Introduction The small joints of the hands and feet play a central role in the diagnosis and classification of arthropathy. Ultrasound can be used to assess involvement in areas that are clinically occult as well as determine the precise structures involved. Whilst a systematic approach should include a full examination of extra-articular structures, including skin, subcutaneous tissue, the tendon and tendon sheath, the enthesis and associated bursae, this article focuses on intra-articular components of disease, specifically the early detection and classification of effusion, synovitis and erosions. In the patient with arthritis, serial examination can assess current activity and disease distribution, as well as monitoring progression or therapeutic response. Technique 1 Fig. 1 a b arrow arrowhead c d  1 1 2 1 Fig. 2 arrowheads arrow  1 2 1 3 4 Normal anatomy Metacarpo-phalangeal joints 3 3 Fig. 3 a asterisks arrowheads c open arrowhead arrow open arrow b c MC PP  3 1 4 3 Fig. 4 arrow MC PP  5 6 7 8 For these reasons, others use the point of maximal joint distension and compare it with the normal measurement for that location. This, however, varies with the area of the joint being examined, but most authors agree that an increase in joint dimension of more than 1 mm above normal is sufficient to suggest abnormality. On the extensor aspect of the MCPJs, the proximal recess can be measured from bone to the deep surface of the extensor tendon, a distance of approximately 2.5 mm. The distal recess is much smaller, measuring less than 1.5 mm. In the transverse plane, the synovial space is limited on its lateral and medial sides, proximally by the extensor hood and distally by the extensor slips. There should be no posterior bulging of the hood or slips; this finding would suggest focal synovial disease. On the flexor side, the proximal recess is more distended and easier to visualise than the distal recess. The proximal recess measures approximately 3 mm from bone to the deep surface of the tendon. This space includes fat, capsule and two synovial layers, so the space may be increased by hypertrophy of any of these. On high-resolution equipment, the joint cavity itself can be identified between surrounding fat and connective tissue. Occasionally, a small quantity of fluid can be identified, separating the two synovial layers. With increasing practice, ultrasonologists quickly gain an appreciation of the range of normal for the small joints of the hands and feet. The bony contours of the metacarpal head and proximal phalanx should be smooth, though it is not uncommon to identify a normal indentation on the dorsal aspect of the metacarpal head. This depression is smooth, well demarcated and has reflective bone at its base. It occurs at the site of the fused growth plate and is not associated with overlying synovial thickening. These features allow differentiation from a metacarpal head erosion. Interphalangeal joints 5 Fig. 5 arrow asterisk PP MP  1 6 Fig. 6 arrow open arrow PP MP R2P  Measuring the joint space on the radial and ulnar side is difficult. This is due to the variable appearances of the ulnar and radial collateral ligaments due to anisotropy. For this reason it is often best to measure from the bone surface to the outer aspect of the collateral ligament. Under normal circumstances this distance is less than 2.5 mm. 7 Fig. 7 arrow arrowheads  The small joints of the forefoot Technique Normal anatomy 8 Fig. 8 asterisk arrow MT PP  Ultrasound pathology Intra-articular pathology Effusion and synovitis 9 Definitions 9 10 Fig. 9 arrowheads 6 PP MP  Fig. 10 arrowheads asterisk  Classifying abnormal 11 12 13 Fig. 11 MC PP  Fig. 12 asterisk MC PP  Fig. 13 asterisk arrow  10 6 7 Angiogenesis 11 12 13 14 15 14 Fig. 14 arrow asterisk PP MP  There are two ways of measuring synovial blood flow, power Doppler ultrasonography (PDUS) and colour flow Doppler ultrasonography (CFDUS). Power Doppler is more sensitive but also more prone to artefact. It is sensitive to intravenously administered contrast agents which can be used to augment very slow rates of flow. The software that measures power Doppler is proprietary; therefore, results from one manufacturer may not be comparable with another. There is also a concern that serial studies on the same equipment may be incomparable following software or platform upgrades. 