Rheumatoid Arthritis and Tuberculous Arthritis: Differentiating MRI Features

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1 Musculoskeletal Imaging Original Research Choi et al. MRI of Rheumatoid and Tuberculous Arthritis Musculoskeletal Imaging Original Research Jung-Ah Choi 1,2 Sung Hye Koh 3 Sung-Hwan Hong 2 Yong Hwan Koh 2 Ja-Young Choi 2 Heung Sik Kang 1,2 Choi JA, Koh SH, Hong SH, Koh YH, Choi JY, Kang HS Keywords: arthritis, MRI, rheumatoid arthritis, tuberculosis, tuberculous arthritis DOI: /AJR Received November 25, 2008; accepted after revision January 9, Department of Radiology, Seoul National University Bundang Hospital, 300 Gumi-dong, Bundang-gu, Seongnam, Gyeongi-Do, , Korea. Address correspondence to H. S. Kang (kanghs@radcom.snu.ac.kr). 2 Department of Radiology, Seoul National University College of Medicine, Seoul, Korea. 3 Department of Radiology, Hallym Sacred Heart Hospital, Hallym University School of Medicine, Pyeongchon, Gyeongi-Do, Korea. AJR 2009; 193: X/09/ American Roentgen Ray Society Rheumatoid Arthritis and Tuberculous Arthritis: Differentiating MRI Features OBJECTIVE. The purpose of our study was to determine the MRI findings of rheumatoid arthritis (RA) and tuberculous arthritis, with emphasis on differential diagnostic features. MATERIALS AND METHODS. MR images of 63 joints in 62 patients with clinically or pathologically proven RA (36 joints in 35 patients) or tuberculous arthritis (27 joints in 27 patients) were evaluated retrospectively with regard to pattern and degree of synovial thickening, size of bone erosions, rim enhancement at bone erosions, degree of bone marrow and periarticular soft-tissue edema, and presence and number of extraarticular cystic masses. MRI findings were compared between RA and tuberculous arthritis by statistical analysis using kappa statistics, the Mann-Whitney U test, linear-by-linear association, and the chi-square test. RESULTS. Nonuniform and greater degree of synovial thickening was more frequent in RA (p < 0.01); the thicker the synovial membrane, the greater the likelihood of RA (p < 0.01). Bone erosions of tuberculous arthritis were larger (p < 0.01), and the likelihood of tuberculous arthritis increased proportionally to the increment of size of the bone erosions (p < 0.01). Rim enhancement at bone erosion was more frequent in tuberculous arthritis (p < 0.01). Extraarticular cystic masses were more frequently seen and more numerous in tuberculous arthritis (p < 0.01). CONCLUSION. Uniform synovial thickening, large size of bone erosion, rim enhancement at site of bone erosion, and extraarticular cystic masses were more frequent and more numerous in tuberculous arthritis. MRI may be helpful in the differentiation between RA and tuberculous arthritis. T uberculous arthritis is generally a monoarticular disease that typically involves the spine or largeand medium-sized joints, such as the hip and knee. Rheumatoid arthritis (RA) is usually a symmetric polyarticular disease with common involvement of peripheral joints. However, RA may also manifest as a monoarticular disease and persist in such a fashion for a long time [1]. RA and tuberculous arthritis may have similar clinical characteristics, consisting of a chronic course with periarticular soft-tissue swelling [2], as well as similar radiologic findings, such as periarticular osteoporosis, bone erosion, and presence of joint effusion, rendering differential diagnosis difficult [2]. Actually, there have been reports of cases in which tuberculous arthritis was misdiagnosed as RA or juvenile RA [3 5]. The value of MRI has been advocated in each of the two diseases because of its superior soft-tissue contrast and its ability to depict changes in cartilage, liga- ments, and synovial tissue, especially in the evaluation of early changes [6, 7]. With MRI, both diseases have been reported to reveal heterogeneous low signal intensity in the joint because of rice