Dr Vijay Rao, Consultant Rheumatologist, Manipal Hospital Bengaluru elaborates on the use and benefits of ultrasound to evaluate and monitor rheumatoid arthritis
Musculoskeletal (MSK) ultrasound was once the sole province of the radiologist but in recent years it has been introduced into routine clinical practice by an increasing number of rheumatologists. It is a powerful tool not only for evaluating joint and soft tissue pathology but also for facilitating interventions such as aspiration and injection. Its applications continue to grow.
A traditional method of monitoring the joint disease of patients with rheumatoid is X-rays, whereby images are produced by exposing photographic film (radiographs). This technique has proven useful for doctors to follow the course of joint destruction. The early development of discrete bony destruction (erosions) is associated with more severe rheumatoid. While standard X-ray radiographs contribute substantially to the clinical evaluation of rheumatoid arthritis, they do lack some sensitivity early in the course of the disease. This means that substantial joint destruction must happen before changes on the standard X-ray test become apparent.
Modern treatment for rheumatoid arthritis is frequently directed at early disease. Accordingly, efforts to establish methods for early diagnosis of the disease have increased. Several radiographic imaging modalities have been explored, including magnetic resonance imaging (MRI) and ultrasonography. MRI scanning has been found to be sensitive as an indicator of early rheumatoid joint destruction, but it is very expensive and not widely available. Ultrasonography is an attractive method of imaging because of its low cost, absence of harmful radiation, and rapidity of imaging. Recent advances in ultrasound image technology have allowed the development of sonographic equipment for imaging inflamed joints in patients with rheumatoid arthritis.
History
The first report on the use of ultrasound in a rheumatology clinic – to assess the knee in rheumatoid arthritis (RA) – was published over 30 years ago. Since then, further technical advances and the falling cost of equipment have paved the way for MSK ultrasound to become an integral part of routine clinical practice. Today, training in ultrasound is a compulsory part of rheumatology postgraduate medical education in Germany and Italy and courses as well as programmes have been established in most American and European countries. In 2005 up to 90 per cent of UK rheumatologists reported that they used ultrasound in the management of rheumatology patients, with 30 per cent performing it themselves.
Advantages
Ultrasound allows real-time imaging that can be carried out in the clinic or at the bedside. It is non-invasive and non-radioactive. Hence it allows the assessment of several joints in a relatively short space of time. The running costs are low and it is largely immune to the metal artefacts that can cause difficulties with magnetic resonance imaging (MRI) and computerised tomography (CT). Ultrasound also enhances the doctor–patient consultation as it provides an immediate visual aid to help educate the patient about their disease.
Disadvantages
These advantages are counterbalanced by the initial cost of equipment, the time and cost of training and the lack of available time to perform ultrasound in a busy clinic. Ultrasound cannot see into or beyond bone. It has limited resolution for deeper joints (such as the hip) and the patient’s body habitus may sometimes make examination difficult. There are also valid concerns about the standardisation of examinations by different ultrasonographers and how best to assess and certify competency.
Technical aspects
Ultrasound uses reflected pulses of high frequency sound to assess soft tissue, cartilage, bone surfaces and fluid-containing structures. The basic principle of ultrasound is that the denser the material the sound wave is passing through, the more reflective it is and the whiter (or echoic) it appears on screen. Water is the least reflective body material. Sound waves pass straight through water and it appears black (or anechoic) on screen. Grey-scale or B-mode ultrasound displays the different intensities of echoes in black, white and shades of grey
(Figure 1). Doppler ultrasound uses the principle that sound waves increase in frequency when they reflect from objects (such as red blood cells) that are moving towards the transducer (red signal) and decrease when they are moving away from the transducer (blue signal). Power Doppler ultrasound measures the amplitude of the Doppler signal (which is determined by the volume of blood flow) and superimposes it on the grey-scale image, thereby depicting increased micro vascular blood flow (Figure 2 ).
