The annual incidence of sarcoidosis ranges from 1 to 15 cases per 100,000 people, varying significantly by geographical region, with the highest rates observed in Northern Europe (11–15 per 100,000) and the lowest in Eastern Asia (0.5–1 per 100,000) [1]. Sarcoidosis is a multifaceted inflammatory disorder characterized by the formation of noncaseating granulomas and a variable clinical presentation [[2], [3], [4], [5]]. While the pulmonary system is most commonly involved, accounting for approximately 90 % of cases, sarcoidosis frequently presents with a spectrum of extra-thoracic manifestations that can be challenging to diagnose accurately [1,6]. These include cardiac, neurologic, abdominal, and musculoskeletal involvement, among others, underscoring its status as a true multisystem disease [1,6,7].

The pathophysiology of sarcoidosis remains poorly understood, though it is believed to arise from an interplay between environmental triggers and genetic predispositions, resulting in an aberrant immune response [8]. The heterogeneity of sarcoidosis is further complicated by its unpredictable clinical course, which ranges from asymptomatic and self-limiting disease to chronic, progressive, and debilitating organ dysfunction [9,10]. This variability necessitates a comprehensive imaging approach to accurately diagnose and monitor the disease, as imaging plays a central role in identifying typical and atypical features of organ involvement, guiding biopsies, and assessing therapeutic response [3,9,10]. This review aims to extensively examine the imaging findings across various modalities and organ systems in sarcoidosis.

Sarcoidosis has a broad spectrum of clinical manifestations that vary depending on the organ systems involved (Fig. 1). Pulmonary involvement is the most common and hallmark feature, often presenting with dry cough, dyspnea, and chest discomfort [1,11]. However, up to 10–15 % of patients may be asymptomatic, with the diagnosis made incidentally through imaging studies [1,11].

Extra-thoracic manifestations in 15–30 % of cases involving the skin, eyes, liver, and nervous system [7,12]. Dermatological signs, such as erythema nodosum and lupus pernio, are diagnostic clues [1,13]. Ocular sarcoidosis, present in 15–20 % of patients and up to 79 % in some reports, frequently manifests as uveitis and, if untreated, can lead to severe complications like cataracts and vision loss [[14], [15], [16], [17], [18]]. While 20–25 % of sarcoidosis patients have cardiac involvement, only 5 % may become symptomatic [[19], [20], [21]]; it may manifest with arrhythmias, conduction abnormalities, or heart failure [22,23]. Neurological involvement, known as neurosarcoidosis (NS), affects the central and peripheral nervous systems in 5–10 % of cases and can present as cranial nerve palsies, seizures, or hydrocephalus, complicating the diagnostic process [24,25]. While sarcoidosis is traditionally recognized as a multisystem disorder, approximately 10 %–20 % of NS cases occur without evidence of systemic sarcoidosis. The neurological symptoms are the initial presentation in 50 %–70 % of the individuals [[25], [26], [27], [28], [29]]. A summary of the clinical and pathological features that support a diagnosis of sarcoidosis is summarized in Fig. 2 [30].

The immune-mediated formation of noncaseating granulomas is the hallmark of sarcoidosis; its pathogenesis involves an interplay of genetic predisposition, environmental exposures, and immune dysregulation [8,31]. Genetic studies have identified associations with human leukocyte surface antigen (HLA) as early as 1973 [32], showing that HLA-A7 was twice as prevalent in sarcoidosis patients compared to healthy individuals (52 % vs. 27 %). Subsequent studies identified additional associations, such as specific HLA class II alleles; Löfgren's syndrome exemplifies a phenotype with a relatively uniform presentation linked to a specific genetic predisposition, particularly the HLA-DRB1∗03 allele [[33], [34], [35]]. Additionally, the HLA-DRB1∗04 allele is linked to manifestations such as hypercalcemia and Heerfordt syndrome—a presentation involving uveitis, salivary gland enlargement, and cranial nerve palsy [36,37]. A similar role of environmental exposures has been identified; epidemiological studies highlighted associations with farming, mold, and other occupational exposures [[38], [39], [40], [41], [42], [43], [44]]. Infections, stress, and immunologic triggers like immune checkpoint inhibitors have been proposed as modulators of disease onset and activity [[45], [46], [47]]. The precise antigenic triggers remain under investigation; however, several candidates, such as Mycobacterium tuberculosis, Cutibacterium acnes, and inorganic agents like silica and pesticides, have been implicated [[48], [49], [50], [51], [52]]. Moreover, regulatory T-cell dysfunction and excessive mammalian target of rapamycin (mTOR and the Rac1 signaling pathways) [53] have been implicated in the persistence of granulomas [[54], [55], [56], [57], [58], [59]].

