156 Ultrasound Elastography in Musculoskeletal Radiology: Past, Present, and Future Žiga Snoj, MD, PhD1,2 C. H. Wu, MD3 M.S. Taljanovic, MD4 I. Dumić-Čule, MD5 E. E. Drakonaki, MD6 Andrea S. Klauser, MD7 1 Radiology Institute, University Medical Centre Ljubljana, Ljubljana, Address for correspondence Žiga Snoj, MD, PhD, Radiology Institute, Slovenia University Medical Centre Ljubljana, Ljubljana, Slovenia 2 Faculty of Medicine, University of Ljubljana, Ljubljana, Slovenia (e-mail: [email protected]). 3 Department of Physical Medicine and Rehabilitation, National Taiwan University Hospital College of Medicine, National Taiwan University, Taipei, Taiwan 4 Department of Medical Imaging, University of Arizona, Business, SimonMed Imaging, Scottsdale, Arizona 5 Department of Diagnostic and Interventional Radiology, University Hospital, Zagreb, Croatia 6 Medical School of the European University, Cyprus 7 Department of Radiology, Division of Rheumatology and Sports Imaging, Medical University Innsbruck, Innsbruck, Austria Semin Musculoskelet Radiol 2020;24:156–166. Abstract Ultrasound elastography (USE) is becoming an important adjunct tool in the evaluation of various musculoskeletal (MSK) traumatic conditions and diseases, with an increasing number of applications and publications in recent years. This rapidly evolving technique enhances the conventional ultrasound (US) examination by providing information on the elastic properties of tissue alongside the morphological and Keywords vascular information obtained from B-mode US and Doppler imaging. Those perform- ► ultrasound ing USE must have basic knowledge of its proper imaging techniques and limitations. In ► elastography this review article, we place the USE in historical perspective and discuss basic ► musculoskeletal techniques and current applications of USE in the evaluation of various traumatic ► quantitative imaging and pathologic conditions of fasciae, nerves, muscles, tendons, ligaments, and MSK ► shear deformation soft tissue masses. Ultrasound elastography (USE) is a method that evaluates The Past tissue elasticity in real time during ultrasound (US) imag- ing.1,2 It has added a new dimension to US and is considered Palpation is a qualitative diagnostic technique that has been This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. the most important advance in US technology since Doppler used in medical practice for thousands of years. Hippocrates was imaging implementation.1,2 The technique has a wide range reported to use palpation for battle injuries, identifying that if of applications in radiology including musculoskeletal (MSK) the bone is not visible, it can assist in locating the weapon mark, soft tissue imaging.1,2 This article reviews the history and and, for head injuries, it can determine whether the cranium physics of this technique, focuses on current applications in beneath the flesh is strong or weak.3 Palpation is still used today the assessment of muscle, tendon, nerve, ligaments, and soft and is an essential component of any clinical evaluation. How- tissue masses, and discusses future potential applications ever, palpation is based on subjective evaluation with limited and technical considerations. disease detection and limited organ accessibility.3 Issue Theme Musculoskeletal Copyright © 2020 by Thieme Medical DOI https://doi.org/ Ultrasound Update; Guest Editor, Publishers, Inc., 333 Seventh Avenue, 10.1055/s-0039-3402746. Alberto Stefano Tagliafico, MD New York, NY 10001, USA. ISSN 1089-7860. Tel: +1(212) 760-0888. Ultrasound Elastography in MSK Radiology Snoj et al. 157 Elastography is a field in science that aims to overcome these calculate local strains within the region of interest (ROI).9 In limitations by providing noninvasive imaging of tissue mechan- some systems, semiquantitative tools are provided to analyze ical properties. The US evolution has come to the point where image characteristics as a strain ratio.3 This is an index of real-time elastography is possible and now integrated in com- relative elasticity of target ROI versus reference ROI (usually the merciallyavailable US machines.Along the pathof the evolution subcutaneous fat layer).3,7 of US, many important steps need to be noted as the foundation fi for modern MSK radiology. The rst report on MSK sonography Shear Wave Elastography was published in 1958.4 In 1972, the first B-scan image of a human joint was published describing the use of US imaging in A principle exploited in SWE is that directional force leads to differentiating Baker’s cysts from thrombophlebitis.4 The first tissue shear deformation propagating as a shear wave.3 SWE is report of using US-guided joint aspiration was published in a type ofdynamicelastographyand