Sequential magnetic resonance monitoring of pulmonary flow with endovascular stents placed across the pulmonary valve in growing swine

Background-Patients with endovascular stent implantation for the treatment of right ventricular outflow tract obstruction are often left with incomple te relief of the obstruction and significant pulmonary regurgitation. A non invasive and reproducible method for monitoring such patients is desirable. MRI in the presence of a stent, however, has to overcome the problem of po tential metallic artifacts. Methods and Results-Under x-ray fluoroscopic guidance, endovascular nitinol stents were placed across the pulmonary valve in 6 young pigs to induce pu lmonary regurgitation. Five additional pigs served as controls. Initial MRI was performed after 2 days (13.5 +/-1.8 kg) and follow-up after 3 months ( 32 +/-2.9 kg). Pulmonary flow volumes and regurgitant fraction were quantif ied by velocity-encoded cine (VEC) MRI through (VEC-TS) and distal to (VEC- DS) the stent. VEC-TS was compared with VEC-DS and volumetric measurements of left and right ventricular stroke volumes provided by cine MRI ("gold st andard"). Antegrade and retrograde pulmonary flow volumes by VEC-TS were sl ightly but significantly less than those with VEC-DS and cine MRL Excellent correlations (r >0.97) for phasic pulmonary flow volumes as measured by VE C-TS and VEC-DS were shown. Pulmonary regurgitant fraction increased from 3 2.8 +/- 15% to 49.6 +/- 17% (P <0.05) over the course of 3 months with VEC- TS. Conclusions-MRI demonstrates the progression of pulmonary regurgitation in growing swine. VEC MRI has the ability to quantify pulmonary blood flow ins ide the lumen of nitinol stents. MRI appears to be ideally suited for monit oring patients with endovascular nitinol stents in the pulmonary artery or pulmonary valve position.