bodies, cartilage fragments, and hemorrhage [8, 9]. Furthermore, the distinction by virtue of the number of joints involved does not always hold true because early asymmetric oligoarticular involvement of RA is not rare [10], and polyarticular involvement in tuberculous arthritis has been reported [11]. Although there has been a report comparing RA and tuberculous arthritis of the knee using MRI [12], this report compared a limited number of cases and only cases involving the knee; therefore, a comprehensive investigation of MRI in those two articular diseases, especially comparing the MRI findings of the two diseases, has not been undertaken to our knowledge. The purpose of this study was to determine MRI findings of RA and tuberculous AJR:193, November

2 Choi et al. arthritis with special emphasis on findings that allow their differentiation. Materials and Methods Subjects and MRI MR images of 63 joints in 62 patients (32 female and 30 male; age range, 4 76 years) that were clinically and pathologically diagnosed as either RA (36 cases in 35 patients) or tuberculous arthritis (27 cases in 27 patients) were evaluated. The cases were retrieved retrospectively by a computer-based database program that searched on patients by final diagnosis of RA or tuberculous arthritis during a period from October 1998 to September Medical records were reviewed, and all patients had undergone MRI because of monoarticular disease with joint manifestation for the first time with symptom durations varying from 3 to 40 months, so the clinician had no prior knowledge of the disease of the patient. Final diagnoses were made on the basis of clinical history, physical examinations, radiographic survey, synovial biopsy, culture, tuberculosis polymerase chain reaction, and serologic studies. The study design was approved by our institutional review board; informed consent was not required at the time of study. MRI was performed with various MR units, including 1- and 1.5-T scanners, but in all patients, spin-echo T1-weighted (TR range/te range, /10 30) and fast spin-echo T2-weighted (2,500 4,500/60 108) images with or without fat suppression, along with gadolinium-enhanced spin-echo T1-weighted images were acquired [13, 14]. Although imaging planes were variable, at least two orthogonal plane images were available, including transverse images in all patients. The field of view varied from to mm and the matrix from to ; various coils were used, including wrist, shoulder, knee, head, and body surface coils. Analysis of MR Images MR images were analyzed retrospectively and independently by three experienced musculoskeletal radiologists without prior information about patient history, findings of other imaging studies, and final diagnosis. MR images were analyzed regarding the following findings: uniformity and degree of synovial thickening, size of and enhancement around bone erosion, degree of marrow edema, degree of soft-tissue edema, and the presence of extraarticular cystic masses. One of the three observers determined the number of extraarticular cystic masses, which were defined as any cystic masses located outside the joint regardless of their contiguity with the joint. The degree of synovial thickening after IV gadolinium administration was TABLE 1: Age and Sex Distribution of Rheumatoid Arthritis and Tuberculous Arthritis Characteristic Tuberculous Arthritis Rheumatoid Arthritis Total No. of Patients Mean Age (y) classified into four grades according to the maximal thickness of enhancing synovium: grade 0, 0~3 mm; grade 1, 3.1~6 mm; grade 2, 6.1~9 mm; and grade 3, > 9 mm. Uniform synovial thickening was considered to be present when the maximal thickness of synovium measured less than two times the minimal thickness after IV gadolinium administration. Size of the largest bone erosion in longest diameter was documented. The size of the bone erosions was classified into five grades: grade 0, no erosion; grade 1, < 6 