Abstract
Time-resolved (TR) techniques provide a means of discriminating photons based on their time-of-flight. Since early arriving photons have a lower probability of probing deeper tissue than photons with long time-of-flight, time-windowing has been suggested as a method for improving depth sensitivity. However, TR measurements also contain instrument contributions (instrument-response-function, IRF), which cause temporal broadening of the measured temporal point-spread function (TPSF) compared to the true distribution of times-of-flight (DTOF). The purpose of this study was to investigate the influence of the IRF on the depth sensitivity of TR measurements. TPSFs were acquired on homogeneous and two-layer tissue-mimicking phantoms with varying optical properties. The measured IRF and TPSFs were deconvolved using a stable algorithm to recover the DTOFs. The microscopic Beer-Lambert law was applied to the TPSFs and DTOFs to obtain depth-resolved absorption changes. In contrast to the DTOF, the latest part of the TPSF was not the most sensitive to absorption changes in the lower layer, which was confirmed by computer simulations. The improved depth sensitivity of the DTOF was illustrated in a pig model of the adult human head. Specifically, it was shown that dynamic absorption changes obtained from the late part of the DTOFs recovered from TPSFs acquired by probes positioned on the scalp were similar to absorption changes measured directly on the brain. These results collectively demonstrate that this method improves the depth sensitivity of TR measurements by removing the effects of the IRF.
Highlights
Light in the near-infrared range is weakly absorbed by biological tissue, which enables deeper lying organs such as the brain or muscle to be monitored by probes placed on the skin [1,2,3,4,5]
The early Δμa values correspond to a pathlength of 325 mm for the temporal point-spread function (TPSF) data and 174 mm for the distribution of times-of-flight (DTOF) data; the middle Δμa values correspond to pathlengths of 710 and 459 mm for the TPSF and DTOF data, respectively; and late Δμa values correspond to pathlengths of 1092 and 741 mm for the TPSF and DTOF data, respectively
Absorption changes recovered from simulated and experimental TPSFs were compared to changes obtained using the corresponding DTOF data
Summary
Light in the near-infrared range is weakly absorbed by biological tissue, which enables deeper lying organs such as the brain or muscle to be monitored by probes placed on the skin [1,2,3,4,5]. Near-infrared spectroscopy (NIRS) provides a noninvasive approach for monitoring tissue function due to the unique absorption properties of oxygenated and deoxygenated haemoglobin [6]. Despite these advantages, the accuracy of NIRS, even when used for monitoring relative tissue oxygenation, is challenged by light absorption in superficial tissue [7,8,9,10]. Methods for separating the contributions from different tissue layers typically rely on multi-distance measurements, which take advantage of the increased depth penetration of photons detected at larger source–detector separations [11]. The distribution of times-of-flight (DTOF) of photons provides a means of separating absorption changes in different tissue layers since light travelling through superficial tissue is detected earlier than light that penetrates the deeper layers [15,16]
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