Psoriatic arthritis (PsA) is an inflammatory joint disease characterized by arthritis and often enthesitis in patients with psoriasis, presenting a wide range of manifestations in various patterns. Imaging procedures are primarily conventional radiography, ultrasonography (US), and magnetic resonance imaging (MRI); other modalities such as computed tomography are not used routinely. Imaging is an integral part of management of PsA. In this article, we provide an overview of the status, virtues, and limitations of imaging modalities in PsA, focusing on radiography, US, and MRI.
Key points
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Radiography, ultrasonography (US), and MRI are integral parts of psoriatic arthritis (PsA) management, and are increasingly used in clinical trials.
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Computed tomography is the gold standard for evaluating bone changes in arthritis, but is rarely used owing to ionizing radiation.
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Conventional radiography allows fast, feasible, and relatively inexpensive assessment of the cumulative skeletal damage in PsA.
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US can visualize the peripheral joint and entheses involved in PsA in high resolution and can guide invasive procedures.
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MRI allows detailed visualization of all structures involved in PsA, and is sensitive for peripheral and axial disease manifestations.
Introduction
Psoriatic arthritis (PsA) is a chronic inflammatory joint disease associated with the skin disease psoriasis. It is characterized by arthritis, enthesitis, and/or dactylitis; cutaneous involvement may be subtle. PsA was first described as a distinct rheumatic disease in the 1950s and subsequently in the 1970s as part of the concept of spondyloarthropathy (SpA). Until the 1950s, an inflammatory arthritis occurring in the presence of psoriasis was believed to represent rheumatoid arthritis (RA) occurring coincidentally with psoriasis. However, PsA is usually seronegative for rheumatoid factor, and today, acknowledging that diagnosing PsA can be difficult owing to its varying presentation, suspicion should be raised in a patient with psoriasis and signs of arthritis, for example, RA-like arthritis, asymmetrical arthritis, involvement of the distal interphalangeal (DIP) joints or inflammatory back pain in the absence of rheumatoid factor. Occasionally, arthritis may precede the development of psoriasis. Main differential diagnoses include RA, SpA including ankylosing spondylitis (AS), osteoarthritis (OA), gout, and fibromyalgia. A range of imaging methods can be used in suspected or established PsA and provide important information on the disease process.
Different imaging procedures are used in PsA, primarily conventional radiography, ultrasonography (US) and MRI, all having different virtues and limitations. Radiography is the mainstay in imaging in inflammatory joint diseases, but is not able to detect the inflammatory changes that are the earliest disease manifestations. The reference method for assessing structural damage is computed tomography (CT), but it is rarely used in arthritides owing to ionizing radiation. US and MRI allow direct visualization of both early inflammatory and destructive joint changes. Other imaging modalities, such as scintigraphy, PET, single photon emission CT, and dual-emission x-ray absorptiometry, are available, but their roles in the diagnosis and management of PsA are limited, and they are not described further. Research into improved techniques and novel imaging methods such as whole-body MRI, various nuclear medicine techniques, and optical imaging are exciting future options.
Introduction
Psoriatic arthritis (PsA) is a chronic inflammatory joint disease associated with the skin disease psoriasis. It is characterized by arthritis, enthesitis, and/or dactylitis; cutaneous involvement may be subtle. PsA was first described as a distinct rheumatic disease in the 1950s and subsequently in the 1970s as part of the concept of spondyloarthropathy (SpA). Until the 1950s, an inflammatory arthritis occurring in the presence of psoriasis was believed to represent rheumatoid arthritis (RA) occurring coincidentally with psoriasis. However, PsA is usually seronegative for rheumatoid factor, and today, acknowledging that diagnosing PsA can be difficult owing to its varying presentation, suspicion should be raised in a patient with psoriasis and signs of arthritis, for example, RA-like arthritis, asymmetrical arthritis, involvement of the distal interphalangeal (DIP) joints or inflammatory back pain in the absence of rheumatoid factor. Occasionally, arthritis may precede the development of psoriasis. Main differential diagnoses include RA, SpA including ankylosing spondylitis (AS), osteoarthritis (OA), gout, and fibromyalgia. A range of imaging methods can be used in suspected or established PsA and provide important information on the disease process.
