Musculoskeletal Ultrasound
Sean W. Mulvaney
Sean N. Martin
INTRODUCTION
Musculoskeletal (MSK) ultrasound (US) is a rapidly emerging imaging modality in the field of sports medicine. It is a powerful tool in both the diagnosis and management of the injured athlete. MSK US is unique in that it provides previously unavailable point-of-care diagnostic imaging, completed by the clinician, and with the patient still in the examination room (3). This technology is the result of continued improvement of broadband high-frequency transducers and new developments in signal-processing software that have improved the image quality for evaluating MSK structures (10).
Advantages of US imaging:
Portable, point-of-care imaging.
Direct, dynamic visualization of injured structure, aiding in a pathoanatomic diagnosis:
□ Many pathologic conditions are only apparent while a structure is in motion.
□ Magnetic resonance imaging (MRI) only allows the injured tissue to be viewed in one position.
Relatively inexpensive.
Focus: able to focus on the area of interest to a degree not possible with MRI.
Side-to-side comparison views are simple to obtain for anatomic variants/growth plate comparison.
Low-risk visualization of soft tissue and superficial bone:
□ No risk associated with iodinated contrast.
□ No risk associated with MRI contrast (i.e., gadopentetate dimeglumine contrast resulting in gadolinium-associated nephrogenic systemic fibrosis).
□ No risks associated with ionizing radiation.
Ability to accurately image soft tissue structures despite metallic implants.
Immediate correlation of imaging with history and physical.
Real-time visualization, allowing accurate percutaneous procedures.
□ Increases accuracy of medication placement.
□ Allows determination of the exact pain generator in cases when multiple pathologic structures are visualized.
Disadvantages of US imaging:
Significant time, effort, and cost investment to become proficient.
□ Many providers initially rely upon attendance at dedicated MSK US courses to learn directly from industry leaders.
Although US can rule-in a fracture, it can neither rule-out nor characterize a fracture.
Due to reliance on direct visualization, US can be hindered by intervening bony structures. Intraarticular pathology is incompletely visible on US exam.
Labral pathology is incompletely visible on US.
Even if a surgical lesion is identifiable on US, many orthopedic surgeons are more comfortable with MRI imaging to justify surgery, for presurgery planning, and for use as a reference during surgery.
US is more limited with obese patients.
PHYSICS
The US image is the product of sound waves that have been emitted by piezoelectric crystals on the US transducer, passed through tissues, some of which are reflected back to the transducer.
Echo signature of bone, ligaments, tendons, muscles, and other soft tissues is based on the degree of absorption versus reflection of the sound waves.
The US image is based on reflected sound or US waves. The processor translates the echo signatures of the US waves into a real-time two-dimensional image.
Attenuation is the name for all interactions that decrease the intensity of the US beam except reflection. Attenuation includes the effects of scattering and absorption that reduce the US wave amplitude.
Constant = frequency × wavelength:
The constant used is the velocity of sound in tissue (dermis, adipose, muscle, tendon, cartilage, and ligament), approximately 1,540 meters per second.
Because the position of any object measured with US can be determined only to an accuracy of about one wavelength, the limitation on image resolution is imposed by wavelength.
The resolution required in diagnostic US is 1 mm or less.
□ Therefore: frequency = constant/wavelength
Frequency (MHz) = 1,540 meters per second · 0.001 m
1.54 × 106 meters per second = 1.54 MHz = the minimum usable frequency for MSK US applications, which will yield a resolution of about 1 mm before significant attenuation degrades this.
US technology allows for a degree of lateral resolution, which is defined as the ability to visualize and differentiate structures parallel to the transducer footprint and determined by:
Hardware: Chiefly the number of sound-emitting crystals on the transducer. This is a fixed number determined at the time the transducer is manufactured. The number of crystals determines the potential number of lines of sight (LOS) and is one of the primary quantitative indicators of the quality of a transducer.
Software: By sequencing of the sound emission by the crystals, the software is able to activate the sound-emitting crystals in different time sequences to generate multiple sound wave fronts. By having the computing power to process these multiple reflecting LOS, the software is able to generate a significantly more detailed two-dimensional image on the display screen than if all the crystals fired all only in one sequence. This sequence processing is referred to as “multibeam” or “crossbeam” imaging.
Axial resolution allows for clearer visualization of structures at varying depths, in the same axis as the transducer. Axial resolution can be increased or decreased by adjusting the frequency.
Frequency refers to the time between emissions of sound waves.
