Version May 2024
INTRODUCTION — Ultrasonography (US), also referred to as ultrasound imaging or sonography, is an imaging modality that uses reflected pulses of high-frequency (ultrasonic) sound waves to assess soft tissues, cartilage, bone surfaces, and fluid-containing structures. US imaging, once the sole province of radiologists, is now widely used by generalists and specialists across a range of medical fields, including rheumatology, sports medicine, obstetrics and gynecology, surgery, and emergency medicine.
The basic nomenclature, principles of application, and technical considerations of musculoskeletal US are discussed here. Other imaging modalities used to diagnose disorders of the musculoskeletal system and guidelines for selecting imaging studies (eg, plain film radiography, computed tomography [CT], magnetic resonance imaging [MRI], and US) for selected musculoskeletal problems are presented separately. (See "Imaging techniques for evaluation of the painful joint" and "Imaging evaluation of the painful hip in adults" and "Radiologic evaluation of the painful shoulder in adults" and "Musculoskeletal ultrasonography: Clinical applications".)
Performance of the musculoskeletal ultrasound examination for specific parts of the body is reviewed in detail separately:
●Upper extremity (see "Musculoskeletal ultrasound of the shoulder" and "Musculoskeletal ultrasound of the elbow" and "Musculoskeletal ultrasound of the wrist")
●Lower extremity (see "Musculoskeletal ultrasound of the hip" and "Musculoskeletal ultrasound of the knee" and "Musculoskeletal ultrasound of the ankle and hindfoot")
NOMENCLATURE AND CONVENTIONS — Various terms are used to describe US equipment, transducer and image orientation, normal and abnormal features in acquired images, and artifacts [1-4].
Types of ultrasonography
B-mode ultrasonography — Brightness (B)-mode US and grayscale US are terms indicating the same technique. They are used interchangeably.
Grayscale ultrasonography — Depending on the various tissues under the transducer footprint, grayscale US includes different intensities of echoes and displays them in black, white, and various shades of gray. Full digital processing of the returned echoes creates an image of the anatomy of the region of interest as well as the structural background for Doppler US. The full range of grayscale US images can be seen in the examples found throughout this topic, including the following.
Doppler ultrasonography — Doppler US relies technologically on the Doppler principle, which states that sound waves increase in frequency when they reflect from objects (eg, red blood cells) moving toward the transducer and decrease when they reflect from objects moving away.
Color Doppler ultrasonography — Color Doppler US applies the Doppler effect combined with real-time imaging. The information from Doppler US is integrated in the grayscale image as a color signal. This signal indicates the direction of blood flow. Red signals indicate flow that is directed toward the US transducer while blue signals indicate flow that is directed away from the transducer, as seen in the following example (image 1).
Duplex ultrasonography — Duplex US is the combination of real-time imaging and Doppler US. It depicts both the anatomical image with color signals and the Doppler curves. In addition, this technique allows for estimation of the velocity of flow from a combination of the Doppler frequency shift and the beam angle.
Power Doppler ultrasonography — Power Doppler US displays the total integrated Doppler signal in color (image 2 and image 3 and movie 1). The sensitivity of power Doppler US for small vessels and for slow blood flow is greater than duplex US. Power Doppler US shows hyperemia in inflamed tissues. The amount of vascularity present is usually assessed subjectively from the power Doppler US image. The percentage flow on power Doppler US can be scored semiquantitatively on a scale of 0 to 3. Grade 0 represents no signal on power Doppler US, 1 represents up to four single signals or confluent signals, 2 represents signals present in less than 50 percent of the Doppler box, and 3 represents >50 percent. The number of Doppler signals may vary from machine to machine. In a study comparing the Doppler sensitivity of six different types of machines, power Doppler was more sensitive on one-half of the machines, whereas color Doppler was more sensitive on the other half, using both factory settings and study settings [5].
Elastography US measures tissue displacement or strain as a response to an external compression force. It is based on the assumption that the strain is smaller in harder than in softer tissues. Color coding is assigned to the elastographic images depending on the magnitude of the strain, ranging from red (soft tissue) to blue (stiff tissue). As yet, the role of elastography in the assessment of certain musculoskeletal disorders is under investigation.
Quantitative ultrasonography — B-mode or grayscale US provides largely qualitative information about anatomic structures. In contrast, quantitative US is intended to provide quantified assessments of tissue based on physical phenomena associated with the propagation of US waves within the tissue [6,7]. It can provide information about tissues beyond the resolution limitations of grayscale US. Quantitative US has been used for years to assess bone density, but its use in other tissues (eg, liver, lung, nerve) is expanding.
