PRINCIPLES Of MSK U/S ARE VERY EASY , WE WILL DISCUSS IT AT THIS ARTICLE
THANKS TO DR .AHMED . F. ABOGAMAL FOR HIS GREAT EFFORT
The Sonographic Revolution
For many years the rheumatologist adopted to fight against time to diagnose and follow most of the rheumatic diseases, this fighting continues to occur with no winners until the sonographic revolution has occurred in rheumatology.
Ultrasonography has been shown to be capable of substantially changing the traditional approach to most clinical problems in daily rheumatological practice because of its ability to allow a quick, safe and inexpensive access to otherwise undetectable anatomical information on the early targets of most rheumatic diseases.
Ultrasonography completes the physical examination in a rapid and accurate manner and allows the very early detection of a wide spectrum of pathological findings involving different anatomical structures in the musculoskeletal system.1
Although musculoskeletal ultrasound may be considered as one of the latest developments in ultrasound applications, it was first used, as far back as 1958, in the assessment of the acoustic attenuation of musculoskeletal tissues by K.T Dussik et al.2, 3
Dussik had also been the first to use ultrasound as a medical imaging modality in 1942. 2, 4
Ultrasound developed slowly until 1960 when Donald and colleagues produced the first automatic scanner.2
For the next decade, ultrasound was predominantly limited to the evaluation of abdominal and pelvic diseases.
By 1972 the first B-scan image of a joint was reported in the differentiation of a Baker’s cyst and thrombophlebitis.5
Graf, in 1980, published his landmark paper on the use of ultrasound in the diagnosis of congenital hip joint dislocation.6
In 1988, L. De Flaviis described ultrasound of the hand in rheumatoid patients including erosions, 10 years after Cooperberg described features of synovial thickening and joint effusion in the rheumatoid knee.7, 8
Since this time, particularly in the past decade, there has not only been a rapid development in ultrasound technology, but also widespread use of ultrasound in the investigation of musculoskeletal disorders to the point where it is now firmly established as a key imaging modality.
BASICS OF MUSCULOSKELETAL ULTRASOUND
Ultrasound uses high-frequency sound waves to acquire images for the soft tissues, bones, and nerves of the body.
The ultrasound machine sends an electrical signal to the transducer, which is connected (via a cable) to the monitor and computer processing unit.
This system results in the production of sound waves that are transmitted from the transducer to the soft tissues of the body through acoustic transmission gel.9
The sound waves interact with the tissue interfaces and some reflect back toward the transducer on the surface of the skin.
The ultrasound image is produced from this reflection, which converts sound waves to an electrical current.9
The ultrasound machine then measures the amplitude of the returning electrical signal and records its travel time to determine the depth of the reflected structure.
As the machine generates and records the amplitudes and travel times of the sound beams, it uses the computer software to produce a two-dimensional image of the scanned structures.10
The transducer is an essential element of US equipment, responsible for the generation of a US beam and the detection of returning echoes.
It greatly influences spatial resolution, penetration and signal-to-noise ratio, and in turn its selection affects the quality of the US image.9
Frequency of the Transducer:
To produce an image of optimal quality, it is important to select the proper transducer for the area of interest.
A transducer is described by the range of sound wave frequencies it can produce (MHz).
Higher-frequency transducers produce higher resolution images, but at the expense of beam penetration, because higher-frequency sound is more rapidly absorbed by the tissues.
Lower-frequency transducers are more capable of assessing deeper structures, but they have lower resolution.9
Shape of the Transducer foot plate:
Transducers also can be described as linear or curvilinear.
Linear transducers produce a sound wave that is propagated in a straight line perpendicular to the transducer surface (Fig.1).
Such transducers are best for evaluating linear structures like tendons.
Curvilinear transducers produce an arched beam and are less commonly used, but have some advantages in evaluating deeper structures (Fig. 2) and the small-footprint linear array L shaped transducer (hockey-stick transducer) which is suitable for evaluation of the small joints (Fig. 3).