mm; grade 2, 6~10 mm; grade 3, 11~15 mm; and grade 4, > 15 mm on T1-weighted images. Enhancement around bone erosion was determined on gadolinium-enhanced images as either present or absent. The number of erosions was not counted. Bone marrow edema was classified into four grades after calculation of the extent of marrow edema for the joint as ([maximal distance from articular margin to outer margin of signal change of bone marrow on sagittal or coronal image / maximal diameter of articular surface on axial image] 100[%]) into the following four grades: grade 0, no edema; grade 1, 1~25%; grade 2, 26~50%; and grade 3, > 50%. Soft-tissue edema was classified into three grades after measuring the maximal vertical distance from the outer margin of joint capsule to the outer margin of periarticular soft-tissue edema on coronal or sagittal image as follows: grade 0, no edema; grade 1, 0.1~1 cm; and grade 2, > 1 cm. No. of Patients Mean Age (y) No. of Patients Mean Age (y) Female Male Total no. of patients Median age (y) Age range (y) TABLE 2: Distribution of Locations of Rheumatoid Arthritis and Tuberculous Arthritis Location Rheumatoid Arthritis (%) Tuberculous Arthritis (%) Total (%) Shoulder 2 (5.6) 0 (0) 2 (3.2) Elbow 7 (19.4) 2 (7.4) 9 (14.3) Wrist 12 (33.3) 4 (14.8) 16 (25.4) Hip 0 (0) 4 (14.8) 4 (6.3) Knee 11 (30.6) 13 (48.1) 24 (38.1) Ankle 4 (11.1) 4 (14.8) 8 (12.7) Total 36 (100) 27 (100) 63 (100) Note Data are numbers of cases (%). Bone marrow and soft-tissue edema were evaluated on T2-weighted sequences. Statistical Analysis Statistical analysis was performed using a commercially available software (SPSS for Windows, version ). Using kappa statistics, interobserver agreement was evaluated for uniformity of synovial thickening, degree of synovial thickening, rim enhancement around bone erosions, bone marrow edema, soft-tissue edema, and the presence of extraarticular cystic masses. Interobserver agreement was considered poor for kappa values equal to or less than 0.200, fair for values of , moderate for values of , substantial for values of , or almost perfect for values of If the kappa value of an item was less than 0.2 in one or more pairs between the three observers, the item was considered to be improper for use as an objective sign for differential evaluation of the joint [15, 16]. The Mann-Whitney U test was used for comparing two groups for the degree of synovial thickening, size of bone erosion, and bone marrow edema. Linear-by-linear association was calculated for the statistically significant items for evaluation of likelihood. The chi-square test was used for comparing two groups for nonuniform synovial thickening, rim enhancement of bone erosion, and presence of extraarticular cystic mass. A 1348 AJR:193, November 2009

3 MRI of Rheumatoid and Tuberculous Arthritis p value of 0.05 was considered to indicate statistical significance. Results The mean age of the 62 patients was 43.3 years (45 years for those with RA and 42 years for those with tuberculous arthritis; median age, 44.5 years). The distribution by age and sex and the location of the evaluated joints of both RA and tuberculous arthritis groups are listed in Tables 1 and 2, respectively. The case of a 4-year-old patient represented juvenile RA. The results of kappa statistics for evaluation of interobserver agreement are summarized in Table 3. Interobserver agreement was generally moderate. The kappa value of interobserver agreement for the degree of soft-tissue edema was less than in one pair and less than in all three pairs between the three observers. Thus, the degree of soft-tissue edema was eliminated from further statistical analysis. The results of image analysis for uneven synovial thickening, the degree of synovial thickening, the size of bone erosion, the presence of rim enhancement along the erosions, the degree of bone marrow edema, the degree of soft-tissue edema, and the presence and number of extraarticular