Dissociation between 2-[18F]fluoro-2-deoxy-D-glucose positron emission computed tomography, ultrasound and clinical assessments in patients with non-severe rheumatoid arthritis, including remission

Background Inflammation of patients joints with severe disease activity of rheumatoid arthritis (RA) has already been visualized and quantified by 2-[18F]fluoro-2-deoxy-D-glucose positron emission computed tomography ([18F] FDG PET/CT), but little is known about the metabolic status and its relationship with clinical and ultrasonography (US) metrology in patients with low/moderate activity or in remission. Methods Clinical assessments [based on 28-joint disease activity score (DAS28-CRP) and Clinical Disease Activity Index (CDAI)], [18F] FDG PET/CT, US and X-ray were performed on 63 RA patients classified into remission or low/moderate or severe disease activity groups. PET/CT was visually and then semi-quantitatively analysed by determining the standardized uptake value (SUV) of positive joints. Results Of the 1764 joints, 21.1% were tender only, 13.7% swollen only, 27.6% tender or swollen, 7.3% tender and swollen, 20.5% PET/CT-positive and 8.6% US-positive. PET and US measurements were correlated, albeit with poor concordance. The positive predictive value of PET/CT for clinical evaluation (tender and/or swollen) was low, whereas its negative predictive value was high. Highly significant differences were found with the number of PET/CT-positive joints and with cumulative SUV between “severe” and “non-severe” patients (including those in remission and those with low/moderate activity) and not between those classified as “remission” and “non-remission” or “remission” and “low/moderate activity”. Moreover, the correlation between PET/CT measurements and clinical activity was positive only in the CDAI severe disease group. In patients in remission or with low/moderate activity, only 20–30% of joints were PET/CT-negative. In remission, PET/CT and US were positive in different joints, and PET/CT-positive but US-negative joints mainly exhibited RA (38.1%) or normal (49.2%) and not osteoarthritic (12.7%) X-ray patterns. Conclusions [18F] FDG PET/CT was effective at distinguishing patients with severely active disease from other patients. In non-severe RA patients, including those in remission, PET/CT results are discordant from US and clinical observations. A longitudinal analysis is needed to explore the clinical relevance of such infra-clinical disease. Supplementary Information The online version contains supplementary material available at 10.1186/s41927-021-00196-1.


Background
Since the use of biologic agents in the therapeutic armamentarium against rheumatoid arthritis (RA), low disease activity (LDA) and remission are common goals for better outcomes, including less radiographic progression [1]. In clinical trials, remission is often defined as a 28joint disease activity score (DAS 28 ) < 2.6. However, this target is achieved in only 30-40% of patients [2,3]. Within this group, the disease remains active in a significant proportion of patients, as observed in the DREAM registry, where 31.1% of patients had a swollen joint count ≥2 [3] and experienced joint damage progression [4]. Furthermore, imaging studies have shown at least one synovitis in 33-73% of patients in remission by ultrasound (US) and in up to 96% of them by magnetic resonance imaging (MRI) [5]. Surprisingly, even the most stringent remission criteria, such as ACR/EULAR Boolean remission [6], did not decrease the prevalence of US-diagnosed synovitis [7][8][9][10][11]. Although the relevance of US, specifically power Doppler activity, in clinical remission is widely accepted for driving radiologic progression [8][9][10][11][12][13][14] and future clinical flares [7,10,13,15], the relevance of other imaging techniques for assessing synovitis remains poorly understood, and discordance between predictors of clinical and US remission indicates complex interactions between them [9,16].
We and others [17][18][19][20][21][22][23][24] have shown that 2-[ 18 F]fluoro-2-deoxy-D-glucose positron emission computed tomography ([ 18 F] FDG PET/CT) is able to detect and quantify inflammation in RA synovitis. The number of PETpositive joints among the 28 joints of the DAS and the cumulative standard uptake value (CSUV) of these PETpositive joints were highly correlated with clinical status: number of swollen and tender joints, erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) parameters [22,24] and also with US data such as the number of US-positive joints and the cumulative synovial thickness [22]. Roivanen et al. [21] reported that up to 90% of the joints were rated positively by clinical evaluation (swollen and tender) and also by [ 18 F] FDG PET, whereas a proportion of 75% was quoted by Elzinga [19]. However, in these studies, correlations between PET parameters and DAS 28 scores were obtained in RA patients who had for all [20,22] or a large majority [21,24] severe disease activity. The main objectives of the present study were (1) to correlate PET/CT parameters to US and clinical measurements among RA patients and (2) to analyse variations in PET/CT parameters based on disease activity (remission, low/moderate activity or severe activity) defined by two classic composite indices, DAS 28 -CRP and Clinical Disease Activity Index (CDAI) scores.