Granuloma formation begins with the activation of antigen-presenting cells, including alveolar macrophages and dendritic cells, in genetically predisposed individuals [[60], [61], [62], [63], [64]]. These cells process and present antigens to CD4+ T-helper cells, triggering a Th1/Th17 response, with the subsequent release of pro-inflammatory cytokines, such as interferon-gamma (IFN-γ), interleukin-2 (IL-2), IL-12, and tumor necrosis factor-alpha (TNF-α) [[64], [65], [66], [67], [68]]. Eventually, granulomas will be formed, consisting of macrophages, epithelioid cells, and multinucleated giant cells surrounded by lymphocytes and fibroblasts. Over time, granulomatous inflammation can transition to fibrosis, leading to irreversible organ damage.

Based on radiologic findings, the Scadding staging system categorizes pulmonary sarcoidosis into five stages [69]. Stage 0, representing 8–10 % of cases, shows no significant functional impairment [[69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80]]. Stage I, the most common (40–51 %), demonstrates minimal functional deficits, including reductions in forced vital capacity (FVC) (4 %), forced expiratory volume in the first second (FEV1) (8 %), and diffusing capacity of the lung for carbon monoxide (DLCO) (13 %), with radiographic resolution reported in 49–82 % of cases and mortality between 0 and 9 % [[69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80]]. Additional imaging features of thoracic sarcoidosis are discussed in later sections. Stage II, encompassing 29–40 %, exhibits moderate impairments with reductions in FVC (8–16 %), FEV1 (16 %), DLCO (27 %), and total lung capacity (TLC) (5 %), alongside a decline in radiographic resolution (31–68 %) and a mortality rate of 5–11 % [[69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80]]. Stages III (12–20 %) and IV (2–5 %) are associated with severe impairments, including reductions in FVC (20 %), FEV1 (26 %), DLCO (45 %), and TLC (22 %), coupled with low radiographic resolution (10–38 % in Stage III and 0 % in Stage IV) and high mortality rates (12–18 % for Stage III and 16–17 % for Stage IV) (Fig. 3) [[69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80]].

Chest radiography is a cornerstone in the initial evaluation of pulmonary sarcoidosis. The Scadding staging system classifies findings into five stages (Fig. 3, Fig. 4), with Stage 0 normal and Stage IV indicating advanced fibrosis [69,80]. However, the sensitivity of radiographs is limited in detecting subtle parenchymal or mediastinal abnormalities, necessitating advanced modalities for comprehensive evaluation [81,82].

High-resolution computed tomography (HRCT) stands as the preferred imaging tool for evaluating pulmonary sarcoidosis owing to its exceptional sensitivity in detecting hallmark features [81,82]. The characteristic perilymphatic distribution of micronodules, commonly observed along bronchovascular bundles, interlobular septa, and subpleural regions, is a key finding that aids diagnosis [83] (Fig. 5, Fig. 6). In advanced stages, HRCT reveals architectural distortion, reticulations, and traction bronchiectasis, reflecting progressive fibrosis [84,85]. Among the distinctive patterns, the “galaxy sign"—a cluster of central coalescent nodules surrounded by smaller satellite nodules—can strongly suggest sarcoidosis (Fig. 7), though it is not unique to the disease [86]. Furthermore, HRCT provides a detailed evaluation of mediastinal and hilar lymphadenopathy, which may or may not demonstrate calcification [4] (Fig. 8).

While non-gated chest magnetic resonance imaging (MRI) has limited utility in evaluating thoracic sarcoidosis, gated cardiac MRI plays a central role in the assessment of cardiac sarcoidosis (CS), providing unparalleled detail in characterizing myocardial involvement. Late gadolinium enhancement (LGE), a hallmark feature on cardiac MRI, reveals myocardial inflammation or fibrosis [19,20] (Fig. 9, Fig. 10). The LGE typically presents in a non-vascular pattern, most often with a patchy, mid-myocardial distribution along the basal septum and lateral walls of the left ventricle, with the endocardium often spared [19,20,87]. Such findings are not merely diagnostic but also prognostic, as they are strongly associated with major adverse cardiac events.

In addition to detecting LGE, cardiac MRI provides valuable functional and structural information that could indicate disease progression. This includes assessing ejection fraction and identifying myocardial thinning or aneurysm formation, which may signal disease progression [19,20]. Myocardial edema, highlighted on T2-weighted sequences, indicates active inflammation and provides a direct target for guiding immunosuppressive therapy [19,20].

