CONVENTIONAL NUCLEAR MEDICINE
Conventional nuclear medicine techniques are divided basically into two-dimensional planar scans and three-dimensional single photon emission computed tomography (SPECT), which permits reconstruction of images in sagittal, coronal and axial planes[1-3]. SPECT images allow improved localization of the site of uptake (e.g., for differentiating involvement of the facets or pedicle of a vertebra), and increases sensitivity and specificity[1-3]. Hybrid SPECT/computed tomography (CT) imaging, which allows morphological and functional data to be acquired and fused, increases even more the diagnostic accuracy of Nuclear Medicine studies because it provides anatomical localization of SPECT findings.
Different radionuclides, including Technetium-99m (99mTc), Gallium-67 (67Ga), Indium-111 (111In) and Iodine-123 (123I) have been used in studies for RA and will be reviewed in the following sections.
Amongst the different radionuclides available, 99mTc is presently the most commonly used[3,5]. For the evaluation of bone diseases, there are different radiopharmaceuticals available including 99mTc labeled hydroxy methylene diphosphonate (HDP), dicarboxy propane diphosphonate (DPD) and methylene-diphosphonate (MDP), with the latter being the most commonly used[1-3]. After intravenous injection, 99mTc-MDP circulates in the vascular system, then equilibrates to the extravascular space and, subsequently, accumulates in the bone. These three phases may be evaluated in a bone scintigraphy, which has high sensitivity but low specificity. In many cases, distinction between degenerative, inflammatory and metastatic bone processes may be difficult[1-3,6]. In RA, bone scintigraphy has a certain degree of usefulness and may allow identification of arthritic joints[7,8]. However, planar scintigraphy and SPECT have the limited spatial resolution in comparison to radiography and magnetic resonance imaging (MRI)[3,5].
Bachkaus et al performed a prospective study comparing clinical evaluation, conventional radiography, ultrasound, three-phase 99mTc-MDP bone scintigraphy and MRI in 60 patients with different types of arthritis including RA, arthritis related to connective tissue disease and spondylarthropathy. They found that clinical assessment, scintigraphy, ultrasound and MRI were more sensitive than radiography in identifying inflammatory processes and destructive joint lesions. However, scintigraphy had limited specificity.
Recently, in an attempt to improve the specificity of SPECT images, Ostendorf et al studied the application of a multipinhole SPECT (MPH-SPECT), originally created for small animal imaging. Six human subjects were studied after injection of 99mTc-DPD: 3 with established RA, 1 with early RA, 1 with osteoarthritis (OA) and 1 healthy volunteer. The authors reported better identification of anatomic landmarks with MPH-SPECT in contrast to planar scintigraphies, but comparison with other methods such as MRI was limited.
In a second study by the same group, the clinically dominant hands of 13 subjects with initial RA, nine with initial OA and five control subjects were evaluated by MPH-SPECT and skeletal scintigraphy. MRI was carried out in RA subjects, and these images were later fused with MPH-SPECT. Bone scintigraphy identified 26 articulations with augmented uptake while MPH-SPECT detected 80 joints. MPH-SPECT indicated a central tracer uptake in RA (10 out of 13 patients) and an eccentric pattern in OA (7 out of 9 patients). Uptake in MPH-SPECT matched areas of marrow edema and destruction in MRI in 11 out of 13 patients.
Buchbender et al compared 3 tesla MRI with 99mTc-DPD scintigraphies using MPH-SPECT in 10 early RA patients. Visual and region of interest (ROI) analyses of MPH-SPECT images were carried out. The authors reported that MPH-SPECT detected higher rates of inflammatory bone involvement compared to MRI.
The accumulation of 67Ga-citrate into inflammatory is complex and involves different mechanisms. It binds to transferrin and suffers extravasation in areas of inflammation where vascular permeability is increased[12,13]. Moreover, 67Ga suffers cross-chelation to lactoferrin, a protein released which is taken up by macrophages and also binds to siderophores, low-molecular-weight products of bacteria[12,13].
Even though 67Ga-citrate scintigraphy has good sensitivity detection of inflammation and has been used in the evaluation of RA[3,13-15], there are numerous drawbacks with this technique. 67Ga scintigraphy leads to relatively elevated radiation burden because of its physical half-life and high-energy gamma radiation (91-393 keV). It also has elevated background activity and slower imaging times. Additionally, it cannot precisely differentiate inflammation from infection or neoplasias.
