Optimization of Ultrasound Backscatter Spectroscopy to assess neurotoxic effects of anesthesia in the newborn non-human primate brain

Studies in animal models have demonstrated that long exposures to anesthetics can induce apoptosis in the newborn and young developing brain. These effects have not been confirmed in humans due to the lack of a non-invasive, practical in vivo imaging tool with the ability to detect these changes. Following the success of using Ultrasound Backscatter Spectroscopy (UBS) to monitor in vivo cell death in breast tumors, we aimed at applying UBS to assess neurotoxicity of the anesthetic Sevoflurane (SEVO) in a non-human primate (NHP) model. Sixteen 2–7 day-old Rhesus macaques were exposed for 5 hours to SEVO. Ultrasound scanning was done with a phased array transducer on a clinical ultrasound scanner operated at 10 MHz. Data consisting of 10–15 frames of radiofrequency (RF) echo signals from coronal views of the thalamus were obtained at 0.5 and 6.0 h after initiating exposure. The UBS parameter “Effective Scatterer Size” (ESS) was estimated by fitting a scattering form factor (FF) model to the FF measured from RF echo signals. The approach involved analyzing the frequency-dependence of the measured FF to characterize scattering sources and selecting the FF model based on a χ(2) goodness of fit criterion. To assess data quality, a rigorous acceptance criterion based on the analysis of prevalence of diffuse scattering (an assumption in the estimation of ESS) was established. ESS changes after exposure to SEVO were compared to changes in a control group of 5 primates for which ultrasound data were acquired at 0 min and 10 min (no apoptosis expected). Over the entire data set, the average measured FF at 0.5 h and 6.0 h monotonically decreased with frequency, justifying fitting a single form factor over the analysis bandwidth. χ(2) values of an (inhomogeneous continuum) Gaussian FF model were one fifth those of the discrete fluid sphere model, suggesting that a continuum scatterer model better represents ultrasound scattering in the young Rhesus brain. After applying the data quality criterion, only 5 out of 16 subjects from the apoptotic group, and 5 out of 5 subjects from the control group, fulfilled the acceptance criteria. All of the subjects in the apoptotic group that passed the acceptance criterion showed a significant ESS reduction at 6.0 h. These changes (−6.4% [95% Confidence Interval −14.3% to −3.3%]) were larger than those in the control group (−0.8% [95% Confidence Interval −2.0% to 1.5%]). Data with low prevalence of diffuse scattering corresponded to possibly biased results. Thus, ESS has potential to detect changes in brain microstructure related to anesthesia-induced apoptosis.