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Introduction to Musculoskeletal Ultrasound

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Authors

Shrut Patel MD
Sports Medicine Fellow
Geisinger Commonwealth School of Medicine
Department of Orthopaedics and Sports Medicine
Geisinger Wyoming Valley Medical Center
Wilkes-Barre, Pennsylvania

Brandon G. Tunis PT, DPT, RMSK
Geisinger Orthopedics and Sports Medicine

What is ultrasound?

  • Ultrasound (US) is the conversion of electrical energy into sound waves through a medium called piezoelectric crystals[1]Jacobson J. Fundamentals of Musculoskeletal Ultrasound. Elsevier Health Sciences; 2017..
  • The sound waves are utilized to produce an image.
example of musculoskeletal ultrasound

Clinical Utility

  • Ultrasound serves an extension to the clinical exam and offers diagnostic value in patients presenting with unknown disease[2]Cuccurullo S. Physical Medicine and Rehabilitation Board Review Book. Springer Publishing; 2019.
  • Ultrasound can be utilized to varying degrees to evaluate any organ system and a wide variety of clinical pathology
  • Ultrasound can be performed as point-of-care (POCUS) by physicians or via formal diagnostic studies by trained ultrasound technicians
  • Additionally ultrasound is utilized therapeutically in physical therapy for chronic inflammation (ex. bursitis or calcific tendinitis), musculoskeletal pain, degenerative arthritis and contracture (ex. adhesive capsulitis, shoulder periarthritis, and hip contracture), and subacute trauma. Ultrasound utilizes sound waves to help generate a thermal effect or heat, which can penetrate up to 8 cm in depth. Heat absorption is greatest at the bone-muscle soft tissue interface. The thermal energy produced increases the distensibility of collagen fibers which makes mobilization of soft tissue easier. Ultrasound thermal effect is also used to drive medications transdermally by increasing cell permeability, a process called phonophoresis.
  • Because ultrasound is dynamic and allows you to visualize structures in real time, it can be used for a wide variety of procedures.
  • This is especially true in the management of musculoskeletal pathology, where ultrasound allows easy visualization of muscle, tendon, bursa and bone for procedural intervention.

Advantages

  • US is a quick, portable, and effective way to evaluate pathology
  • Allows for dynamic, real-time evaluation compared to static images with other imaging modalities (i.e. MRI, CT, Radiographs)
  • Allows for contralateral comparisons to unaffected limb
  • Less expensive than other imaging modalities[3]Bierig SM, Jones A. Accuracy and cost comparison of ultrasound versus alternative imaging modalities, including CT, MR, PET, and angiography. J Diagn Med Sonogr. 2009;25(3):138-144.[4]Parker L, Nazarian LN, Carrino JA, et al. Musculoskeletal imaging: medicare use, costs, and potential for cost substitution. J Am Coll Radiol. 2008;5(3):182-188.[5]Md Ms, N. S. J., Frcpc, A. R. M., & Mpa, K. P. M. (2019). Point of Care Ultrasound (2nd ed.). Elsevier.
  • Image quality not significantly impacted by metal artifact
  • Allows for visualization of soft tissue pathology with US transducer and can receive immediate feedback
  • Enables opportunity to perform minimally-invasive interventional procedures
  • Imaging modality with no radiation compared to CT and radiography.[6]Center for Devices, Radiological Health. Medical X-ray imaging. Fda.gov. Published 2021. Accessed September 1, 2021. … Continue reading[7]Center for Devices, Radiological Health. Ultrasound Imaging. Fda.gov. Published 2020. Accessed September 1, 2021. https://www.fda.gov/radiation-emitting-products/medical-imaging/ultrasound-imaging[8]Facr HWM. Learning Radiology: Recognizing the Basics. 4th ed. Elsevier; 2019.

Disadvantages

  • User dependent, thus requires significant amount of experience and expertise
  • In obese patients, there is limited visualization for deep tissue/structures, such as intra-articular pathology
  • Limited by certain pathologies such as heavy calcification and air

How does US work?

  • The US machine consists of two main components: the computer and the transducer.
  • The transducer is a handheld device that is placed on the patient. The transducer contains piezoelectric crystals which convert the electrical signal into sound waves. The sound waves are emitted through the transducer which then penetrate through bodily tissues and generate echoes to form images.
    • The generation of sound waves is called the reverse piezoelectric effect.
    • The images generated from the echo is the piezoelectric effect.
Ultrasound machine with a screen and transducer
Illustration of ultrasound machine with a screen and transducer
    • The echo or echogenicity (brightness or darkness) of the image will depend upon the characteristics of the soft tissue and how much it reflects or absorbs the sound waves.
  • Sound waves are measured in frequencies
    • Inverse relationship between frequency and wavelength/depth
      • Low frequencies allow for greater tissue penetration but poorer image quality
      • High frequencies allow for decreased tissue penetration but higher image quality
  • US transducers will create thin cuts/slices (similar to MRIs and CTs) of 3-D images and display them in 2-D. It is important to evaluate a structure in at least multiple planes or axes in order to avoid missing pathology.