Different imaging procedures are used in PsA, primarily conventional radiography, ultrasonography (US) and MRI, all having different virtues and limitations. Radiography is the mainstay in imaging in inflammatory joint diseases, but is not able to detect the inflammatory changes that are the earliest disease manifestations. The reference method for assessing structural damage is computed tomography (CT), but it is rarely used in arthritides owing to ionizing radiation. US and MRI allow direct visualization of both early inflammatory and destructive joint changes. Other imaging modalities, such as scintigraphy, PET, single photon emission CT, and dual-emission x-ray absorptiometry, are available, but their roles in the diagnosis and management of PsA are limited, and they are not described further. Research into improved techniques and novel imaging methods such as whole-body MRI, various nuclear medicine techniques, and optical imaging are exciting future options.
Conventional radiography
Conventional radiography is the most widely used imaging method in PsA, and provides a record of the cumulative joint damage. It has a very high spatial resolution, and shows the skeletal structure well, whereas visualization of soft tissue is rarely of value on top of clinical examination. Radiography is a fast, feasible, reliable, and relatively inexpensive procedure allowing assessment of a large number of joints. However, radiography is a 2-dimensional visualization of a 3-dimensional anatomy, resulting in suboptimal delineation of many parts of bone, owing to projectional superimposition, particularly in anatomically complex joints. Radiography of peripheral joints requires a small dose of ionizing radiation. For evaluating the axial skeleton the dose is greater, and this concern contributes to favoring MRI, particularly in younger patients. Except that radiography has been digitalized over the last decades, no major technical advances have appeared in many years.
Peripheral Psoriatic Arthritis
Although PsA shows similarities with RA there are major differences, for example, in the type and site of lesions as well as the joints involved. Although RA is characterized by mainly osteodestructive lesions, PsA involves both osteodestructive and osteoproliferative manifestations ( Fig. 1 ). In particular, juxtaarticular new bone formation, appearing as ill-defined ossification near joint margins on radiography of the hand or foot, are characteristic, and this finding is an important part of the CASPAR classification criteria for PsA. The presence of joint damage on radiography has been shown to be an independent predictor of radiographic progression. Structural damage can be detected and progression followed by radiography, but radiography is less sensitive to erosive damage than MRI. In PsA trials, radiographic joint damage is an established, important outcome measure of structural progression, and several radiographic scoring methods have been developed, most often as modifications of RA methods. The Sharp–van der Heijde modified scoring method for PsA is the most frequently used method in PsA trials, and scores bone erosion and joint space narrowing (indirect measure of loss of cartilage) in hands and feet, although bone proliferation is not assessed. In contrast, the Ratingen method for PsA scores bone proliferation, in addition to scoring bone erosion.
Axial Psoriatic Arthritis
For axial disease, no specific scoring systems for PsA exist, but scoring systems for AS can be applied. For the sacroiliac joints (SIJ), the radiographic part of the modified New York 1984 criteria for AS may be applied to diagnose radiographic sacroiliitis, bearing in mind that MRI is more sensitive, whereas for the spine, the modified Stoke AS spine score is the generally preferred and most sensitive radiographic method for registration of structural damage progression in AS.
A clear consensus on scoring method does not exist currently, but defining the optimal use of radiographic scores is on the research agenda of the European League Against Rheumatism (EULAR). Despite its limitations, radiography of peripheral joints should generally be performed routinely when there is suspicion of PsA, because it may be of use in the differential diagnosis and provide a basis before more modern imaging, as US and/or MRI is performed. In case of suspicion of axial disease, the recent EULAR recommendations for the use of imaging in SpA recommends initial radiography of the SIJ, unless there is a short disease duration and/or young age, in which case MRI may be performed initially. In the future, radiography will probably continue to play a central role in PsA, as a fast, easily accessible, and relatively inexpensive imaging method.
Computed tomography
CT visualizes calcified tissue with high resolution and can be considered the standard reference for assessing structural damage in inflammatory arthritides, owing to its excellent depiction of bone structures ( Fig. 2 ). By using multidetector CT, 2-dimensional images can be viewed in any given plane. However, CT is unable to detect active inflammatory lesions, and involves ionizing radiation.