Study design and patients
This cross-sectional study, approved by the ethics committee of our hospital (B70720108722), included 63 patients fulfilling the ACR/EULAR 2010 criteria for RA [25] from July 2010 to April 2012. Written informed consent was obtained from each patient. All assessments were performed on the same day by the same independent experienced investigator unaware of the other results: the clinical evaluation and biological test were performed in the morning followed by US and then PET/CT evaluation. X-rays were available as routine controls and were performed at a maximum of 6 weeks after the study day. The patient (PtGA) and the physician (PGA) global assessments were determined using a visual analogue scale (VAS) (0-100 mm) as well as the Health Assessment Questionnaire (HAQ) [26]. Disease activity was evaluated using the DAS 28 -CRP (without the PGA) [27] and the CDAI (with the PGA and without CRP) [28]. Each patient was categorized as in remission (DAS 28 -CRP ≤ 2.6 or CDAI≤2.8), in low to moderate disease activity (2.6 < DAS 28 -CRP ≤ 5.1, 2.8 < CDAI≤22), or in severe disease activity (DAS 28 -CRP > 5.1, CDAI> 22) [29]. The number of joints that were solely tender (T), solely swollen (S), "tender or swollen" (T/S) and "tender and swollen" (T&S) was recorded.

[ 18 F] FDG PET/CT imaging
The PET/CT studies were performed using a Gemini BigBore scanner (Philips Medical Systems, Cleveland, OH, USA). The patients fasted for 4 h and were injected with [ 18 F] FDG (4 MBq/kg body weight with a maximum of 370 MBq) through an indwelling catheter placed in the median cubital vein and flushed with 5 cc of saline solution afterwards. Blood glucose levels were lower than 140 mg/dl. The uptake time was 60 min, and the image acquisition sequence was as follows: first, a scout view CT, followed by a PET emission study that included the knees, hands, wrists, elbows and shoulders, with 2 min per bed position for a total scanning time that ranged from 14 to 18 min. Finally, a low-dose CT (5-mm slice thickness, tube voltage 120 Kv, tube current-time product 80 mAs) was performed over these joints. Figure 1 presents an exemplative picture of [ 18 F] FDG PET/CT imaging of the knees and hands. The hands and wrists were positioned and fixated, arms down, on a dedicated Plexiglas device to avoid movements between the PET and CT acquisitions. PET images were reconstructed using an iterative list mode time-of-flight algorithm, and corrections for attenuation, dead-time, random and scatter events were applied. The images were first visually analysed, and joints were considered positive for synovitis when the [ 18 F] FDG uptake was increased compared to the background in areas corresponding to joint synovium on CT, i.e., either when thickened synovium was recognized on CT or in locations corresponding anatomically to synovium, excluding uptake in other structures such as muscle and tendons. The [ 18 F] FDG uptake was then quantified using the maximum standardized uptake value (SUVmax). In PET-positive joints according to the visual analysis, the SUVmax was obtained by drawing a region of interest (ROI) over the most active synovial area identified. When no synovitis was identified, ROIs were placed in the corresponding areas on the CT and drawn around the appropriate joint: at the dorsal surface of the radius (on top of the lunate) for the wrists, over the lateral recess at the level of the midpatella for the knees and for the small joints as metacarpophalangeal (MCP) or proximal interphalangeal (PIP) joints. A global metabolic assessment was obtained through the number of PETpositive joints (visual evaluation) and the sum of all SUVmax values from the positive joints (cumulative SUV, CSUV).
Ultrasound and X-ray examinations US assessments were performed using a B-mode multifrequency 10-14.0 MHz transducer (Logiq 9) (GE Healthcare, Milwaukee, WI, USA). US positioning for the wrists, MCP and PIP joints and for the knees has been described elsewhere [22]. Proximal and distal radiohumeral recesses and posterior recess were studied for the elbows. The glenohumeral joint, with a posterior transverse view, was studied for the shoulders. Synovial measurements were systematically carried out perpendicular to the great axis and at the point of greatest thickness. A cut-off for US positivity was defined as synovitis that was at least 1-mm thick (3 mm for the shoulders) based on US determinations in healthy controls, described elsewhere [22]. In joints where 2 (wrists) or 3 (elbows, knees) scans were obtained, the joint was considered positive if at least one measurement was positive. The cumulative synovial thickness (CST), i.e., the sum of thicknesses of all US-positive joints, is the addition of all (single or multiple) synovial measurements performed. X-rays were obtained for peripheral joints (PIPs, MCPs and wrists).

