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Musculoskeletal Ultrasonography
1
What are some benefits of ultrasonography (US) for musculoskeletal applications?
There are multiple benefits and unique advantages that US offers in musculoskeletal imaging. US does not involve the use of ionizing radiation, which is especially attractive when imaging pediatric and pregnant patients, and there are no absolute contraindications to diagnostic US. In addition, US has a higher spatial tissue resolution than magnetic resonance imaging (MRI) and computed tomography (CT), and it allows for real-time dynamic examination of anatomic sites with suspected abnormality. US can effectively be used to image patients with surgical hardware that leads to artifacts on CT and MRI, and Doppler US can provide important physiologic information about tissue blood flow. In particular, US is useful to distinguish whether a lesion is cystic or solid and to guide biopsies and therapeutic injections.
2
What are the common indications for musculoskeletal US?
Musculoskeletal US may be used for a wide range of indications. Some of the most common indications include evaluation for tendon pathology in the rotator cuff, quadriceps, patellar, Achilles, and elbow common flexor/extensor tendons. It is also commonly used to assess for ligamentous injury in the anterior talofibular ligament of the ankle, medial and lateral collateral ligaments of the knee, and ulnar collateral ligament (UCL) of the thumb. Additional common indications include the evaluation of bursal abnormalities (Baker cyst, subacromial-subdeltoid bursitis/calcific bursitis), muscle injury or hematoma, and joint effusions. US can be used to evaluate palpable masses and is also extremely useful for real-time interventional guidance, allowing direct visualization of the needle within the target.
3
When is musculoskeletal US not utilized?
Musculoskeletal US is significantly limited for the evaluation of internal joint derangement and is not used as the sole modality to evaluate for deep ligamentous injury (e.g., the anterior and posterior cruciate ligaments), labral pathology, cartilage injury, and meniscal injury. US is also not utilized to evaluate bone tumors because the ultrasound beam cannot effectively penetrate deep structures or cortical bone.
4
How are images oriented in musculoskeletal US?
The convention in musculoskeletal US imaging is to have the notch of the transducer directed to the patient's head or the patient's left side. Images are described as being in “long axis” or “short axis” to the structure being imaged, corresponding to “longitudinal” and “transverse” planes in body imaging. When describing the orientation of the transducer to a tendon, the terms long axis and short axis are preferred, because tendons and ligaments travel obliquely around the joints. For example, if one is imaging the posterior tibialis tendon in the ankle, which courses vertically proximal to the medial malleolus but then travels anteriorly distal to the medial malleolus, the transducer would be oriented longitudinally with respect to the body, and then transversely as it follows the tendon distally. Meanwhile, the tendon is being imaged in “long axis” the entire time.
5
What are the commonly used transducers in musculoskeletal US?
The transducer frequency is one important determinant of image quality. The higher the frequency, the greater the spatial resolution of the images and the lower the degree of tissue penetration. It is important to select the transducer with the highest frequency that still allows penetration to the depth of the structure being imaged. Linear probes are preferable to curved probes for musculoskeletal US because the ultrasound waves are propagated parallel to the transducer surface, limiting artifacts during evaluation of linear structures such as tendons. The linear 12-5 MHz is the workhorse transducer for musculoskeletal US, while the linear 17-5 MHz transducer may be used for the evaluation of more superficial structures. The compact or “hockey stick” linear transducer is ideally suited for the evaluation of small joints and guiding procedures performed on the distal extremities where sharp contours allow for limited contact with the probe surface. The 5-9 MHz curvilinear transducer is less commonly used in musculoskeletal US but has a role in the evaluation of deep structures or patients with larger body habitus.
6
What is anisotropy, and how can this be used to differentiate among soft tissue structures?
Anisotropy refers to the variation of ultrasound interaction with fibrillar tissues and is primarily observed when evaluating tendons and ligaments. For example, when the ultrasound beam is directed perpendicular to a structure such as a tendon, the typical hyperechoic fibrillar appearance is generated. The tendon will appear speckled and hyperechoic in the short axis and striated and hyperechoic in the long axis ( Figure 63-1 ). Upon slight angulation of the transducer probe from this perpendicular plane, the hyperechoic appearance of the tendon will become hypoechoic. This artifact may be confused for pathology as tendon and ligament abnormalities are often hypoechoic. Therefore, when evaluating a tendon or ligament, it is important to focus on the segment that is perpendicular to the ultrasound beam to exclude anisotropy as the cause of a focal hypoechoic defect. Anisotropy can be helpful in differentiating among normal soft tissue structures, especially in the ankle or wrist, where a hyperechoic tendon or ligament is often surrounded by hyperechoic soft tissues. Toggling or “heel-toeing” the transducer along the tendon or ligament will cause the structure to become hypoechoic, providing greater contrast from a background of hyperechoic fat which does not possess anisotropy ( Figure 63-2 ).
7
What is the normal sonographic appearance of various soft tissues (skin, subcutaneous fat, fascia, cortical bone, hyaline cartilage, fibrocartilage, and muscle)?
The epidermis and dermis of the skin typically appear as a thin hyperechoic layer. Subcutaneous fat is hypoechoic with thin linear echogenic septations paralleling the skin surface. Fascia appears as a thin, linear hyperechoic layer with variable thickness. Cortical bone is very hyperechoic with posterior acoustic shadowing. Hyaline cartilage, which covers the articular surface of bone, is uniformly hypoechoic. This is in contrast to the fibrocartilaginous labrum and meniscus, which are hyperechoic ( Figure 63-3 ). Normal muscle tissue appears relatively hypoechoic with interspersed hyperechoic fibroadipose septa; in long axis, normal muscle has a pennate, or featherlike, appearance. In short axis, muscle demonstrates a characteristic “starry sky” appearance ( Figure 63-4 ).
8
What is the normal echotexture of tendons and ligaments?
Normal tendons are hyperechoic with a signature fibrillar echotexture. Ligaments are also hyperechoic but can be differentiated from tendons due to their more compact, finely striated echotexture. Additionally, ligaments may be discerned from tendons as they connect two osseous structures ( Figure 63-5 ).
9
What is the normal echotexture of peripheral nerves?
Peripheral nerves demonstrate a fascicular appearance in which individual nerve fascicles are hypoechoic. The hypoechoic fascicles are surrounded by hyperechoic connective tissue referred to as epineurium. Large peripheral nerves will often be surrounded by a rim of hyperechoic fat. Short axis imaging is often helpful to clarify that an interrogated structure indeed represents a nerve, because peripheral nerves display a characteristic speckled or honeycomb appearance in short axis ( Figure 63-6 ).
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