S. E. Wayson1, D. Dalecki2, N. A. Wilson1,2 1University Of Rochester, Department Of Surgery, Rochester, NY, USA 2University Of Rochester, Department Of Biomedical Engineering, Rochester, NY, USA
Introduction: Anorectal malformations (ARMs) are congenital disorders that impair development of the rectum, anal canal, and anal sphincter. After surgical correction, fecal continence depends on the structure and function of the anal sphincter complex as microstructure impacts contraction strength. However, no current imaging technologies can non-invasively and quantitatively characterize anal sphincter structure. Parameters derived using quantitative ultrasound (US), such as the integrated backscatter coefficient (IBC), can estimate tissue structural properties. The IBC estimates scattering strength and exhibits angular dependence in tissues with aligned structure, thereby providing a measure of tissue organization. Our objective is to develop quantitative US techniques to characterize anal sphincter microstructural organization, based on the guiding hypothesis that the IBC as a function of insonification angle can detect fiber alignment in an anal sphincter acoustic phantom.
Methods: Two anal sphincter phantoms were fabricated using 10% gelatin, 5% graphite powder, and embedded 100-μm diameter suture. Muscle fascicles were modeled using concentrically arranged suture and disorganized fascicles were modeled using disorganized suture. Backscattered echoes were acquired at 19 insonification angles using a 58-MHz single-element US transducer. IBC images and the mean IBC for a region-of-interest (ROI) were computed at each angle. Gaussian curves were fitted to the data. To characterize differences in IBC curves between the aligned and disorganized phantoms, the maximum IBC (IBCmax) was used to quantify scattering strength and the IBC change within 8° of IBCmax (ΔIBC±4°) was used to quantify angular dependence of scattering strength.
Results: IBC images at angles of 90°, 94°, and 98° are shown for the aligned (Fig. 1A) and disorganized (Fig. 1B) phantoms. IBC magnitude was angular dependent for the aligned phantom. The disorganized phantom showed weaker scattering and less angular dependence compared to the aligned phantom. Figure 1C presents IBC estimates as a function of angle for both phantoms. The IBCmax and ΔIBC±4° were greater in the aligned compared to the disorganized phantom.
Conclusion: This work demonstrates proof-of-concept that quantitative US and IBC as a function of insonification angle can be used to characterize differences in structural organization in anal sphincter phantoms. These findings suggest that scattering strength and the angular dependence of scattering strength is reduced in disorganized compared to aligned structures. This work establishes the foundation for a dedicated preclinical quantitative US technique to characterize anal sphincter microstructure.