Transducers

linear transducer 2
  • Linear
    • Uses high frequency of 5-15MHZ
    • Ideal for superficial structures (hand, shoulder, elbow, wrist)
    • Up to 6 cm scan depth
compact linear transducer 2
  • Compact linear
    • Uses very high frequency >10MHz
    • Ideal for very superficial structures (fingers, superficial nerves)
    • Up to 2-3 cm scan depth
curvilinear transducer 2
  • Curvilinear
    • Uses low frequency of 2-5 MHz
    • Ideal for deeper structures (hip joint, glenohumeral joint – particularly helpful with obese patients)
    • Up to 30cm scan depth
phased array transducer 2
  • Phased Array
    • Not commonly used for musculoskeletal evaluation
    • Uses low frequency of 1-5 MHz
    • Ideal for deeper obstructed structures (heart, inferior vena cava)
    • Up to 35cm scan depth

Ultrasound Terminology

sound wave characteristics
Sound wave characteristics
ultrasound high frequency vs low frequency
High frequency vs low frequency[9]Image courtesy of Dr Vi Dinh, MD, pocus101.com, “Basic Principles of Ultrasound Physics and Artifacts Made Easy”.
  • Wavelength: distance between consecutive successive crests or troughs of a wave
  • Frequency: number of waves per unit of time
    • Measured in hertz (Hz) = one wave per second.
    • Megahertz (MHz) = 1,000,000 Hz = 1,000,000 waves per second
  • As sound waves pass through tissue, different types of tissues will generate distinct images based upon how much sound wave energy is absorbed or reflected.
ultrasound adjustment of the focal zone

Adjustment of focal zone. Image A demonstrates a large focal zone. Note the arrows marking the smaller focal zone in B[10]McDonald, Shelley, et al. “Basic appearance of ultrasound structures and pitfalls.” Physical medicine and rehabilitation clinics of North America 21.3 (2010): 461.

