¹⁸F FDG PET-CT with CT Aortogram of Infected Ascending Aortic Pseudoaneurysm in a Patient with Pyrexia of Unknown Origin

INTRODUCTION

Pseudoaneurysm of the thoracic aorta is an uncommon but serious complication after cardiac surgery, with an estimated incidence of 0.65–2%.^[1,2] A pseudoaneurysm arises when disruption of the aortic wall occurs, often at sites of suture lines, cannulation, clamping, or cannulation, with containment of the haematoma by the adventitia or mediastinal tissues, rather than by all three layers as in a true aneurysm.^[1-3] The ascending aorta is the least common segment to be involved. Predisposing factors include prior cardiac surgery (valve replacement, coronary artery bypass graft [CABG]), prosthetic material, mediastinal infection, and prior aortic manipulation.^[2-4]

Diagnosis of an infected pseudoaneurysm is challenging. Clinical presentation may be nonspecific (fever, malaise), and non-contrast imaging may only show a mediastinal soft-tissue or fluid collection, failing to demonstrate vascular communication or active infection.^[3-5] Hybrid imaging using ¹⁸F fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) combined with contrast-enhanced CT aortogram offers the advantage of simultaneous anatomical and metabolic assessment, enabling detection of vascular lesions as well as periaortic infection.^[3,6-8]

Here, we present a case of an infected ascending aortic pseudoaneurysm diagnosed on contrast-enhanced ¹⁸F FDG PET/CT in a patient with pyrexia of unknown origin (PUO) and prior complex cardiac surgery, illustrating the value of vascular imaging in nuclear medicine protocols.

CASE REPORT

A 68-year-old male with type 2 diabetes mellitus presented with persistent fever, chills, malaise, and anorexia of several weeks’ duration. His history was significant for CABG and triple valve replacement performed one year prior. There was no clinical or surgical wound infection at the sternotomy site.

Initial investigations included Dengue NS1 antigen testing (negative) and multiple sets of blood cultures (sterile). A non-contrast CT scan of the chest showed a soft tissue lesion anterior to the ascending aorta; however, the origin (vascular vs non-vascular) remained unclear.

Given the prior cardiac surgery and persistent PUO, a whole-body ¹⁸F FDG PET-CT was performed using a modified protocol: first, an arterial-phase contrast-enhanced CT aortogram was performed using the bolus tracking method, followed by whole-body PET-CT acquisition 60 minutes after administration of 10 mCi (≈ 370 MBq) of ¹⁸F FDG.

In Fig 1. (A) shows whole body PET MIP (maximum intensity projection) image. Fused images revealed postoperative changes, including prosthetic cardiac valves. A saccular outpouching measuring approximately 3.6 × 3.7 × 3.6 cm (AP x TR x CC) was noted, arising from the anterior wall of the ascending aorta via a narrow neck (~2.5mm). The sac opacified with contrast on arterial-phase CT (blue arrow in B and C), and washout was observed in the delayed images (D & E), confirming luminal communication. The aneurysm wall appeared thin. Surrounding the sac was a periaortic soft-tissue collection showing elevated FDG uptake (SUVmax 4.5), indicative of active inflammation or infection (F & G). The lesion abutted adjacent mediastinal structures including the right atrium, superior vena cava, right main pulmonary artery, left brachiocephalic vein, and ascending aorta. Additional mildly FDG-avid mediastinal and supraclavicular lymph nodes were present (H & I). There was no evidence of contrast extravasation or frank rupture.

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Fig 1: (A) ¹⁸F FDG whole body PET MIP (maximum intensity projection) image. Fused images reveal postoperative changes, including prosthetic cardiac valves; (B and C) Show a saccular outpouching measuring approximately 3.6 × 3.7 × 3.6 cm (AP × TR × CC) arising from the anterior wall of the ascending aorta via a narrow neck (~2.5 mm) (blue arrow in B); (D and E) Show the sac opacified with contrast on arterial-phase CT and washout in the delayed images, confirming luminal communication. The aneurysm wall appeared thin; (F and G) Show a periaortic soft-tissue collection surrounding the sac with peripheral increased FDG uptake (SUVmax 4.5), indicative of active inflammation or infection. The lesion abutted adjacent mediastinal structures, including the right atrium, superior vena cava, right main pulmonary artery, left brachiocephalic vein, and ascending aorta; (H and I) Show additional mildly FDG-avid mediastinal and supraclavicular lymph nodes. FDG: Fluorodeoxyglucose; PET: Positron emission tomography; MIP: Maximum intensity projection; CT: Computed tomography; SUVmax: Maximum standardised uptake value; AP × TR × CC: Anteriorposterior x transverse x craniocaudal

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A diagnosis of an infected ascending aortic pseudoaneurysm was made. The patient was commenced on broad-spectrum intravenous antibiotics (ertapenem and teicoplanin) and advised of cardiac surgery. On day 5, ertapenem was discontinued due to suspected neurotoxicity (tremors, altered sensorium). Despite supportive care, the patient’s condition worsened, leading to acute decompensated heart failure and pulmonary oedema.

DISCUSSION

PInfected pseudoaneurysm of the ascending aorta is a rare and often delayed complication after cardiac surgery, which may present months or even years postoperatively.^[1,2,5] The clinical picture may be subtle, with non-specific symptoms like fever, malaise, or anorexia, often classified as pyrexia of unknown origin. Routine non-contrast imaging may detect mediastinal soft-tissue abnormalities but cannot reliably identify vascular communication or confirm infection.^[5]

Contrast-enhanced CT aortogram is the standard for detecting pseudoaneurysm neck, sac morphology, and contrast flow, but does not provide information on metabolic activity or inflammation. Conversely, ¹⁸F FDG PET-CT identifies hypermetabolic inflammatory/infective processes but lacks detailed anatomical resolution of vascular structures.^[3-7] Combining both modalities – as in contrast-enhanced PET-CT with CT aortogram – thus offers a comprehensive evaluation, enabling simultaneous assessment of vascular anatomy and infective activity.

In this patient, the hybrid imaging approach was crucial: it confirmed the presence of a saccular pseudoaneurysm, demonstrated its communication with the aortic lumen, and identified a surrounding FDG-avid collection, strongly suggesting active infection. Without contrast-enhanced CT and PET, this might have been misdiagnosed as a nonvascular postoperative collection, delaying diagnosis or leading to inappropriate management.

Review of recent nuclear medicine literature supports the use of PET/CT in suspected aortic graft or native aorta infections; increased FDG uptake in grafts or aneurysm walls has been correlated with infection rather than sterile postoperative changes.^[4-7]

Given the high risk associated with infected pseudoaneurysm (including rupture, mediastinitis, sepsis, and death), early diagnosis is critical.^[1,2] This case emphasises that in patients with prior cardiac surgery presenting with unexplained fever or mediastinal abnormalities, contrast-enhanced ¹⁸F FDG PET-CT should be strongly considered.

CONCLUSION

Contrast-enhanced ¹⁸F FDG PET/CT with CT aortogram protocol provides a powerful diagnostic modality for detecting infected ascending aortic pseudoaneurysm in postoperative cardiac patients presenting with pyrexia of unknown origin. Incorporation of vascular imaging protocols into PET/CT studies should be considered in such clinical scenarios to enable early diagnosis and timely surgical referral.