cystic masses are listed in Table 4. The results of comparison between the two groups using mean values of all data sets created by the three observers by statistical analysis for each item are summarized in Table 5. Because the degree of synovial thickening and size of bone erosion were statistically significant in differentiation of the two diseases and classified into grades (i.e., quantified), linear-by-linear association was calculated for evaluation of trend for both items (Table 5). Nonuniform synovial thickening was noted in 72.2%, 86.1%, and 80.6% of RA cases by three independent observers (Figs. 1 3) and in 44.4%, 55.6%, and 44.4% of tuberculous arthritis cases, whereas uniform synovial thickening was observed in 27.8%, 13.9%, and 19.4% of RA and in 55.6%, 44.4%, and 55.6% of tuberculous arthritis (Figs. 4 7). Therefore, nonuniform synovial thickening was more frequent in RA than in tuberculous arthritis (p < 0.01). Grade 0 synovial thickening was evident in 55.6%, 44.4%, and 44.4% of tuberculous arthritis cases (Fig. 4) and in 27.8%, 16.7%, and 27.8% of RA. In contrast, grade 3 synovial thickening was noted in 44.4%, 22.2%, and 36.1% of RA cases (Figs. 3 and 8) and in TABLE 3: Kappa Statistics for Evaluation of Interobserver Agreement Feature Evaluated Observers 1 vs 2 Observers 2 vs 3 Observers 3 vs 1 Uneven synovial thickening Degree of synovial thickening Size of bone erosion Rim enhancement of erosion Bone marrow edema Soft-tissue edema Extraarticular cystic mass TABLE 4: Comparison of Rheumatoid Arthritis and Tuberculous Arthritis on MRI by Three Independent Observers Grade Observer 1 Observer 2 Observer 3 RA (%) TB (%) RA (%) TB (%) RA (%) TB (%) Uneven synovial thickening Yes 26 (72.2) 12 (44.4) 31 (86.1) 15 (55.6) 29 (80.6) 12 (44.4) No 10 (27.8) 15 (55.6) 5 (13.9) 12 (44.4) 7 (19.4) 15 (55.6) Degree of synovial thickening 0 10 (27.8) 15 (55.6) 6 (16.7) 12 (44.4) 10 (27.8) 12 (44.4) 1 3 (8.3) 6 (22.2) 7 (19.4) 10 (37.0) 6 (16.7) 4 (14.8) 2 7 (19.4) 4 (14.8) 15 (41.7) 3 (11.1) 7 (19.4) 7 (25.9) 3 16 (44.4) 2 (7.4) 8 (22.2) 2 (7.4) 13 (36.1) 4 (14.8) Size of bone erosion 0 11 (30.6) 2 (7.4) 13 (36.1) 3 (11.1) 12 (33.3) 2 (7.4) 1 4 (11.1) 2 (7.4) 7 (19.4) 1 (3.7) 0 (0) 1 (3.7) 2 9 (25.0) 2 (7.4) 2 (5.6) 7 (25.9) 13 (36.1) 2 (7.4) 3 5 (13.9) 7 (25.9) 6 (16.7) 9 (33.3) 9 (25.0) 10 (37.0) 4 7 (19.4) 14 (51.9) 3 (8.3) 6 (22.2) 2 (5.6) 12 (44.4) Rim enhancement of erosion Yes 5 (13.9) 14 (51.9) 2 (5.6) 18 (66.7) 4 (11.1) 21(77.8) No 31 (86.1) 13 (48.1) 34 (94.4) 9 (33.3) 32 (88.9) 6 (22.2) Bone marrow edema 0 13 (36.1) 6 (22.2) 19 (52.8) 7 (25.9) 16 (44.4) 4 (14.8) 1 6 (16.7) 5 (18.5) 3 (8.3) 2 (7.4) 2 (5.6) 4 (14.8) 2 2 (5.5) 0 (0.0) 5 (13.9) 6 (22.2) 5 (13.9) 4 (14.8) 3 15 (41.7) 16 (59.3) 9 (25.0) 12 (44.4) 13 (36.1) 15 (55.6) Soft-tissue edema 0 13 (36.1) 10 (37.0) 29 (80.6) 16 (59.3) 13 (36.1) 8 (29.6) 1 10 (27.8) 9 (33.3) 3 (8.3) 11 (40.7) 9 (25.0) 7 (25.9) 2 13 (36.1) 8 (29.6) 4 (11.1) 0 (0) 14 (38.9) 12 (44.4) Extraarticular cystic mass Yes 6 (16.7) 21(77.8) 7 (19.4) 19 (70.4) 7 (19.4) 21(77.8) No 30 (83.3) 6 (22.2) 29 (80.6) 8 (29.6) 29 (80.6) 6 (22.2) Note Data are numbers of cases (%). RA = rheumatoid arthritis, TB = tuberculous arthritis. AJR:193, November

4 Choi et al. 7.4%, 7.4%, and 14.8% of tuberculous arthritis. Therefore, the involved synovial membrane in RA generally was thicker than in tuberculous arthritis (p < 0.01). The p value of linear-by-linear association of degree of synovial thickening was less than 0.01; therefore, the thicker the synovial membrane, the more likely the diagnosis of RA. With regard to the presence of bone erosions, bone erosions were observed in 48.1%, 88.9%, and 92.6% of tuberculous arthritis cases (Fig. 4) and in 69.4%, 63.9%, and TABLE 5: Comparison of Rheumatoid Arthritis and Tuberculous Arthritis by Mean Data of Three Sets of Three Observers and Linear-by-Linear Association (Trend) Mean Data a Comparison Result p Linear-by-Linear