MRI is an established imaging technique for the detection of early inflammatory changes in RA joints.17
Musculoskeletal US is a rapidly emerging technique, especially for the detection of soft tissue lesions in inflammatory rheumatoid diseases.13,18
Several studies have proved the ability of US and MRI in the sensitive detection of synovitis and early erosive lesions,12,19,20,21
which is of major importance in the early treatment of RA. However, with the exception of a 2 year follow up study involving mainly RA and psoriatic arthritis finger joints2
there are no longitudinal long term data for US, while MRI has been proved to be sensitive for the follow up analysis of bone damage.22,23
No data on the importance of US findings for later radiographic or functional status are available (that is, the prognostic value of US in RA is unknown).13
Some indirect support for a predictive value of US is provided by the high agreement with MRI findings.1,2,11,24
In this study we collected the first long term data comparing CR, US, and MRI for the detection of bone erosions and synovitis in RA finger joints. During long term DMARD treatment we saw a significant reduction of synovitis with both US and MRI, but the reduction was more evident with US. In CE a reduction of finger joint swelling was also assessed, although CE seemed clearly to underestimate the presence of synovitis in RA joints, which was also shown in previous studies.1,24,25
US was sensitive for the detection of very small fluid accumulations and proved better than MRI, especially in the PIP joints, which might explain the slightly higher percentage of joints affected by synovitis which were detected by US.
Follow up analysis of finger joint synovitis is an important task in controlling the effectiveness of treatment. However, for long term outcome, a precise analysis of erosions is essential. Although we saw a decrease in synovitis during long term DMARD treatment, we found an increase of finger joints with erosions with all imaging techniques. The difference between baseline and 7 year follow up was highly significant as detected by CR and US, but there was no significant difference with MRI between both time points.
A possible reason for the small number of erosions detected by US at baseline is the distinct difference in the quality of the US devices (fig 2). With a significantly lower resolution and the need to use an acoustic standoff pad it is likely that US missed a number of erosions at baseline. Also, it should be mentioned that neither ulnar nor radial aspects of the joints were evaluated by US, raising the possibility that some erosions might have been missed. A high number of erosions were already detected at baseline visit by MRI1
and owing to our binary evaluation system no statistically significant further progression of MRI erosions was shown because of a ceiling effect (truncation of data because a score of 1 could not progress further as it is already the highest possible score),26
which may help to explain our current results. Although it could be shown that low field MRI (0.2 Tesla) has similar sensitivities for bone erosions to those obtained with 1.5 Tesla MRI devices,27
a better detection of bone erosions might have been achieved by using higher field MRI (>0.2 Tesla) devices.
The selection of patients with established disease (mean disease duration 14.7 years) might have had an impact on the study outcome. Our data are in agreement with the 2 year follow up study,2
which recorded an increase of erosions with clinical improvement and regression of synovitis, but now more pronounced with MRI than with US. An increase in erosions in the wrist as detected by MRI despite clinical improvement was also described by McQueen et al
Another aim of our study was to determine whether radiographically occult finger joint erosions, previously detected by US and MRI, would be detected by CR 7 years later. Nearly every second erosion previously detected by MRI presented on CR 7 years later, showing that erosions could actually be predicted by MRI. The reason why not all formerly detected erosions were seen later in the disease course by CR might be explained by healing processes, reader error, and technical limitations of x
US initially detected erosions in nine joints, of which two erosions were seen in radiography at follow up. Again, we explain this rather low percentage of erosions at baseline by the low quality of the US images 7 years ago. However, another explanation lies in the fundamental difference in the way in which CR and MRI generate image contrast. This is outlined by Peterfy,29
but basically, the lucency associated with erosions on radiographs is due to the loss of cortical bone, not trabecular bone. CR is quite insensitive to trabecular bone loss. Consequently, the often larger intramedullary component of an erosion is typically not visible on radiographs. MRI, however, provides excellent delineation of abnormalities in the marrow space, and therefore can detect even large intramedullary erosions that are radiographically occult. Many of these erosions may not become detectable on radiographs even after many years.
For MCP joints, more erosions were detected by MRI than by US and CR. The distribution of finger joint erosions shows that MCP IV is less often affected than MCP II, III, and V, as also described by Wakefield et al
Interestingly, we saw a more frequent and distinct affection of PIP compared with MCP joints by US than by CR and MRI, which is in agreement with recently presented data.30,31
This underlines the importance of potentially including PIP joints in common scoring systems both for US and MRI—for example, the RA‐MRI scoring system developed by the OMERACT MRI in an RA working group.32
US has some advantages over MRI, because it is easier to perform examinations of all finger joints with US, although documentation is time consuming. In MRI examinations, especially in the dedicated systems which we used in this study, the region to be examined has to be defined before the examination is performed (for example, focus on PIP and MCP joints or wrist and MCP joints) owing to field of view and coil limitations. However, when limited to a field of view, it would probably be more useful to include the wrist and MCP joints rather than solely the PIP and MCP joints. On the other hand, with MRI, images can be obtained by a technician, whereas with US, several readers are needed to interpret and score both synovitis and erosions at a later time point.
For each method, two experienced readers reached consensus about synovitis and erosion findings, except for US which was evaluated by one experienced examiner at baseline. Although this might have had an influence on the interpretation of the pathological findings, we have recently shown that US performed by experienced examiners giving good interreader agreement results.15,33