Lipomatous hypertrophy of the interatrial septum

Non-encapsulated fatty overgrowth of the interatrial septum, typically in obese elderly patients, classically sparing the fossa ovalis and giving a “dumbbell/waist” shape. Benign but can look scary + be FDG-avid.

Imaging features

CT

• Low attenuation fat in interatrial septum (≈ –80 to –120 HU)
• Marked thickening (cutoff often >2 cm)
• Characteristic dumbbell shape (best seen in short axis reconstruction)
• Spares fossa ovalis → central thin waist
• May extend to RA/RV roof; no mass effect capsule or invasion

MRI

• High T1 / high T2 with India-ink or fat-sat signal drop
• No enhancement after contrast (other than thin septal vessels)
• Confirms fat and helps exclude solid tumour

Echo

• Echogenic thickened septum, sparing fossa ovalis
• Bulky septum may protrude into RA more than LA

PET

• Can be FDG-avid (brown-fat–like metabolism) → classic pitfall for “cardiac malignancy”.

Differentials – how to tell apart

• Atrial lipoma

  • Encapsulated, discrete mass, may not spare fossa ovalis
  • No “waisted dumbbell” shape.

• Cardiac metastasis / sarcoma / lymphoma

  • Soft-tissue attenuation, not fat density
  • Infiltrative, enhancing, often multifocal or with pericardial/adjacent involvement.

• Thrombus

  • Non-enhancing soft tissue, usually in appendage
  • No macroscopic fat, often associated with AF, low flow.

Associations & clinical angle

• Elderly, obese patients; often with atrial arrhythmias (esp. AF)
• Rarely SVC/RA inflow obstruction, conduction disturbance.