Statistical analysis
The results are generally expressed as the mean ± standard deviation (SD). Correlation coefficients were calculated to measure the association between PET/CT and clinical or US parameters. Spearman correlations were used for skewed distributions. Concordance between methods was quantified by the intraclass coefficient (ICC). Ordinal logistic regression was used to assess the relationship between disease activity categories based on the DAS 28 -CRP or CDAI (remission, low/moderate and severe disease activity) and PET/CT number of positive joints and CSUV. A test was performed to determine whether all three disease severity categories were distinguishable. If this was not the case, a classic logistic regression analysis was applied, and optimal Youden cutoff values were determined from the receiver operating characteristic (ROC) curve method. The results were considered significant at the 5% level (p < 0.05). All statistical analyses were performed with SAS (version 9.4).

Relationships between PET/CT and clinical measurements
Joint positivity on PET/CT was compared to the clinical evaluation ("tender", "swollen", "tender or swollen", "tender and swollen") for each joint. The diagnostic efficacy of PET/CT (sensitivity, specificity, and positive and negative predictive values) is presented in Table 2. PET/ CT sensitivity was low with respect to clinical measurements: only 58.9% of the joints that were both tender and swollen and only 35.8% of the joints that were tender or swollen were positive on PET/CT. Specificity was higher: 82.6% of the joints that were not tender or not swollen, and 85.4% of the joints that were neither tender nor swollen were PET-negative (Table 2). In accordance with these data, the positive predictive value of PET/CT was low, while the negative predictive value was as high as 96.2% in tender and swollen joints. In other words, when PET/CT was negative, the probability that the articulation was not tender and/or not swollen was high; however, when PET/CT was positive, the probability that this articulation was tender and/or swollen was of poor value.

