In vivo noninvasive systemic myography of acute systemic vasoactivity in female pregnant mice

Altered vasoactivity is a major characteristic of cardiovascular and oncological diseases, and many therapies are therefore targeted to the vasculature. Therapeutics which are selective for the diseased vasculature are ideal, but whole-body selectivity of a therapeutic is challenging to assess in practice. Vessel myography is used to determine the functional mechanisms and evaluate pharmacological responses of vascularly-targeted therapeutics. However, myography can only be performed on ex vivo sections of individual arteries. We have developed methods for implementation of spherical-view photoacoustic tomography for non-invasive and in vivo myography. Using photoacoustic tomography, we demonstrate the measurement of acute vascular reactivity in the systemic vasculature and the placenta of pregnant mice in response to two vasodilators. Photoacoustic tomography simultaneously captured the significant acute vasodilation of major arteries and detected the selective vasoactivity of the maternal-fetal vasculature. Photoacoustic tomography has the potential to provide invaluable preclinical information on vascular response that cannot be obtained by other established methods.

Methods
In this dataset, we acquired 3D photoacoustic tomography image volumes of pregnant CD-1 mice (Gestational day 16) to demonstrate in vivo vasodilation during pregnancy. The 3D volumes were acquired using the TriTom photoacoustic tomography system (Photosound Technologies Inc., Houston, TX) every 38 seconds with a 3 cm3 volume of the abdomen of the animals. Each volume captured the systemic and fetal vasculature and placentas.  

We treated the animals with the vasodilators sildenafil (sildenafil citrate, PHR1807, Sigma-Aldrich) and the G protein-coupled receptor G-1 (100089335, Cayman Chemical, Inc.). A volume of 0.1 mL of either G-1 (100 µg/kg of body weight in 5% DMSO and PBS), sildenafil (1 mg/kg body weight in PBS), or control PBS was administered through the right jugular vein catheter, followed by a flush of 0.1 mL PBS, while imaging continuously.

B-mode ultrasound imaging (Vevo 2100, FUJIFILM VisualSonics, Inc., Toronto) was used to validate the extent of vasodilation and response time, using a single artery and G-1. The cross-sectional change in diameter of the artery was monitored continuously before, during, and post-drug administration for 1 hour. The diameter of the artery was measured manually using the measurement tools in the VevoLab software.

The 3D images from the photoacoustic tomography system were acquired before, during, and after drug administration at 808 nm, the isosbestic point of the optical absorption of hemoglobin and oxyhemoglobin for 30 minutes. The laser fluence at the surface of the mouse skin was calculated to be 0.27, 0.325, and 0.142 mJ/cm2 for 690, 808, and 890 nm wavelengths respectively. The photoacoustic images were reconstructed using a standard modified back-projection algorithm (TriTom reconstruction software V3.0.3). After reconstruction, additional processing steps were performed in Matlab V2021b. In this 3D volumetric image, we monitored the systemic arteries including the internal thoracic artery, iliac artery branching from the abdominal aorta as well as uterine artery. In addition, we monitored multiple placentas with their fetuses where spiral arteries feed the placenta and umbilical cord connecting the fetal side of the placenta to the fetus.

To measure the diameter of the artery, we applied a Frangi vesselness filter to enhance the vascular signal in the 3D images. The 3D images were visualized in Amira V6.0.1 (Thermo Fisher Scientific, Waltham, MA). We manually segmented the 3D volume of the internal thoracic arteries, iliac artery, and superficial EPA arteries and measured the arterial diameter. To measure the photoacoustic signal intensity of the placenta, fetal vasculature, and uterine artery volumes, we manually segmented the 3D volumes from 3D images using MATLAB.