The use of musculoskeletal ultrasound (US) has been in the realm of radiology and physiatric medicine specialists for over 50 years following the foundation of the American Institute for Ultrasound in Medicine (AIUM) in 1951.1 The AIUM is a “multidisciplinary association dedicated to advancing the safe and effective use of ultrasound in medicine through professional and public education, research, development of guidelines, and accreditation.”2 With these principles in place to promote this mission, the AIUM, in collaboration with the American College of Radiology (ACR), developed the AIUM Practice Guideline for the Performance of the Musculoskeletal Ultrasound Examination, effective as of October 1, 2007. Practice guidelines of the AIUM are intended to provide the medical ultrasound community with guidelines for the performance and recording of high-quality ultrasound examinations.
Initially, diagnostic ultrasound applications were limited due to poor resolution and lack of real-time imaging capability.3 During the subsequent years, physiatrists began to lead the medical community with the use of therapeutic ultrasound techniques.4 In the 1980s, with the use of real-time ultrasonographic imaging and detailed anatomic imaging, diagnostic musculoskeletal ultrasound became capable of fully evaluating the musculoskeletal system. With equipment cost reductions and resolution improvements, this field has expanded to various clinical practices that diagnose and treat musculoskeletal disorders. Many practitioners have now incorporated diagnostic ultrasound to diagnose pathology in tendons, nerves, ligaments, joint disorders and, subsequently, for use in performing therapeutic procedures with ultrasound-guidance techniques.
Fundamental Concepts in Musculoskeletal Ultrasound
Musculoskeletal ultrasound involves the use of high-frequency sound waves (3-17 MHz) to image soft tissues and bony structures in the body for the purposes of diagnosing pathology or guiding real-time interventional procedures. Using high-resolution scanning produces detailed anatomic images of tendons, nerves, ligaments, joint capsules, muscles and other structures in the body. Practitioners may now use ultrasound guidance to diagnose tendonosis, partial- or full-thickness tendon tears, nerve entrapments, muscle strains, ligament sprains and joint effusions—as well as guide real-time interventional procedures for treatment modalities.
Some basic terminology used in ultrasound lexicon:5,6
Echotexture refers to the coarseness or non-homogeneity of an object.
Echogenicity refers to the ability of tissue to reflect ultrasound waves back toward the transducer and produce an echo. The higher the echogenicity of tissues, the brighter they appear on ultrasound imaging.
Hyperechoic structures are seen as brighter on conventional US imaging relative to surrounding structures due to higher reflectivity of the US beam.
Isoechoic structures of interest are seen as bright as surrounding structures on conventional US imaging due to similar reflectivity to the US beam.
Hypoechoic structures are seen as darker relative to the surrounding structures on conventional US imaging due to the US beam being reflected to a lesser extent.
Anechoic structures that lack internal reflectors fail to reflect the US beam to the transducer and are seen as homogeneously black on imaging.
Longitudinal structure is imaged along the long axis.
Transverse structure is imaged perpendicular to the long axis.
Shadowing is the relative lack of echoes deep in an echogenic structure due to attenuation of the ultrasound beam (e.g., to large calcifications, bone, gas, metal).
Posterior acoustical enhancement is the brighter appearance of tissues deep in an area where there are few strong reflectors to attenuate the sound beam (e.g., simple fluid is anechoic since there are no internal reflectors to produce echoes). Thus, the sound beam that passes through the fluid is stronger than when at the same depth in soft tissue.
Anisotropy is the effect of the beam not being reflected back to the transducer when the probe is not perpendicular to the structure being evaluated (e.g., an angled beam on bone would create an anechoic artifact since the beam is reflected at the angle of incidence away from the transducer).
Ultrasound Imaging Advantages
Musculoskeletal ultrasound provides several distinct advantages in relation to basic radiography (X-rays), computed tomography (CT) and magnetic resonance imaging (MRI)—especially in focused musculoskeletal and neurological examinations.1,7 Ultrasound is a hands-on, dynamic, and interactive examination which allows the practitioner to use real-time high-resolution soft tissue imaging. It also facilitates dynamic examination of anatomic structures while interacting with the patient during the conduct of the imaging study. US imaging is minimally affected by metal artifacts (e.g., cochlear implants, hardware, or pacemakers) and can also be used in certain patients that are contraindicated for MRI imaging (e.g., claustrophobic or obese patients). US imaging facilitates the ability to guide minimally-invasive, interventional procedures (e.g., intraarticular injections and aspirations). It also enables rapid contralateral limb examination for comparison studies. The obvious advantages of US—such as portability, relatively low cost compared to other imaging, lack of radiation risk, and no known contraindications—are good reasons to consider using this modality.
Practitioners, however, must also recognize several notable disadvantages in musculoskeletal ultrasound.1,7 Its most important limitations lie in its limited field of view and limited penetration thus potentially resulting in incomplete evaluation of bony and joint anatomy. Yet, ultrasound provides a very high quality picture of a relatively small area so that clinicians should use US to confirm or characterize pathological changes within a defined body region. From an equipment standpoint, musculoskeletal ultrasound study is also limited by the variable quality and variable expense of the US equipment. From the operator/examiner standpoint, musculoskeletal ultrasound study is limited by the examiner’s skill level, a lack of educational infrastructure and, as yet, a lack of certification or accreditation process in this early phase of musculoskeletal imaging.