Association Uneven synovial thickening RA > TB < 0.01 Degree of synovial thickening RA > TB < 0.01 < 0.01 Size of bone erosion RA < TB < 0.01 > 0.05 Rim enhancement of bone erosion RA < TB < 0.01 Degree of bone marrow edema NS Extraarticular cystic mass RA < TB < 0.01 Note Dash ( ) indicates not applicable. NS = not significant. a (Each case of observers ) / 3. Fig year-old woman with rheumatoid arthritis. Fat-saturated, gadolinium-enhanced sagittal T1- weighted MR image of knee shows uneven and thick (grade 2 in all observers) synovium (arrows). Fig year-old girl with rheumatoid arthritis. Fatsaturated axial T1-weighted image of elbow shows uneven and thick (grade 2 in all observers) synovial hypertrophy (arrows). More markedly thickened synovium in olecranon fossa and surrounding radial neck is probably due to synovial redundancy. 66.7% of RA cases. Furthermore, the size of bone erosions was greater in tuberculous arthritis than in RA (p < 0.01). The p value of linear association of the size of the bone erosion was greater than Rim enhancement around bone erosions was more frequent in tuberculous arthritis (51.9%, 66.7%, and 77.8%) (Figs. 4 and 6) than in RA (13.9%, 5.6%, and 11.1%), and the difference was statistically significant (p < 0.01). Grade 3 bone marrow edema was noted in 59.3%, 44.4%, and 55.6% of tuberculous arthritis cases by the three observers (Figs. 5 and 7), and in 41.7%, 25.0%, and 36.1% of RA (Fig. 3). The difference of marrow edema in the two groups was not clinically significant (p > 0.05). Extraarticular cystic masses were more frequent in tuberculous arthritis (Figs. 5 7) than in RA, and the difference was statistically significant (p < 0.01). Fig year-old woman with rheumatoid arthritis. Fat-saturated, gadolinium-enhanced sagittal T1- weighted image of ankle shows uneven and thick (grade 3 in all observers) synovial enhancement (arrow) and extensive bone marrow enhancement (grade 3 in all observers, arrowheads) of calcaneus. A B Fig year-old man with tuberculous arthritis. A and B, Unenhanced (A) and contrast-enhanced (B) axial T1-weighted images of knee show rim enhancement of bone erosions (arrowheads, B) at medial tibial plateau AJR:193, November 2009

5 MRI of Rheumatoid and Tuberculous Arthritis Fig year-old woman with tuberculous arthritis. Fat-saturated, gadolinium-enhanced sagittal T1- weighted image of elbow shows even synovial thickening (grade 1 in two observers and grade 0 in one observer, arrowhead) of elbow joint with bone marrow enhancement (grade 3 in all observers). Cystic mass, possibly resulting from expansion of joint capsule, with rim enhancement is seen at anteroinferior portion of radial neck (arrow). Fig year-old woman with tuberculous arthritis of ankle. Fat-saturated gadoliniumenhanced sagittal T1-weighted image shows even and thin (grade 1) synovial thickening of ankle and intertarsal joint. Subchondral bone marrow shows heterogeneous enhancement (arrowheads, grade 3 in two observers and grade 2 in one observer). Cold abscess at posterior aspect of ankle (arrows) shows even thin rim enhancement and faint softtissue enhancement. Discussion Monoarticular disease is most commonly caused by an infectious disease process but can also be of noninfectious origin, such as metabolic or immunologic. In some cases, it is difficult to differentiate the origin of a monoarticular disease [17]. Differential diagnoses include infectious arthritides, such as pyogenic or tuberculous arthritis, or noninfectious diseases, such as synovial osteochondromatosis, pigmented villonodular synovitis, rheumatoid arthritis, gout, and hemophilic arthropathy, among many others. Appropriate treatment after accurate diagnosis is crucial for good prognosis and preservation of joint function; final pathologic diagnosis usually requires culture of synovial fluid or synovial biopsy or both. RA is a chronic inflammatory arthritis, affecting about 1% of the population