Structural evaluation of porcine heart valve prostheses with radiofrequency ultrasound

Background: Bioprosthetic heart valve use is limited by progressive degeneration. Early degenerative changes are often occult, making assessment of tissue integrity difficult. Ultrasound tissue characterization may detect alterations in tissue structure and allow early detection of leaflet degeneration. Methods: Using a modified echocardiographic unit (Acuson), radiofrequency (RF) integrated backscatter amplitude (IBA) (∫|RF|dt) was measured in 38 leaflets from nine explanted and six control porcine valves. Regions of interest in each leaflet were studied using four ultrasound frequencies. Radiographic gray scale mean and leaflet thickness were measured at each region of interest. Percent collagen and mineral were calculated for each region of interest using color-image processing of histologic sections and compared to IBA. Results: IBA values for control vs. explanted leaflets were (mean value±standard deviation): 8.2±4.69 dB vs. -4.7±4.64 dB at 7.0 MHz; -5.8±4.34 dB vs. -3.1±5.34 dB at 5.0 MHz; -3.8±3.38 dB vs. -2.1±3.18 dB at 3.5 MHz; and -9.0±4.58 dB vs. -7.1±4.25 dB at 2.5 MHz. Collagen content was 27.7±8.50% vs. 33.2±10.90%, mineral content was 0.1±0.10% vs. 2.1±4.30%, and radiographic gray scale mean was 150.6±1.96 vs. 145.3±5.14 for control vs. explanted leaflets, respectively. For control and explanted leaflets IBA, collagen content, mineral content, and radiographic gray scale mean were different (control vs. explanted P<0.05). Leaflet thickness was also noted to be different between the two groups. IBA was different among explanted leaflets with low, medium, and high mineral content. Conclusion: IBA was found to be a useful technique to differentiate normal from explanted porcine prosthetic valves in vitro. This method may be useful in the serial assessment of bioprosthetic leaflet degenerative properties in vivo.