SOFT TISSUE SCAN

 I.  Introduction and Indications

Many patients come to the emergency department with complaints related to the soft tissues, e.g., infection, injury, and abnormal masses. Traditionally, computed tomography and magnetic resonance have been used when imaging studies are needed in these patients; however, ultrasound is generally more readily available and in some instances is the preferred imaging study.(1)
The most common use of bedside ultrasound in patients with soft-tissue abnormalities is in the evaluation of infections, including cellulitis, abscess, and necrotizing fasciitis. Other soft-tissue indications include the evaluation of cysts and lymph nodes. Ultrasound is also used to locate foreign bodies.
Although the majority of abscesses are treated with incision and drainage, in certain cases, usually because of cosmesis, treatment with needle aspiration and antibiotics may be an option.(3,7) Ozseker and colleagues(7) found that ultrasound-guided aspiration and irrigation of breast abscesses was preferred to surgical drainage for abscesses with a diameter less than 3 cm.(7) Ultrasound provides dynamic real-time guidance for needle aspiration, resulting in increased success.

II.  Anatomy

The structures that are imaged in soft-tissue bedside ultrasound are primarily the skin, subcutaneous tissue, fascia, and muscle.
The skin consists of two layers: the superficial epidermis and the deeper, thicker dermis. The subcutaneous tissue, located beneath the dermis, consists of connective tissue septa and fat lobules.(2) Fascia, a deeper structure, is a dense, fibrous membrane.(3) Muscle consists of long muscle fibers grouped together into fascicles. Various layers of connective tissue surround the individual muscle fibers, fascicles, and entire muscles.

III.  Scanning Technique, Normal Findings and Common Variants

Equipment
Selection of the appropriate transducer is an important aspect of soft-tissue bedside ultrasonography. Because most structures to be imaged are relatively superficial, a high-frequency (7–12 MHz) linear transducer is most useful and provides a good balance between imaging depth and resolution. On occasion, deeper structures need to be visualized; in these cases, a lower-frequency transducer is required. If a lower-frequency linear transducer is not available, then a curvilinear transducer may be used (Figure 1).

Figure 1: To image deep structures such as the thigh muscles as shown here,
a low frequency curvilinear transducer may be used.

Scanning Technique
The sonographer’s grip on the transducer is especially important in soft-tissue ultrasound because fine, controlled movements of the transducer are often required. The transducer should be held such that part of the hand, such as the small finger or ulnar aspect, rests against the patient’s body to stabilize the transducer.(4)
In many cases, it is helpful to begin scanning a short distance away from the area of interest to gain an appreciation of the appearance of the normal, uninvolved anatomy. Then the transducer can be slid toward the area of interest. It can be helpful for the sonographer to place a finger of the hand not holding the transducer on the area of interest and then slide the transducer until it contacts the finger.
Images should be obtained in both longitudinal and transverse planes, which will provide the most information and allow accurate localization.(5) It may be helpful to view the contralateral side of the patient’s body to obtain information about the normal appearance of structures.(5)
Normal Findings
With the equipment that is typically used for bedside ultrasonography, the epidermis and dermis cannot be differentiated. They appear together as a thin, hyperechoic layer.(2) The subcutaneous layer appears hypoechoic on ultrasound, with two components: hypoechoic fat interspersed with hyperechoic linear echoes running mostly parallel to the skin, which represent connective tissue septa (Figure 2).(2) Veins and nerves may be visualized within the subcutaneous layer.