Technology and Terminology

Various terms are used to describe ultrasonographic equipment and images. B-mode US refers to brightness mode, and it allows real-time imaging. B-mode US is the precursor to gray-scale US and is somewhat limited beyond differentiating fluid from solid; consequently, it has largely been replaced by gray-scale US. Gray-scale US differentiates between intensities of echoes and displays them in black, white, and various shades of gray, which improves not only the resolution of the images but the ability to distinguish between different types of tissue. However, even gray-scale US cannot differentiate between fibrous synovial tissue and active synovitis; such a differentiation requires characterization of blood flow. Color Doppler US uses the principle that sound waves increase in frequency when they reflect from objects moving toward the transducer and decrease when they reflect from objects moving away. This is combined with real-time imaging to indicate the presence and direction of blood flow. Red signals indicate flow toward the transducer and blue signals indicate blood flow away from the probe. Power Doppler US has increased sensitivity for imaging small vessels and slow blood flow, which better demonstrates hyperemia and can help differentiate between inflammatory (hyperemic) and scar tissue. Power Doppler US may help visualize neovascularization or angiogenesis in inflamed or otherwise affected tissues (e.g., chronic tendinosis). Both color and power Doppler US allow one to clearly differentiate cystic lesions from vessels. (See also Chapter 225, Emergency Department, Hospitalist, and Office Ultrasonography [Clinical Ultrasonography], for discussions of principles of ultrasound and beginner scanning. Quality assurance, credentialing, and liability are also discussed.)

MSK US should be performed with a high-resolution linear-array transducer with frequencies between 7.5 and 20 MHz. The lower frequencies allow visualization of the deeper structures (e.g., hip joint), whereas the higher frequencies are better for superficial structures (e.g., finger joints). In addition to gray-scale US, machines with tissue harmonic imaging or compound imaging are useful for musculoskeletal applications. Advances in ultrasonographic equipment mirror changes in computer technology, and the actual machine used does not have to be the latest version. Although the newer machines have more features and are more portable, they are more expensive. To reduce startup costs, instead of purchasing a new machine, it may be possible to purchase a high-frequency linear-array transducer for a machine already owned or to purchase a used machine when someone else upgrades his or her equipment. Standoff probe attachments are available to enhance resolution in certain older equipment; they are also useful with most equipment when scanning very superficial structures.

Orientation and Anatomy

Ultrasonographic scans are defined by two views oriented perpendicular to one another, the transverse/axial/short-axis view and the longitudinal/long-axis view (Fig. 185-1). Long-axis images are further defined as sagittal or coronal. Understanding anatomy as viewed by ultrasonography is a “learned skill” that takes both patience and practice to acquire. One must keep two principles in mind. First, three-dimensional structures are seen on a screen in only two dimensions. Second, a 90-degree turn of the probe will change the orientation of a two-dimensional view from axial (short axis) to longitudinal (long axis) or vice versa.

Figure 185-1 Short-axis (transverse/axial) and long-axis (longitudinal/sagittal/coronal) views in ultrasonographic scanning.

Scanning

The position of the probe in the practitioner’s hand is variable; many hold it as a large pencil or like a computer mouse (Fig. 185-2). Artifact is minimized by keeping the probe as perpendicular as possible to the tissue being scanned. By convention, solid or echogenic tissue or structures are whiter on the image, whereas fluid or fluid-filled tissue or structures are darker, hypoechoic, or echolucent. Table 185-1 shows some common superficial anatomic structures seen with MSK US as well as the common views used to scan these structures.

Figure 185-2 Hand and probe position.

Indications

See Table 185-2.

TABLE 185-2 Summary of Musculoskeletal Ultrasonography Uses

Precautions and Contraindications

Diagnostic MSK US is a safe imaging modality with no exposure to radiation, no risks of a large magnet disrupting vascular clips, and no issues related to claustrophobia. The fact that ultrasonography has been used for years in maternal-fetal medicine with no documented ill effects speaks to its safety. Therefore, the major precaution is related to the imaging abilities; this procedure is highly operator and interpreter dependent. The sensitivity and specificity are related to the ability to visualize the individual structures and, once these are seen, to distinguish normal from abnormal anatomy. It is quite easy to visualize and diagnose an inflamed biceps tendon; however, it can be quite difficult to differentiate between chronic hamstring inflammation and a high hamstring injury. Clinicians must know their limitations and caution must be used to avoid scanning, diagnosing, or performing procedures beyond their skills.

Although procedures performed using ultrasonographic guidance are safer than blindly performing injections and aspirations, whenever a needle enters the skin, there are inherent risks of pain, bleeding, infection, and damage to underlying or surrounding tissues. No absolute contraindications to MSK US exist, but similar to all procedures, using good judgment and proper techniques will protect you and your patient.

