Quantitative hemodynamic evaluation in children with coarctation of aorta: phase contrast cardiovascular MRI versus computational fluid dynamics

Pressure gradient across coarctation of aorta (CoA) is conventionally computed from phase contrast magnetic resonance imaging (PC-MRI) by applying the Bernoulli equation to the peak blood flow velocity measurement obtained just distal to the aortic narrowing. In order to test the validity and accuracy of the Bernoulli flow assumptions of negligible viscous forces in assessment of pressure gradients across the coarctation, we sought to determine pressure information from patient-specific computational fluid dynamics (CFD) simulation, modeling Newtonian, viscous, incompressible blood flow under steady and pulsatile inflow conditions. The transient high velocity jet observed though a moderate thoracic aortic coarctation model (65% area reduction) reconstructed from magnetic resonance angiography scans of an 8-year old female patient provided for the 2012 STACOM CFD challenge, was studied over a cardiac cycle under patient-specific flow conditions. Descending aorta hemodynamics was contrasted with a geometrically and dynamically comparable normal aorta simulation. The peak velocity of the modeled CoA jet (6.99 m/s) was observed to occur ~2 cm distal to the site of coarctation. The magnitude of this velocity was found to be similar to appropriately dynamically scaled clinical observations (6.00±0.6 m/s) of peak velocity obtained from PC-MRI data on three pre-surgical CoA patients, evaluated at Children's Hospital of Pittsburgh. Bernoulli pressure gradient across the CoA computed using the CFD velocity field at the peak-systole instant of pulsatile flow grossly overestimated the true gradient predicted from CFD (30 mm Hg) when unsteady jet wake effects were more pronounced, but underestimated the CFD pressure gradient at steady time-averaged inflow conditions (5.8 mm Hg). Based on this pilot study, CFD determined flow fields are a more reliable clinical indicator of pressure gradient which considers viscous flow and complex jet wake interactions affecting hemodynamics downstream of CoA.