On cardiac positron emission tomography (PET), CS patterns of active inflammation can appear as focal or focal-on-diffuse patterns of hypermetabolic/increased fluorodeoxyglucose (FDG) activity (Fig. 11), which reflects active inflammation, and these features help differentiate active cardiac sarcoidosis from cardiomyopathies without active inflammation, such as inactive sarcoidosis or scarring from prior myocarditis [19,20]. Whole-body PET also evaluates extracardiac involvement, guiding biopsy sites and therapeutic decisions [4,19,20]. Optimal image quality requires meticulous patient preparation, including prolonged fasting and a low-carbohydrate diet to suppress physiological myocardial uptake [4,19,20]. This imaging modality helps distinguish CS from other infiltrative cardiomyopathies and provides critical insights into disease activity. Beyond cardiac involvement, whole-body PET allows for assessing extracardiac sarcoidosis, aiding in selecting biopsy sites, and informing therapeutic strategies.

The selection of an imaging modality for cardiac sarcoidosis remains an area of ongoing debate, as there is no consensus regarding the optimal initial test [88]. Both cardiac MRI and FDG-PET offer distinct advantages and are often used complementarily. The decision on which modality to employ should be guided by clinical context, patient characteristics, and the specific diagnostic objective. According to the joint procedural position statement on imaging in cardiac sarcoidosis [88], – from the European Association of Nuclear Medicine, the European Association of Cardiovascular Imaging, and the American Society of Nuclear Cardiology-cardiac MRI should be the preferred first-line modality when available, given its ability to detect early fibrosis and structural abnormalities.

Cardiac MRI is generally favored as the initial imaging modality due to its superior sensitivity in detecting cardiac sarcoidosis (95 % vs. 84 %; P = 0.002) compared with FDG PET, while both modalities demonstrate similar specificity (85 % vs. 82 %; P = 0.85) [89]. Late gadolinium enhancement on MRI, particularly in a nonischemic pattern, is a hallmark feature of cardiac sarcoidosis and provides high diagnostic yield. Additionally, the inclusion of T2-weighted imaging further enhances MRI sensitivity (99 % vs. 88 %; P = 0.001), making it particularly useful in early disease detection [89]. Given its high sensitivity, cardiac MRI should be considered the first-line test for suspected cardiac sarcoidosis when available. A negative MRI result in a patient with low to intermediate pretest probability of disease typically excludes the diagnosis, obviating the need for further evaluation. However, in cases where clinical suspicion remains high despite a negative MRI, FDG-PET may be warranted to assess for active inflammation that might not yet have caused myocardial scarring [90,91].

FDG-PET, on the other hand, is advantageous in evaluating disease activity and monitoring response to therapy. Unlike MRI, which primarily detects structural myocardial changes and fibrosis, PET reflects metabolic activity and inflammation [91,92]. Importantly, FDG-PET sensitivity is influenced by prior anti-inflammatory therapy, with studies showing higher sensitivity in patients not yet treated. Therefore, when the clinical indication is to confirm active disease or assess treatment response, FDG PET is preferred. Moreover, FDG PET is often the modality of choice in patients with implanted cardiac devices, where MRI quality may be compromised due to artifact.

Although less sensitive, echocardiography remains a valuable screening tool for assessing CS. Findings include left ventricular regional wall motion abnormalities, septal thinning, and diastolic dysfunction. Doppler studies may reveal pulmonary hypertension, a common complication of advanced sarcoidosis [4,19,20,93].

Sato et al. provided a multimodality imaging approach for diagnosing CS, a simplified version of the updated Japanese Circulation Society guidelines (Fig. 12) [94,95]. Using their approach, they correctly identified 43 % of patients with isolated CS who experienced a higher incidence of cardiovascular events during the follow-up [95].

Abdominal sarcoidosis is a relatively common extrapulmonary manifestation of sarcoidosis, reported in up to 70 % of cases with systemic involvement [12]. It frequently affects the liver, spleen, lymph nodes and, less commonly, the kidneys, pancreas, and gastrointestinal tract [12]. Although most patients are asymptomatic, its manifestations can significantly alter disease prognosis and management.

Hepatic sarcoidosis is among the most frequent abdominal manifestations of sarcoidosis, with imaging abnormalities reported in 50–80 % of cases [12,96]. On CT, common findings include hepatomegaly and hypodense nodules (Fig. 13), which may range from scattered granulomas to larger, confluent lesions [[97], [98], [99], [100]]. Chronic inflammation in advanced disease can lead to fibrosis, cirrhosis, and portal hypertension.

On MRI, nodular lesions in hepatic sarcoidosis, typically measuring 5–20 mm, are characteristically hypointense on all sequences, most prominently on T2-weighted fat-saturated images 12,101. This distinct T2 hypointensity aids in differentiating these lesions from metastases and inflammatory diseases, which are generally hyperintense on T2-weighted fat-saturated images [12]. These nodules exhibit less enhancement than the surrounding liver parenchyma on gadolinium-enhanced T1-weighted images 12,101. Additional MRI findings include irregular hepatic contour and high periportal signal intensity, which may reflect granulomatous inflammation or fibrosis 12,102.