Ultrasound adjustment of the focal zone 2
Poorer resolution when focal zone is positioned superficially compared to when it is redirected to the area of interest
  • Focal Zone: Area where the sound waves/beam is most sharp producing the highest quality image
Acoustic impedence of different body tissues and organs
Acoustic impedence of different body tissues and organs[11]Chan V, Perlas A. Basics of ultrasound imaging. In: Atlas of Ultrasound-Guided Procedures in Interventional Pain Management. Springer New York; 2011:13-19.
  • Acoustic Impedance (AI): physical property of specific bodily tissue and how much resistance the sound waves have as they travel through.
    • Dependent upon density of the tissue and speed of the sound waves
    • Larger AI = stronger and clearer reflection
Ultrasound examples of attenuation
Ultrasound examples of attenuation.
  • Attenuation: the weakening of sound wave intensity as it passes through medium/tissue.
    • Absorption – echo energy lost as heat
    • Reflection – if sound waves are not directly perpendicular to the tissue of interest, then some will reflect away from the transducer
Ultrasound long axis and short axis
Procedure being performed with the ultrasound in long axis (A, B) and short axis (C, D)[12]Keskin H, Keskin F, Aydin P, Guler MA, Ahiskalioglu A. Syringe-free, long-axis in-plane versus short-axis classic out-of-plane approach for ultrasound-guided internal jugular vein catheter placement … Continue reading.
Ultrasound echogenecity
Echogenicity is a spectrum from hyperechoic (white) to anechoic (black)[13]Image courtesy of Dr van der Plas, startradiology.com, “Ultrasound Technique”
  • Ultrasound Planes:
    • Sagittal and Coronal planes are referred to as the longitudinal/long axis plane.
    • Longitudinal/Long Axis = transducer is oriented lengthwise/parallel to the structure of interested
    • Transverse plan is referred to as the short axis plane.
    • Short Axis = transducer is oriented across/perpendicular to the structure of interest
  • Echogenicity: ability of a structure to reflect sound waves and produce an image/echo
    • Hyperechoic: white in appearance, structure is highly reflective and is brighter than surrounding tissue
    • Isoechoic: light gray in appearance, structure appears the same as surrounding tissue
    • Hypoechoic: dark gray in appearance, structure is minimally reflective and is darker than surrounding tissue
    • Anechoic: black in appearance, structure is non-reflective
Example of anisotropy
Anisotropy (yellow arrow) seen at different probe positions (blue box)
Ultrasound of the Achilles Tendon in short axis demonstrating Anisotropy
  • Anisotropy – artifact image generated based upon direction of the sound beam on the structure of interest
    • For structures to be visualized with most optimal clarity, the incident sound waves exiting the transducer must be oriented directly perpendicular to the target structure. When this occurs, the reflected sound waves are able to bounce directly back to the transducer generating a clear image
    • If the incident sound waves are slightly angled relative to the target tissue, the transducer will not receive the full reflection/echo, thus producing an obscured image which appears unintentionally dark. This is an example of anisotropy.
  • Echotexture – describes the internal pattern of a particular tissue, such as tendon or muscle.
Example of ultrasound gain
Example of gain adjustment low (A) and high (B)
  • Gain – the “brightness” of the image, which refers to degree of amplification of returning echoes
    • Time Gain Compensation – helps account of tissue attenuation by increase the amplification (gain) with time (depth) to produce uniform brightness along the entire image
  • Power – energy emitted from the transducer
    • Operate with minimum power and maximum gain
Video demonstration of compressibility of the brachial artery (non-compressible) and vein (compressible)
  • Compressibility – used to describe and identify vascular structures. May also help describe cystic lesions, masses, or other fluid filled structures
Color doppler of the brachial artery and vein
  • Doppler – used to help identify vascular structures. Doppler mode demonstrates a shift in frequency of returning sound waves.
    • In regard to vascular structures, the frequency of sound waves is higher when blood is flowing toward the transducer compared to a slower frequency when blood is flowing away.
    • Neovascularization: May also be used with non-vascular structures to determine presence of blood flow or neovascularization, which would demonstrate a pathological state
M-mode demonstration on cardiac ultrasound
  • Motion mode or M-mode – used to analyze the movement of structures over time. A still image is acquired and then the M-mode is engaged to evaluate for movement of structures. M-mode is useful in measuring cardiac chamber size or heart valve motion. 
Posterior acoustic shadowing illustration
Illustration of posterior acoustic shadowing with ultrasound example
US demonstrating an echogenic CBD stone with strong posterior acoustic shadowing
  • Posterior acoustic shadowing – loss of sound wave beyond a particular tissue due to the highly reflective nature of that tissue 
Posterior acoustic enhancement illustration
Illustration of posterior acoustic enhancement with ultrasound example
Bladder ultrasound demonstrating posterior acoustic enhancement as it propagates through the hypoechoic fluid
  • Posterior acoustic enhancement – sound wave propagates through less reflective tissue, such as with fluid, and then deeper tissues become hyperechoic/bright
    • Sound waves propagate relatively unimpeded through less reflective structures such as fluid/cyst. Time gain compensation overcorrects creating an area of hyperechogenicity deep to the structure.
Ultrasound reverberation illustration
Illustration of reverberation with ultrasound example
Reverberation seen on lung ultrasound with the linear echoes extending deep to the lung tissue.
  • Reverberation – Occurs when the incident sound wave encounters two highly echogenic parallel structures resulting in the echo being passed back and forth over a period of time. 
    • The ultrasound machine misinterprets this reflection as a series of linear echoes extending deep beyond the structure. Commonly seen with needles or metal implants.
Ultrasound transducer movement
The 4 cardinal movements of scanning
  • Scanning Terminology
    • Toggle/Tilting – angle transducer from side to side
    • Heel-toe maneuver – transducer is rocked or angled along its long axis 
    • Translate – transducer is moved to a new location while maintaining perpendicular angle on the skin
    • Sweep – transducer is moved side to side while maintaining a stable hand position
Ultrasound illustration in plane
Illustration of in plane technique. Note the probe axis is aligned over the needle in the same vector[14]Moore CL, Copel JA. Point-of-care ultrasonography. N Engl J Med. 2011;364(8):749-757.
Ultrasound illustration out of plane
Illustration of out of plane technique. Note the probe axis is aligned over the needle in a perpendicular axis[15]Moore CL, Copel JA. Point-of-care ultrasonography. N Engl J Med. 2011;364(8):749-757.
  • Procedural Terminology
    • Needle orientation is described in two ways: longitudinal approach (in-plane) or transverse approach (out-of-plane).
      • In-plane approach is when the needle is placed lengthwise along/parallel to the transducer and structure of interest. 
        • Needle tip is inserted along the center of the short side of the transducer. 
        • In this approach, the needle will appear as a bright line as the ultrasound beam will pick up the shaft of the needle.
        • Flatter angles of needle penetration will result in better visualization as more sound waves will reflect off the needle and back to the transducer
      • Out-of-plane approach is when the needle is placed on the short axis/perpendicular to the long side of the transducer.
        • Needle tip is inserted along the center of the long side of the transducer. 
        • In this approach, the needle will appear as a bright dot as the ultrasound beam will pick up the cross section of the needle instead of the entire shaft.  
Video demonstrating needle in plane parallel and at an angle. The more parallel the needle is to the probe, the better it is visualized.
  • As the needle travels through the soft tissue, the transducer and/or needle may need to be adjusted for improved visualization. When making adjustments to better visualize the needle in relation to the target structure, it is recommended that one moves either the transducer or needle and not both. The transducer may be adjusted with the heel-toe maneuver, toggling, translation, sweeping, or a combination of each for improved needle visualization. 

