Ultrasound elastography for musculoskeletal applications

Abstract

Ultrasound elastography (EUS) is a method to assess the mechanical properties of tissue, by applying stress and detecting tissue displacement using ultrasound. There are several EUS techniques used in clinical practice; strain (compression) EUS is the most common technique that allows real-time visualisation of the elastographic map on the screen. There is increasing evidence that EUS can be used to measure the mechanical properties of musculoskeletal tissue in clinical practice, with the future potential for early diagnosis to both guide and monitor therapy. This review describes the various EUS techniques available for clinical use, presents the published evidence on musculoskeletal applications of EUS and discusses the technical issues, limitations and future perspectives of this method in the assessment of the musculoskeletal system.

Figures

Figure 1

(a, b) Longitudinal and (c, d) transverse free-hand strain elastograms of the middle third of asymptomatic Achilles tendons (T) showing the two distinct ultrasound elastography patterns of normal tendons. Type 1 (a, c) tendons appear homogeneously stiff, (green/blue) with no distinct soft (red) areas. Type 2 (b, d) tendons appear considerably inhomogeneous with soft (red) areas (longitudinal bands or spots), which did not correspond to any changes in B-mode ultrasound. The retro-Achilles fat appears as a mosaic of green, red and blue. Note the red areas at the lateral and medial sides of the tendon in the transverse plane (c, d), corresponding to an artefact, secondary to difficulty in stabilising the hand-held transducer.

Figure 2

(a) Longitudinal colour Doppler ultrasound image and (b) free-hand strain elastogram of a 23-year-old recreational runner with insertional Achilles tendinopathy. There is hypoechogenicity and neovascularity at the tendinopathic area [asterisk in (a)], which appears softer (red) compared with the stiff (blue/green) normal-appearing remaining tendon (T). The retro-Achilles fat appears as a mosaic of various levels of stiffness. (c) The elastogram of a normal (asymptomatic) Achilles tendon calcaneal insertion is presented for comparison. The retrocalcaneal bursa (asterisk) appears considerably softer (red) compared with the stiffer tendon (green/yellow). Note the presence of soft (red) foci beneath the calcaneal bone (C), which correspond to artefacts. LT, left.

Figure 3

Longitudinal shear wave elastograms of a normal (a) Achilles and (c) patella tendon, as well as (b, d) a case of distal patella tendinopathy in a 23-year-old football player. The elasticity qualitative and quantitative scale is presented at the upper right corner of the images. Measurements (mean, minimum, maximum and standard deviation) within the circular region of interest (ROI) are presented in kilopascals ranging from 0 (dark blue) to 300 (dark red). (a, c) The normal Achilles and patella tendons (T) appear as homogeneous stiff (red) structures, as opposed to fat, which is homogeneously soft (blue). (a) The mean stiffness of a representative area at the mid-portion of the Achilles free tendon is 300 kPa. (d) In the case of distal patella tendinopathy, the tendinopathic area appears hypoechoic with neovascularity (asterisk). (b) In the corresponding elastogram, the abnormal area appears softer (blue; mean elasticity 40.94 kPa) compared with the stiffer normal tendon (red; mean elasticity 261.16 kPa). The small amount of fluid in the deep infrapatella bursa appears softer than the tendinopathic area (blue, mean elasticity 34.38 kPa).

Figure 4

Axial free-hand strain elastograms of (a) normal relaxed calf muscles and (b) vastus lateralis muscle. The medial and lateral heads of gastrocnemius (M and L, respectively), as well as the vastus lateralis (VL), appear as a mosaic of intermediate or increased stiffness (green or blue colour, respectively), with scattered softer (red) areas near muscle boundaries. The subcutaneous fat appears soft (red/yellow). The homogeneous stiff appearance of soleus (S) in (a) is probably because of inadequate B-mode and ultrasound elastography data due to increased depth.

Figure 5

The impact of gel on the strain elastograms. (a, b) Longitudinal and (c, d) axial elastograms of the same asymptomatic Achilles tendon (T). The inclusion of a small amount of gel in the elastogram (b, d) results in a homogeneously stiffer tendon without areas of distinct softening (red), which are evident when no gel is included (a, c). The level of pressure and the ultrasound elastography settings were kept stable.