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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5779_Библиотеки_им_академика_М_И_Перельмана
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1
Introduction to
Musculoskeletal Ultrasound
Imaging
Gina A. Ciavarra, MD
INTRODUCTION TO ULTR ASO UN D PHYSICS
Ultrasound is distinct from other imaging modalities in that it uses sound waves rather than
ionizing radiation to create images. One of the major components of the ultrasound machine is
the transducer, or probe, which serves as the primary contact point between the patient and the
sonologist as well as the ultrasound machine. A transducer is any device that converts one form
of energy into another. In the case of the ultrasound transducer, electrical energy is converted
to ultrasound waves, and vice versa.
elements (crystals), which produce the piezoelectric effect that allows this conversion between
electrical signal and ultrasound energy. When an electrical signal is transmitted to the transducer,
the crystals vibrate at a particular frequency. As the crystals vibrate, a sound wave is created and
transmitted into the patient. After the sound wave reflects back to the transducer from the patient,
the crystals again vibrate, causing an electrical signal, which can then be converted into an ultrasound image. The crystals can be designed so that they will vibrate at a specified frequency. Within
the patient, the sound waves may be reflected at soft tissue interfaces back toward the transducer as
described, absorbed by the tissue interface, or refracted.
wave as it passes through tissue interfaces that exhibit differing ultrasound propagation speeds (eg,
fluid to muscle or soft tissue to bone).
the transducer that are converted into the electrical signals that ultimately become an ultrasound
image. Ultrasound waves transmitted perpendicular to the surface of the object of interest will
be reflected more than nonperpendicular waves. Therefore, for optimal imaging, the structure of
interest should be imaged at an angle perpendicular to the probe.
Coupling or acoustic transmission gel is used to promote the transmission of sound waves
into the patient. The gel is positioned between the patient’s skin and the transducer to eliminate
air between the 2 surfaces. The introduction of air between the transducer and patient impedes
transmission of the sound waves because the air causes ref lection of the sound waves at the skin’s
surface, thus preventing the waves from entering the patient and resulting in a degraded image or
no image at all.
3
1,2
The ultrasound probe comprises multiple piezoelectric
1,3
Refraction is the bending of the sound
3
It is those ultrasound waves, or echoes, reflected back to
3
Rawat M.
- 1 -
Atlas of Musculoskeletal Ultrasound of the E xtremities (pp 1-12).
© 2021 SLACK Incorporated.
2 Chapter 1
ULTR AS OU ND EQUIPMENT/PROBES
When selecting a transducer, the sonologist has to consider what is the optimal frequency of
the transducer based on the depth of the structure of interest because the frequency determines
the image quality and penetration. Each transducer possesses the capability to produce a range of
sound frequencies measured in megahertz (MHz). Higher-frequency transducers are able to produce images of higher resolution. Unfortunately, this comes at the expense of penetration of the
sound waves through the tissues. Thus, high frequency transducers are best suited to assess more
superficial structures (eg, tendons of the hand and wrist). Lower-frequency transducers are better
able to penetrate the soft tissues and therefore better suited to evaluate deeper structures (eg, hip
joint); however, this results in lower resolution (Figure 1-1).
B
1,3
A
Figure 1-1. Ultrasound images obtained with different
transducers. (A) Hip injection with curvilinear probe.
Note the conical shape of the beam and curvature at the
surface (white arrow). (B) Patellar tendon (white arrow)
obtained with a linear probe. Note the rectangular shape
of the beam. (C) Ulnar nerve (N) perineural injection
obtained with a hockey stick probe. The small surface
and curvature of the bone (B) make this probe an ideal
choice. Needle (white arrow) and anesthetic (a).
C
Additional considerations include the shape and size of the transducer. The 2 most common
types of transducer designations are linear and curvilinear. The linear transducer is optimal for
assessment of linear structures (eg, tendons and ligaments) and for use along relatively flat surfaces
such as the extremities. The sound waves transmitted from the linear probe propagate parallel
to the probe surface and result in a rectangular image. With a curvilinear probe, the ultrasound
waves transmit in a radial path from the probe surface, resulting in a wider field-of-view (FOV)
image as compared with the linear probe. An additional type of linear probe with a small footprint
(sometimes called a hockey stick probe due to its shape) is especially useful in imaging small structures in the hand, wrist, foot, and ankle (Figure 1-2).
1-3
Introduction to Musculoskeletal Ultrasound Imaging 3
A B
Figure 1-2. (A) Curvilinear, (B) linear, and (C) hockey stick (small footprint) ultrasound transducers.
The availability of the different probes and capabilities of individual ultrasound machines
vary based on size, power, and cost. Larger ultrasound systems have more powerful computing
capabilities and allow for the use of ultrahigh frequency transducers, resulting in high-resolution
images of superb quality. Smaller, more portable machines, which may be the size of a briefcase,
are also available. These have the advantages of lower cost and portability but are more limited
in their ability to produce high-resolution images and have fewer advanced applications (eg, realtime fusion with computed tomography scan/magnetic resonance imaging). As the technology
continues to advance, the technological differences between the 2 types of machines will become
less pronounced.
