Musculoskeletal Ultrasound: A Primer for Primary Care
Musculoskeletal ultrasound (MSK US) has been around for more than 50 years, since the foundation of the American Institute for Ultrasound in Medicine (AIUM) in 1951.1 Initial efforts centered around diagnostic ultrasound applications, but they were limited due to poor resolution and lack of real-time imaging capability.2 In subsequent years, however, physiatrists began to lead the medical community with the use of therapeutic ultrasound techniques.3 In the 1980s, with the use of real-time ultrasonographic imaging and detailed anatomic imaging, diagnostic MSK US became capable of fully evaluating the musculoskeletal system. In 2012, the AIUM released a revised version of its Practice Guideline for the Performance of the Musculoskeletal Ultrasound Examination, which provided the medical ultrasound community with guidelines for the performance and recording of high-quality ultrasound examinations.4
Recently, with equipment cost reductions and resolution improvements, this field has expanded to various clinical practices that diagnose and treat musculoskeletal disorders. This article will discuss some concepts in MSK US that will be helpful for the practicing physician.
MSK US involves the use of high-frequency sound waves (3-17 MHz) to image soft tissues and bony structures in the body. 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. Table 1 contains basic terminology used in the ultrasound lexicon.5,6
US Imaging Advantages
MSK US provides several distinct advantages in relation to basic radiography (x-rays), computed tomography, and magnetic resonance imaging (MRI)—especially in focused MSK and neurological examinations.1,7 Because MSK US is performed in real time, it allows the practitioner to see high-resolution soft tissue imaging while interacting with the patient during the conduct of the imaging study. US imaging is minimally affected by metal artifacts (eg, cochlear implants, hardware, or pacemakers) and also can be used in certain patients who have contraindications to MRI imaging (eg, claustrophobic or obese patients). US imaging facilitates the ability to guide minimally invasive, interventional procedures (eg, intra-articular 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 of MSK US.1,7 The most important limitations lie in its limited field of view and penetration, which potentially can result in incomplete evaluation of bony and joint anatomy. From an equipment standpoint, MSK US also is limited by the variable quality and variable expense of the equipment. From the operator/examiner standpoint, MSK US is limited by the examiner’s skill level and a lack of educational infrastructure. It is also in the early processes of certification and accreditation.
To generate US waveforms, the machine generates an electric current to crystals inside the transducer, which, in turn, vibrate. The vibrating crystals generate a sinusoidal sound wave. The transformation of electrical energy to mechanical energy—known as piezoelectricity—can be expressed in terms of frequency, wavelength, amplitude, and propagation speed. Through the use of ultrasound coupling gel, sound waves travel into the body until they encounter an acoustic interface, which reflects the wave. The reflected sound wave is detected by the transducer using a “reverse piezoelectric effect” to transform the mechanical sound energy wave to electrical signals for processing. By alternately generating and recording the amplitudes and travel times of sound beams (also known as “pulsed US”), the US machine can use sophisticated computer software to generate the black and white, two-dimensional image of the body part. An acoustic interface that reflects a large amount of sound energy will appear brighter on the monitor as compared to less reflective interfaces, which appear darker. For example, a large amount of sound energy is reflected at the interface between bone and muscle, resulting in bone appearing bright (or white) on the monitor screen. Most importantly, it is important to understand that all US images are not based on the absolute material properties of a tissue but rather on the relative material properties of that tissue compared with adjacent regions being studied or viewed.
Diagnostic Applications of MSK US
Basic, normal MSK anatomy should be reviewed in detail to provide in-depth knowledge of normal and abnormal MSK anatomy on the US examination. A basic and fundamental introduction of anatomy is reviewed in Table 2.8
US scanning generates a 2-dimensional view of a 3-dimensional structure. The ability to skillfully manipulate the transducer using specific movements (sliding, tilting, rotating, and heel-toeing) ensures that the targeted structures are investigated fully. The transducer must be moved fully through the entire range of the structure to scan completely and avoid errors of omission. Anisotropy is a major pitfall of inexperienced practitioners; this occurs when an otherwise normal, smooth structure appears “dark” on US imaging because the beam didn’t encounter the structure perpendicular to the plane of the structure.1,5-7,9 A beam that encounters the tendon perpendicular to the surface will be reflected backward and toward the transducer, while a beam encountering the surface at any angle is reflected obliquely and away from the transducer. The tendon appears bright (hyperechoic) in the former case, while the tendon appears artifactually dark (hypoechoic) in the latter case. During the MSK examination, the examiner should avoid anisotropy by continually manipulating the transducer to direct the generated beam perpendicular to the target structure. With experience, the practitioner will develop scanning skills for image optimization, and transducer manipulations (sliding and rotating) will become automatic and effortless. To facilitate the learning process, US manufacturers have established presets for various MSK applications.