the popliteal fossa appear hyperechoic in the image below right.   There are numerous theories as to this phenomenon, but in general, it is believed to be related to depth of the nerves, the amount of fat and connective tissue surrounding the nerves, as well as the fat and stroma within the nerves themselves.


References

Gray AT. Atlas of ultrasound-guided regional anesthesia. Philadelphia, PA. Saunders, Elsevier; 2010:45-67.

Pollard BA, Chan VW. An introductory curriculum for ultrasound-guided regional anesthesia: a learner’s guide. Toronto. University of Toronto Press Inc.; 2009:23-28.

Sites B D, Brull R, Chan VW, et al. Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. part I: understanding the basic principles of ultrasound physics and machine operations. Reg Anesth Pain Med. 2007;32(5):412-418.

Chan VW, Abbas S, Brull R, et al. Ultrasound imaging for regional anesthesia; a practical guide. 3rd ed. Toronto. Toronto Printing Company; 2010:11-12.

Bigeleisen PE, ed, Orebaugh SL, Moayeri N, et al. Ultrasound-guided regional anesthesia ad pain medicine. Baltimore, MD. Lippincott Williams & Wilkins; 2010:26-33.

 

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