Transducer orientation and image presentation — US scans are defined by two primary views, namely, a transverse view or short axis (image 4) and a longitudinal view or long axis (image 5). The following images provide examples of these two views in the metacarpophalangeal joint (image 4 and image 5), wrist (image 6), and proximal calf (image 7 and image 8). Transverse views are similar to axial views obtained by computed tomography (CT) scan or by magnetic resonance imaging (MRI).
The following orientations of patient, ultrasonographer, and images are suggested standards to aid image acquisition, presentation, and communication of findings [1,8,9]:
●The patient sits or lies at the right side of the sonographer.
●The sonographer looks at the patient from a caudal (or distal) to cranial (or proximal) orientation.
●The upper part of the US image corresponds to anatomic areas closer to the transducer. This is anterior (ventral) if the patient is supine.
●The lower part of the image is the area more distal to the transducer. This is posterior (dorsal) if the patient is supine.
●The left side of the image is the left side from the perspective of the sonographer. Thus, the left side of the image represents the right side of the body if the patient is supine or is sitting facing the examiner. Conversely, the left side of the image represents the left side of the body if the patient is prone or is sitting facing away from the examiner.
For standardization purposes, some ultrasonographers prefer to localize the medial (ulnar, tibial) anatomical area seen on transverse images always on the left side of the image and the lateral (radial, fibular) anatomical area on the right side of the image to better compare findings of both extremities. However, the author and others prefer to place the transducer in such a way that the left side of the image always corresponds to the left side of the patient from the perspective of the examiner.
Most musculoskeletal ultrasonographers prefer that the longitudinal view displays proximal (cranial) anatomy to the left side of the screen and distal (caudal) anatomical structures to the right (image 5). This orientation also facilitates recognition of anatomic and pathologic structures in publications.
Transducer manipulation — To obtain optimal images, the sonographer should be familiar with a few basic techniques for manipulating the transducer. These include sliding, compressing (one or both ends of the footprint), rotating, tilting, and rocking. First, the sonographer should sit comfortably; it is difficult to perform a good examination while uncomfortable. The hypothenar eminence of the hand holding the transducer should rest firmly on the patient's skin. The transducer can be manipulated to obtain the desired image as follows (figure 1):
●Sliding is the most common movement performed. It involves sliding the transducer along an area, usually while searching for a specific structure or pathology. Sliding alone does not involve a change in the angle of the transducer with the tissue being examined.
Sliding can be performed in the long axis (longitudinal or sagittal plane) or short axis (transverse plane). When performed in the short axis, this maneuver is sometimes referred to as "sweeping."
●Compression means pressing the transducer further into the tissue. It is performed to gain greater visibility of deeper structures or to improve the focus of such structures.
●Rotating the transducer means changing its orientation from longitudinal to transverse or vice versa.
●Tilting (or "fanning") is the preferred maneuver to avoid producing anisotropy of structures (eg, tendons). It is performed by gently moving the upper part of the transducer along its long axis while maintaining contact at the same point.
●Rocking the transducer is like tilting but is performed in the transverse axis.
Echogenicity and tissue appearance — Echogenicity or echotexture of various musculoskeletal tissues is assessed from the appearance on the display. The echogenicity of a tissue depends not only upon the characteristic of that tissue but also upon the transducer frequency. Echogenicity is also affected by the position of the transducer. The characteristics of various elements of the musculoskeletal system as they appear when assessed with transducer frequencies between 5 to 18 MHz are described below.
Bone surface — Bone surface is typically hyperechoic (ie, bright) with posterior acoustic shadowing, as in the following example (image 9). Cortical irregularities can be depicted by US. As US waves are reflected by bone, US does not provide any information of anatomical structures that are localized below an intact bone surface. A good example of the hyperechoic bone surface is apparent at the deepest bright line in the axial view from the lateral aspect of the shoulder. Bone erosions appear as regular or irregular discontinuities of the cortical surface.
Cartilage — Hyaline cartilage is anechoic (ie, black). Hyaline cartilage (ie, articular cartilage) lies directly adjacent to the bone surface. The normal surface of hyaline cartilage is regular (image 10). Degenerated cartilage may have increased echogenicity and may have an irregular surface. Fibrocartilage (eg, labrum of the glenohumeral joint or knee meniscus) is relatively hyperechoic, although less so than bone.
Synovium — The width of normal synovium is too small to be visualized by US. The grayscale echogenicity of hypertrophic synovium is hypoechoic. Synovium in healthy persons usually does not exhibit color Doppler or power Doppler signals. However, US equipment with a high sensitivity for flow signals may show minor flow even in the absence of joint disease.
