Document Type
Article
Publication Date
2019
Abstract
It has been found that approximately 45% of all patients who undergo heart surgery suffer a lack of brain oxygenation at some point during the operation. Unfortunately, monitoring cerebral oxygen saturation non-invasively in adults during surgery is extremely difficult. While near-infrared spectroscopy (NIRS) can be used to monitor cerebral oxygenation in neonates, extending it to adults is challenging due to the increased thickness of the extra-cerebral layers in adult heads.Two potential NIRS methods for monitoring brain oxygen saturation in adults are continuous-wave (CW) and time-resolved (TR) NIRS. The primary objective of this project was to investigate which of these two NIRS methods is the best suited for detecting changes in adult cerebral oxygen saturation. This was achieved by i) developing a robust methodology for the segmentation and mesh generation from 3D MRI images of an adult head, ii) simulating CW-NIRS and TR-NIRS light propagation in the generated head mesh, and iii) comparing the sensitivity of both NIRS signals to changes in cerebral oxygenation. More specifically, a volumetric mesh was generated from 3D MRI images which were segmented into 4 tissue-types, consisting of the skin, skull, cerebrospinal fluid, and brain tissue. Photon propagation was simulated in the mesh for wavelengths ranging from 650 nm to 947 nm, using brain oxygen saturation levels ranging from 40% to 70%. These in silico experiments were designed to mimic typical measurements from CW-NIRS and TR-NIRS devices and were analyzed to determine the effectiveness of each modality at monitoring brain oxygen saturation. The greatest difference in the values for the CW-NIRS was found at 696 nm. From the 70% brain oxygenation to the 40%, there was a difference of approximately 3%. A 0.004% difference was found at the isosbestic point (798 nm). For the 696 nm wavelength in the TR-NIRS, there was a difference of approximately 56%, with a maximum difference of 72.6% at 657 nm. Similar to the CW-NIRS, TR-NIRS had a 0% difference at the isosbestic point of 798 nm. Additionally, the TR-NIRS was unaffected by source detector distance, while CW-NIRS signal improved with distance from the source. We found that TR-NIRS is more sensitive to changes in brain oxygen saturation in adults than CW-NIRS. Thus, while CW-NIRS is effective in neonates, the extra-cerebral layers in adults are too thick to make it a viable option. These simulations also suggest that TR-NIRS will be more appropriate than CW-NIRS in monitoring cerebral oxygenation in adult cardiac surgery patients.
Notes
Regional Winner in the Medical Sciences category.