Relationships between PET/CT parameters and disease activity threshold
The number of PET/CT-positive joints and CSUV were analysed based on disease activity categories (based on the DAS 28 -CRP or CDAI) and illustrated in Fig. 3 and Supplementary Table 1 (e.g., there were 3.6 ± 5.4 PET/ CT-positive joints in those with DAS28-CRP remission, 4.7 ± 6.7 joints in those with low/moderate activity, and 13.6 ± 11.2 joints in those with severe disease activity). An ordinal logistic regression evidenced a significant relationship between the mean number of PET/CT-positive joints or the CSUV and clinical disease activity (Supplementary Table 1). However, "remission" and "low/moderate" disease activity categories could not be dissociated by PET/CT. Moreover, 27.3% of patients without any metabolic activity were observed in both remission subgroups defined by the DAS 28 -CRP and CDAI, while 25.8 and 27% of patients were observed in the DAS 28 -CRP and CDAI low/moderate activity subgroups (p = 0.99 for both DAS 28 -CRP and CDAI subgroups), indicating that PET/CT was unable to discern remission and low/moderate activity (Table 3).
Thus, the two categories remission and low/moderate activity were merged, and a classic logistic regression analysis was performed between patients with severe and non-severe (including remission and low/moderate activity) disease activity (Table 4): highly significant differences were found in the number of PET/CT-positive joints and in CSUV. The optimal threshold for identifying RA patients with clinically and biologically severe disease was at least 8 PET/CT-positive joints and a CSUV ≥17.8 for the DAS 28 -CRP and 6.8 and 15.0 for the CDAI, respectively. Disease activity thresholds were also studied by dividing RA patients into remission and nonremission categories (including low/moderate and severe disease), but no significant differences were observed in terms of the number of PET/CT-positive joints and CSUV (data not shown). Table 3 shows that 16/22 and 8/11 patients in remission (according to DAS 28 -CRP and CDAI, respectively) were nonetheless PET/CT-positive for at least one joint. Of interest, 12/22 (54.5%) and 6/11 (54.5%) patients also had at least one US-positive joint, despite being in remission (data not shown). As an illustration regarding the patients in remission based on the DAS 28 -CRP, at the joint level, 75 joints were PET/CT-positive, and 24  Overall, there was a significant correlation between the metabolic measurements (number of positive joints and CSUV) and the clinical assessments (DAS 28 -CRP and CDAI) ( Table 5). However, when classifying patients in remission, low/moderate or severe categories, this significant correlation between PET and clinical assessment was observed only in the subjects with severe activity according to CDAI and not in the RA patients in the low/ moderate disease activity or remission groups.

Relationship between X-rays and PET/CT
The 63 peripheral joints (PIPs, MCPs and wrist) that were PET-positive but US-negative in the 16 patients in  Table 2 Diagnostic efficacy of PET/CT in the 1764 joints analyzed. Values are expressed in percent with 95% confidence interval. The item "swollen" includes "swollen only" and "tender and swollen". The item "tender" includes "tender only" and "tender and swollen". The item "tender or swollen" includes "tender only", "swollen only" and "tender and swollen". The item "tender and swollen" includes only a joint that is simultaneously tender and swollen" clinical remission (DAS28-CRP < 2.6) were characterized with X-ray. Features of RA, i.e., symmetrical joint narrowing, bone erosion or demineralization, and of OA, i.e., asymmetrical joint narrowing, subchondral sclerosis, or osteophytes, were recorded. The results were consistent with RA in 24/63 joints (38,1%) and with OA in 8/63 joints (12.7%) with OA signs. In 31/63 joints (49.2%), Xrays were normal. In particular, RA/OA/normal features were described in 7/0/24 of the 31 PIPs, 12/6/5 of the 23 MCPs and 5/2/2 of the 9 wrists. Considering the corresponding clinical status, none of these joints were tender or swollen.