Commonly used transducers in musculoskeletal ultrasound include the high frequency (20–7 MHz), linear transducer, and the low- to medium frequency (5–2 MHz) curvilinear array transducer.
The examiner should choose the highest-frequency transducer that can adequately image the target structure at the appropriate depth.9
Fig.1: Linear transducers are best for evaluating linear structures like tendons (coated from Jacobson JA, 9).
Fig.2: Curvilinear transducers are used for evaluating deeper structures such as hip, piriformis muscle, and sciatic nerve (coated from Jacobson JA, 9).
Fig.3: The small-footprint linear array transducer is also called a hockey-stick transducer and has higher frequency compared with other transducers. This transducer is used for evaluating small superficial structures such as peripheral nerves (coated from Jacobson JA, 9).
Reducing the width and thickness of the US beam to be limited to the area of interest has definite advantages in terms of contrast and spatial resolution.
In modern linear-array transducers’, focusing is currently not obtained by means of a fixed lens as in the old mechanical sector probes in which degrading of the image quality occurred at a short distance from the focal zone.
Focusing is now produced electronically by activating a series of elements in the array with appropriate delays, so that the trigger pulses to the inner elements are delayed with respect to the pulses to the outer ones.
In this way a curved wave front results from constructive interference bringing the US beam toward a focus. Recently, the introduction and refinement of matrix (1.5D probes) transducers led to further progress.
In these transducers, the single row of long piezoelectric elements found in a conventional probe is replaced by more layers (three to seven) incorporated into a single thin layer to produce parallel rows of short elements.
The slice thickness of the US image is improved by performing dynamic focusing in the elevation plane.
This leads to better spatial and contrast resolution and reduction of partial- volume averaging artifacts.11 A less expensive alternative to 1.5D probes is the use of peculiar acoustic lenses – Hanafy lenses –placed in front of the piezoelectric elements.
The Hanafy lens has nonuniform thickness and resonance properties: it produces a narrow and uniform image slice thickness and, simultaneously, a very broad bandwidth pulse.
The inner portion of the lens is thinner, resonates at higher frequencies and focuses in the near field, whereas its outer portions resonate at lower frequency and are focused in both transmission and reception at the deepest part of the image providing better penetration.12
Fig 4: Hanafy lens (Acuson-Siemens) coated from Claudon et al., 12.
Basic Scanning Techniques:
At first it is important to use enough acoustic transmission gel.
After that the examiner should begin by holding the transducer between the thumb and fingers of their dominant hand, positioning the end of the transducer near the ulnar side of the hand.
It is essential to securely stabilize the transducer on the patient’s body, with either the palm of the imaging hand or the small finger.
After the transducer is properly placed on the patient’s skin, a rectangular image is produced on the monitor.
The top of the image represents the tissues in contact with the transducer; the bottom of the image displays the deeper structures.
The left-to right orientation of the image can be altered using a button on the machine or by rotating the transducer 180 degree (fig 5).9
The image on the screen can be further adjusted and optimized by changing the sound-beam penetration using the depth controls.
Selection of the proper transducer is also critical in evaluating the depth of a structure.
There is an inverse relation between the frequency of the transducer and its depth of penetration, such that higher-frequency transducers are usually best to view superficial structures, whereas lower-frequency transducers are used for deeper structures.10, 13
Fig.5: (A, B) Orientation of the image. The top of the image usually shows the shallow structures and the bottom of the image, the deeper structures. The notch (arrow) of the probe usually represents the left side of the image (asterisk), but an image can be inverted by controls on the machine and it is important to confirm the side of the transducer with the side of the image on the screen. During examinations, t is important to be consistent. Many find it helpful to align the left side of the image with the examiner’s left side, coated from Jacobson JA, 2007.9
The next step in imaging is adjusting the focal zone of the ultrasound beam.
The number of focal zones should be reduced to span the area of interest.
The zone feature is usually displayed on the side of the image as a series of cursors or symbols.
