Mycotic Aneurysm - Imaging

Background

Imaging Modalities

1. CT Angiography (CTA) - First-Line for Most Locations

• Saccular and lobulated vessel contours - eccentric or multilobulated shape, rather than smooth fusiform
• Perianeurysmal soft tissue inflammation - manifested as perivascular contrast enhancement / inflammatory mass
• Intramural or perivascular air - pathognomonic when present
• Perianeurysmal fluid collection
• Eccentric, rapidly enlarging aneurysm in the context of fever/infection

"Features pointing to an infectious etiology of an aneurysm include saccular, eccentric, or multi-lobulated shape surrounded by inflammatory mass, intramural or perivascular air, or fluid. These features are well demonstrated with CT angiography."

• Fuster and Hurst's The Heart, 15th Edition

2. MR Angiography (MRA) / MRI

• MRI with gadolinium enhancement: higher sensitivity for small lesions, microbleeds, cerebritis, and brain abscesses (critical in IE workup)
• MRI detects microbleeds (round T2* hypointensities ≤10 mm) in up to 80% of IE patients on imaging
• For intracranial MAs specifically: MRI/MRA has approximately 40% sensitivity - significantly lower than DSA for small distal aneurysms
• Cerebral imaging is mandatory (CT or MRI) for any suspicion of neurological complication in IE patients

3. Digital Subtraction Angiography (DSA) - Gold Standard for Intracranial MAs

• Distal location - beyond M2 (MCA), P1 (PCA), or equivalent; peripheral rather than at the circle of Willis
• Multiplicity - multiple aneurysms are characteristic (vs. saccular berry aneurysms)
• Fusiform or irregular shape
• Change in size on follow-up - new aneurysm appearance or growth on repeat DSA during antibiotic treatment
• Located at arterial bifurcations (sites of flow turbulence where emboli lodge in the vasa vasorum)

4. Duplex Ultrasound

• Useful for peripheral artery mycotic aneurysms (femoral, popliteal, radial/ulnar)
• Can detect pulsatile mass, confirm flow within aneurysm sac
• Color Doppler demonstrates swirling "yin-yang" pattern
• Limited for deep visceral or intracranial locations

5. Nuclear Medicine / FDG-PET

• FDG-PET (often combined as PET-CT) is primarily used to detect graft infection and aortic/large vessel mycotic aneurysms
• Useful in cases where CT findings are equivocal - metabolic activity in the aneurysm wall suggests active infection
• Particularly helpful for mycotic aortic aneurysms and post-endovascular graft infection surveillance

6. Echocardiography

• Transthoracic echocardiography (TTE) is the preferred modality for coronary artery mycotic aneurysms
• Transesophageal echocardiography (TEE) is essential for the underlying IE diagnosis and source identification

Imaging by Location

Key Imaging Differentiators from Non-Infectious Aneurysms

Serial Imaging - Critical Point

Recent Evidence

Pathophysiology of Mycotic Aneurysm Formation: How Infected Emboli Weaken the Arterial Wall

Overview of the Sequence

Step 1 - The Source: Vegetations on the Valve

Step 2 - Two Routes of Entry into the Arterial Wall

Route A: Occlusion of the Vasa Vasorum (primary and most likely)

"The most likely cause is impaction of infected material in the vasa vasorum of the artery, with resulting destruction of the arterial wall."

• Bradley and Daroff's Neurology in Clinical Practice

Route B: Intraluminal Impaction

Route C: Contiguous Spread

Step 3 - Bacterial Infection of the Wall Layers

1. Bacterial toxins and enzymes (particularly from Staphylococcus spp.) are released directly, degrading the extracellular matrix of the vessel wall.
2. Pattern recognition by toll-like receptors on resident vascular cells triggers a local inflammatory response.
3. Pro-inflammatory cytokines (IL-1β, TNF-α, IL-6) are released by endothelial cells, smooth muscle cells, and macrophages - recruiting neutrophils and monocytes into the wall.