worldwide [6], that leads to cartilage and bone destruction and eventually to loss of function. Tuberculous arthritis is second in frequency to spinal involvement by tuberculosis; osteoarticular tuberculosis is estimated to occur in 1.5 3% of patients with musculoskeletal tuberculosis [18]. The incidence of extrapulmonary tuberculosis has been increasing over the past decades because of increasing travel and immigration; an increasing number of elderly, chronically ill, or immunosuppressed patients; and an increasing number of patients with AIDS [17]. Both RA and tuberculous arthritis may have not only similar clinical manifestations, but also similar radiographic findings, such as periarticular osteopenia, marginal erosions with relatively late sparing of the joint space, and increased joint effusion, which may render differential diagnosis difficult in some cases [2, 19]. Grossly, both RA and tuberculous arthritis have chronic inflammatory hypertrophied synovium. However, on microscopic examination, the synovium of RA shows infiltration by plasma cells and lymphocytes with or without lymphoid follicles and fibrin deposits often seen close to the synovial lining or within the stroma [20], whereas the synovium in tuberculous arthritis contains necrotic and fibrinlike material, caseous areas, and collections of leukocytes and mononuclear phagocytes [2]. RA consists of articular manifestations of a disease of immunoregulatory dysfunction that are linked by complex processes. In addition to lymphocytes and plasma cells, many different cells, as Fig year-old woman with tuberculous arthritis and combined tenosynovitis and cold abscess. Fatsaturated gadolinium-enhanced T1-weighted axial scan shows even and thin (grade 0 in three observers) enhancement in synovium of wrist and tendon sheath (arrowheads) associated with rim enhancement of fluid collection (arrows) at volar aspect of radius. Bone marrow (grade 3) and soft-tissue edema (grade 1 in two observers and grade 2 in one observer) adjacent to synovium show heterogeneous enhancement. Fig year-old man with rheumatoid arthritis. Fat-saturated gadolinium-enhanced coronal T1-weighted image of wrist shows ovoid mass formation of synovium (arrowheads) at distal radioulnar joint level. well as their variable products (cytokines), contribute to the disease process. Lymphocytes, plasma cells, and a large number of plump luxuriant cells in the synovium of RA produce bulky, hypervascular, proliferative lesions, almost resembling a tumor, in response to the proliferative factors generated AJR:193, November

6 Choi et al. by the activated immune response. In fact, synovial tissue taken from a rheumatoid knee may weigh up to 500 g or more, in contrast to synovium in a normal knee, which in total usually weighs less than 5 g [9]. In our patients, synovial thickness in RA was variable in the joint space. Prominent thickening, even mass formation of more than 1 cm, was frequently seen at sites of synovial redundancy (Fig. 8) or at sites with more capacious space in a joint in RA (Figs. 1 3 and 8). This may be explained by the fact that sufficient space allows more room for tissue accumulation in joints without an immediate increase in pressure or displacement of other tissue, permitting bulky proliferative growth [9]. In contrast, tuberculous arthritis is characterized by synovial inflammation and formation of granulation tissue that are accompanied by immune responses with no proliferative factor, so thick and uneven or bulky proliferation of synovium and bone erosion, which are seen more frequently in RA, are usually not seen in tuberculous arthritis. Instead, even and relatively thin synovium and rim enhancement are observed in tuberculous arthritis. Bone change of RA occurs after proliferative pannus extension over the cartilage and cartilaginous destruction by enzymatic, phagocytic, and mechanical processes. But in tuberculous arthritis, cartilaginous destruction