99mTc and 111In-labeled leukocytes
Leukocytes may be labeled with 99mTc or 111In-oxine for detection of inflammatory and infectious diseases[3,17-20]. Al-Janabi et al labeled leukocytes with 99mTc in subjects with RA and found a 50%-80% decrease in leukocyte uptake after local steroid injection into eight out of nine painful knees, which showed clinical response. Gaál et al performed 99mTc-hexamethylpropylene amine oxime (99mTc-HMPAO) labeled leukocyte scintigraphy in 21 patients with RA. A significant association was seen between the uptake in hands and feet and clinical evaluation. Thurlings et al performed two scintigraphies after injection of 99mTc-HMPAO labeled monocytes in eight RA patients, with a two-week interval. Arthroscopic biopsies were performed one day after the second scintigraphy and synovial macrophage infiltration was evaluated by immunohistochemical staining. The number of scintigraphically positive joints was significantly associated with the number of activated macrophages in the synovium.
Appelboom et al investigated the use of 99mTc labeled ciprofloxacin (Infecton scintigraphy) in 106 patients, 17 of them with RA. Subjects received an intravenous injection of 99mTc-ciprofloxacin and whole body scans were acquired after 4 h. Augmented uptake was seen in 12 patients with RA. Association between clinically inflamed joints and articular 99mTc-ciprofloxacin uptake was observed. The authors concluded that the radiotracer was not specific for infection and could potentially identify the presence of inflammation in joints and monitor their response to treatment.
99mTc-labeled human immunoglobulin G
Labeling polyclonal human immunoglobulin G (HIG) with 99mTc allows evaluation of inflammation and infection. Different groups have suggested that these exams may have higher sensitivity than clinical assessment, bone scintigraphy and labeled leukocyte scintigraphy[25,26]. However, similar to radiotracers like 67Ga, the exam has limited specificity.
99mTc and 111In-anti-E-selectin
Chapman et al evaluated the biodistribution of 111In-labeled anti-E-selectin monoclonal antibodies in 14 subjects with RA and compared it with 111In-labeled polyclonal HIG in 6 of these patients. 111In-anti-E-selectin resulted in better sensitivity and image intensity and more focal localization in synovium.
The same group published another study where they used 111In-anti-E-selectin and 99mTc-labeled polyclonal HIG in 11 patients with RA. Scintigraphic images were compared with clinical scores. The authors reported that 111In-anti-E-selectin had greater sensitivity and specificity than 99mTc-HIG. However, the necessity of performing 24 h images with 111In-anti-E-selectin led to the development of a 99mTc-labeled tracer. In this study, the authors performed scintigraphies 4 h and 20-24 h after 111In- or 99mTc-anti-E-selectin injection in a group of 10 patients with RA. They concluded that they led to similar diagnostic accuracy, what favored the use of the 99mTc-labeled tracer. In another group of 16 RA patients, 99mTc-anti-E-selectin was compared with 99mTc-HDP 4h after injection. Although 99mTc-anti-E-selectin seemed to have in vivo instability, as indicated by thyroidal and intestinal uptake, 99mTc-anti-E-selectin was better than 99mTc-HDP (88% vs 57%) in terms of accuracy. Inactive or normal joints didn’t show uptake of 99mTc-anti-E-selectin.
Vanhagen et al studied the articulations of 14 subjects with ongoing RA, 4 with intense OA, and 30 controls. The somatostatin analog 125I-Tyr3-octreotide was used for in vitro somatostatin receptor autoradiography and the somatostatin analog 111In-DTPA-D-Phe1-octreotide was used for scintigraphy. A total of 76% of tender and of augmented joints of the subjects with RA were identified by nuclear medicine scans. The authors found that joint uptake was associated with the amount of pain and swelling. In vitro autoradiography of the synovial membranes indicated somatostatin receptors in 2 of the RA patients. In subjects with OA, joint uptake was considerably poorer than in subjects with RA, while the ones of control subjects didn’t exhibit uptake.
Marcus et al studied the biodistribution of a 99mTc-labeled murine monoclonal antibody (Muromonab, Orthoclone OKT3®), specific for T lymphocyte glycoprotein CD3 receptor. Seven patients with RA and two with psoriatic arthritis were included. Scintigraphies of the whole-body and of the articulations were carried out. All joints with intermediate to intense pain showed intermediate to high uptake, while all asymptomatic joints and joints with mild or minimal pain had normal images. Of note, two patients had side effects (shaking chills and neck pain) after 99mTc-OKT-3 injection.