Step 4 - The Central Destructive Mechanism: Matrix Metalloproteinases (MMPs)

• Recruited neutrophils and macrophages, activated by cytokine signaling, release matrix metalloproteinases (MMPs) - particularly MMP-2, MMP-3, MMP-9, and MMP-12.
• MMPs are zinc-dependent proteolytic enzymes that degrade the structural proteins of the arterial wall: elastin (the elastic fiber backbone of the media) and collagen (the tensile-strength scaffold of all three layers).
• Normally, MMP activity is controlled by TIMPs (tissue inhibitors of metalloproteinases). In an infected arterial wall, the inflammatory state upregulates MMPs while reducing TIMPs, creating an unchecked proteolytic environment.
• The PGE2-cAMP signaling pathway, activated by inflammatory cytokines, amplifies MMP production.

"Bacterial infection triggers the release of proinflammatory cytokines, which attract neutrophils. These cells activate matrix metalloproteinases, which contribute to the focal breakdown of the blood vessel wall. The degree of metalloproteinase activity correlates with the risk of future aneurysm rupture."

• StatPearls: Mycotic Aneurysm (2025)

Step 5 - Structural Failure of the Arterial Wall

Step 6 - Immune Complex Deposition (Additional Mechanism)

Step 7 - Why Intracranial Arteries Are Particularly Vulnerable

• No external elastic lamina (absent or poorly developed)
• Thin adventitia compared to systemic vessels
• No vasa vasorum in the smallest branches - meaning emboli lodge directly in the arterial lumen and invade the intima
• Located at bifurcation points where flow turbulence and eddy currents cause endothelial shear stress, facilitating embolus impaction and bacterial adherence

Summary Diagram (Conceptual)

Valve vegetation (IE)
        ↓
Septic embolus shed into circulation
        ↓
    ┌──────────────────────────────┐
    │ Lodges in vasa vasorum       │  ← Primary route
    │ (infarcts wall from outside) │
    └──────────────────────────────┘
    ┌──────────────────────────────┐
    │ Occludes arterial lumen      │  ← Secondary route
    │ (bacteria invade intima)     │
    └──────────────────────────────┘
        ↓
Bacteria establish in adventitia/media
        ↓
Cytokines (IL-1β, TNF-α) → recruit neutrophils & macrophages
        ↓
MMP release (MMP-2, -3, -9, -12) + TIMP suppression
        ↓
Elastin + collagen degradation → transmural necrosis/abscess
        ↓
Wall cannot withstand arterial pulsation
        ↓
Aneurysmal dilation → RUPTURE (SAH, intracerebral hemorrhage, or hemorrhage into the relevant territory)

Why Some Resolve and Others Rupture

• Effective antibiotics can sterilize the wall before irreversible structural damage occurs - explaining why ~30% of intracranial MAs resolve on antibiotic therapy alone with serial imaging.
• MMP activity level may be the key prognostic variable - higher activity correlates with rupture risk.
• Aneurysm growth on follow-up imaging is the clearest predictor of imminent rupture, which is why serial DSA during antibiotic treatment is standard practice.

Viva: Mycotic Aneurysms

Suprasellar Mass and Craniopharyngioma - Imaging

Anatomy of the Suprasellar Region - Why It Matters

Suprasellar Mass - Differential Diagnosis Framework

• Children: Craniopharyngioma, optic/hypothalamic glioma, germinoma
• Adults: Pituitary macroadenoma, meningioma, craniopharyngioma (papillary type)

Craniopharyngioma

Origin and Types

CT Imaging

Adamantinomatous CP on CT:

• Hypodense cystic component in the suprasellar cistern - often multilocular
• Calcification - present in ~90%; can be peripheral "eggshell" calcification of the cyst wall, or irregular nodular foci within the solid component
• Solid enhancing nodule after contrast
• The classic CT triad: cyst + solid nodule + calcification
• May extend into the sella inferiorly or the third ventricle superiorly
• Obstructive hydrocephalus when third ventricle is compressed

Papillary CP on CT:

• Isodense solid suprasellar mass - lacks the cystic/calcified appearance
• Typically no calcification
• Homogeneous enhancement after contrast
• Often located in the third ventricle floor

MRI Imaging - Gold Standard for Characterization

Signal characteristics of Adamantinomatous CP:

"Cystic components are typically hyperintense and less commonly isointense to CSF on T1-weighted images. Multiple cysts with varying signal intensities are characteristic. Solid components have variable signal intensities and usually enhance with contrast."

