Diagnostic ultrasound in the musculoskeletal system

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  • Diagnostic Ultrasound in The Musculoskeletal System

    David J. Wilson

    Introduction

    Diagnostic ultrasound has become an established investigation titr disorders of the heart. abdominal organs and foetus. The technique is best suited to the study of soft tissues that do not contain gas. It is therefore a little surprising that there has not been greater .Ipplication to the musculoskeletal system.

    Over the last few years there has been a rapid improvement in image quality combined with reduc- tion in size of the apparatus. The latest machines are easily moved and may be used wherever there is a I? amp power point. Costs have remained low compared to other forms of imaging : an ultrasound room would be around a quarter of the price of a plain X-Ray. room. The cheapest machines are equivalent to the price of a larse car although some orthopaedic applications require higher quality and this figure might be trebled

    This paper describes the current state of musculos- keletal applications of diagnostic ultrasound and leads to the conclusion that there is a strong case for the provision of a clinical service in those centres that deal with these disorders.

    Physical Basis

    Primydv\

    High frequency sound waves are transmitted through soft tissues and reflected by boundaries where there is an abrupt change in density or texture. Fluid provides a medium that transmits sound with little reflection

    David J. Wilson MBBS, B!Sc. MRCP, FRCR Department of Radiology. Nuffield Orthopedic Centre, Windmill Road. Heading- ton. Oxford OX? 71 D

    and relativ elv little attenuation. Tissue, air and tissue, bone interfaces reflect such a large proportion of the sound that transmission is effectively blocked.

    tltrasound pulses applied to soft tissue w-ill generate echoes that indicate the type of interface by intensity and its depth by the time taken for the echo to occur. As the direction of the pulse is known an image may be generated to display the location and character of the interfaces.

    A$ the sound IS transmitted there IS a progressive attenuation of the pulse. The echoes reflected from identical interfaces will appear weaker at greater depth. To compensate for this the returned signal may be ampiified progressively depending on the time after the pulse was produced.

    I~ltrasound equipment produces the qound by applying an electrical pulse to a piezoelectric crystal mounted within a hand-held probe. The echoes are detected by the same crystal during the pause between pulses. The received sound is converted back to an electrical impulse and this may bc digitised for computed .malysis and display. The higher the frequency the better the re\olution of the system; however the sound is more rapidly attenuated and useful penetration reduced. In practice this means high frequency (7 IO MHz) for fine detail in superficial tissues and lower frequency (7 5 MHz1 fr>r deeper structures.

    A stationary probe gives information about a thin column of tissue below the contact area. If displayed on an oscilloscope this is termed an A scan and is used mainly for adjusting the controls 111 a static scanner. Rapid repetition of the pulses allows the beam of ultrasound to be swept across the patient to create a cross-sectional image. A hand held sweep paints a static image on the display and is termed a B scan This has the advantage of covering either small

  • 42 DIAGNOSTIC ULTRASOUND IN THE MUSCULOSKELETAL SYSTEM

    or large areas of tissue. For example a whole limb may be displayed on one image. If the sweep is mechanised either by oscillating the crystal or by electronically linking an array of small crystals, a cross-sectional image may be rapidly refreshed giving a real time or apparently moving image. The area covered is limited by the size of the probe and the shape of the displayed image depends on the type of swept beam. Mechanical and electronic sector scanners give a pie shaped image whilst electronic-phased array gives a rectan- gular picture. There are no known side effects or risks involved in the use of ultrasound at diagnostic frequencies and power.

    Application

    The main advantage of a real time machine is that by moving the probe around the operator builds up a three-dimensional concept of the structures below. This is such an improvement on the single or static image that the older B scans are now rarely used in general ultrasound departments. Unfortunately the advantage is only apparent to the operator; even an experienced ultrasonographer finds the moving image produced by another hand difficult to interpret. Taking hold of the probe adds another dimension and is essential to accurate diagnosis. This means that the operator must also make the diagnosis. Unlike X-Ray examinations there is little to be gained by study of someone elses films. Here lies the main disadvantage of diagnostic ultrasound; it is totally dependent on a skilled operator and is therefore expensive in terms of a radiologists time.

