Musculoskeletal Ultrasound 

 

Basic Anatomy 

Basic MSK US Anatomy; From Introduction to Bedside Ultrasound

Soft Tissue

Soft tissue layers can be divided into superficial and deep portions. The superficial portion consists of the epidermis/dermis and the subcutaneous tissue. The epidermis/dermis will appear as a solid hyperechoic line of varying thickness at the very top of the screen. The subcutaneous tissue is relatively hypoechoic with intermittent lace like stranding. The deep portion consists of muscles, tendons, ligaments, bones and joint spaces. Notice that the muscular fascia, which is a hyperechoic line, separates the superficial from deep portion. Nerves and blood vessels will often run in this area. Muscle appears hyperechoic relative to subcutaneous tissue with striations that run parallel in a particular direction. At the bottom of the image we appreciate bone which is a smooth hyperechoic line with posterior shadowing given its strong reflector properties. 

Longitudinal View of the Supraspinatus Tendon; From Wiki Commons

Tendon

Tendons will share a similar fibrillar appearance to muscle however given that they are more dense, they will appear more hyperechoic relatively. This is appreciated in the longitudinal view of the supraspinatus tendon seen to the left.

 

Transverse View of Median Nerve; From UCSF Nerve Atlas

Nerve

Nerves take on a unique appearance on ultrasound. Specifically in a transverse plane, their pattern can be described as a honeycomb, which is a direct result of alternating hypoechoic axonal areas and hyperechoic peri-/epineurium, Note the honeycomb appearance in the transverse image of the median nerve seen to the left.  

Basic Concepts

Probe: The majority of the time you will be using a high-frequency linear probe. This is ideal since most structures will be superficial and you will want high resolution for proper tissue differentiation. You may decide to use the curvilinear probe when evaluating the hip or performing a nerve block in a larger person. 

Positioning: This will vary quite significantly depending on what you are scanning. The key is to make sure the patient is in a comfortable position and that you are supporting the extremity you are examining to avoid unnecessary movement and artifact. 

Orientation: You may find yourself on the left side of the patient, at the head of the bed, or even at the patient's backside. Regardless of the patient, be sure to keep your probe marker on the same side relative to yourself so you never confuse yourself with orientation. The area of interest can then be systematically assessed in two orthogonal planes. 

Compare: Remember that your patients have two of everything. While you may not initially be certain about potential pathology you see, it is helpful to compare your examination to the contralateral side to help with deciding normal versus abnormal. 

Dynamic imaging: Musculoskeletal ultrasound should be interactive, allowing you to assess function and procedure efficacy in real time. For example, when assessing for tendon injury you should be ranging the extremity while observing for tendon abnormalities, or monitoring your needle advance into a fracture site for a hematoma block. 

 

Water bath for small part evaluation; From Introduction to Bedside Ultrasound

 

Small part imaging: You may find it difficult to assess a finger tendon or small joint given the size of the body part, the superficial nature of the structure of interest, or even patient discomfort. The water bath will be your best friend here and will allow for excellent visualization of your anatomy as well as less pain for the patient. You may also try a plentiful quantity of gel, however this can get messy. 

Anisotropy: This is a key concept to understand in MSK ultrasound. Anisotropy translates to mean "directionally dependent" and is a result of the fibrillar nature of structures such as tendons and nerves (and to a lesser extent muscle). This quality leads to the ultrasound image varying depending on the angle between the structure of interest and the ultrasound beam (also known as the angle of insonation). For example, when the angle of insonation is perpendicular to a tendon, the tendon appear bright and normal, however as the angle varies, the tendon can take on a hypoechoic appearance. Do not be fooled by this when evaluating for a tear, as you may just be appreciating this phenomenon. However if you do notice that a muscle or tendon does not display the property of anisotropy, this can be a sign of underlying muscle or tendon pathology. For a quick video explanation of this phenomenon, click here. 


Where do I scan? This is usually fairly easy, simply ask the patient where it hurts and have them point to that spot with one finger. If you start here, you will find the pathology the majority of the time without issue. 

Literature

FOAMed Curriculum