Ultrasonography (US) of hepatic sarcoidosis typically reveals increased parenchymal echogenicity and a coarse hepatic texture, occasionally accompanied by discrete nodules 102. In longstanding cases, calcifications may be observed 12102. Nodules, representing the coalescence of granulomas, can be diffusely distributed throughout the liver and range in size from a few millimeters to several centimeters 102. These nodules are often hypoechoic on US and appear hypodense on CT. However, they may be echogenic (Fig. 13) depending on the liver's baseline echogenicity and the extent of fibrosis within the granulomas 102. Focal nodules, identified in approximately 5 % of patients, may cause contour irregularities 103. Due to the nonspecific nature of these imaging findings, tissue biopsy is required for definitive diagnosis.

While hepatic sarcoidosis is a common abdominal manifestation, its imaging findings are not pathognomonic, and a broad differential diagnosis should be considered, particularly in newly diagnosed patients. Granulomatous liver diseases such as tuberculosis, histoplasmosis, brucellosis, and primary biliary cholangitis can present with similar nodular patterns on CT and MRI, necessitating clinical correlation. Likewise, hepatic lymphoma and metastases may mimic sarcoid involvement, particularly when nodules are hypodense on CT or hypointense on T2-weighted MRI. FDG-PET, although valuable for assessing systemic sarcoidosis activity, lacks specificity, as other inflammatory or neoplastic conditions can also demonstrate increased metabolic activity. Given these overlapping imaging features, liver biopsy may be required to confirm hepatic sarcoidosis and exclude alternative diagnoses, especially when imaging findings are atypical or when there is a discordance between radiologic and clinical presentation.

Splenic involvement in sarcoidosis occurs in approximately 24–53 % of cases and is often associated with concurrent hepatic or pulmonary involvement 104. Splenomegaly is the most common imaging finding and may present with homogeneous infiltration or as multiple discrete nodules scattered throughout the spleen 105,106. These nodules are more frequently observed on abdominal CT than hepatic nodules, appearing as hypodense, non-enhancing lesions (Fig. 14, Fig. 15). In advanced cases, these nodules may coalesce, leading to larger, irregular masses that mimic infectious or neoplastic processes 103. Differential diagnoses for hypodense splenic nodules include tuberculosis, lymphoma, metastases, abscesses, and opportunistic infections like candidiasis in immunocompromised patients.

MRI findings in splenic sarcoidosis are similar to hepatic involvement, with nodules appearing hypointense on both T1-and T2-weighted sequences 4,12,101,102 (Fig. 14). Similarly, US reveals splenic nodules as hypoechoic lesions, typically discrete but capable of coalescing as they enlarge. Contour irregularities of the splenic surface may also be observed in advanced disease 4,12,101,102. On FDG-PET, the splenic lesions have hypermetabolic activity with increased FDG uptake (Fig. 6).

Gastrointestinal (GI) involvement in sarcoidosis is exceedingly rare, occurring in less than 10 % of cases, often identified incidentally during autopsy or imaging 107. The stomach is the most commonly affected organ, with granulomatous infiltration typically localized to the antrum 107. Gastric sarcoidosis can present as either a localized or diffuse form, each with distinct imaging and clinical manifestations. In the localized form, granulomas infiltrate the mucosa, leading to ulcers, gastritis, nodular irregularities, or friable mucosa. These changes may mimic polyps or other benign lesions 108. Diffuse involvement is characterized by fibrosis, significant mucosal thickening, and rigidity, which may narrow the lumen and cause gastric outlet obstruction 107,109-112. CT imaging often reveals thickened gastric folds with mucosal irregularity (Fig. 16) or a segmental “linitis plastica-like” appearance, necessitating differentiation from gastric carcinoma 111. PET/CT may be employed to identify active inflammation in symptomatic cases or guide biopsy in ambiguous scenarios.

The small intestine is the least commonly affected portion of the GI tract 110. Involvement may manifest as intestinal obstruction caused by extrinsic compression from enlarged mesenteric lymph nodes or luminal narrowing due to cicatrizing granulomas 108,113. These findings, though rare, can resemble Crohn's disease, tuberculosis, or other inflammatory disorders. Diagnostic clues favoring sarcoidosis include elevated serum angiotensin-converting enzyme levels and superficial mucosal involvement without the transmural inflammation typically seen in Crohn's disease 114.

In the large bowel, the sigmoid colon is the most frequently affected site 110. Colonic sarcoidosis may occur even in the absence of visible mucosal abnormalities. Imaging findings include aphthous erosions, polyps, ulcers, obstructive lesions, and stenosis 115-118. External compression by significantly enlarged lymph nodes is a common cause of intestinal obstruction, underscoring the importance of detailed imaging.

