British Journal of Radiology 74 (2001),183-185 © 2001 The British Institute of Radiology
The use of beam angulation to overcome anisotropy when viewing human tendon with high frequency linear array ultrasound
D J A Connolly, MRCP, FRCR1,
L Berman, MRCP, FRCR2 and
E G McNally, FRCPI, FRCR1
1 Department of Radiology, Nuffield Orthopaedic Centre, Windmill Road, Headington, Oxford OX3 7LD
2 Addenbroke's Hospital, Hills Road, Cambridge CB2 2QQ, UK
Correspondence: Dr E G McNally
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Abstract
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Anisotropy is the property of tendons, nerves and muscles to vary in their ultrasound appearance depending on the angle of insonation of the incident ultrasound beam. Loss of reflectivity in tendons may also denote underlying disease. We describe beam angulation, a simple technique available with most modern ultrasound machines, which allows the operator to overcome the potential pitfall of anisotropy in ultrasound assessment of peripheral tendons.
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Introduction
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The use of ultrasound in musculoskeletal radiology is increasing. Ultrasound is of particular use in assessing injury to superficial structures such as tendons and ligaments.
Normal tendon appears as a predominantly reflective fibrous structure bordered by a reflective sheath [1]. Inflamed tendon loses this normal reflectivity and appears dark. Crass et al [1] found that normal tendon reflectivity was angle dependent and termed this phenomenon anisotropy. The normal appearance of tendon depends upon the incident ultrasound beam being perpendicular to the tendon [2, 3]. Change of the incident beam angle produces a reduction in tendon echogenicity relative to that of muscle and may mimic tendinitis. This alteration can occur with an angle of incidence from the perpendicular as little as 2°. Miles [4] examined equine tendon and discovered that anisotropy could be elucidated no matter which direction the incident beam was angled.
The difference in echogenicity of soft tissues such as tendon and myocardium when scanned with a perpendicular linear array transducer beam is thought to be related to differences in collagen and hence to the linear elastic properties of the tissue (Young's modulus) [5, 6]. Recent work and debate has assessed methods for quantifying anisotropy using statistically based correlation techniques [7, 8].
Tendinosis is the degenerative change of tendons such as occurs in patellar tendinosis and Achilles tendon degeneration. Tendinitis is an inflammatory condition, often associated with a thickened tendon [9]. Both entities appear as poorly reflective intratendinous lesions. Decreased reflectivity of tendons resulting from anisotropy will obscure textural detail and mimic these conditions [9].
The superficial position of the structures being examined means that even slight angulation of the probe to overcome anisotropy may impair contact between the probe and the skin surface. Most major ultrasound manufacturers now include a beam angulation package in the specifications of their latest ultrasound machines (Personal communications, GE, Siemens, Toshiba, Ecoscan). This facility can be used to overcome anisotropy and therefore to improve visualization of superficial tendons [10].
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Method
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The high frequency linear array transducer is applied to the skin and the tendon is visualized as normal. If a focal area of relatively lower echogenicity is identified within the tendon, the beam is angled electronically through its full range so that the incident beam is perpendicular to the region of previously low echogenicity. If the previously low echogenicity area within the tendon becomes more echoic and eventually isoechoic to the rest of the tendon, then tendon disease can be excluded. The incident beam may be angled incrementally in both directions until optimal visualization of the entire tendon has been achieved (Figures 1
and 2).
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Figure 1. Ultrasound with a high frequency transducer of the biceps tendon demonstrating how an area of reduced echogenicity within the tendon (arrows in (a)) that mimics a focal area of tendon injury is revealed to represent normal tendon when the beam is angled perpendicular to the different regions of the tendon (b,c).
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Figure 2. High frequency linear array ultrasound of the flexor digital tendon at the proximal interphalangeal joint of the right index finger. An area of low tendon echogenicity when the beam is oblique to the area of interest (a) is demonstrated to have normal tendon echogenicity when the incident beam is angulated perpendicular to the tendon without compromising skin contact (b–d).
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Discussion
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Ultrasound is increasingly being used as the diagnostic medium for assessment of tendon injuries and tendinitis [11, 12]. Common indications include assessment of the biceps tendon, rotator cuff, patellar tendon and Achilles tendon, where the sonographer must ensure that low echogenicity regions within tendons are not merely the result of anisotropy [13]. A lesion can only be confirmed if a poorly reflective area remains when the angle of insonation is perpendicular to the long axis of the tendon. It is not always practicable to position the probe head perpendicular to the tendon being examined, for example with the finger flexors. In such cases, utilization of stand-off, large quantities of jelly or water-baths have all been described. A simpler technique is to use a facility that is available with most modern ultrasound machines and that helps the sonographer to assess the integrity of superficial tendons more clearly without compromising possibly tenuous skin-to-transducer contact. Even where the probe may be angled to give better access, electronic angulation is often easier.
Beam angulation occurs at the transducer level. The transducer is composed of a series of elements, each of which contributes to the entire beam. If the elements are stimulated together, a parallel beam is produced. If a graded delay on the stimulation pulse is placed across the array of elements, an angled beam is produced (Figure 3). At the operator level, the degree of angulation is usually fixed, but on some equipment the angles can be varied incrementally. The ideal angle depends on the tendon being examined and the body habitus of the patient. By combining probe angulation with manipulation the ideal angle is easily achieved in the majority of cases.
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Figure 3. (a) Simultaneous stimulation of each crystal in the array results in a perpendicular propagated sound wave. (b) Graded stimulation results in an angulated propagated wave.
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We therefore advise the use of beam angulation in ultrasound of tendons and ligaments to overcome the potential diagnostic pitfall of anisotropy.
Received for publication May 25, 2000.
Revision received October 9, 2000.
Accepted for publication December 5, 2000.
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References
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Abstract
Introduction
