The influence of a clear layer on near-infrared spectrophotometry measurements using a liquid neonatal head phantom

It is difficult to test near-infrared spectrophotometry instruments in vivo . Therefore we constructed a liquid phantom which mimics the neonatal head. It consists of a spherical 3.5 mm thick layer of silicone rubber simulatin g skin and bone and a 0.5 mm thick clear layer of polypropylene imitating c erebrospinal fluid. It acts as container for a liquid solution with Intrali pid(R), 60 mu mol l(-1) haemoglobin and yeast. The solution was oxygenated using oxygen and then deoxygenated by the yeast. From the instrumental (Cri tikon 2020) algorithm, we found that with increasing scattering (0.5%, 1%, 1.5% and 2% Intralipid(R) concentration) the reading was increasingly offse t from the expected value of 0 mu mol l(-1) by 55.7, 68.6, 76.5 and 80.4 mu mol l(-1) (oxyhaemoglobin) and 16.0, 24.4, 29.6 and 31.7 mu mol l(-1) (deo xyhaemoglobin). This reduced the range of the oxygen saturation reading fro m the expected 100% to 31.5, 21.1, 14.3 and 11.5%. Haemoglobin concentratio n changes were increasingly underestimated by a factor of two to four. For a second algorithm based on the diffusion approximation the offsets were sm aller: oxyhaemoglobin 11.4, 17.8, 22.5 and 25.1 mu mol l(-1) and deoxyhaemo globin 1.3, 3.4, 5.2 and 6.0 mu mol l(-1). The range of the oxygen saturati on reading was higher: 41.3, 29.2, 23.4 and 16.6%. Concentration changes we re underestimated by a factor of six to ten. This study demonstrates the ne ed to develop algorithms which take into consideration anatomical structure s.