INTRODUCTION — Ultrasonography (US), also referred to as ultrasound imaging or sonography, is an imaging modality that utilizes reflected pulses of high-frequency (ultrasonic) sound waves to assess soft tissues, cartilage, bone surfaces, and fluid-containing structures. US imaging, at one time the sole province of radiologists, has become now widely available in rheumatology clinics and other ambulatory and emergency settings.
The nomenclature, technical considerations, validity, and reliability of musculoskeletal US are discussed here. Imaging modalities generally used to diagnose disorders of the musculoskeletal system and guidelines for selecting imaging studies (eg, plain film radiography, computed tomography [CT scan], 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".)
The use of US to screen for, diagnose, and monitor the response of osteoporosis to treatment requires dedicated, special-purpose devices rather than US imaging systems to assess bone mineral content. (See "Screening for osteoporosis in postmenopausal women and men", section on 'Other techniques not used for screening'.)
NOMENCLATURE AND CONVENTIONS — Various terms are used to describe ultrasonographic (US) equipment, transducer and image orientation, normal and abnormal features in acquired images, and artifacts.
Types of ultrasonography
B-mode US — Brightness (B)-mode US or Grayscale US are terms indicating the same technique. They are used interchangeably.
Grayscale US — Depending on the various tissues under the transducer foot print, 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.
Doppler US — 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 US — Color Doppler US (CD-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 probe while blue signals indicate flow that is directed away from the probe.
Duplex US — 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 US — Power Doppler ultrasonography (PDUS) displays the total integrated Doppler signal in color. The sensitivity of PDUS for small vessels and for slow blood flow is greater than duplex US. PDUS shows hyperemia in inflamed tissues. The amount of vascularity present is usually assessed subjectively from the PDUS image. The percentage flow on PDUS can be scored semi-quantitatively on a scale of 0 to 3. Grade 0 represents no signal on PDUS, 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 amount of Doppler signal 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 half of the machines, whereas color Doppler was more sensitive on the other half, using both factory settings and study settings [1].
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.
Transducer orientation and image presentation — US scans are defined by two views, namely a transverse view or short axis (image 1) and a longitudinal view or long axis (image 2). 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 [2,3]:
●The patient sits or lies at the right side of the sonographer.
●The sonographer looks at the patient from a caudal to rostral orientation.
●The upper part of the US image corresponds to anatomic areas closer to the probe. This is anterior (ventral) if the patient is supine.
●The lower part of the image is the area more distal to the probe. 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 sonographers 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. The author and others, though, prefer to place the probe in such a way that the left side of the image corresponds to the left side of the patient from the perspective of the examiner.
Most ultrasonographers in rheumatology prefer the longitudinal view displays proximal (cranial or rostral) anatomy to the left side of the screen and the distal (caudal) anatomical structures to the right (image 2). This orientation also facilitates recognition of anatomic and pathologic structures in publications.
Echogenicity — 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. The characteristics of various elements of the musculoskeletal system as they appear when assessed with probe frequencies between 5 to 18 MHz are described below:
Bone surface — Bone surface is typically echoic (ie, bright) with posterior acoustic shadowing. Cortical irregularities can be depicted by US. As ultrasonic 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 echoic 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) is directly adjacent to the bone surface. The normal surface of hyaline cartilage is regular. Degenerated cartilage may have increased echogenicity and may have an irregular surface. Fibrocartilage (eg, the labrum of the glenohumeral joint or a knee meniscus) is hyperechoic.
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.
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 mid-echoic. Similar to the connective tissue, subcutaneous fat is also mid-echoic 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. They are slightly hyperechoic if localized perpendicular to the probe. Hypoechogenicity of tendons is an artifact that is referred to as anisotropy. The hypoechogenicity is not due to pathology but is due to scattering of the beam that is not perpendicular to the tendon surface. Scattered sound waves are not captured by the probe, and thus the tendon appears dark. Beam obliquity will result in artifactual decreased echogenicity, and the tendon then appears black.
Nerves — Nerves are similar in US 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") and is less fibrillar. Nerves also display anisotropy, although to a lesser degree than tendons.
Muscles — Muscles are predominantly hypoechoic but are sometimes mid- or hyperechoic according to 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 imperceptible ultrasonographically.
Ligaments — Ligaments have a similar echotexture to that of tendons and are 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.
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.
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.
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 prostheses or a needle) introduced into the tissue in the US beam path. Reverberation can affect color and power Doppler imaging [4].
Edge shadows — The term “edge shadows” 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 3).
Enhancement — Enhanced through-transmission is most commonly seen deep to fluid-filled structures (image 4 and image 5).
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 6).
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) [4]. 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 beyond the range of the PRF.
TECHNICAL CONSIDERATIONS — The technical equipment used for musculoskeletal ultrasonography (US) is essentially the same as that used in other medical disciplines. Transducers are preferably linear array transducers, not curved array. Due to anatomical reasons, curved array transducers may be used for examining the hip and the axillary regions.
