In a 56-year-old male with a prior traumatic aortic tear repaired with an aortic graft, who is an intravenous drug user with recurrent Serratia bacteremia, what is the best imaging study to rule out infection of the aortic graft?

Best Imaging Study for Suspected Infected Aortic Graft

CT angiography (CTA) is the best initial imaging study to rule out infected aortic graft in this patient, with strong consideration for 18F-FDG PET/CT if CTA is nondiagnostic or equivocal given the high clinical suspicion. 1

Algorithmic Approach to Imaging

First-Line Imaging: CTA

The 2016 American Heart Association guidelines clearly establish CTA as the initial imaging procedure for suspected intra-abdominal vascular graft infection (VGI) in the absence of recent manipulations 1. For this patient with a thoracic aortic graft from remote trauma (38 years ago), CTA should be performed first because it:

• Has sensitivity of 85-100% and specificity of 85-94% for VGI 1
• Rapidly identifies perigraft fluid collections, anastomotic pseudoaneurysms, and extent of infection
• Defines vascular anatomy for potential surgical planning
• Allows CT-guided aspiration of fluid collections for culture (critical for identifying Serratia as the source) 1

Key CTA findings suggesting infection:

• Perigraft fluid collection (not attributable to recent surgery since graft is 38 years old)
• Anastomotic pseudoaneurysm
• Perigraft soft tissue inflammation or gas 1

Second-Line Imaging: 18F-FDG PET/CT

If CTA is nondiagnostic or equivocal—which is common in intracavitary grafts—18F-FDG PET/CT should be obtained 1, 2. The 2025 EANM/SNMMI guidelines and 2016 AHA statement both support PET/CT as a valuable second-line test for vascular graft infections 1, 2.

Critical protocol requirements for PET/CT in this case:

• Myocardial suppression preparation is mandatory (high-fat, low-carbohydrate diet or prolonged fasting) to accurately assess thoracic aortic grafts 2
• Acquisition from head to lower limbs to identify infection source and assess for dissemination 2
• Delayed imaging may improve sensitivity for detecting graft infection 2

PET/CT interpretation for infected grafts:

• Positive findings: focal uptake of any intensity (score 4-6 on standardized scoring) 2
• SUVmax values typically >6.0 with focal uptake pattern suggest active infection requiring treatment 3
• In proven aortic graft infection, median SUVmax is 7.1 when starting antibiotics 3

Why This Approach for This Specific Patient

This 56-year-old IV drug user with recurrent Serratia bacteremia has extremely high pretest probability for infected aortic graft:

1. IV drug use is a major risk factor for Serratia bacteremia with high rates of endovascular complications (12% endocarditis rate in IV drug users with Serratia) 4
2. Recurrent bacteremia strongly suggests an endovascular source that hasn't been eradicated
3. The 38-year-old graft provides ample time for biofilm formation and chronic low-grade infection

Alternative Imaging Modalities (Lower Priority)

MRI/MRA can be considered if:

• Contrast-induced kidney injury is a concern
• CTA is nondiagnostic
• Sensitivity 68-85%, specificity 97-100% 1
• Better soft tissue resolution than CT but more expensive and time-consuming

Indium-111 labeled WBC scan:

• Should NOT be used alone 1
• Only considered in combination with other modalities if CTA and PET/CT are indeterminate
• Falsely positive in early postoperative period (not relevant here)
• Decreased sensitivity if patient is currently on antibiotics 1
• Inferior to both CTA and MRI for detecting aortic graft infection 5, 6

Ultrasound:

• Limited utility for intracavitary/thoracic grafts 1
• More useful for extracavitary (peripheral) graft infections
• Cannot adequately visualize thoracic aorta

Critical Pitfalls to Avoid

1. Do not rely on clinical findings alone—VGI presentation is often nonspecific, and this patient's recurrent bacteremia may be the only clue 1

2. Do not perform indium scan as first-line imaging—it has inferior performance compared to CTA and is particularly poor when used alone 1, 6

3. If PET/CT is performed, ensure proper myocardial suppression—failure to suppress myocardial uptake will obscure thoracic aortic graft assessment 2

4. If fluid collection is identified on CTA, obtain CT-guided aspiration for culture—microbiological confirmation of Serratia in perigraft fluid is diagnostic 1

5. Consider ESBL activity—36% of Serratia isolates show ESBL production in IV drug users; this impacts surgical planning and antibiotic selection 4

Clinical Decision Points

If CTA shows definitive infection (perigraft fluid, gas, pseudoaneurysm):

• Proceed directly to multidisciplinary surgical planning
• Obtain cultures via CT-guided aspiration if safe
• Start empiric broad-spectrum antibiotics (consider cefepime or carbapenem given ESBL risk) 4

If CTA is negative or equivocal but clinical suspicion remains high (recurrent Serratia bacteremia in IV drug user):

• Obtain 18F-FDG PET/CT with proper myocardial suppression 1, 2
• PET/CT has high negative predictive value and can confidently exclude infection if negative

If both CTA and PET/CT are negative:

• Consider alternative sources (endocarditis of native valves, other endovascular sites)
• MRI may be considered as third-line imaging 1