11 16 15 16 Fig. 15 blue red MT  Fig. 16 Screen capture during colour Doppler examination of the finger , showing automated calculation of resistive index  CFDUS is less sensitive to motion artefact and is a more objective measurement than is PDUS. Problems include difficulties in acquiring a good trace when angiogenesis is sparse and maintaining the fix as measurements are made, particularly if the equipment freezes the image during measurement. The use of beam steering can sometimes help with vessel fixation. CFDUS is insensitive to contrast agents, which diminishes its usefulness in patients with subtle or early synovitis. Changes in RI can also be induced by increasing probe pressure and by changes in temperature. Serial studies should be carried out at controlled temperature. In most cases this can be achieved simply by ensuring the patient has not just come from an abnormally hot or cold environment to room temperature, though some argue that the use of a water bath is the only means of ensuring reliable temperature control. Probe pressure can be controlled in the usual manner by ensuring that there is always a layer of contact gel between probe and skin. 17 18 19 Ultrasound contrast agents 20 Because ultrasound involves assessment of a relatively small field of view, the short duration of optimal activity of micro-bubbles following a bolus injection means that there will be different concentrations in the synovium of a joint viewed at the beginning of an ultrasound examination compared with one examined at the end. This compares poorly with the single-time snapshot that can be achieved with intravenous administration of MRI contrast agent, where uptake in all of the joints and tendons within the larger field of view can be assessed simultaneously. There are other disadvantages of bolus administration of ultrasound contrast agents, which include a blooming artefact that can occur due to the sudden arrival of a large quantity of micro-bubbles into the ultrasound field and uneven destruction of micro-bubbles by the US wave acting on the large bolus itself. Administering the contrast agents by infusion rather than bolus injection can overcome some of these issues and yield more stable enhancement for up to 20 min. Three-dimensional ultrasound, with maximum intensity projection algorithms, may also help in overcoming some of these disadvantages and provide a better single-time snapshot image of synovial enhancement. 11 21 16 22 Bone erosions 17 18 23 13 Fig. 17 open arrow arrowhead 13 arrow asterisk MT PP  Fig. 18 arrowheads MC arrow  23 1 24 25 24 26 Erosions in gout are larger, also irregular, and lie further away from the joint. Erosions must be distinguished from other causes of peri-articular bone irregularities. Entheseal new bone formation at the site of tendon and ligament insertions occur in seronegative arthritis and can give a similar appearance. They are particularly prevalent at the DIPJs, where erosive changes related to osteoarthritis are also found. Small joint US following treatment 27 17 28 29 30 31 33 31 32 30 34 Clinical application of ultrasound in small joint arthritis Ultrasound versus clinical examination 35 36 35 37 Ultrasound versus other imaging techniques 23 38 23 24 39 25 8 40 41 45 46 47 Advantages and disadvantages of ultrasound 38 19 20 21 48 Fig. 19 asterisks DCR PCR  Fig. 20 arrow MT PP  Fig. 21 arrowheads  Conversely, ultrasound has difficulty with large deep joints, superficial joints when there are significant deformities, and in the assessment of articular cartilage. US is a relatively time-consuming procedure, especially if multiple joints are examined. The images acquired can be re-read and reclassified by other readers at a later date, but there is no ability to identify abnormalities overlooked and not imaged at the time of the original examination. MR images can be obtained remotely, reviewed, re-read and scored at a later date and preserved for serial comparison. Increasing use of ultrasound video-loops and 3-D ultrasound may assist in this regard in the future. MRI demonstrates single-time shot synovial enhancement in the entire field of view, which can include all the important small joints of the hands and wrists as well as adjacent tendons and ligaments. Although the time taken to review MRI images of the hand and wrist for diagnostic purposes is probably shorter than the time taken to complete an ultrasound examination, a full MRI score such as the rheumatoid arthritis MRI scoring system (RAMRIS), which is a more close approximation to ultrasound, takes considerably longer. Despite this, there is a strong argument that, for research studies of different therapeutic regimens in patients with rheumatoid arthritis, plain films and MRI provide more robust serial assessment and will remain the gold standard for some time. Conclusions In routine rheumatological practice, ultrasound of the small joints of the hands and feet accurately detects occult synovial disease and allows classification of the extent and activity of synovitis as well as superior detection of erosions in comparison with radiography. US can guide intra-articular therapy as well as assess its response and the response to systemic treatment. Future goals for ultrasound trials need to include further validation studies, studies of diagnostic and therapeutic impact and longer term outcomes from clinical and therapeutic decisions based on the ultrasound examinations.