Tissue Features

Elbow US Calcific tendonitis
Elbow ultrasound showing calcific tendinopathy or pathologic bone deposition in the tendon.
Elbow US Normal Bone
Elbow ultrasound showing normal bone contour
  • Bone/Calcium Deposits: highly reflective of sound, therefore will be hyperechoic and bright with posterior acoustic shadowing
Shoulder US biceps tendon in long axis
Shoulder ultrasound showing normal biceps tendon in long axis[16]Gil-Agudo Á, Solís-Mozos M, Crespo-Ruiz B, et al. Echographic and kinetic changes in the shoulder joint after manual wheelchair propulsion under two different workload settings. Front Bioeng … Continue reading
Shoulder US supraspinatus in short axis
Shoulder ultrasound showing normal supraspinatus tendon in short axis
  • Tendons: linear strands of fibers that are hyperechoic/bright
    • Short axis view – “broom-end” or echogenic dots appearance
    • Long axis view – “paint-brush” or fibrillar appearance
    • Of note, tendon is highly susceptible to anisotropy, so it must be scrutinized thoroughly before deciding if pathology is present.
Wrist US nerve in long axis
Long axis of a nerve
Thigh US femoral nerve in short axis
Short axis view of the femoral nerve (FN)[17]Image courtesy of Nysora.com, “Ultrasound-Guided Fascia Iliaca Nerve Block”
  • Nerves: tubular structure with mixed echogenicity (hyperechoic fascicles and hypoechoic background)
    • Short axis view – “honeycomb” appearance
    • Long axis view – Fascicular appearance
Elbow US UCL Ligament
Elbow ultrasound of the Ulnar Collateral Ligament (UCL).
Elbow US UCL Ligament
Elbow ultrasound of the Ulnar Collateral Ligament (UCL).
  • Ligaments: similar to tendons in that it is hyperechoic, but is more compact fibrillar pattern than tendons
    • However, ligaments appear less uniform and ordered given their weaved collage bundles that restrict forces in many directions. Overall, ligaments are described as more compact and fibrillar relative to tendon.
  • Skin:  slightly hyperechoic
US muscle long axis
Ultrasound showing muscle in long axis (yellow arrow). Note that the tendon is labeled (blue arrow) and more muscle caught at an oblique angle (red)
Thigh US muscle short axis
Thigh ultrasound showing quadriceps muscle in short axis and the so-called “starry night” appearance.
  • Muscle: mixed echogenicity (hypoechoic regions with hyperechoic linear septae)
    • Short axis view – “starry night” appearance
    • Long axis view – parallel lines
  • Fat: hypoechoic with areas of hyperechoic septated lines
Knee US various views of articular cartilage
Ultrasound showing various measurements of normal femoral articular cartilage[18]Herrera H GA, Llinás PJ, Flórez L, et al. Medición ecográfica del grosor del cartílago femoral en rodilla de jóvenes universitarios sanos. Rev Esp Cir Ortop Traumatol. 2020;64(4):244-250.
  • Cartilage: hypoechoic/anechoic
  • Fluid: anechoic
    • Can be septated or loculated, presenting as multiple walled off structures
Knee US color doppler popliteal fossa
Color doppler of the popliteal fossa[19]Fernandez-Cuadros, Marcos, et al. “Basics of Ultrasound for Musculoskeletal Evaluation in the Rehabilitation Settings: Review of the Literature.” Middle East Journal of Rehabilitation and … Continue reading
US color power doppler unknown soft tissue lesion
Power color doppler of unknown soft tissue lesion. Note imaging appearance is concerning with multifocal peripheral vascularity[20]Smith E, Azzopardi C, Thaker S, Botchu R, Gupta H. Power Doppler in musculoskeletal ultrasound: uses, pitfalls and principles to overcome its shortcomings. J Ultrasound. 2021;24(2):151-156.
  • Color doppler: provides information regarding direction of movement.
    • Used to determine vascular flow
    • Movement toward transducer – red
    • Movement away – blue
    • Note that red and blue do not mean arterial and venous respectively
  • Color Power Doppler: Denotes power/amplitude of US rather than direction of movement
    • Sensitive to movement in ANY direction
    • Single color – red/orange
    • Used to identify neovascularization/hyperemia due to inflammation

References

References[+]