IgG4-related coronary arteritis may be suspected in patients presenting with cardiac symptoms and with history of biopsy proven IgG4-related disease in other major organs, such as the pancreas, lacrimal and parotid glands [3] , kidneys, orbits [4] , skin, lymph nodes, etc. Biopsy of small-to-medium sized vessels to look for lymphoplasmacytic infiltration and IgG4 deposits is impossible unless the patient happens to undergo coronary artery bypass grafting [5] . Although conventional coronary angiography can detect luminal stenosis and aneurysm formation, CT angiography has the advantage in allowing direct visualization of vessel walls and mural thickening [6] .

Our case was unusual in having no preceding cardiac symptoms and the IgG4-related disease seemed to be limited to the coronary arteries only. The incidental finding of the cardiac pseudotumour on echocardiogram prompted us to evaluate his cardiac condition further with MRI and CT coronary angiogram. The finding of the “pigs-in-a blanket” sign on CT scan was pathognomonic of IgG4-related coronary arteritis [7] . Unfortunately, histopathological confirmation was not available because the disease was limited to coronary arteries, which were not amenable to biopsy. However, according to the 2019 American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) classification criteria for IgG4-RD, biopsy for histopathologic confirmation is not required when the diagnosis is straightforward on the basis of clinical, serologic and radiologic findings [9] .

Traditionally ¹⁸F-FDG positron emission tomography/computed tomography (PET/CT) is used to assess the extent of inflammation in IgG4-RD and to monitor disease activity after treatment. ¹⁸F-FDG PET/CT had a sensitivity of 85.7% and specificity of 66.1% for diagnosing IgG4-RD. However, false-negative finding of ¹⁸F-FDG PET was also noted in some studies [10] [11] . In IgG4-RD, fibrosis produced by the large number of fibroblasts is a major histopathologic feature. Therefore, a recently introduced PET agent targeting fibroblast activation protein, ⁶⁸Ga-fibroblast activation protein inhibitor (⁶⁸Ga-FAPI), has been used to assess IgG4-RD. Unlike ¹⁸F-FDG which accumulates in cancer cells and active inflammatory lesions, ⁶⁸Ga-FAPI uptake is associated with the degree of fibrosis and is more specific for IgG4-RD. There is evidence that ⁶⁸Ga-FAPI PET/CT had a higher positive rate than ¹⁸F-FDG PET/CT in the detection of IgG4-RD in the pancreas, bile duct/liver, and salivary gland [12] . In our patient, although both ¹⁸F-FDG and ⁶⁸Ga-FAPI PET/CT showed involvement of the coronary arteritis, there were differences in tracer uptake in other organs. There was non-specific FDG uptake in the aorta and lymph nodes which were not ⁶⁸Ga-FAPI avid while a small ⁶⁸Ga-FAPI-avid focus at pancreatic head was not detectable on the ¹⁸F-FDG scan. Even though pancreas is a common organ to be involved in IgG4-RD, the clinical significance of this small ⁶⁸Ga-FAPI-avid focus remained unclear.

Glucocorticoid therapy is an effective treatment for IgG4-related disease but many IgG4-RD patients relapse during or after glucocorticoid taper and the long-term use of glucocorticoids may lead to undesirable side-effects such as obesity, hypertension, diabetes, osteoporosis and hyperlipidaemia, especially in elderly patients. In addition, in patients who are hepatits B carriers, like our patient, glucocorticoids may cause reactivation of fulminant hepatitis. Traditional disease-modifying anti-rheumatic drugs (DMARDs) had been used for IgG4-RD but are generally ineffective [13] . In IgG4-RD, excessive production of plasmablasts from activated CD20+ B lymphocytes together with CD4 cytotoxic T-lymphocytes is believed to be responsible for the lymphoplasmacytic infiltration, tissue damage and storiform fibrosis at sites of inflammation. B-lymphocyte depletion therapy with anti-CD20 monoclonal antibodies (Rituximab) has been shown to be effective for induction therapy and treatment of relapses in IgG4-RD [14] [15] . Rituximab is well tolerated despite potential side effects such as infections, hypersensitivity and hypogammaglobulinaemia. Clinical response defined by improvement or resolution of symptomatic organ involvement is high [14] . However, despite good clinical improvement, radiological normalization is seen in less than one-third of patients by conventional radiologic imaging (CT or MRI) and in less than half of patients by ¹⁸F-FDG PET/CT imaging [14] . Persistent mural thickening of the coronary arteries in the repeat CT scan of our patient probably represented irreversible scar-fibrotic lesions due to long-standing chronic inflammation.