Peripheral Psoriatic Arthritis
Few scientific studies using CT in arthritides were available until fairly recently. In peripheral joints, high-resolution CT and micro-CT have been applied in research for detailed examination of bone damages in hands with PsA. Studies using micro-CT scans revealed Ω-shaped erosions in the metacarpophalangeal joints of PsA patients compared with RA, where the U-shaped erosions were noted. Also, this technique documented new bone formation at entheses in both psoriasis patients without arthritis and PsA patients, a pattern that differed from that observed in OA. However, in clinical practice CT has no routine role in examination of peripheral small joints in PsA.
Axial Psoriatic Arthritis
In the SIJ, CT depicts bone erosion, sclerosis, and joint space alterations, including ankylosis, very well but has minimal role in clinical practice because of the ionizing radiation provided and because MRI has been shown to be nearly as good in detecting SIJ bone changes. In the spine, CT is not used routine, but is helpful if radiography is negative in a patient with PsA with suspected vertebral fracture. Overall, CT is rarely used in PsA, but is an option for both axial and peripheral disease if radiography is inconclusive and MRI is unavailable or contraindicated.
Ultrasonography
US allows for high-resolution visualization of all structures involved in peripheral arthritis, and is sensitive for peripheral but not axial disease manifestations. Most attention has so far been given to the SpA group as a whole, because this patient group shows similar features in peripheral arthritis; however, in recent years studies have also been published solely on PsA patients. In the following, the US technique and findings in PsA are reviewed and the potential prognostic and diagnostic value and the use for monitoring disease activity are addressed. Finally, new potential techniques are discussed.
Ultrasound Technique and Findings
US is suitable for the evaluation of structural changes as well as changes in perfusion in joints, tendons and other soft tissues. It can easily be performed by trained rheumatologists in relation to the clinical examination, and allows guidance of invasive procedures. US examinations are performed using B-mode US, most often in combination with color or power Doppler US. In Doppler US, color information is superimposed on the grayscale image displaying the reflection of the US by the moving erythrocytes thereby providing information about perfusion and tissue vascularization. The Doppler choice depends on the most sensitive modality, which may vary from model to model. In situations where the Doppler modality is less sensitive, US contrast agents in the form of microbubbles may be used to enhance the scattering reflection of erythrocytes by amplifying the Doppler signal, thereby increasing the sensitivity of the Doppler examination to low velocity flow. US has limited access in some areas and for some pathologies because it cannot penetrate bone. This limitation results in lower sensitivity than MRI and CT for diagnosing bone erosions and an inability to diagnose osteitis. The main disadvantages of US versus MRI are that it is operator dependent and, for optimal information, there is a need for a trained investigator. Especially for Doppler US, there is also intermachine variability.
Ultrasonography for Diagnosing Peripheral Involvement
In peripheral PsA, US is able to detect both joint involvement (synovitis and erosions) and extraarticular involvement like bursitis, tenosynovitis, and enthesitis. US seems to be more sensitive than clinical examination for the detection of synovitis, tenosynovitis, and enthesitis in patients with PsA. In 2005, consensus definitions for US-related pathologies independent of disease were published, including definitions for synovitis, erosions, tenosynovitis, and enthesopathy, enthesopathy was further elaborated in 2014, when The Outcome Measures in Rheumatology (OMERACT) US group published consensus-based definitions for US elementary lesions for enthesitis.
Synovitis
The appearance of peripheral joint involvement in PsA is nonspecific and has not received as much attention as entheseal changes ; the diagnostic value is solely in detecting joint inflammation. The occurrence of synovitis in proximal interphalangeal and metacarpophalangeal joints in PsA patients compared with RA patients are less frequent, although there is a predominance of synovitis in DIP joints in PsA. This may not be the case in early PsA, where DIP involvement is found in fewer than 50% of patients investigated. US can also detect subclinical synovitis, which is very common in early PsA, most frequently involving the wrist, knee, and/or metatarsophalangeal joints. Similar findings have been found in psoriasis patients with joint symptoms, where up to 50% of clinically inactive joints had positive US findings for synovitis with or without Doppler activity.