Higher frequencies result in greater axial resolution, resulting in a more detailed US image; however, higher frequencies have less tissue penetration (the ability to see deeper structures clearly). High-frequency sound waves have less penetration because they are subject to more rapid attenuation of sound waves by refraction and absorption of sound waves by tissues (reflectors).
Lower frequencies result in less axial resolution, resulting in a less detailed US image; however, lower frequency sound waves have greater tissue penetration because they are less subject to attenuation by reflectors.
Higher frequency = higher resolution and less penetration (more attenuation). Use higher frequencies to view relatively shallow structures with greater detail.
Lower frequency = less resolution and higher penetration (less attenuation). Use lower frequencies to view deeper structures.
Contemporary transducer technology emits multiple (broadband) frequencies near simultaneously to generate the most useful real-time US image by maximizing potential resolution (using some higher frequencies) and penetration (using some lower frequencies). The particular mix of high and low frequencies may be manually or automatically selected depending on the particular US machine used. The mix of frequencies is based on the depth of the structure of interest, with the frequencies emitted for deeper structures viewed weighted toward lower frequencies and vice versa with superficial structures.
There are two commonly used transducer styles that feature frequency ranges that are useful for MSK US, with the choice between the two dependent on the particular application.
All transducers operate within a range of frequencies as determined by the manufacturer.
□ Broadband high-frequency linear transducer: Most universal in application. Appropriate for superficial and medium-depth applications such as hand, knee, and shoulder. These transducers have a higher degree of resolution compared to curvilinear transducers at more superficial depths.
□ Broadband low-frequency curvilinear transducer: Appropriate for viewing deeper structures such as hip, spine, and shoulders in large individuals.
TERMINOLOGY
Anisotropy
US best interprets echo signatures when the transducer is positioned directly perpendicular to a structure.
The artificial lack of signal (artifact) caused by loss of perpendicular positioning is termed anisotropy and is a significant and omnipresent challenge in MSK US.
The physical explanation for this phenomenon remains largely undetermined; however, it is associated with viewing relatively fibrillar structures, and the effects of anisotropy are more pronounced as structures become more fibrillar.
Anisotropy is a commonly encountered artifact when visualizing bundles of fibers (ligaments, tendons, muscle) or fascicles (nerves) as they curve (with loss of perpendicular positioning of the transducer) and can be easily misinterpreted as a tear. It affects higher density (more fibrillar) structures before less fibrillar structures.
□ Ligament > tendon > muscle > nerve
Knowledge of the relative anisotropic nature of structures aids the scanner in identification of anatomy. For example, when viewing the carpal tunnel in a transverse axis, as the operator tilts the probe, the carpal ligament signal will drop out first, then the tendons, then the muscle, with the median nerve as the last fibrillar structure in view.
A tilt as small as 3-7 degrees away from a perpendicular plane can produce anisotropy and can be seen in both the longitudinal and transverse planes.
This effect is particularly evident when scanning a curved structure, such as a tendon insertion, and can be corrected by positioning the probe to remain perpendicular to the curvature.
Volume averaging
US transducers emit a shaft of sound about 1 mm thick. This 1-mm three-dimensional shaft of sound gradually spreads and becomes wider as it penetrates tissue. The reflected sound waves from this three-dimensional shaft of sound are then processed by the computer to render a two-dimensional image.
The resultant generated image is an average of the total volume of the reflected signal.
Disruption of normal echo signature of a structure is often not translated into the image until more than 50% of the 1-mm shaft is abnormal. Even if 75% of a tendon in a given image is disrupted, it may still appear to be grossly intact, although relatively hypoechoic compared to the other parts of the tendon.
□ This is a crucial concept when it comes to interpretation of partial-thickness tears.
If a defect is suspected, turn the transducer orthogonally to assess for relative hypoechoic areas consistent with partial tears.
Aside from traditional longitudinal and transverse scanning axis, oblique approaches should be undertaken when attempting to delineate a partial-thickness tear.
An understanding of volume averaging is crucial to successful visualization of US-guided needles in longitudinal axis.
□ Because needles are relatively narrow, if the needle is located within the outer 25% of the projected sound shaft, the signal from the needle will be “volume averaged out,” and no needle will be seen on the screen.
□ The needle must be in the middle 50% of the projected shaft of sound to be visible; this requirement becomes more exacting with deeper injections.
□ A clinician’s eye dominance needs to be accounted for when attempting to place a needle under a transducer. What looks to be the centerline of the probe may actually be slightly to the left or right of centerline.
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