There are several basic requirements for successfully introducing ultrasound into your clinical practice: A basic knowledge of the physics of ultrasound, a detailed knowledge of relevant anatomy, an ability to evaluate ultrasound findings in a clinical setting, ready access to ultrasound equipment, suitable equipment for imaging small joints and access to a mentor.
Equipment
There has been a progressive improvement in imaging definition and the size, portability and cost of sonographic equipment over the last 10 years. There are several important considerations while selecting a machine such as:
Cost: Cost relates directly to image resolution and quality but the cost of equipment is falling and the quality is improving. A basic system can be purchased for approximately £30,000 but a top-of-the-range machine, with several different transducers, may cost as much as £150,000. It is a good rule of thumb to try and purchase the best machine that you can afford, but before you buy, insist on borrowing a demonstration model for several days so that you can try it out on a number of your patients. This way you will be able to compare the machine’s performance with its competitors in a number of different clinical situations.
Image resolution and quality: The choice of probe size and frequency depends on the size and depth of the structures of interest. A higher frequency probe (10–20 MHz) will have a smaller field of view with high resolution but poor tissue penetration, making it ideal for small superficial structures. The reverse is true for lower-frequency probes (<7.5 MHz). Modern machines are equipped with multi-frequency transducers. 3D probes are now available but are not yet widely used in general rheumatological practice.
Transducer design: Probes may be annular, radial or linear. Linear array transducers are the preferred option for most MSK scanning.
Equipment size and portability: Portability is an advantage for multi-site use. However, larger, less mobile systems can achieve better image quality for not much more cost.
Colour and power Doppler: These options are essential. It is important to test the Doppler tool on a system before purchasing to see if it is sensitive enough for the detection of small joint synovitis. A good sign of a very sensitive system is the ability to detect blood flow in the normal nail bed or small distal arterioles in normal fingers.
Software options : On most machines, a variety of software options are available to enable the user to develop a personalised system. Most MSK ultrasonographers will be happy with a basic package but more advanced options include panoramic imaging, contrast-enhanced software, vascular packages and 3D scanning.
Other equipment: Acoustic gel will be needed for general scanning, and sterile gel with a probe sheath for ultrasound-guided injection. Many variables can influence the image obtained with ultrasound, including the type of machine, transducer settings, transducer pressure and patient position. It is best to adopt a standard scanning protocol to ensure reproducibility. All images should be interpreted in the light of the clinical history and examination.
Uses of ultrasound in rheumatology
Ultrasound has many uses in the diagnosis and management of MSK disorders. It can:
(a) measure the extent of anatomical damage and inflammation in early arthritis,
(b) assess the course of inflammatory disease,
(c) determine therapy efficacy, and
(d) allow direct guidance for joint and soft tissue injection.
MSK ultrasound is consistently superior to clinical examination at a variety of locations, even in the performance of basic clinical skills such as detecting the presence of knee effusion. However, MSK ultrasound complements clinical examination but does not replace it and all findings should be interpreted in the light of the clinical examination. A significant advantage of MSK ultrasound over MRI, CT and scintigraphy is the ability to hone in on the area of symptoms or clinical abnormality with the ultrasound probe immediately after clinical examination. This has the further advantage of improving the operator’s knowledge of regional and functional anatomy, leading to a better understanding of pathological processes and improved clinical examination skills.
Conclusion
MSK ultrasound has revolutionised the practice of many rheumatologists in the past decade but significant resources are required to establish a credible service. The initial investment in equipment and training is substantial and it can be difficult to find time in a busy outpatient clinic to perform ultrasound. However, the application of ultrasound has the potential to deliver accurate and early diagnoses, monitor disease and facilitate intervention in the clinic to the benefit of our patients. Appropriate training is essential and standardisation of training and assessment is well under way. Issues of concern regarding validity and reproducibility are being addressed. MSK ultrasound is a continuously expanding field and the advent of increasingly powerful machines and 3D ultrasound is likely to further extend its applications in rheumatology.
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