2
C
SCAN TECHNIQUE/IMAGE APPEARANCE
When the transducer is placed on the patient and an image is created, the more superficial
structures are those closest to the transducer along the superior aspect of the image. The deeper
structures, farthest from the transducer, are along the inferior aspect of the image. Typically,
when imaging a structure in long axis, the convention is to have the more proximal aspect of the
structure to the left side of the image and the more distal aspect to the right. The most important
point is to be consistent across imaging studies. The left and right sides of the image can be easily switched by using the invert button on the ultrasound machine or by rotating the probe 180
degrees.
ization of the structure in question. This begins even before initiating the study with appropriate
probe selection. As discussed, higher-frequency transducers are preferred for the evaluation of
more superficial structures (eg, hand, wrist), whereas lower-frequency transducers are preferred
for deeper structures (eg, hamstring origins, hip joint). Linear transducers are preferred unless
evaluating a deeper structure, where the lower-frequency curvilinear probe is preferable.
available on the ultrasound machine. First, the depth of the ultrasound beam should be adjusted.
This is accomplished by ensuring that the object of interest is centered within the image. Second,
the number of focal zones should be adjusted. An ultrasound beam is narrowed, or focused, not
simply at one point but over a range of depths also known as a focal zone. More specifically, a
focal zone is defined as a range of depths over which the ultrasound beams are most narrowed (or
focused). The number of focal zones should be adjusted to include the fewest number of focal zones
while encompassing the entire region of interest being scanned such that the ultrasound beam
2
Once oriented, the sonologist should optimize the image to improve the resolution and visual-
1,3
After selecting the appropriate probe, the image should be optimized using some of the buttons
4 Chapter 1
is most focused on the target structure, improving image resolution. Once the number of focal
zones is selected, the depth should also be adjusted to optimize evaluation of the object of interest
as previously discussed (Figure 1-3). Finally, the gain, or image brightness, should be modified to
improve the image quality. Because different tissues attenuate the sound waves to varying degrees,
adjusting the gain can optimize the image to account for these differences, resulting in a more
uniform appearance of the image.
2,3
B
A
D
Figure 1-3. Adjusting the depth and focal zones (median
nerve). (A) Suboptimal depth (D) and focal zone (FZ)
result in a blurry image with poorly centered median
C
due to suboptimal focal zone (FZ). (C) Optimal focal zone (FZ) improves blurriness of the median nerve (MN), but
poor depth (D) selection results in poorly centered nerve. (D) Both depth (D) and focal zone (FZ) are optimized with
improved image quality of the median nerve (MN).
nerve (MN). (B) Depth (D) has been improved for better
centering of the nerve (MN), but image remains blurry
Introduction to Musculoskeletal Ultrasound Imaging 5
TERMINOLOGY
In evaluating the musculoskeletal system, it is important to be aware of the principal terms
used to describe and differentiate the various structures. The term hyperechoic or echogenic is
used to describe structures that appear bright on the image in the musculoskeletal system. These
structures include normal tendons and the surface of the bone, as well as soft tissue calcifications.
The term hypoechoic refers to structures that produce fewer reflected echoes and appear less bright
within the image. These structures include muscle, certain soft tissue masses, and complex fluid
collections. A structure that produces no echoes is termed anechoic and appears black within the
image. Simple cysts or fluid collections are typically anechoic. The term isoechoic is used to refer
to a structure that is of similar echogenicity to the surrounding structures. An example of an
isoechoic structure may include a lipoma within the subcutaneous fat (Figure 1-4).
2-5
A
C
Figure 1-4. Echogenicity in ultrasound. (A) This foreign
body (FB) is hyperechoic, or bright. (B) This soft tissue
sarcoma (S) is hypoechoic, or less bright, due to fewer
reflected echoes. (C) A simple cyst (small white C) is
anechoic, producing no echoes. (D) Lobules of fat within
this Morel-Lavallée lesion (L) may be isoechoic to the
adjacent subcutaneous fat (F).
B
D
6 Chapter 1
NORMAL STRUCTURES
The appearance of normal structures in the musculoskeletal system will be discussed in more
depth in later chapters. In order to understand one of the more relevant artifacts in musculoskeletal imaging, however, it is important to be cognizant of the normal appearance of tendons.
Tendons comprise multiple individual, longitudinally oriented, parallel collagen fibers that are
tightly bundled, resulting in a fibrillary pattern on ultrasound. This results in the characteristic
hyperechoic appearance of tendons when the ultrasound beam is oriented 90 degrees to the tendon
(Figure 1-5).
Figure 1-5. Normal tendon. The
biceps tendon (BT) imaged in long
axis demonstrates the characteristic
fibrillary pattern of tendons and is
hyperechoic.