Equipment

NOTE: Linear-array transducers do have one significant limitation: they accentuate anisotropy. This hypoechogenic artifact of the tendon insertions can look like a tendon tear (Fig. 185-3); it is due to the lack of divergent beam geometry.

Figure 185-3 Anisotropy (arrow) of supraspinatus insertion on greater tuberosity (GT). HUM, humerus.

Preprocedure Patient Education and Preparation

Diagnostic ultrasonography, and any possible ultrasonography-guided procedure, should be briefly explained to the patient (or representative) before obtaining consent verbally and often in writing. The approximate length of time to accomplish the procedure, the few risks involved (e.g., irritation from ultrasonic gel, having to remain still during the examination, missed diagnosis), the amount of discomfort to expect from an ultrasonography-directed procedure, the benefits of having this procedure, and any alternatives should be explained. For an ultrasonography-guided procedure, any additional risks should be explained (e.g., risks of infection, injury to a nearby structure such as a vessel or nerve, scar formation, tendon rupture, hematoma) as well as any alternatives. Any other aspects should be addressed, depending on the practitioner’s personal style (e.g., some clinicians curtail use of nonsteroidal anti-inflammatory drugs [NSAIDs] or aspirin before certain procedures [e.g., dry-needling tendon]; other clinicians use topical ethyl chloride “cold” spray before needling instead of injecting an anesthetic). The cost should also be explained, especially for Medicare patients; a Medicare waiver should be signed. There may be benefit to discussing postprocedure expectations at this point, especially those regarding activity, physical therapy, and restrictions. The patient should know that the room lights may be dimmed to improve visualization of the screen. (See the sample forms online at www.expertconsult.com for checklists for diagnostic and ultrasonography-guided procedures.)

Technique

Overall

1 There are several prerequisites essential to performing adequate MSK US; these should be ensured before scanning. Enough time should have been allocated to provide the imaging and perform the procedure (10 to 60 minutes). There must be adequate support staff, and they can help prepare the patient (e.g., obtain informed consent, appropriately uncover the body part to be imaged), turn on the machine, and record the patient’s demographic information. Often an assistant is needed to operate the machine during a procedure because the clinician’s hands may be occupied with the probe and needle. Both the patient and clinician should be positioned in a manner that will be comfortable for the duration of the procedure. Any additional questions should be answered before starting the procedure.

2 Apply ultrasonic gel to the patient’s skin. If planning an ultrasonography-guided percutaneous procedure, certain additional steps should be taken.

• Apply a sterile cover over the probe/transducer (a sterile glove can be used). Some clinicians apply a small ribbon of gel to the probe before applying the sterile cover.

• Cleanse the area of skin with antiseptic solution.

• Apply to the area any of the following:

Sterile ultrasonic gel

Packet of sterile KY Jelly

Betadine-impregnated gel (if you are not using a disposable probe cover, this will stain the transducer)

• Apply sterile probe to site.

NOTE: Preferably a sterile probe cover should be over the probe.

3 Capture basic images (Table 185-3 shows some common sonographic pathologic findings):

• Identify large and obvious structures first.

• Get into the habit of viewing most structures in both transverse and longitudinal planes.

• When needed, capture images of the area in question in both static and dynamic positions (i.e., relaxed and contracted or flexed and extended).

• Consider using color or power Doppler to look for blood flow in low- to high-flow states to assess for inflammation or neovascularization, or to differentiate between a cyst and a vessel.

• When performing percutaneous needling (aspiration/injection or dry needling), try to obtain and document images. Because the clinician’s hands will be occupied with the probe and syringe (Fig. 185-4), an assistant may be needed to run the machine.

4 Postprocedure: Once the procedure is completed, wipe the ultrasonic gel off of the patient and the probe.

• It is important to remove the gel before it dries.

• If a percutaneous procedure has been performed, remove and discard the sterile cover. Clean the probe head with manufacturer’s approved cleaning solution.

• If blood, body fluids, or Betadine somehow gets onto the probe surface, wash it with soap and water and then follow the manufacturer’s recommendation to disinfect the probe.

TABLE 185-3 Common Sonographic Pathologic Findings

Figure 185-4 Example of positioning for an ultrasonography-guided injection.

NOTE: Povidone–iodine, if allowed to stay on the probe even for a short length of time, will discolor the probe and may shorten its working life.

• Complete and save any documentation on the ultrasonography unit and in the patient’s chart.

Specific

Shoulder

1 To examine the biceps tendon, the patient’s hand should be in the supine position and resting on the thigh or a pillow on the examination table with the arm slightly rotated externally. To localize the tendon, it can be scanned transversely, from where it emerges under the acromion, and followed distally to where it inserts. The tendon should then be scanned longitudinally. While scanning, determine whether the tendon is properly located in the bicipital groove, or if it can be subluxated or dislocated. Also, attempt to determine whether it is torn or disrupted.

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