Synovial fluid — Normal synovial fluid is anechoic material within a joint. It is displaceable, is compressible, and does not exhibit Doppler signal. Synovial fluid is more easily detected when present in increased amounts, as in inflamed joints or tendon sheaths, as in the following examples (image 11 and image 12 and image 13).
Joint capsule — The joint capsule is the anatomical structure that forms the boundary between the hypoechoic synovium, the anechoic synovial fluid, or the anechoic cartilage and the periarticular soft tissues, which are usually isoechoic. Similar to the connective tissue, subcutaneous fat is also isoechoic and is slightly irregular. It usually appears slightly less echoic (ie, hypoechoic) than the surrounding connective tissue.
Tendons — Tendons are characterized by a fine internal fibrillar pattern, as seen in the following examples (image 5 and image 14 and image 15 and image 16). They are slightly hyperechoic if visualized with a transducer held perpendicular to the tendon. Hypoechogenicity of normal tendons is an artifact referred to as anisotropy that occurs due to scattering of the beam when the transducer is not positioned perpendicularly to the tendon surface. Scattered sound waves are not captured by the transducer, and thus, the tendon appears dark. Beam obliquity causes artifactually decreased echogenicity, and the tendon appears black. The differential diagnosis of a hypoechoic tendon includes various causes of tendon pathology.
Nerves — On US, nerves are similar in appearance to tendons. Within the nerve, the nerve bundles appear slightly hypoechoic and are surrounded by hyperechoic connective tissue. Their transverse US appearance is more dotted ("honeycombing") (image 17 and image 18) and less fibrillar than tendons. Nerves also display anisotropy, although to a lesser degree than tendons.
Muscles — Muscles are predominantly hypoechoic but are sometimes iso- or hyperechoic depending on the transducer orientation. Fine intramuscular hyperechoic lines represent the epi- and perimysium; thicker hyperechoic lines represent septae and investing fascia.
Bursae — Bursae are hypoechoic or anechoic depending upon the structures that prevail in the bursae. The small amount of fluid present in a normal bursa may be seen with high-frequency transducers.
Ligaments — Ligaments have a similar echotexture to that of tendons and are slightly hyperechoic. However, if they consist of several layers, the fibrillar pattern may run in different directions.
Resolution — Resolution refers to both axial and lateral (horizontal) resolution. Axial resolution is the ability of the US beam to distinguish two objects that lie in the line of the US beam at different depths. Lateral or horizontal resolution refers to the minimum lateral distance between two objects that can be differentiated and visualized on the display when they lie side to side.
Axial resolution — Axial resolution is determined primarily by the frequency of the US signal. Higher frequencies (hence, shorter wavelengths) produce better axial resolution for more superficial structures.
Lateral resolution — Due to widening of the sound beam with increasing tissue depth, the lateral resolution decreases with depth. High-frequency transducers that are generally used for musculoskeletal US reach an axial resolution of up to 0.1 mm and a lateral resolution of 0.2 mm. Twenty-MHz transducers reach an axial resolution power of 0.04 mm.
Time or B gain correction — The US beam strength weakens with increasing depth due to a combination of absorption and scattering of the tissues. Time or B gain correction is used to allow similarly echogenic structures at different depths to appear on the display with roughly equivalent intensity. Time-gain compensation applies increasing amplification to the signal generated by echoes returning to the transducer using an exponential function based on the time of flight. The examiner can modify the time gain to optimize the visualization of the tissues in the beam path.
Imaging artifacts
Refraction — Refraction is an artifact depicting real structures at the wrong position resulting from bending of the US wave between two materials; this phenomenon may be minimized by keeping the incident beam as close as possible to perpendicular to the surfaces of interest.
Reflection — Reflection occurs when US waves hit tissue and bounce back at the same angle but in a different direction. Reflection occurs at boundaries between two different tissues and at tissue interfaces. Tissue boundaries that are at 90 degrees to the US wave act as perfect reflectors, whereas tissue boundaries that have a lesser angle or are parallel to the US wave are poor reflectors. These reflected waves generate an echo detected by the transducer when the reflected angles lie within its field of view.
Reverberation — Bouncing of the beam back and forth between the transducer and the object gives rise to multiple echoes. Reverberation produces repetitive echoes below a structure (eg, below a metal object such as a prosthesis or a needle) introduced into the tissue in the US beam path. Reverberation can affect color and power Doppler imaging [10].
Edge shadows — The term "edge shadow" is used to describe the US phenomenon of hypoechoic areas behind the edge of spherical structures. Originally, this artifact was described in relation to fluid-filled, rounded structures, but it is also seen with solid structures (image 19). One technique that can be used to help distinguish an edge shadow from fluid (eg, edema) is to compress the structure and see if the fluid displaces.