Older machines offer single-frequency transducers, like those of 5 MHz (usually a curved array one), 7.5 MHz, and 10 MHz, the latter 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 MHz to 10 MHz or 7.5 MHz to 18 MHz, with some probes exhibiting frequencies up to 22 MHz. A high-frequency transducer with a small footprint is useful to examine small joints or small arteries. As noted above, the higher is the frequency, the shorter is the wavelength of the US pulse and the better is 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 probe are, therefore, insufficient to accurately assess ligaments of 1 to 2 mm thick. Higher frequency probes (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 three-dimensional shape of an object, by a single sweep of a transducer. Four-dimensional US demonstrates the dynamic motion of 3D imaging. Three-dimensional US may become important in delineating the size of erosions or the extent of synovitis. Contrast-enhanced sonography is being assessed as an aid to detecting active synovitis, but its roles in clinical research and in clinical practice have not been defined.
VALIDATION, RELIABILITY, AND STANDARDIZATION — A growing number of studies on validity and reliability of ultrasonography (US) in patients with rheumatic disease have been published. As a first step in the iterative process of standardization, an international group of experts in the musculoskeletal US field proposed consensus-based definitions of rheumatoid arthritis erosions, synovitis, tenosynovitis, tendon damage, and enthesitis [5,6]. Studies performed thus far suggest moderate to good intraobserver reproducibility and variable agreement between observers, which depends, in part, upon the joint examined and upon the feature of disease that is being assessed. In addition, studies have shown a good US sensitiveness to change when treating synovitis in RA patients [7,8]. Additional data are needed on these test characteristics in patients with various musculoskeletal disorders [9]. The following studies are illustrative:
●In one study, the second metacarpophalangeal joints of 55 patients with rheumatoid arthritis were assessed ultrasonographically by two examiners [10]. The intraobserver kappa statistic for the detection of cortical bone erosions on the second metacarpophalangeal joints of 55 rheumatoid arthritis patients was 0.75, and the interobserver kappa for agreement between two observers was 0.76, implying substantial agreement among the observers. These kappas are comparable with those of radiologists scoring lesions on mammograms [11]. All bone erosions that were detected sonographically but that were not visible on radiography corresponded to magnetic resonance imaging (MRI) abnormalities (ie, were true positive findings).
●In another study, 14 experts examined four regions (shoulder, hand, knee, and foot) in four patients and compared the acquired images with previously performed MRI for synovitis, erosions, bursitis, and tendonitis [12]. The overall kappa was 0.76. However, there was considerable difference between joints, with the kappa statistic ranging from a high of 0.76 for the shoulder to a low of 0.28 for the ankle/toes. Kappa values for erosions, bursitis, and tendonitis were high, but agreement for synovitis was low. Sensitivity and specificity of the US images compared with MRI was moderate to good.
●In another study, 23 experts acquired and interpreted images of four joints (shoulder, hand/wrist, knee, and ankle/foot), without a set of predefined definitions or scanning method [13]. Overall agreements were 91 percent for joint effusion/synovitis and tendon lesions, 87 percent for cortical abnormalities, 84 percent for tenosynovitis, 83.5 percent for bursitis, and 83 percent for power Doppler signal; kappa values were good for the wrist/hand and knee (0.61 and 0.60) and were fair for the shoulder and ankle/foot (0.50 and 0.54).
●Another experiment assessed the intra- and interobserver reliability regarding synovitis of small hand joints in RA patients. Seventeen rheumatologist experts in US scored a sequence of 86 images of metacarpophalangeal, proximal interphalangeal, metatarsophalangeal, and wrist joints for the presence or absence of gray scale and power Doppler synovitis according to a predefined (OMERACT) definition. In the follow-up experiment, the experts acquired images of 32 metacarpophalangeal and 32 proximal interphalangeal joints on eight patients with rheumatoid arthritis. Results showed good agreement both between the interobserver and intraobserver interpretation of static images, but the kappa values for the acquisition of images were low [14].
●US has been shown to be a valid and reproducible technique for detecting synovitis of the knee, and is more sensitive than clinical examination [15]. As an example, a small observational study of US assessment of patients with knee osteoarthritis found that US was reliable for detecting osteophytes and protrusion of the medial meniscus, as well as for detecting synovitis [16].
●Other studies concerned the reliability of US detection of shoulder disease [17] and tenosynovitis in RA [18], gout [19,20], and pseudogout [21].
More data are needed regarding the responsiveness to change for US synovitis and tenosynovitis in RA, and for validation of these observations, before we can draw a more definite conclusion about the position of US in the clinical trial management of RA patients, if any. (See "Clinical manifestations of rheumatoid arthritis", section on 'Ultrasonography' and "General principles and overview of management of rheumatoid arthritis in adults", section on 'Clinical assessment of disease and related testing'.)
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
●Technologic advances have allowed real-time use of ultrasonography (US) to image tendons, bursae, ligaments, cartilage, synovium, synovial fluid, nerves, 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.)
●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.)
●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' above.)
●Axial and lateral resolutions are generally superior with higher-frequency US. Systems used for musculoskeletal US have typical axial and horizontal resolutions of 0.1 mm and 0.2 mm, respectively. (See 'Resolution' above.)
●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 used for musculoskeletal imaging typically employ multifrequency or broadband linear array probes. The transducer frequency and probe size are determined by the size and depth of the structures of interest. A higher frequency and smaller probe are used for smaller, superficial structures while lower frequencies and larger probes are used for deeper and bigger structures. (See 'Technical considerations' above.)
●Studies of US imaging in rheumatic diseases suggest a moderate to good intraobserver reproducibility and variable interobserver agreement that depends, in part, upon the joint examined and upon the feature of disease that is being assessed. (See 'Validation, reliability, and standardization' above.)