Our group of research developed another technique for labeling OKT3 with 99mTc and also investigated its use to evaluate disease activity in subjects with RA. A total of 38 patients with RA functional classes II and III according to American College of Rheumatology criteria were evaluated. Planar anterior scans of the patients’ metacarpophalangeal and interphalangeal joints, shoulders, elbows, wrists and knees were carried out 1 h and 3 h after the infusion of 99mTc-OKT3. Significant association (P < 0.05) was found between the 99mTc-OKT3 uptake and swollen or tender joints and the visual analogue scale. It was possible to distinguish subjects in remission from subjects with active synovitis. On the other hand, no association was seen between 99mTc-OKT3 uptake and the patients’ duration of disease, gender and age or erythrocyte sedimentation rate.
In a continuation of the previous report, we have studied 1232 joints from 44 patients with RA were evaluated 1 h and 3 h after injection of anti-CD3 antibody labeled with 99mTc and compared with another 812 joints from 33 patients with juvenile idiopathic arthritis (JIA), OA or gouty arthritis (GA). RA and JIA showed high uptake at the first scan, which augmented after 3 h. In OA, uptake was minimal or absent. Therefore, it was possible to distinguish RA and JIA from OA and GA. However, it was not possible to distinguish subjects with RA in remission from those with OA.
Becket et al performed three-phase bone scans with 99mTc-HDP and scintigraphies with an anti-CD4 antibody named MAX.16H5 labeled with 99mTc. Six patients with RA were included prospectively and five of them received 99mTc-anti-CD4 scans after 1.5 h, 4 h and 24 h. In all patients, affected joints could be distinctively imaged at as early as 1.5 h. The authors reported that uptake in affected joints was associated with clinical signs and early 99mTc-MDP weakly uptake. However, it was not clear if late uptake of the radiotracer differed from control immunoglobulins.
To evaluate this aspect, the same group later included eight patients with severe, active RA to perform scintigraphies with 99mTc-labeled anti-CD4 or polyclonal HIG, with five of them receiving both radiotracers. Scintigraphies of the whole-body and of the joints were carried out after 1, 4 and 24 h. The authors found that 99mTc-anti-CD4 had higher target-to-background ratio in knee and elbow joints, suggesting higher specificity than 99mTc-HIG.
Malviya et al labeled Rituximab, an anti-CD20 antibody (MabThera®), with 99mTc in 20 patients with chronic inflammatory diseases and acquired scintigraphies after 6 h and 20 h. Five of the patients had RA and presented uptake of the radiotracer in known lesioned joints. Nonetheless, such uptake was variable and not all patients showed uptake in each clinically positive joint.
99mTc-anti-tumor necrosis factor-alpha
Chianelli et al labeled Infliximab (Remicade®), a chimeric mouse/human anti-tumor necrosis factor alpha (anti-TNF-alpha) antibody, with 99mTc and included seven RA patients eligible to receive intra-articular Infliximab therapy for scintigraphic evaluation previously and 3 mo following the therapy. Planar scans of the joints were carried out 3, 6 and 24 h after intravenous infusion of 99mTc-Infliximab. Post-treatment scans indicated that the uptake disappeared in 1 joint, was reduced considerably in 2, was faintly in 4 and remained unchanged in 2. The authors suggested 99mTc-Infliximab could potentially aid in the choice of those subjects who would profit most from treatment with unlabeled Infliximab and provide a more objective assessment of immunotherapy efficacy.
A study from our group of research compared whole body and hand/wrist scintigraphies after injection of 99mTc-anti-TNF-α with clinical examination and MRI of wrists joints and hands in subjects with active RA. Eight subjects with active RA and one healthy volunteer were included. With MRI considered as the gold standard, the sensitivity and specificity of scintigraphy was 89.9% and 97.3%, respectively, while pain and edema had sensitivity of 65.3% and 59.2% and specificity of 75.2% and 95.3%, respectively.
123I-IL-1 receptor antagonists
Barrera et al studied the biodistribution of 123I labeled interleukin-1 receptor antagonist (IL-1ra) in four subjects with RA. A comparison of scintigraphies acquired with 123I-IL-1ra and those acquired with a non-specific radiopharmaceutical was made. Although the authors found that labelled IL-1ra allowed the identification of synovial disease in subjects with RA this process did not seem to occur by specific binding.
POSITRON EMISSION TOMOGRAPHY
The radionuclides that have been used for Positron Emission Tomography (PET) include fluorine-18 (18F), carbon-11 (11C) and iodine-124 (124I). PET has two to three times higher spatial resolution than SPECT and permits quantification of standardized uptake value (SUV)[40-42]. In the following sections the studies that used PET for RA monitoring are reviewed.