    A static or B scanner is relatively clumsy and difficult to use. The controls are more complex and the apparatus rather bulky. Single sections do not give the same look around ability as a real time machine. Despite these draw-backs the facility to examine the full length of a limb and the chance to produce serial sections at fixed intervals mean that there is still a significant role for the static scanner in musculoskeletal applications.

    Those wishing to start a service in an orthopaedic or rheumatological centre would be best advised to purchase a modern real time machine with at least two probes. One at a high frequency (7.5 or 10 MHz) for small parts work and one at an intermediate frequency (3.5 or 5 MHz) for deeper structures. Mechanical sector probes would be preferred due to their small contact area with the skin, especially important in rounded structures like the limbs. Electronic sector scanners are not yet able to provide the high frequency option but would be a viable alternative for lower frequencies. A B scanner would be a useful addition and might well be available from a general ultrasound department where they have fallen into disuse.

    Techniques

    Preparation

    Ultrasound equipment may look intimidating at first sight and a word of warning before sending the patient for examination often allays undue anxiety.

    Fragile or very sick patients may be examined on the ward as most of the equipment is transportable. However this should not be a routine as the constant movement would increase the risk of damaging the apparatus.

    Ultrasound cannot penetrate plaster or wound dressings and these should be removed. Sector scan- ners only require a small port of contact and a window cut in a plaster is often sufficient to examine an adequate area of a limb. Thin adhesive plastic dressings are not a barrier to sound and provided that there are no air bubbles they may be left in place. Indeed their use is recommended if post-operative ultrasound examination is anticipated.

    Traction and splintage equipment may create problems when trying to examine by B scanning but rarely causes any difficulty for real time studies. The patient should be left on their normal bed for convenience and comfort.

    Methods qfE.uamination

    The type of examination must be tailored to the individual clinical problem with due account of the limitations of the technique. Comparison with other imaging modalities is essential. There is a strong case for the routine use of computed tomography with ultrasound for soft tissue masses and extra-osseous extension of bone tumours.

    Methods are largely the same as those used in general ultrasound. The following additional points apply to examination of the musculoskeletal system.

    Throughout all examinations the opposite side should be examined for comparison. Many potential pitfalls and artefacts mimicking disease may be avoided by this simple practice.

    Bone reflects the majority of transmitted sound and may be seen as a sudden loss of echoes bordered by a bright line. This might be interpreted as a fluid-filled area by the unwary especially as reverberation artefact often gives ghost images below the bony contour. Normally a fluid collection would show acoustic enhancement where the beam has traversed an area of fluid with relatively little attenuation compared to adjacent soft tissue. The virtually total block to sound at bone surfaces means that adjacent fluid collections may fail to show such enhancement. This is particu- larly important in the examination of joints for effusions. Comparison of a suspect area with a known area of fluid at the same depth without changing the gain settings, often resolves the difficult case. Conve- nient fluid areas include adjacent blood vessels or the urinary bladder.

  • Solid tumouls with a homogeneous texture may mimic fluid b> giving an area of low echoes and posterior acoustic enhancement. This phenomenon is particularly striking in tumours of neural or lymphatic tissue [Fig. 11. Again comparison with known fluid areas may resolve the problem. Fat is relatively echogenic compared to muscle but it should be noted that many liposarcomas contain surprisingly little true fdt and this affects their ultrasound appearance. Muscle groups and tendons may be differentiated by selective patient movements whilst observing the motion on a real time image.

    Most ultrasound equipment is capable of measuring distance and area by cursors displayed on the screen. These may be used to measure size of fluid collections and masses. Serial sections made with a B scanner have been used to calculate volumes; however this is time-consuming and simple measurement of the maximum dimension in three planes is usuall> sufficient . -l.

    Hard copies ;lre of value to the