The depth and length of the focal zone position should match the position of the structure of interest.
The examiner may adjust the gain to either increase or decrease the brightness of the echo image.
A standardized approach to the study of the various anatomical regions is advisable even if the choice of the acoustic window is conditioned by the specific diagnostic need.14
Each anatomical area should be explored on various scan planes so as to obtain all of the necessary information.
An in-depth knowledge of the anatomy of the region examined is required for the musculoskeletal US. 14
Advantages and Disadvantages of Musculoskeletal Ultrasound:
When comparing US with other musculoskeletal imaging techniques (radiography, computed tomography, and magnetic resonance imaging), ultrasound offers numerous advantages.
Perhaps the most important of these is that ultrasound offers a hands-on, dynamic examination, allowing the clinician to image in real-time, while interacting with the patient.15
The clinician can ask the patient for assistance during the examination and may accurately localize painful areas by palpating with the transducer and seeking patient feedback.16
The sonographer can also ask the patient to reproduce the abnormal or painful event while scanning dynamically.
Furthermore, ultrasound is less expensive when compared to CT or MRI, can be repeated, and US can be used to compare the finding in the diseased side with the normal side of the same patient in the same session.
Ultrasound transmits no radiation to the patient or user, and thus may be used in special populations, including pregnant women and those with metal implants or other sources of imaging artifacts. 16
In addition, the enhanced sensitivity for detecting low-velocity flow in small tendon and synovium vessels achieved by recent colour Doppler and power Doppler techniques has led to the incorporation of Doppler US in rheumatology.
US allows an immediate correlation between imaging findings and clinical data, which improves diagnosis and management of patients with a range of rheumatic diseases from inflammatory arthritis, vasculitis or osteoarthritis to soft-tissue diseases.16-20
US has demonstrated more sensitivity than clinical evaluation in assessing joint inflammation.21-26
In addition, ultrasound is a bedside tool for performing accurate and safe musculoskeletal injections.27
On the other hand, the primary disadvantage of ultrasound is that it is operator dependent and there is a steep learning curve. Numerous examinations must be performed to establish a reliable diagnostic algorithm.28
Ultrasonographic Tissue Appearance:
Musculoskeletal ultrasound can be used to identify muscles, tendons, ligaments, nerves, bone surface and vessels, at a resolution of less than0.1 mm.
These structures can be characterized and described by their echogenicity. 29
When there is a large difference in impedance at the interface between tissues the sound beam is strongly reflected, creating a bright echo on the image. This process is described as hyperechoic.
If an image displays no echo and is black, it is referred to as anechoic, and when a weak or low echo is produced, it is termed hypoechoic.
When the echogenicity of a structure appears equal to an adjacent structure, it can be described as isoechoic or of equal echogenicity. 29
In general, when interpreting musculoskeletal ultrasound, a practical order of echogenicity is: bone/ligament/tendon/nerve/muscle.
In addition to their echogenicity, tissues are also described by their echotexture, which refers to their internal echo pattern.
Echotexture varies based on whether a structure is imaged longitudinally or transversely.30, 31
Definitions of Basic ultrasound terminology:
Echotexture refers to the coarseness or non-homogeneity of an object.
Echogenicity refers to the ability of tissue to reflect ultrasound waves back toward the transducer and produce an echo. The higher the echogenicity of tissues, the brighter they appear on ultrasound imaging.
Hyperechoic structures are seen as brighter on conventional US imaging relative to surrounding structures due to higher reflectivity of the US beam.
Isoechoic structures of interest are seen as bright as surrounding structures on conventional US imaging due to similar reflectivity to the US beam.
Hypoechoic structures are seen as darker relative to the surrounding structures on conventional US imaging due to the US beam being reflected to a lesser extent.
Anechoic structures that lack internal reflectors fail to reflect the US beam to the transducer and are seen as homogeneously black on imaging.
Longitudinal structure is imaged along the long axis.
Transverse structure is imaged perpendicular to the long axis.
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Chapter One Principles of ultrasound
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