2,4,5
IMAGING ARTIFACTS IN ULTR ASOU ND
Although there are a number of sonographic artifacts, there are several important artifacts
that the sonologist should be aware of in order to make an accurate diagnosis, as well as to avoid
mistaking artifact for pathology:
• The most well-recognized artifact in musculoskeletal imaging is anisotropy. The normal
appearance of tendons is hyperechoic with a fibrillar pattern.
vidual fibers that comprise the tendon. This appearance occurs when the tendon is imaged at
an angle perpendicular to the ultrasound beam. If the beam is positioned at an angle less than
or greater than 90 degrees, the tendon will appear falsely hypoechoic, mimicking pathology.
This characteristic of tendons is known as anisotropy (Figure 1-6). Ligaments also exhibit
anisotropy when imaged at an angle other than 90 degrees.
A
Figure 1-6. Anisotropy. (A) The supraspinatus tendon is imaged with the transducer beam perpendicular to the
tendon (T) and (B) at an angle less than 90 degrees, resulting in loss of fibrillar pattern (white arrow) and mimicking
tendinosis.
B
4,5
This results from the indi-
2,4,5
Introduction to Musculoskeletal Ultrasound Imaging 7
• Another artifact is shadowing. This is the result of reflection, absorption, or refraction of an
ultrasound beam at an interface. This produces a dark (hypoechoic) or anechoic region deep
to the interface, which may obscure structures within the path of the shadow. Within the
musculoskeletal system, this occurs most commonly with bone or calcification (Figure 1-7).
Figure 1-7. Shadowing. Calcification
(Cal) within the rectus femoris tendon
produces a dark area (white arrow)
deep to it due to reflection and/or
absorption of ultrasound beams at
the surface.
6,7
• Posterior acoustic enhancement occurs when imaging fluid or certain soft tissue tumors.
These structures result in decreased attenuation of the ultrasound beam when compared with
the surrounding tissues. Accordingly, the tissues deep to the fluid will appear more echogenic
because more of the incident beam will pass through (Figure 1-8).
3,6
Figure 1-8. Posterior acoustic
enhancement. Note how the tissues
deep to the cyst (C) overlying the
radius (Rad) are brighter (large white
arrow), with septation in cyst (small
white arrow).
8 Chapter 1
• Finally, posterior reverberation and ring-down artifact are 2 additional artifacts that one may
encounter in the musculoskeletal system. Reverberation (Figure 1-9) occurs when the sound
beam reflects repeatedly between the transducer and a parallel surface, such as bone, or a
metal object, such as a foreign body or surgical device (eg, clip, fixation plate). The resulting
images display as multiple linear echoes deep to the imaged surface at equally spaced intervals. A subtype of reverberation is known as ring-down artifact. When the reflection of the
sound beam is highly efficient, such as with gas bubbles in fluid or metal hardware, a series
of bright echoes will be displayed in deeper tissues posterior to the bubble or metal structure
(Figure 1-10).
Figure 1-9. Reverberation artifact.
Sound beam reflects between the
transducer and the surface of the
metacarpal bone with multiple
linear echoes (white arrows).
(MC = metacarpal of thumb;
PP = proximal phalanx of thumb;
UCL = ulnar collateral ligament .)
3,6,7
Figure 1-10. Ring-down artifact.
Series of bright echoes (white arrows)
deep to metal screws (SC).
Although there are other artifacts that may be encountered in ultrasound, the aforementioned
are the most commonly encountered when imaging the musculoskeletal system.
Introduction to Musculoskeletal Ultrasound Imaging 9
ENHANCED ULT RAS OU ND TECHNIQUES
More recent enhancements in ultrasound imaging have allowed for further reduction in imaging artifacts. Compound or spatial compound imaging has allowed for improved imaging at tissue
boundaries or the edge of the structure of interest. With compound imaging, multiple images
are created in succession and merged to form a single image. This allows for better evaluation of
curved structures. Compound imaging also reduces speckle or “noise” in the image, resulting in a
smoother imaging appearance (Figure 1-11).
3,8
A
Figure 1-11. Compound imaging. Use of compound imaging in (A) results in a smoother image of the supraspinatus
tendon (ST), as compared with (B), which demonstrates speckle, or noise.
B
When an ultrasound beam enters the patient, it creates multiple echoes as it interacts with
the tissues it is traversing. These reflected echoes may be useful to the sonologist to improve the
quality of the image. Tissue harmonic imaging, a feature available on most ultrasound machines,
enhances image quality by incorporating these additional reflected echoes that have been distorted
as they interact with the surrounding tissues. These beams are not contained in the original echo
but may be added to the original echo, strengthening it and thereby resulting in higher image
contrast and an improved signal-to-noise ratio (Figure 1-12).
3,9
A
Figure 1-12. Tissue harmonic imaging. Use of tissue harmonic imaging in (A) results in higher image contrast and
better signal-to-noise ratio with better visualization of the median nerve (MN) than in (B), obtained without tissue
harmonic imaging.
B
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