Secondary Logo

Journal Logo

Letters to the Editor: Letters & Announcements

Limitations in Ultrasound Imaging Techniques in Anesthesia: Obesity and Muscle Atrophy?

Saranteas, Theodosios PhD

Editor(s): Saidman, Lawrence

Author Information
doi: 10.1213/ane.0b013e3181ae09a4
  • Free

To the Editor:

Ota et al.1 concludes that, when using ultrasound guidance (US), the anterior approach to sciatic nerve block is performed as easily and successfully as the posterior approach. The authors stated that, in elderly individuals, sciatic nerve identification is less successful because of muscle atrophy in which fascia may not be distinguishable with US imaging. Additionally, the authors implied that in obese patients, the sciatic nerve is not clearly visualized because of its deep anatomic location. On the basis of above considerations, I would like to comment on a few issues.

Ultrasound imaged muscle bundles are seen as hypoechoic zones, whereas the perimysium and aponeurosis are seen as hyperechoic structures. In the case of muscle atrophy, “hypoechoic” muscle bundles degenerate, whereas perimisium and aponeurosis remain intact. The atrophic muscles, depicted as hyperechoic structures, reflect US energy, thus decreasing the ability of the US beam to penetrate in deeper tissues.2,3

In obese patients, because of deep anatomic location of nerves, the US beam travels a greater distance, resulting in beam attenuation. In addition, other factors may affect imaging quality through fat, which are as follows: 1) Exaggerated attenuation, i.e., the adipose tissue, has a nonlinear relationship to frequency as opposed to the usually assumed linear relationship in most biological tissues; 2) Phase aberration of the sound field because of uneven speed of sound in the irregularly-shaped adipose layers. This is due to differing speeds of sound in the overlying, nonhomogeneous tissue above the focus of the transducer; and 3) Reflection because of mismatch of acoustic impedance at the fat/muscle interfaces. When the US beam crosses a boundary between muscle layer and fat, a portion of energy is reflected back to the transducer because of different acoustic velocity between the two tissues (pure fat 1450 m/s and muscle 1580 m/s).4–6

In these cases, image quality may be improved by using different technical approaches which reduce speckling, clutter, or other acoustic artifacts.7 Advanced US imaging techniques, such as compound and harmonic imaging, improve the image because of a reduction of these artifacts. For example, harmonic imaging reduces phase aberration artifacts from overlying tissue and compound imaging reduces similar artifacts by averaging multiple scan lines from different directions.7,8 Additionally, compression of fat, location of the fat in the focus of the transducer, and large beam width of the US signal may improve imaging quality through fat.4,9

Among the major US innovations of recent years, 3D US is the ideal tool to avoid the limitations affecting the diffusion and reliability associated with traditional US.10 However, many studies are required to ascertain its utility in the imaging of nerve structures.

In conclusion, obesity and muscle atrophy mainly increase the number of reflective interfaces not only leading to more echoes but also decreasing incident sound available to penetrate deeper tissues, such as nerves, vessels, or other targeted structures.

Theodosios Saranteas, PhD

2nd Department of Anesthesiology, School of Medicine

University of Athens, Attikon Hospital

Athens, Greece, EU

[email protected]


1. Ota J, Sakura S, Hara K, Saito Y. Ultrasound-guided anterior approach to sciatic nerve block: a comparison with the posterior approach. Anesth Analg 2009;108:660–65
2. Chhem R, Kaplan P, Dussault R. Ultrasonography of the musculoskeletal system. Radiol Clin North Am 1994;32: 275–89
3. Saranteas T, Chantzi C, Iatrou C, Kostopanagiotou G, Dimitriou V. Ultrasound and regional anaesthesia techniques—is there any limitation? Reg Anesth Pain Med 2007;32:546–7
4. Fiegler W, Felix R, Langer M, Schultz E. Fat as a factor affecting resolution in diagnostic ultrasound: possibilities for improving picture quality. Eur J Radiol 1985;5:304–9
5. Feigenbaum H. Physics and instrumentation. In Feingenbaum H, Armstrong WF, Rayan T, eds. Feigenbaum’s echocardiography. Philadelphia: Lippincott William and Wilkins, 2005:12–5
6. Shmulewitz A, Teefey SA, Robinson BS. Factors affecting image quality and diagnostic efficacy in abdominal sonography: a prospective study of 140 patients. J Clin Ultrasound 1993;21:623–30
7. Entrekin RR, Porter BA, Sillesen HH, Wong AD, Cooperberg PL, Fix CH. Real time spatial compound imaging: applications to breast, vascular and musculoskeletal ultrasound. Semin Ultrasound CT MR 2001;22:65–77
8. Shapiro RS, Wagreich J, Parsons RB, Stancato-Pasik A, Yeh HC, Lao R. Tissue harmonic imaging sonography: evaluation of image quality compared with conventional sonography. AJR Am J Roentgenol 1998;171:1203–6
9. Browne JE, Watson AJ, Hoskins PR, Elliott AT. Investigation of the effect of subcutaneous fat on image quality performance of 2D conventional imaging and tissue harmonic. Imaging Ultrasound Med Biol 2005;31:957–64
10. Cimmino M, Grassi W. What is new in ultrasound and magnetic resonance imaging for musculoskeletal disorders? Best Pract Res Clin Rheumatol 2008;22:1141–8
© 2009 International Anesthesia Research Society