Enthesitis
Enthesitis is a very important feature in PsA, and has received most attention in US studies. The consensus-based definitions from 2014 for US enthesitis independent of SpA type was developed to ensure a greater degree of homogeneity in future clinical studies. The elementary lesions of enthesitis included presence of enthesophytes, calcifications, and erosions at the insertion site (chronic changes) and increased thickness, hypoechogenicity, and Doppler activity in the enthesis (inflammatory changes; Fig. 3 ). The diagnostic value of Doppler findings at the enthesis for SpA and PsA is not elucidated fully. Enthesopathy changes such as enthesophytes and bone erosions may be found in weight-bearing entheses owing to mechanical stress. Therefore, the focus for diagnosing SpA has mainly been on the inflammatory components. Doppler activity in grayscale enthesitis changes has been reported to be frequent in patients with SpA, and also specifically in PsA, as compared with patients with mechanical low back pain and healthy controls, but Doppler findings in entheses may also be seen in RA patients, although with less severity and frequency. Subclinical enthesitis is also present in PsA, and US may aid in the classification of the patients as having oligoarthritis or multiple joint involvement.
Tenosynovitis and dactylitis
Some US studies have indicated that US flexor tenosynovitis is the major contributor to clinical dactylitis ; however, there is increasing evidence that other components, such as synovitis and diffuse soft tissue changes, may play a role ( Fig. 4 ). Further studies are needed to define properly the US features of dactylitis.
The Value of Ultrasound for Prediction of Development of Psoriatic Arthritis
The role of US for prognosticating PsA is not established. Entheseal involvement in patients with psoriasis, but without clinical PsA, suggests that enthesitis may be a predictor of development of PsA. Enthesopathy seems to be associated especially with nail involvement, and this may indicate that nail disease is linked to the expression of enthesitis, including subclinical disease. In a cohort of patients with suspected SpA, Doppler activity in at least 1 enthesis provided good predictive value for later diagnosing the patient with SpA.
Monitoring Psoriatic Arthritis by Ultrasonography
Most studies, which aim to monitor treatment response, have applied semiquantitative scoring systems for grayscale and/or Doppler changes. The scoring systems used for joint involvement are either those used in monitoring RA treatment, or semiquantitative scores developed by the respective authors. Disease-modifying antirheumatic drug–resistant knee arthritis in both RA and PsA has responded well to treatment with tumor necrosis factor-α blocker, with a significant decrease in both grayscale and Doppler semiquantitative scores after 12 months. There was no difference in treatment response between RA and PsA patients. For monitoring entheseal involvement in PsA, different combinations of entheses and elementary lesions have been suggested but no consensus exists on a single scoring system. The first to propose a grayscale scoring system of lower limb enthesitis were Balint and colleagues, and since then several other scoring systems have been proposed that also includes Doppler US. More work is needed to develop standardized and responsive US outcome measures in PsA.
Ultrasound for Diagnosing and Monitoring Axial Involvement
The use of US for axial involvement in PsA is limited, primarily because of its inability to penetrate bone. Therefore, US cannot detect osteitis changes, but it may be used to identify the SIJ. It is, however, a deep joint and US can only visualize the superficial, posterior part of the joint. Visualization may be hampered further in obese patients. Although it has been reported that contrast-enhanced Doppler US had a high negative predictive value for sacroiliitis it has no role in diagnosing SIJ involvement in PsA patients but may be used for guiding SIJ injections.
New Techniques in Ultrasonography
Recently, there has been a considerable technological development with high-frequency probes and high-frequency Doppler, which have made it possible to improve the resolution of the grayscale image and the sensitivity to low velocity flow. Some of the new probes allow elastosonography or 3-dimensional US. Elastosonography is a new technique for assessing tissue elasticity, and has a potential application in evaluation of tendon stiffness. Three-dimensional US is a new US modality and seems to be promising in the assessment of joint pathology in inflammatory diseases. One of the advantages may be related to the operator independence owing to image acquisition of infinite 3-dimensional datasets obtained by automated transducer sweeping. Three-dimensional US has been shown to demonstrate enthesitis pathology well, and seems to improve interobserver reliability in the assessment of synovitis and bone erosions. Further studies are needed to validate this technique in PsA. Finally, image fusion allows simultaneous comparison and mapping of US images onto other preacquired images, for example, MRI.
