Myocardial Fiber Mapping of Rat Hearts, Using Apparent Backscatter, with Histologic Validation

Abstract

Myocardial fiber architecture is a physiologically important regulator of ejection fraction, strain, and pressure development. Apparent ultrasonic backscatter has been shown to be a useful method for recreating the myocardial fiber architecture in human-sized sheep hearts, due to the dependence of its amplitude on the relative orientation of a myofiber to the angle of ultrasonic insonification. Thus, the anisotropy of the backscatter signal is linked to, and provides information about, the fiber orientation. In this study, we sought to determine if apparent backscatter could be used to measure myofiber orientation in rodent hearts. Fixed adult rat hearts were imaged intact, and both a transmural cylindrical core and transmural wedge of the LV free wall were imaged. Cylindrical core samples confirmed that backscatter anisotropy could be measured in rat hearts. Ultrasound and histological analysis of transmural myocardial wedge samples confirmed that the apparent backscatter could be reproducibly mapped to fiber orientation (angle of the fiber relative to the direction of insonification). These data provided a quantitative relationship between the apparent backscatter and fiber angle that was applied to whole heart images. Myocardial fiber architecture was successfully measured in rat hearts. Quantifying myocardial fiber architecture using apparent backscatter provides a number of advantages, including its scalable use from rodents to man, its rapid low-cost acquisition, and minimal contraindications. The method outlined in this study provides a method for investigators to begin detailed assessments of how the myocardial fiber architecture changes in pre-clinical disease models, which can be immediately translated into the clinic.

Collagenase treatment reduces the anisotropy of ultrasonic backscatter in rat myocardium by reducing collagen crosslinks

“…Prior to digestion, the most intense ultrasonic backscatter was visualized in areas of the myocardium where myofibers aligned perpendicular to the angle of insonification: backscatter intensity was comparatively reduced in areas of the myocardium with predominately parallel myofibers. This fiber‐alignment determinant of the anisotropy of ultrasonic backscatter has been previously validated (Milne et al., 2012, 2016, 2019). Throughout collagenase digestion, backscatter intensity increased disproportionally in areas where myofibers aligned parallel to the angle of insonification.…”

“…The highest backscatter intensity is generated where myofibers are aligned perpendicular to the angle of insonification and is lowest where myofibers are parallel. We, and others, have shown that anisotropy of ultrasonic backscatter relates directly to the alignment of myofibers, evidenced in both short and long‐axis ultrasonic images, of small and large mammals (Holland et al., 2005; Milne et al., 2012, 2016, 2019). Thus, myocardial backscatter is anisotropic, or variable, depending on the orientation of the ultrasound source and the myocardial myofiber (mesostructure) alignment.…”

“…Receiver gain was set to 60 dB and time gain compensation (TGC) was set to zero for each heart. These gain settings were previously established to maintain a linear relationship between backscatter and gain (Milne et al., 2019). Once the transducer was aligned to obtain short axis (parasternal‐like alignment) 3D images, the experimental procedure began.…”

“…Sex was not considered a biological variable because a treatment (collagenase) effect on a biological sample (heart) was being assessed; use of more than one sex could have led to unnecessary duplication if the treatment in this initial study did not produce an effect. The study utilized female rates for consistency with previous studies by our lab group when assessing backscatter intensity anisotropy (Milne et al, 2019). Animal use was approved by the Institutional Animal Use and Care Committee of Wayne State University.…”

Estimating cardiac active tension from wall motion—An inverse problem of cardiac biomechanics

“…In particular, we are aiming at clinical cine MRI acquisitions. The rule‐based assumption of cardiac fiber orientation could in future be personalized by diffusion tensor MRI 7 or quantified by echocardiography 8,9 . Since in case of using clinical data, there is no ground truth available to validate the method, we decided to first determine the accuracy of the reconstruction of active tension and its sensitivity to variation of model parameters using synthetic data.…”