Sarcoidosis is also a recognized cause of noninfectious granulomatous appendicitis, presenting as an enlarged appendix with indistinct walls. On CT, findings may include an abnormally large appendix with soft tissue density but without periappendiceal fat stranding or fluid 119. The presence of enlarged regional lymph nodes should raise suspicion for granulomatous diseases such as sarcoidosis, particularly in the absence of typical inflammatory changes 119.

Lymphadenopathy is a common feature of abdominal sarcoidosis, found in ∼30 % of patients, with the porta hepatis, para-aortic, and celiac regions being the most frequently affected sites 97,103. On CT (Fig. 15), these lymph nodes are typically homogeneously hypodense and measure less than 2 cm in diameter, appearing more discrete and less confluent than those seen in lymphoma [97]. Retrocrural lymph node involvement, which is more characteristic of lymphoma, is comparatively rare in sarcoidosis. 120.

Peritoneal involvement in sarcoidosis is exceedingly rare but may present as exudative ascites, multiple granulomatous nodules scattered across the peritoneum, or solitary lesions 121,122. CT and MRI findings include soft-tissue attenuation and nodular or diffuse peritoneal thickening, which can mimic peritoneal carcinomatosis or infectious etiologies such as tuberculosis or fungal infections 108,113. A definitive diagnosis often requires a peritoneal biopsy. On FDG-PET, lymph nodes and peritoneal lesion will have hypermetabolic activity with increased FDG uptake (Fig. 15).

Renal involvement in sarcoidosis is observed in 7–40 % of cases, with nephrocalcinosis and nephrolithiasis being the most frequently reported manifestations 123,124. These conditions arise from hypercalcemia and hypercalciuria, caused by excessive calcitriol production in extrarenal granulomas 125-127. CT is useful in detecting these calcifications, in addition, there can be a striated nephrogram pattern indicative of interstitial nephritis. US and CT are the primary imaging modalities for diagnosing nephrocalcinosis, with CT demonstrating higher specificity (83–89 %) compared to US (66–71 %) 128. Sensitivity values are also high for both modalities, ranging from 85 to 90 % for US and 81–86 % for CT, making them indispensable tools in the evaluation of renal sarcoidosis 128.

Direct granulomatous infiltration of the kidneys is an uncommon manifestation of sarcoidosis, often resembling other renal pathologies such as lymphoma or metastases, which complicates diagnosis 129. This diffuse process can be difficult to assess on CT and US, FDG-PET can be a good problem-solving tool with diffuse increased FDG uptake seen in the kidney or kidneys (Fig. 17). Granulomatous pseudotumors, a rare form of renal sarcoidosis, can present as focal, exophytic nodules that may be singular or multiple and unilateral or bilateral 130-132. These lesions appear echogenic on ultrasonography, while on CT, they are hypo-, iso-, or hyperdense in unenhanced images but hypo-enhancing after contrast administration 130,131,133. On MRI, these lesions typically demonstrate poor delineation from the surrounding parenchyma, appearing isointense or slightly heterogeneous on T1-and T2-weighted images 130,131,133. Following gadolinium contrast administration, the lesions show reduced enhancement compared to normal renal cortex, a feature that aids in differentiating them from other renal pathologies, such as clear cell renal cell carcinoma 134.

Pancreatic involvement in sarcoidosis is rare, occurring in 1–3 % of cases with systemic disease, typically identified during autopsy 135. While it is often asymptomatic, symptomatic cases may present with abdominal pain, nausea, vomiting, weight loss, or obstructive jaundice 135. Pancreatic sarcoidosis can manifest as direct tissue infiltration, ductal obstruction, or constrictive peripancreatic lymphadenopathy 132.

Imaging findings vary based on the form of involvement. CT often reveals hypodense pancreatic lesions, occasionally accompanied by ductal dilatation. MRI further characterizes these abnormalities, showing mildly hyperintense lesions on T2-weighted images and hypointense regions on T1-weighted sequences 136.

Focal pancreatic masses, most commonly located in the head of the pancreas, are seen in approximately half of the cases and often mimic pancreatic carcinoma 136. These masses may necessitate tissue sampling to rule out malignancy. Unlike cancer, however, cholangiograms in sarcoidosis typically demonstrate a long, smoothly tapered narrowing of the pancreatic duct rather than the abrupt termination characteristic of tumors 137. Diffuse nodular infiltration of the pancreas, reported in the other half, further highlights the diverse presentation of pancreatic sarcoidosis 132.

Musculoskeletal involvement is a recognized manifestation of sarcoidosis, occurring in one-quarter to one-third of patients 138. Sarcoid primarily affects the joints, muscles, and bones, with varying presentations and severities. These manifestations can occur independently or in conjunction with other systemic involvements.

