Which imaging finding is most consistent with collateral flow into an aneurysm sac?

• A. Monophasic waveform with broad systolic upstroke and diastolic forward flow
• B. Triphasic waveform with exaggerated systolic peak
• C. Biphasic waveform with reversed diastolic flow
• D. High-resistance waveform with minimal diastolic flow

Answer: (A)

Collateral inflow into an aneurysm sac creates a low-resistance circuit feeding the sac, so the feeding arteries show a monophasic Doppler pattern with a broad systolic upstroke and continuous forward flow in diastole. This persistent diastolic forward flow reflects the low resistance of the collateral supply sustaining flow into the sac. The other waveform patterns represent higher downstream resistance or proximal obstruction and would not indicate ongoing collateral feeding of the aneurysm sac. Therefore, the described monophasic waveform with diastolic forward flow best fits collateral inflow into the sac.