2-deoxy-2-(18F) fluoro-D-glucose (18F-FDG) allows evaluation of tissue metabolism. 18F-FDG accumulation in inflammatory and infectious diseases is based on its increased uptake by polymorphonuclear leukocytes, which adopt glucose after becoming activated. The transportation of 18F-FDG is intermediated by glucose transporters (GLUT), which are also to a higher amount present on the cell membrane of inflammatory and infectious cells. RA is an autoimmune disease, which is associated with systemic and chronic inflammation of the joints, resulting in synovitis and pannus formation, both leading to increased 18F-FDG uptake.
Polisson et al published a seminal report where 18F-FDG PET and MRI were carried out in 2 RA patients with active synovitis in the carpus at baseline and after 14 wk of treatment. In comparison with baseline, there was marked improvement in clinical parameters and decrease in synovial volume measured by MRI and 18F-FDG uptake measured by PET.
The same group published later another study where 18F-FDG PET and gadolinium-enhanced MRI of the wrist were carried out prospectively in 12 subjects under anti-inflammatory treatment in different moments: without drugs for 2 wk and after 2 and 12 wk of treatment. They found that MRI and 18F-FDG PET were strongly correlated with clinical findings in wrists, and concluded that these techniques permitted quantification of alterations in joint inflammation. In addition to these reports, other articles have indicated the capability of 18F-FDG PET to identify alterations in disease activity, but few have shown it can foretell clinical results[45,46].
Nonetheless, one of the most important breakthroughs in the field of Nuclear Medicine has been the advent of PET/CT hybrid imaging, which allows concomitant acquisition of morphologic and functional information, increasing both sensitivity and specificity of findings. Initial case studies suggested that 18F-FDG PET/CT correctly identifies articular and extra-articular inflammatory areas[47-49]. Kubota et al performed 18F-FDG PET/CT in 18 subjects with RA and evaluated uptake in the atlanto-axial, shoulder, elbow, wrist, carpal, knee and hip joints and in axillary lymph nodes. The total uptake score for all joints was significantly associated with C-reactive protein level. Furthermore, 18F-FDG uptake score of painful/swollen joints were greater than not painful/swollen joints and significantly distinct between subjects in remission and those with active inflammation. Roivainen et al studied 17 subjects with active RA that started to receive disease-modifying antirheumatic drugs. Disease activity was clinically evaluated at screening, at baseline and after 2, 4, 8 and 12 wk of therapy, while 18F-FDG PET/CT of all joints was carried out at baseline and after 2 and 4 wk of therapy. 18F-FDG maximum SUV decreased in 76% and 81% at weeks 2 and 4 in comparison to baseline. The percentage of decline in 18F-FDG activity was associated with disease activity at week 12 and with variations in C-reactive protein levels and erythrocyte sedimentation rate.
More recently, fusion of PET and MRI has been developed. Chaudhari et al performed an extremity 18F-FDG PET/CT immediately after MRI at baseline and 5 wk after TNF-alpha inhibitor therapy in a 57-year-old female with RA. CT was later used for PET/MRI fusion. The authors reported that PET uptake decreased significantly in the synovium and at sites of erosions and clinical exam at 3 mo corroborated a positive response to therapy. Then, Miese et al reported on the first hybrid hand PET/MRI in intial RA, demonstrating augmented 18F-FDG uptake occurred in synovitis.
Roivainen et al included 10 subjects with inflammatory disorders of the joints, two of them with RA, in a study that compared 11C-choline and 18F-FDG PET with contrast-enhanced MRI. The authors found that the uptake of 18F-FDG as well as 11C-choline had good correlation with synovial volume measured in MRI and suggested 11C-choline could be a promising radiotracer for quantitative assessment of disease activity.
11C-(R)-PK11195 is a radiotracer that suffers macrophage binding. Van der Laken et al studied the knees of 11 RA patients using 11C-(R)-PK11195 PET imaging and arthoscopic assessment of the knee with greatest inflammation in all subjects. The authors found that 11C-(R)-PK11195 had significantly increased uptake in inflamed joints. Moreover, uptake in non-inflamed knees of RA subjects was considerably greater than in the knees of controls, indicating the existence of subclinical RA activity.
Tran et al included six patients in a study to evaluate the distribution of 124I labeled Rituximab. One patient was excluded due to adverse effects after injection of the unlabeled drug. Whole body PET/CT was carried out in 5 subjects at 10 min, 24 h, 48 h and 72-96 h. Evaluation was carried out based on visual analyses and correlated with disease activity. Accumulation in joints occurred only after 24 h, in 4 out of 5 patients. The authors reported that several exams had uptake in clinically normal joints while a few joints with clinical arthritis had no uptake, but no quantification or comparison with other imaging methods was performed.