Sarcoid arthropathy encompasses a spectrum of joint involvement, broadly categorized into acute and chronic forms, each with distinct clinical and imaging features. These manifestations are reported in 10–38 % of patients with sarcoidosis and may occur in isolation or alongside systemic disease 139,140.

Acute sarcoid arthritis is more common, affecting up to 40 % of patients 140,141. It frequently occurs early in the disease course, often presenting as part of Löfgren's syndrome—a triad of bilateral hilar lymphadenopathy, erythema nodosum, and acute polyarthritis 142,143. This type of arthritis typically involves the ankles bilaterally, occasionally extending to other large joints of the lower extremities, with mild, migratory, and transient pain. Monoarthritis or oligoarthritis is less common.

Conventional radiographic findings in acute sarcoid arthritis are typically nonspecific, demonstrating periarticular soft tissue swelling without significant joint destruction. US may demonstrate joint effusions or tenosynovitis, though extensive synovitis is rare 144. MRI may show mild synovial enhancement (Fig. 18) or bone marrow edema.

Chronic sarcoid arthritis is rare, occurring in only 0.2 % of cases, and is often associated with systemic manifestations, including pulmonary sarcoidosis, lupus pernio, chronic uveitis, and tenosynovitis 145,146. It typically presents as persistent oligoarthritis or polyarthritis, with occasional dactylitis or Jaccoud-type arthropathy, which is characterized by joint deformity without erosive changes 147. Chronic arthritis may progress to joint destruction and is less likely to resolve spontaneously 146. Radiographic findings in chronic sarcoid arthritis may include joint space narrowing, subchondral bone demineralization, and erosions, particularly in advanced disease 148. Soft tissue swelling and periarticular infiltration are often observed 148. MRI frequently demonstrates tenosynovitis and synovitis, especially in the extensor tendons of the fingers 149.

Muscle involvement in sarcoidosis is relatively common, with up to 80 % of patients demonstrating muscular involvement, though most remain asymptomatic 150,151. Symptomatic presentations occur in about 1 % of cases and are classified into four main patterns: nodular, acute myositis, chronic myopathic, and smoldering types 152. These patterns differ in their clinical presentation, imaging features, and response to treatment.

1

Chronic Myopathic Sarcoidosis

Chronic myopathy is the most common symptomatic manifestation, accounting for approximately 86 % of cases 140,153. It typically presents with progressive, symmetrical proximal muscle weakness and atrophy, resembling muscular dystrophy 154,155. Despite its insidious onset, serum muscle enzymes may remain normal or slightly elevated 140. Fatigue and exercise intolerance are common due to the systemic inflammatory state driven by elevated cytokines such as TNF-α, IL-6, and IFN-γ 140,156.Nodular sarcoid myopathy, seen in about 3 % of cases, is characterized by painful, palpable nodules without significant motor deficits 140,153,157. These nodules are tumor-like lesions often found in the extremity musculature. The condition follows a relapsing-remitting course, and treatment with immunosuppressants is commonly effective 157.Acute myositis is the rarest form or muscle involvement, which manifests with diffuse muscle pain, swelling, and proximal muscle weakness 151,158. Diagnosis often requires histopathological confirmation due to nonspecific imaging and laboratory findings 159.

Smoldering myopathy is a newly described subtype presenting with persistent myalgia, without nodules, motor deficits, or muscle atrophy 157. It demonstrates a racial disparity, being less frequently observed among those of African American background 157.

MRI is the preferred imaging modality for assessing sarcoid myopathy. It provides a detailed visualization of muscle involvement and distinguishes between different disease patterns. Nodular myopathy is characterized by star-shaped areas of T2 hyperintensity surrounded by hypointense fibrotic regions 157, while acute myositis presents as diffuse T2 hyperintensity secondary to edema and inflammation 158 (Fig. 19A). Chronic myopathy shows homogeneous hypointense T2 signal alterations consistent with fibrosis 160 as well as muscular atrophy (Fig. 19B). MRI findings are often normal for smoldering myopathy, but muscle biopsy may reveal granulomatous infiltration. FDG-PET, is useful in muscular sarcoidosis as it reveals increased FDG uptake in affected muscles (Fig. 19C), often appearing as streaky or nodular patterns of hypermetabolism. These findings enable precise localization of involved regions, guide biopsy by identifying metabolically active lesions, and help monitor treatment response through changes in FDG uptake over time 161.

