Tissue Echogenicity

Reflection of an ultrasound beam forms the basis for all diagnostic imaging.  Reflection occurs when a sound beam contacts tissues with different densities (acoustic impedance).  Part of the beam reflected back to the transducer and part part of the beam passes through the tissue.  The acoustic impedance (Z) of a tissue describes its stiffness, or resistance against the propagation of sound.  It is equal to density (p) multiplied by propagation velocity (v), where Z = p x v.  The differences in acoustic impedance between tissue boundaries result in reflection, refraction, scattering, and ultimately attenuation of the sound beam.  The amount of reflection of the incidence beam is proportional to the difference of the acoustic impedance between the two tissues, and can be determined by R = [Z2 – Z1 / Z2 + Z1], where R is the ratio of reflected amplitude, and Z1 and Z2 represent the different tissues.  If the two tissues have the same acoustic impedance, their boundary will not produce an echo, and the beam will pass through.  Conversely, if the impedance between the two boundaries is great, the majority of the wave will be reflected back to the transducer, and only a small portion of the sound beam will pass through.  The acoustic impedances of different mediums relevant to diagnostic ultrasound are listed in the Table below:

Acoustic impedance of difference mediums.

Acoustic impedance of difference mediums.

Echogenicity is the amount of reflection caused by varying degrees of acoustic impedance in tissues. Several terms are used to describe tissue echogenicity.  Anechoic refers to those tissues that produce no echo.  This is commonly seen with Rayleigh scattering (discussed later) that occurs within blood vessels where the ultrasound waves are larger than the red blood cells they come in contact with causing a uniform reflection in all directions.  Because there are no echoes returning to the transducer to be evaluated, these structures appear black on the B mode image.  Hyperechoic describes tissue that creates a strong reflection back to the transducer, with only a small amount of the remaining beam continuing through.  These tissues appear bright on B mode.  Fascia and bone are examples of hyperechoic tissue.  Conversely, hypoechoic structures produce weak echoes that appear as varying shades of grey in the B mode image.  Fat is an example of a hypoechoic tissue.  Deep structures may appear hypoechoic as well, because the incident ultrasound beam is weak from attenuation, resulting in diminished returning echoes.  Tissues can also be described as either heterogeneous or homogeneous.  Heterogeneous tissues, such as muscle, are comprised of multiple structures of different acoustic impedances, causing varying degrees of brightness on the B mode image.  Homogeneous tissue on the other hand appears very uniform on the B mode image.  Organs, such as the thyroid gland and liver, are comprised of specialized cells with little variation that result in a consistent brightness on B mode.


References

Aldrich J E. Basic physics of ultrasound imaging. Crit Care Med. 2007;35(5 Suppl):S131-S137.

Zagzebski JA. Physics and instrumentation in Doppler and B-mode ultrasonography. In: Zweibel WJ. Introduction to Vascular Ultrasonography. 4th ed. Philadelphia, PA: W.B. Saunders Company; 2000:17-43.

Marhofer P, Frickey N. Ultrasonographic guidance in pediatric regional anesthesia part 1: Theoretical background. Paed Anaesth. 2006;16(10):1008-1018.

Sites B D, Brull R, Chan V W, 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.

Falyar CR. Ultrasound in anesthesia: applying scientific principles to clinical practice. AANA J. 2010 Aug; 78(4):332-40.

Kremkau F W. Doppler Ultrasound: Principles and Instruments. Philadelphia, PA: W.B. Saunders Company; 1990:5-51.

 

 

 

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