Enhancement — Enhanced through-transmission is most commonly seen deep to fluid-filled structures (image 12 and image 11).
Comet tail — "Comet tail" is used to describe the artifact caused by reverberation, which creates characteristic bands of increased echogenicity deep to the object.
Acoustic shadowing — Acoustic shadowing occurs when the US beam hits a highly reflective surface, like bone, air, calcifications, and calculi. Because little of the beam enters the reflective material, the region beyond it appears hypo- or frankly anechoic (image 9).
Aliasing — Aliasing is a Doppler artifact occurring when the Doppler shifts in frequency, which occurs when the velocities of red blood cells are higher than one-half the pulse repetition frequency (PRF) [10]. This occurs, for example, in areas of stenosis, where the reduced lumen of the vessel is seen with a red-to-blue shift. Red represents flow toward the transducer, within the range of the PRF, and blue velocities are beyond the range of the PRF.
TECHNICAL CONSIDERATIONS — The technical equipment used for musculoskeletal US is essentially the same as that used in other medical disciplines. Transducers are often linear array. For anatomic reasons, curved array transducers may be useful for examining the hip and axillary regions.
Older machines offer single-frequency transducers, like those of 5 (usually a curved array), 7.5, and 10 MHz, the last typically requiring the use of a standoff water pad to image relatively superficial musculoskeletal structures.
US machines are equipped with multifrequency, so-called broadband transducers, usually in the range of 5 to 10 MHz or 7.5 to 18 MHz, with some transducers exhibiting frequencies up to 22 MHz (picture 1). A high-frequency, linear transducer with a small footprint is useful to examine small joints or small arteries (picture 2). As noted above, the higher the frequency, the shorter the wavelength of the US pulse, and the better the axial resolution.
With high frequencies, superficial structures like the temporal arteries, tendons, and small joints (including metacarpophalangeal, proximal interphalangeal, and metatarsophalangeal joints) can be studied. A transducer emitting a beam with a wavelength of 7.5 MHz has an axial resolution of 0.4 mm; the lateral resolution is >0.4 mm. Both axial and lateral resolutions of a 7.5 MHz transducer are, therefore, insufficient to accurately assess ligaments of 1 to 2 mm thickness. Higher frequency transducers (eg, 13 MHz) have an axial resolution of 0.12 mm, and those of 20 MHz have one of 0.038 mm. The disadvantage of higher frequencies is poor tissue penetration.
Conventional machines show two-dimensional pictures in real time of the area of interest. Advances in technology have led to the development of three-dimensional (3D) US for the spatial imaging of structures. High-end machines offer 3D US. It acquires the 3D shape of an object by a single sweep of a transducer. Four-dimensional US demonstrates the dynamic motion of 3D imaging.
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Musculoskeletal ultrasound".)
SUMMARY AND RECOMMENDATIONS
●Ultrasound types – Ultrasonography (US) may be used to image tendons, bursae, ligaments, cartilage, synovium, synovial fluid, nerves, blood vessels, and bone. B-mode, grayscale, Doppler, color Doppler, duplex Doppler, and power Doppler US are different types of US technologies used for musculoskeletal imaging. (See 'Types of ultrasonography' above.)
●Imaging standards – Some standards for orientations of patient, ultrasonographer, and images are suggested to aid in image reading, acquisition, presentation, and communication of findings. (See 'Transducer orientation and image presentation' above and 'Transducer manipulation' above.)
●Tissue appearance – Different musculoskeletal tissues have different appearances on display, ranging from anechoic (black) for homogeneous fluid to brightly echogenic (white) for the bone surface. (See 'Echogenicity and tissue appearance' above.)
●Image resolution – Axial and lateral resolution are generally superior with higher-frequency US. Systems used for musculoskeletal US typically have axial and horizontal resolutions of 0.1 mm and 0.2 mm, respectively. (See 'Resolution' above.)
●Imaging artifacts – Various artifacts may occur and may need to be recognized; these include refraction, reverberation, edge shadows, acoustic shadowing, and aliasing. (See 'Refraction' above and 'Reverberation' above and 'Edge shadows' above and 'Acoustic shadowing' above and 'Aliasing' above.)
●Transducers (probes) – Transducers used for musculoskeletal imaging typically employ multifrequency or broadband linear array transducers. The transducer frequency and size are selected based on the size and depth of the structures of interest. A higher frequency and smaller transducer are used for smaller, superficial structures, while lower frequencies and larger transducers are used for deeper and bigger structures. (See 'Technical considerations' above.)
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