MRI
Conventional MRI allows high-resolution visualization of all structures involved in arthritis, and is sensitive for peripheral and axial disease manifestations. However, MRI in PsA has received less attention than in RA and most knowledge is derived from studies of broader groups of SpA patients, including a limited number of patients with PsA.
MRI Technique and Findings
For visualizing inflammation and structural damage, T1-weighted sequences (signal mainly reflecting fat content and presence of gadolinium contrast) in 1 (axial joints) or 2 (peripheral joints) planes, supplemented by a T2-weighted, fat-suppressed or short tau inversion recovery sequence (signal mainly reflecting water content) in 2 planes are generally performed. Additional acquisition of T1-weighted sequences after intravenous injection of gadolinium-containing contrast agent can aid identification of inflamed tissue in peripheral joints, and can be done with or without fat suppression. A general agreement on which joints to examine with MRI to assess PsA activity and damage has not been established.
The main inflammatory and structural lesions visualized on MRI in peripheral PsA are synovitis, enthesitis, tenosynovitis, periarticular inflammation, bone marrow edema, bone erosion, and bone proliferation. Consensus MRI definitions and suggestions concerning appropriate MRI sequences for use in hands with PsA were published in 2009 by the OMERACT MRI Arthritis Working Group. Administration of a contrast agent is optimal for assessment of synovitis and tenosynovitis in peripheral joints, but can be omitted if the aim is to detect bone marrow edema, bone erosions, and bone proliferations.
The main inflammatory and structural lesions visualized on MRI in axial PsA are bone marrow edema/osteitis, enthesitis, fat infiltration, bone erosion, bone proliferation, and ankylosis. No PsA-specific MRI definitions are published for axial disease, but findings and image sequences used in SpA, are available. Axial enthesitis on MRI lacks a widely accepted definition in PsA and SpA; however, it may be characterized as peripheral enthesitis or as a high signal on short tau inversion recovery sequences in the marrow at the entheseal insertion (or at surrounding tissue).
MRI for Investigating Pathogenesis in Psoriatic Arthritis
Enthesitis is, in addition to the soft tissue changes also seen on US, found to be associated with perientheseal bone marrow edema. Imaging studies have increased the understanding of PsA, and for example, McGonagle and colleagues have proposed that synovitis is a secondary feature to enthesitis. Early MRI studies of PsA patients with dactylitic fingers have concluded that dactylitis is mainly owing to flexor tenosynovitis, but a recent systematic review conclude that it may be caused by inflammation of multiple structures in the fingers and toes, that is, also entheses, synovium, and subcutaneous tissue. In the DIP joints, diffuse inflammation on MRI has been observed to extend to the nail bed, and an association between enthesitis, dactylitis, and nail involvement has been proposed. An MRI study of nails in patients with skin psoriasis found characteristic nail involvement in all patients, and the authors suggested that onychopathy is preceding DIP joint damage in PsA, and that MRI of nails is of diagnostic value in undifferentiated SpA. These findings have not been substantiated further. Future research using improved imaging techniques, such as microscopy high-resolution MRI coils, is needed, including studies on peripheral enthesitis, which may become a priority of the OMERACT MRI Arthritis Working Group.
MRI for Diagnosing Peripheral Psoriatic Arthritis
In symptomatic PsA, synovitis, tenosynovitis, and bone marrow edema are frequent findings. Inflammatory and destructive changes do not have PsA-specific features, and can involve any joint. However, MRI findings may be valuable for discriminating between diagnoses. Bone marrow edema in PsA ( Fig. 5 ) is often located close to the entheses, in contrast with RA, where bone marrow edema often is located close to the capsular attachments, and in OA, where bone marrow lesions are located mainly close to subchondral areas. Bone erosions are seen more often adjacent to collateral ligament insertions in PsA, whereas erosions are more often located centrally in OA. A comparative MRI study of the hand and wrist in PsA and RA patients found bone erosions were more frequent in RA, and periostitis more frequent in PsA. Diaphyseal bone marrow edema and/or enthesitis is much more common in PsA than RA. A few studies of MRI findings in psoriasis patients without arthritis have been published, and all found higher frequency of arthritic and entheseal changes in patients than in controls. These findings suggest that MRI can detect subclinical arthritis.
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