is associated with phagocytic and vascular processes without involvement of proteolytic enzymes [2, 4]. Bone erosion is seen not only after cartilaginous erosion but also after detachment of cartilage due to insinuation of granulation tissue between cartilage and subchondral bone [2]. Nonperipheral subchondral bone erosion was more frequent in tuberculous arthritis in our study, which may have occurred because of this subchondral extension of pannus (Figs. 4 and 7). Intercondylar widening of the distal femur, which was reported by Bohrer [21] to be due to entering of the synovium from the patella, was not seen in our study; instead, intercondylar erosion at the attachment site of the cruciate ligament was frequently seen, especially more frequently in tuberculous arthritis (Fig. 4) than in RA in our study. This may have been due to pannus of the synovial membrane enveloping the cruciate ligaments attaching to the intercondylar area of the knee; in tuberculous arthritis, the tendency toward subchondral extension of granulation tissue and caseation necrosis may have contributed further to the intercondylar erosion, which may explain the larger size of erosions in our study. Extraarticular cold abscess in tuberculous arthritis may also be attributed to nonenzymatic processes [22, 23]. On MRI, a cold abscess is seen as a cystic mass with even, thin rim enhancement, usually not accompanied by prominent surrounding edema (Figs. 6 and 7). Sometimes the differentiation between abscess in tuberculous arthritis and fluid collections in bursa and other extraarticular cystic lesions in RA, such as Baker s cysts, is difficult [6]. In our study, extraarticular cystic mass was more frequently seen in tuberculous arthritis (Figs. 5 7) than in RA, and cases with more than two cystic masses were observed only in tuberculous arthritis. Both tuberculous arthritis and RA are accompanied by bone marrow edema, which is due to increased vascular flow and increased vascular permeability of bone marrow [24]. In our study, the difference of bone marrow edema in RA and tuberculous arthritis was not clinically significant. Our study has several limitations. Different joints were assessed for the two diseases, and the numbers of joint were not matched for the two diseases for example, a large number of wrists were imaged in RA patients, which may have introduced a bias. However, despite the lack of statistical comparison, comparing grossly just the knees and the ankles, of which the number are approximately closely matched, the differences seem to remain according to the two diseases (Figs. 1, 3, 4, and 6). Because of the retrospective nature of the study, the sensitivity, specificity, and positive and negative predictive values of each finding could not be determined. Also, the use of different MRI parameters and different field-strength scanners is another limitation of this study. However, from the results of our study, we may conclude that uneven and thick synovial proliferation was more frequently seen in RA, whereas, even and thin synovium, large bone erosions, rim enhancement around bone erosion, and extraarticular cystic masses were more frequently seen in tuberculous arthritis. Although findings evaluated in this study show some degree of overlap in both RA and tuberculous arthritis, evaluation with MRI may be helpful in differentiation between RA and tuberculous arthritis. References 1. Resnick D. Rheumatoid arthritis. In: Resnick D, ed. Diagnosis of bone and joint disorders, 4th ed. Philadelphia, PA: Saunders, 2002: Resnick D. Osteomyelitis, septic arthritis, and soft tissue infection: organisms. In: Resnick D, ed. Diagnosis of bone and joint disorders, 3rd ed. Philadelphia, PA: Saunders, 1995: Schuchmann L, Pernice W, Hufschmidt C, Adler CP. Tuberculous arthritis: a rare, but important differential diagnosis in juvenile chronic arthritis [in German]. 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