• Brain Imaging with MRI and CT (Cambridge)

Signal characteristics of Papillary CP:

• T1: Iso- to slightly hypointense (no cholesterol/blood content)
• T2: Hyperintense solid mass
• T1 + Gd: Solid, homogeneous enhancement
• No blooming on SWI (no calcification)
• Often appears as a well-demarcated solid mass in the third ventricle floor

MRI Sequences to Request:

1. Sagittal T1 thin-slice (3mm) - best for pituitary stalk, optic chiasm, hypothalamus relationships
2. Coronal T2 - cyst characterization, third ventricle involvement
3. Coronal + sagittal T1 post-Gd - enhancement pattern
4. Axial FLAIR - perilesional edema, hydrocephalus
5. SWI - calcification, hemorrhage
6. DWI - helps distinguish epidermoid (restricted diffusion) from craniopharyngioma (no restriction)

Histopathology Correlate

Differential Diagnosis on Imaging

How to distinguish key suprasellar lesions:

• Adenoma originates from the sella (snowman sign - figure-of-8 expansion through diaphragma sellae); craniopharyngioma is primarily suprasellar
• Adenoma is isointense to grey matter, solid, delayed homogeneous enhancement; rarely calcifies
• Adenoma does NOT have the T1-bright cysts of ACP
• Adenoma may invade the cavernous sinus laterally (Knosp grading)

• RCC is a single ovoid cyst - no enhancing solid nodule, no calcification (or rare thin peripheral calcification)
• T1 signal variable (mucoid: T1 bright; serous: T1 dark)
• No multilocularity; thin or absent wall enhancement
• Key: RCC has no solid nodule and no calcification - the hallmarks of craniopharyngioma

• Meningioma: dural tail sign, dense homogeneous enhancement, isointense T1/T2 to brain, rarely cystic, may show calcification but of different character
• Meningioma arises from tuberculum sellae/planum sphenoidale - not from Rathke's pouch

• Germinoma: isointense, solid, no calcification, homogeneous enhancement
• Germinoma may have elevated CSF/serum β-hCG, AFP
• Germinoma is highly radiosensitive - distinguishing it from craniopharyngioma changes management completely
• DWI: germinoma may show mildly restricted diffusion (hypercellular)

• Glioma: T2 hyperintense without the cystic-calcified morphology of ACP
• Strong association with NF1
• Enhancement variable; may be absent in low-grade glioma

• Pulsation artifact on MRI
• Flow void on T2; MRA/CTA shows connection to ICA or ACoA
• Never calcifies in the craniopharyngioma pattern

Summary: Imaging Signature by Lesion

Clinical Correlations

• Bitemporal hemianopia → optic chiasm compression from above → suprasellar (not sellar) mass
• Diabetes insipidus at presentation → suggests germinoma or Langerhans cell histiocytosis first (pituitary stalk involvement); craniopharyngioma causes DI more often post-operatively
• Growth failure + visual loss in a child → craniopharyngioma until proven otherwise; CT first for calcification
• Pituitary stalk thickening → germinoma, LCH, hypophysitis, TB, sarcoid

Acute Ischemic Stroke: Imaging and Management

Pathophysiology in Brief

• Ischemic core: irreversibly infarcted (CBF <10 mL/100g/min) - dead within minutes
• Penumbra: ischemic but not yet infarcted (CBF 10-20 mL/100g/min) - potentially salvageable with reperfusion

IMAGING

The "Code Stroke" CT Protocol

1. Non-contrast CT (NCCT) → exclude hemorrhage, assess ASPECTS
2. CT Angiography (CTA)   → identify LVO, assess collaterals
3. CT Perfusion (CTP)     → core vs penumbra mismatch (selected patients)

1. Non-Contrast CT (NCCT) - First and Always

"Large cortical infarcts might not be detected by NCCT up to 3 hours from onset, and around 40% of infarcts can be missed by 24 hours."

ASPECTS Score (Alberta Stroke Program Early CT Score)

• Each region scores 1 point if normal
• Subtract 1 point for each region showing early ischemic change
• Score 10 = no ischemic change (ideal)
• Score ≤7 = poor prognosis for independent recovery
• Score ≥6 traditionally required for thrombectomy eligibility (now reassessed - see below)

2. CT Angiography (CTA) - Identify the Occlusion

• Identifies the large vessel occlusion (LVO) site: ICA terminus, M1, M2, basilar artery, etc.
• Collateral circulation grading - good collaterals = larger salvageable penumbra = more time for treatment
• Rules out underlying vascular lesions (dissection, stenosis)
• CTA source images (CTA-ASPECTS) are superior predictors of final infarct size compared to NCCT-ASPECTS, because they directly show the degree of collateral perfusion