based on the measurement 1981 in a septic arthritis work-up.4 In 1994, the first application of the tissue propagation velocity distribution of the direc- of power Doppler was reported with demonstration of soft tional shear wave, produced by the US pulse.3,7 In addition to tissue hyperemia in MSK disease.4 Contemporary elastography qualitative depiction with elastograms, SWE can also quantify was introduced in the imaging field, with applications in the absolute elasticity value of an imaged structure.3 Shear magnetic resonance imaging (MRI) and US.5 In the beginning waves move at a faster rate through harder tissue. The quanti- USE applications were in vitro, and they subsequently evolved tative measurement can be expressed as shear wave velocity in into a real-time tool for in vivo imaging of the distribution of meters per second (m/sec) or as tissue elasticity based on shear tissue strain and elastic modulus. Over time US has become an modulus calculation in kilopascals (kPa).3,10 importantcornerstone ofMSK imaging withUSE striving to find 4 its place in everyday clinical practice. Terminology in Elastography The Present Several physical misconceptions and nomenclatorial inaccu- racies in the literature abound when reporting the elastog- USE has developed rapidly in recent years with an ever- raphy results.10 Stiffness, shear modulus, and shear wave increasing number of publications, mainly due to its wide velocity are distinct physical phenomena and should not be availability on commercial US systems.3 The European Society used interchangeably.10 Thus when reporting elastography of Skeletal Radiology published a group consensus, based on results in kPa, the term shear modulus should be used, and reviewing the literature on USE MSK applications, stating that when reporting elastography results in m/sec, the term shear USE currently has a low level of indication in clinical practice wave velocity should be used.10 but acknowledging it is a promising technique for soft tissue 6 masses and nerve entrapment. Radiologists performing MSK Fascia USE must have basic knowledge of the physics and limitations of the USE techniques used in various tissues. Depending on Interest in fascia has grown in the recent years and is a subject the method of force application and data processing, several ofan increasing number of publications. Elastography research USE methods are available.3 In the field of MSK radiology, has mostly been conducted on plantar fascia; reports of other – compression elastography (CE) and shear wave elastography fascia locations are scarce.11 13 This is understandable because (SWE) are the most commonly used methods. plantar fasciitis is a common pathology and frequently imaged in clinical practice.11 Patients with plantar fasciitis show fi Compression Elastography signi cantly softer plantar fascia compared with healthy individuals, even in those with typical clinical manifestations The principle exploited in CE is that the compression of tissue of plantar fasciitis but without abnormalities on B-mode – produces displacement, or strain.3 In stiffer tissue, the strain is sonography (►Fig. 1).14 16 The experience with other fasciae minimal; in softer tissue, the strain is greater. Tissue displace- is limited, with only a few articles reporting data on thoraco- This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. ment is calculated from repeated manual compression by using lumbar fascia and perimuscular fascia of the posterior thigh ahandheldUStransducer.3 The displacement distribution is and calf.12,13 Langevin et al showed that the shear strain of shown in an elastogram, usually displayed as a color-coded thoracolumbar fascia was 20% lower in human subjects with image superimposed over the B-mode image.7 Red is typically chronic low back pain compared with a control group.12 It was used to depict softer tissue, blue harder tissue, and yellow or shown that USE can depict the effects of manual therapy and green to depict intermediate elasticity.7 It is notable that the thus expand the utility of elastography in fascia evaluation color indicates the relative stiffness of the tissues, and therefore beyond purely diagnostic purposes.13 the color coding can be modified.7,8 Reliability of CE is another challenge because too heavy or too gentle compression can Nerves distortelastogramsdueto nonlinear changes intissue elasticity. Most vendors provide the amount of manual compression as Most nerve USE studies have focused on the median nerve at a visual indicator on the screen.7 For example, a “quality the wrist.17 This is most likely because the median nerve runs factor” > 60 indicates the optimal compression force, whereas superficially with good depiction