Many infectious diseases may have similar clinical manifestations, at least in the early phases, but require different treatments. 2-[18F] FDG-PET/CT can differentiate various types of inflammation including infective endocarditis and large vessel vasculitis [1-3]. In case of coincidence of both inflammatory disease entities, the 2-[18F] FDG-PET/CT is the crucial imaging that leads to proper diagnosing and treatment. Recent publications indicate the possibility of cranial artery inflammation detection during large vessel vasculitis (LVV) by this test [4-8]. We present two cases of female patients who were properly recognized and treated with infective endocarditis (IE) but despite the treatment the inflammatory markers were still elevated. GCA was not expected, where 2-[18F] FDG-PET/CT was performed to check the effects of IE treatment. Surprisingly the study revealed a typical metabolic pattern of GCA in both cases. In the second case, there was a high 2-[18F] FDG accumulation in the extracranial arteries.

The whole body 2-[18F] FDG-PET/CT study was performed 60 minutes after the intravenous injection of 2-[18F] FDG (3.5 MBq per kilogram of body weight), in the supine position. The scanner PET/CT Biograph mCT S (64)-4R (Siemens, Erlangen, Germany) was used with 2.5 minutes of registering per bed position. Images were evaluated visually and semiquantitatively on a dedicated workstation equipped with fusion software (Syngo Via VA30A, Siemens, Erlangen, Germany), which displays PET, CT, and PET/CT fused images.

Both patients were on a ketogenic diet one day before the study.

Case 1

A 62-year-old female patient in the fourth week of targeted antibiotic therapy due to IE of the native aortic leaflet and persistent CRP above the norm (20 ng/ml) was referred to 2- [18F] FDG-PET/CT. Previously she underwent the removal of teeth and teeth implants, which were the source of infection (HACEK culture). The 18F-FDG PET/CT examination revealed a typical pattern of GCA with metabolic activity (grade 3, SUV max 6.8) in the main arteries (aorta, subclavian, axillary, iliac, femoral – TVS 16), without features of IE.

Case 2

An 81-year-old female patient was referred with a suspicion of infective endocarditis (IE) with pathological change on the electrode in ECHO. She also presented heart failure with preserved left ventricular ejection fraction (HFpEF, EF=60%), she was after implantation of a dual-chamber cardiac pacemaker due to sinus node disease, after two ischemic strokes, and occlusion of the left internal carotid artery. She has had a history of breast cancer, after a left- sided mastectomy years ago. There was worsening contact with her and increasing inflammatory markers and positive urine cultures (Klebsiella pneumoniae, MBL-NDM, OXA 48, Proteus mirabilis ESBL, previously Enterococcus Faecium, and Candida glabrata). Also, positive blood culture: Staphylococcus epidermidis, MRCSN. In the treatment there were multiple changes of antibiotic therapy, finally obtaining negative blood and urine cultures, but still presented high CRP values (150-160 ng/dl) when she was referred to 2-[18F] FDG- PET/CT after ketogenic diet preparation. The study revealed no metabolic features of IE, whereas a pattern of high 18F-FDG accumulation (grade 3, SUV max 8.5) in main arteries (aorta, subclavian, axillary, iliac, femoral), typical of GCA. There was also a high 2- [18F] FDG accumulation in the extracranial arteries. The total vascular score (TVS) was 21. Doppler USG was performed, which revealed segmental thickening of the IM complex up to 1.5 mm in the left vertebral artery. Based on the study, GCA treatment was applied with good clinical results. 2-[18F] FDG-PET/CT allowed for proper diagnosis and treatment.

The 2-[18F] FDG-PET/CT study has been extensively explored in oncological indications and the diagnosis of inflammatory disease. Thanks to its metabolic approach it has a unique possibility of noninvasive assessment of inflammatory diseases based on imaging the inflammatory cell activity. 2-[18F] FDG as glucose analog is taken up by living cells. The accumulation of radiopharmaceuticals depends on tissue metabolic activity and typically, it is high in activated inflammatory cells. The retention of 2-[¹⁸F] FDG in the acute phase of inflammation, is present by neutrophils, whereas in chronic one by macrophages and polymorphonuclear leukocytes. The assessment of IE by use of 2-[18F] FDG-PET/CT is based on ESC (European Society of Cardiology) guidelines for the management of endocarditis [9], whereas, assessment of LVV is present in recent EULAR (European League Against Rheumatism) recommendations [10]. Both cases reflect a similar therapeutic dilemma resulting from the lack of appropriate diagnostics. The lack of satisfactory treatment effects was due to the coincidence of two inflammatory processes: IE and LVV, which required the use of a different treatment method.

Figure 1: Case 1. 2-[18F] FDG-PET/CT A, MIP (maximum intensity projection) image shows increased metabolic activity in the main arteries.

Figure 2: Case 2. 2-[18F] FDG-PET/CT A, MIP (maximum intensity projection) image. Fused 2- [18F] FDG-PET/CT: B. coronal slice with an increased uptake of 2-[18F] FDG in the aorta, subclavian, axillary, and common carotid arteries. C. coronal slice of the skull with decreased metabolic activity in the left cerebral hemisphere. D. transverse slice of the skull revealed significantly increased metabolic activity in the cranial arteries.

Conflict of interest: The authors declare no conflict of interest.

Funding: There was no funding received for this paper.

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