Osseous sarcoidosis affects 3–13 % of patients 162,163, commonly involving the small bones of the hands and feet, particularly the phalanges 138. About half of these cases remain asymptomatic and are incidentally detected during systemic evaluations 164. Bone lesions typically present with lytic, permeative (“moth-eaten”) or sclerotic patterns, often involving both cortical and medullary regions 163,165. Radiographs frequently demonstrate lytic lesions with a “moth-eaten” or “lace-like” appearance (Fig. 20), while CT provides detailed visualization of cortical thinning, destruction, or sclerosis. MRI findings vary, with lytic lesions appearing hyperintense on T2-weighted images (Fig. 21), while sclerotic lesions—commonly affecting the axial skeleton—may mimic metastatic disease. PET/CT is valuable for detecting metabolically active lesions (Fig. 22, Fig. 23) and guiding biopsies in ambiguous cases.

Distinct radiological patterns have been classified for sarcoid bone disease 140,166. Type I consists of large cystic lesions that may result in pathological fractures; Type II features multiple small, circumscribed cysts that may create a “lacy” pattern; and Type III is characterized by cortical tunneling that disrupts bone architecture. These forms can coexist in the same bone. Peripheral manifestations such as dactylitis, resembling sausage-shaped digits, occur due to bone and soft tissue involvement, often affecting the second and third phalanges. In rare cases, acro-osteolysis, with nodular densities in terminal phalanges, may develop.

Although less common, axial skeleton involvement can affect the vertebrae, presenting as lytic, sclerotic, or mixed sclerotic and lucent lesions, primarily in the lower thoracic and upper lumbar regions. MRI has proven crucial in identifying multifocal lesions, which appear hypointense on T1-weighted images and hyperintense on T2-weighted sequences, facilitating biopsy site selection and treatment monitoring 167. In the skull, asymmetrical lytic lesions are typical and may also involve the ribs and nasal bones. F-18 FDG PET/CT is highly sensitive for detecting granulomatous bone marrow infiltration in sarcoidosis, showing increased FDG uptake that may present as diffuse or focal patterns in the bone marrow, potentially mimicking metastatic disease 168,169. However, its specificity is limited when sarcoidosis coexists with malignancies that may cause bone metastases, necessitating additional imaging or biopsy for accurate differentiation 168-170.

Cutaneous manifestations of sarcoidosis exhibit a broad range of specific and nonspecific lesions (Table 1) 171-174. Specific lesions, characterized by granulomatous inflammation, include papules, nodules, and plaques, which often display hues ranging from red-brown to purple-brown and are typically asymptomatic. Plaques are frequently well-demarcated, affecting the trunk, extremities, and face, while lupus pernio, a hallmark of chronic and often disfiguring sarcoidosis, prominently involves the central face and ears. Notably, variations such as angiolupoid and verrucous sarcoidosis demonstrate rarer phenotypes with distinctive clinical features, such as prominent telangiectasias or wart-like hyperkeratosis, respectively. Skin pigmentation influences lesion presentation, with subtle erythema and hypopigmented lesions more pronounced in individuals with darker skin tones, highlighting the need for tailored diagnostic approaches 171-174.

Nonspecific lesions, by contrast, reflect the systemic immune response rather than granulomatous infiltration. Erythema nodosum, the most common nonspecific presentation in up to 25 % of patients with sarcoidosis, is marked by tender, erythematous nodules on the shins, often accompanied by systemic symptoms like arthritis or fever 175. While transient, its presence is associated with a favorable prognosis in sarcoidosis and may present as part of Löfgren syndrome, a distinct acute form of the disease 73,176. Other nonspecific eruptions (such as calcinosis cutis, erythema multiforme, or nail clubbing) are less frequent but add to the diverse clinical spectrum, emphasizing the importance of thorough evaluation in diagnosing sarcoidosis.

Evaluation of cutaneous sarcoidosis involves detailed history-taking, physical exams, and targeted tests, including blood panels, chest radiographs, and pulmonary function tests. Cardiac MRI or PET is recommended for cardiac involvement. In addition, cutaneous lesions can demonstrate hypermetabolic/increased FDG uptake on whole-body PET/CT (Fig. 24). Biomarkers like ACE and the soluble interleukin-2 receptor can also aid in assessing disease activity (Table 2) 30,174.

Subcutaneous sarcoidosis, also known as Darier–Roussy sarcoidosis, is an uncommon manifestation of systemic sarcoidosis, reported in only 1.4 %–6 % of cases 177,178. It typically appears as asymptomatic, smooth nodules in the deep dermis and subcutaneous tissue, predominantly on the trunk and arms, often distributed in a linear or sporotrichoid pattern 174. Although more common in individuals of European descent 173, the systemic implications of subcutaneous sarcoidosis remain debated, with some suggesting a link to benign systemic disease 178,179. Imaging can be helpful in further evaluating palpable lesions, particularly US (Fig. 25), in addition to US-guided tissue sampling.

