Technical Considerations

Diagnostic and subsequent interventional examinations are performed using either linear or curved phased array transducers, based on depth and local geometry. Needle selection is based on specific anatomic conditions (i.e., depth and size of the region of interest). We employ a freehand technique in which the basic principle is to ensure needle visualization as a specular reflector (Fig. 1). This relies on orienting the needle so that it is perpendicular (or nearly so) to the insonating beam. The needle then becomes a specular reflector, often having a strong ring-down artifact (7).

Patient positioning to ensure comfort and optimal visualization of the anatomy should first be assessed. It is important to keep in mind that tendons display inherent anisotropy (14). It is, therefore, necessary that the transducer is oriented to maximize tendon echogenicity in order to avoid false interpretation of the tendon as being complex fluid or synovium. An offset may be required at the skin entry point of the needle relative to the transducer to allow for the appropriate needle orientation. Deep structures, such as tendons about the hip, are often better imaged using a curved linear or sector transducer, operating at center frequencies of

FIGURE 1 Ultrasound-guided aspiration of a Baker cyst. (A) Transverse sonogram over the posteromedial aspect of the knee shows a hypoechoic collection in close association with the tendon of the medial head of the gastrocnemius muscle (mhg). A needle (arrow) is present within the cyst, appearing as an echogenic reflector with a strong posterior reverberation artifact. (B) The needle is unchanged in position, and the cyst is observed to be decompressed about the needle tip.

FIGURE 1 Ultrasound-guided aspiration of a Baker cyst. (A) Transverse sonogram over the posteromedial aspect of the knee shows a hypoechoic collection in close association with the tendon of the medial head of the gastrocnemius muscle (mhg). A needle (arrow) is present within the cyst, appearing as an echogenic reflector with a strong posterior reverberation artifact. (B) The needle is unchanged in position, and the cyst is observed to be decompressed about the needle tip.

approximately 3.5 to 7.5 MHz. Superficial, linearly oriented structures, such as in the wrist or ankle, are best approached using a linear array transducer with center frequencies greater than or equal to 10 MHz. These factors should be assessed prior to skin preparation.

In our experience, a short-axis approach affords the best opportunity to avoid intratendi-nous injections. The latter have been associated with collagen breakdown and potential tendon rupture (8,9,11,13). The curvature of the extremity of interest, being greater in short axis, often permits a shorter trajectory and, therefore, greater flexibility in repositioning the needle. This is particularly relevant when performing injections of superficial tendons in the wrist and ankle. Once the transducer is properly positioned, the degree of tendon sheath or bursal distension can be assessed, as well as the needle position relative to the tendon. When positioning the needle into a distended tendon sheath or bursa, the presence of surrounding fluid and/or synovium often provides a standoff to better visualize the needle tip. Alternatively, when injecting into a nondistended structure, a test injection with local anesthetic often permits improved visualization of the needle tip by introducing microbubbles, as well as providing some fluid distension of the sheath or bursa (Fig. 2) (1). The immiscible nature of the steroid-anesthetic mixture may likewise produce temporary contrast effect (Fig. 3).

In every case, the area in question is cleaned with iodine-based solution, and draped with a sterile drape. The transducer is immersed into iodine-based solution and surrounded by a sterile drape; a drape is also placed over portions of the ultra-sound unit. A sonologist or radiologist positions the transducer, while a radiologist positions the needle and performs the procedure. We use 1% lidocaine (Abbot Laboratories, North Chicago, Illinois, U.S.) for local anesthesia. Once the needle is in position, the procedure is undertaken, while imaging in real time. Depending on anatomic location, either a 1.5 in or spinal needle with a stylet is used to administer the anesthesia-corticosteroid mixture typically, [0.5 cc 1% lidocaine, 0.5 cc 0.5% bupivacaine (Sensorcaine-Astra Pharmaceuticals, Westborogh, Massachusetts, U.S.), and 1 cc (40 mg) triamcinolone (Kenalog-Apothecon, a Bristol Myers Squibb Company, Princeton, New Jersey, U.S.), or some equivalent long-acting agent.

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