Ultra high frequency ultrasound: a promising technique to visualize pelvic floor meshes in vivo

The in vivo behaviour of pelvic floor implants and the foreign body response they induce know great interpatient variability, depending on both patient and implant characteristics. The configuration and textural properties of the implant, such as pore size and shape, may change after implantation, which in turn affects the foreign body response1. A method to accurately study pelvic floor implants in vivo, longitudinally and non-invasively, is currently lacking. A recent study demonstrated that ultra high frequency ultrasound (UHFU) offers improved quality of vaginal imaging compared to conventional ultrasound, allowing clear visualization of the vaginal walls2. Here, we investigated whether UHFU also allows for detailed visualization of vaginal mesh. We used an UHFU system (Vevo 3100, Fujifilm VisualSonics Inc., Toronto, ON, Canada) with transducers with respective centre frequencies of 40 MHz (MX550: axial resolution 40 µm) and 20 MHz (MX250: axial resolution 75 µm). Two different mesh constructs were imaged: PP mesh (Restorelle DirectFix Mesh, Coloplast, MN, USA) and a fully biodegradable poly-4-hydroxybutyrate mesh (P4HB ‘Diamond’ construct, Tepha, MA, USA) in two different experimental set-ups. We performed 2-dimensional (2D), cross-sectional imaging and 3-dimensional (3D) brightness (B) mode imaging and measured pore size and cross-sectional fibre surface of the different materials. Imaging with UHFU resulted in clear visualization of PP and P4HB vaginal mesh constructs. 3D-B mode allowed for a 3D reconstruction of the mesh construct and subtraction of the surroundings (Figure 1A, PP mesh. Figure 1B, P4HB mesh). Cross- sectional imaging allowed for accurate measurements of pore size and fibre surface area (Figure 1C). Imaging was also feasible in an in vivo mimicking set up, in which case meshes were positioned in an ex vivo chicken breast (Figure 2). These results suggest that UHFU is a promising technique for non-invasive imaging of pelvic floor implants, allowing for quantification of shrinkage, pore-stability, and degradation of (future) pelvic floor implants. Additionally, UHFU add-ons such as Power Doppler and photoacoustic imaging may facilitate ultrasensitive analysis of parameters of the foreign body response, such as angiogenesis (important for mesh integration), fibrosis (considered the root of many mesh-related adverse events) and bacterial biofilm formation3-5. Hereby, UHFU also has to potential to reduce the sample size of animal studies evaluating newly designed pelvic floor implants, since animals do not have to be sacrificed in order to perform ex vivo analyses of the implanted material at multiple time points. Future studies should ascertain whether in vivo imaging of mesh can be performed in comparable detail as the current in vitro study. Furthermore, the correlation between in vivo imaging and histology should be assessed, as well as the additional value of the mentioned add-ons for imaging angiogenesis, fibrosis and biofilm formation, and their correlation with ex vivo, histological measurements.