A review of approximately 30 case reports between 2000 and 2020 revealed that 15 patients had pulmonary infiltrates on chest CT 180, including the current case. Notably, reported outcomes indicate a favorable prognosis, with most patients experiencing partial or complete recovery 180. This highlights the potential for subcutaneous sarcoidosis to remain localized or contribute to systemic disease with generally good clinical outcomes.

Neurosarcoidosis is a rare but clinically significant manifestation of sarcoidosis, involving the central and peripheral nervous systems in 5–10 % of cases, though autopsy studies suggest subclinical involvement in up to 25 % [24,25]. It often presents as the initial symptom in 50–70 % of affected patients and occurs without systemic sarcoidosis in approximately 10–20 % of cases [[25], [26], [27], [28]]. The most common features include cranial neuropathies (50–75 %), leptomeningitis (10–20 %), myelopathy (5–26 %), and parenchymal brain lesions (up to 50) 25,181. Rarely, manifestations such as pachymeningitis, vascular disease, or hypothalamic/pituitary axis involvement can also occur. Differential diagnoses for each presentation include conditions like multiple sclerosis, infectious diseases (e.g., Lyme disease, tuberculosis), malignancies, and autoimmune disorders, emphasizing the need for thorough clinical, radiologic, and laboratory evaluations to exclude mimickers (Table 3) [25].

The consensus diagnostic criteria for neurosarcoidosis emphasize three levels of certainty: possible, probable, and definite 182. “Possible” neurosarcoidosis is characterized by clinical and imaging findings typical of granulomatous inflammation in the nervous system without pathological confirmation 182. “Probable” neurosarcoidosis requires systemic granulomatous pathology consistent with sarcoidosis 182. “Definite” neurosarcoidosis necessitates both nervous system pathology consistent with sarcoidosis and typical clinical and diagnostic findings, with subtypes depending on the presence or absence of extraneural involvement 182 (Fig. 26).

Evaluating a patient with suspected neurosarcoidosis needs a multimodal approach, including serum studies (e.g., inflammatory markers, autoimmune and infectious serologies), cerebrospinal fluid analysis (e.g., cell counts, protein levels, and pathogen-specific testing), and advanced imaging techniques (e.g., MRI, CT, and PET/CT) [25]. Biopsy remains a cornerstone for definitive diagnosis, supported by electrophysiological tests (EEG, EMG/NCS) and clinical evaluations, such as slit-lamp eye examinations for systemic correlation (Fig. 26) [25].

MRI with and without gadolinium is the cornerstone imaging modality when evaluating suspected CNS neurosarcoidosis. While gadolinium enhancement is not specific to neurosarcoidosis, it is a reliable marker of active disease [25]. Contrast-enhanced sequences are critical in identifying and monitoring CNS involvement. Common MRI findings include parenchymal lesions (Fig. 27), pachy- and leptomeningeal enhancement (Fig. 28, Fig. 29), deep medullary vein engorgement (Fig. 30), and radial perivenular enhancement (Fig. 27), all of which contribute to diagnostic accuracy and therapeutic decision-making 181,183. Moreover, MRI provides valuable insights as a biomarker for assessing response to therapy.

An early diagnostic approach for neurosarcoidosis includes searching for systemic sarcoidosis with histopathologic confirmation, as approximately half of patients with CNS-predominant sarcoidosis present with abnormal chest radiographs during neurologic evaluation 184,185. The evaluation for systemic sarcoidosis should be guided by the patient's clinical presentation and physical examination findings. For patients with suspected occult disease, a minimum workup should include a high-resolution chest CT, a whole-body FDG-PET scan, and a comprehensive ocular examination. These imaging modalities assist in identifying systemic involvement and guide the selection of optimal biopsy sites. Routine laboratory tests should also be performed, such as a complete blood count, liver and renal function panels, and calcium and creatine kinase levels. Although biomarkers like serum angiotensin-converting enzyme, soluble interleukin-2 receptor, and C-reactive protein lack proven utility in diagnosing or managing neurosarcoidosis, they may provide supportive information in select cases.

In summary, sarcoidosis is a multifaceted inflammatory disease requiring a nuanced imaging approach to evaluate its diverse organ involvement. Pulmonary imaging remains central, with high-resolution CT identifying hallmark features like perilymphatic nodules and fibrosis. Advanced modalities, such as FDG-PET and cardiac MRI, are critical for detecting systemic and cardiac involvement, while abdominal MRI and ultrasound delineate hepatic and splenic manifestations. Neurosarcoidosis demands a multimodal strategy, leveraging MRI and PET for accurate diagnosis and monitoring. This manuscript comprehensively synthesizes imaging findings across systems, emphasizing their role in diagnosis, staging, and management.