Discussion
In line with previous work [22], we confirmed that the number of PET/CT-positive joints and the CSUV significantly correlated with the number of US-positive joints, synovial thickness and disease activity based on either the DAS 28 -CRP or CDAI. In addition, PET/CT was quite effective at distinguishing patients with a severely active disease from the others, as a cut-off of 8 for the number of PET-positive joints and 17.8 for the CSUV yielded an area under the curve (AUC) of 0.77 (considering the DAS 28 -CRP as the clinical gold standard). Although the number of hypermetabolic joints and the cumulative SUV tended to be higher with increased clinical severity of the disease, one notable exception should be mentioned. There was no significant difference with the number of PET-positive joints and their CSUV between patients in clinical remission and those with low/moderate disease activity. In both groups, only 25-27% of the patients presented negative PET/CT findings. Clearly, PET/CT results and clinical assessments diverge in nonsevere RA, including remission, in agreement with previous observations made with US and MRI [3][4][5][6][7][8][9].
Comparing the PET and US findings, there were twice as many PET/CT-positive joints as US-positive joints. Furthermore, there was also clear evidence that the PET/CT and US analyses of joints did not concur for those in remission. For example, out of the 22 patients in remission based on the DAS 28 -CRP, 12 were positive with both PET/CT and US, but at the joint level, only 10 of the 75 PET/CT-positive joints (5 wrists, 4 MCPs and one shoulder) were also US-positive. This divergence was also observed for patients in remission according to the strict Boolean-based definition (which is stricter than the DAS 28 -CRP-based definition). In other words, PET/ CT was positive in a significant number of patients with no or low/moderate disease activity based on current clinical scales and was also positive in a significant number of joints that were not considered inflamed according to clinical and US parameters. Two interpretations are possible for this observation. The first would be a higher sensitivity of the metabolic measurements for identifying subclinical joint inflammation. Indeed, in inflammatory diseases, incidental PET/CT findings due to [ 18 F] FDG accumulation are consistently associated with enhanced glycolytic metabolism in inflammatory cellular infiltrates, including activated macrophages, neutrophils and lymphocytes [30]. We may therefore consider that hypermetabolic joints with normal US appearance are joints with an inflammatory component without proliferating synovitis or with a synovitis < 1 mm thickness, which was the cut-off. In a previous series of RA patients with severe disease activity, only 50% of the PIPs and 62% of the MCPs, both tender and swollen, were US-positive using the same cut-off (data not shown) [22]. PET/CT analysis might therefore exhibit greater sensitivity than US. It is noteworthy that in the current series, the PET/CT-positive but US-negative joints within these 16 patients were mostly PIPs (31 joints in 7 patients) and MCPs (23 joints in 7 patients), which are joints that are typically involved in RA. X-ray analysis supports this hypothesis, as 38% of the joints had signs of RA and 49% were normal. An alternative explanation would be to consider those joints and patients as false positive results of PET/CT. It is indeed possible that the joints actually suffer from secondary (MCPs) or primary (PIPs) osteoarthritis. However, only 8/62 (13%) joints, 6 MCPs in 2 patients and 2 wrists in 2 patients had signs of OA. The radiological analysis is thus in favour of the first hypothesis, but a longitudinal follow-up of the patients would be needed to provide definitive evidence. As a limitation, X-rays were only available for peripheral joints (PIPs, MCPs and wrist) with systematic X-ray realization but not for larger joints (knee, elbow, shoulder). In addition, it should be noted that 30% of the studied patients had daily oral glucocorticoid intake, even at low doses, and that our population was composed of 35% of patients in remission (based on the DAS28-CRP) and only 16% of patients  with severe activity; these factors could have influenced our results. A brief report also proposed the use of hybrid PET/magnetic resonance imaging (PET-MRI) to assess inflammatory changes within the synovial tissue in RA [31]. This technique could also be promising to assess remission, mainly because of a higher spatial resolution but also because of the capacity of MRI to detect bone marrow oedema, which is a sign of activity and a risk factor for progression. However, RA was not restricted to the hand, and the use of PET/CT allowed us to obtain a CT view of several other joints, such as the knees, elbows and shoulders.

Conclusion
[ 18 F] FDG PET/CT demonstrated a high specificity and negative predictive value compared to individual clinical evaluation of the joints. Furthermore, PET/CT was effective at differentiating "severe" from "non-severe" patients, although clinical remission was not associated with metabolic remission. Such issues are of high clinical relevance, as PET/CT could possibly identify subclinical and infra-radiological inflammation worthy of treatment to prevent further irrevocable damage to the joints. Further studies are needed to ascertain whether this represents a clinically relevant activity of the disease or secondary degenerative changes.