3. CT Perfusion (CTP) - Core vs Penumbra Mismatch

• Extended thrombolysis window (4.5-9 hours) - EXTEND trial
• Late-window thrombectomy (6-24 hours) - DAWN and DEFUSE-3 trials

4. MRI - More Sensitive, Used When Available

DWI (Diffusion-Weighted Imaging) - Most Important Sequence

• Detects ischemic core within minutes of onset - the earliest sequence to show infarction
• Restricted diffusion (bright on DWI, dark on ADC map) = cytotoxic edema from Na-K ATPase failure
• Sensitivity ~80-95% for acute infarction (note: false negatives exist, especially in posterior fossa, small lacunes, and hyperacute <6 hours)
• DWI-FLAIR mismatch: DWI bright + FLAIR normal = stroke likely within 4.5 hours (used in WAKE-UP trial for unknown-onset stroke treatment)

MRI Sequences in Acute Stroke

5. Imaging Sequence Summary by Time Window

MANAGEMENT

Emergency Assessment - "Time is Brain"

• Door-to-needle time (IV tPA): <60 minutes
• Door-to-groin puncture (thrombectomy): <90 minutes

1. Airway, breathing, circulation - stabilize
2. Blood glucose (hypoglycemia mimics stroke - treat if <60 mg/dL)
3. NCCT + CTA (CT protocol as above)
4. 12-lead ECG (AF is common stroke cause)
5. IV access, bloods: FBC, coagulation, metabolic panel
6. NIHSS scoring

A. Intravenous Thrombolysis (IV tPA / Alteplase)

"The EXTEND study randomized patients presenting between 4.5 and 9 hours after onset - favorable outcome at 90 days was present in more patients in the alteplase group (35.4% vs 29.5%)." - Fuster and Hurst's The Heart, 15th Ed.

Key tPA Contraindications

B. Mechanical Thrombectomy (MT) - LVO Stroke

• Stent retrievers (Solitaire, Trevo): deployed beyond the clot, then withdrawn with clot engaged in mesh
• Large-bore aspiration catheters (Penumbra): direct suction thrombectomy - equivalent to stent retrievers in ASTER and COMPASS trials
• Often combined: aspiration + stent retriever ("Solumbra technique")

Thrombectomy Time Windows and Criteria

"Patients who present in this delayed window are thought to be slow progressors, with leptomeningeal collaterals that help sustain salvageable brain tissue." - Fuster and Hurst's The Heart, 15th Ed.

C. Blood Pressure Management

• Pre-tPA: Must lower BP to <185/110 mmHg before administration (IV labetalol or nicardipine); maintain <180/105 for 24h after
• No thrombolysis/thrombectomy candidate: Permissive hypertension up to 220/120 mmHg for 24-48h (allows autoregulation in penumbra)
• Post-thrombectomy (successful recanalization): Target <180/105 mmHg to reduce risk of reperfusion hemorrhage
• 2025 ESO update (PMID 42095756): Reaffirms cautious approach to BP lowering in acute phase

D. Antiplatelet Therapy

• No LVO/thrombolysis: Aspirin 300 mg loading dose within 48 hours (IST and CAST trials)
• Post-tPA: Hold antiplatelet and anticoagulants for 24 hours (re-image first to exclude hemorrhagic transformation)
• TIA/minor stroke: Dual antiplatelet therapy (aspirin + clopidogrel) for 21 days (POINT and CHANCE trials)

E. Neuroprotective / Supportive Measures

F. Functional Outcomes (Modified Rankin Scale)

Summary Algorithm

Suspected AIS
      ↓
NCCT (immediate) → Hemorrhage? → YES → Treat hemorrhagic stroke
      ↓ NO
ASPECTS score + CTA
      ↓
LVO identified?
   ├─ YES → IV tPA (if eligible, no contraindications) + Thrombectomy
   │                 ↑ 0-24h depending on mismatch imaging
   └─ NO  → IV tPA alone (if <4.5h, or 4.5-9h with CTP mismatch)
            Aspirin 300mg if no tPA/thrombectomy
      ↓
Supportive care: BP, glucose, temp, O2
Early rehab: physio, OT, speech therapy
Secondary prevention: antiplatelet/anticoagulant, statin, risk factor control