New optical sensing technique of tissue viability and blood flow based on nanophotonic iterative multi-plane reflectance measurements

Abstract

Physiological substances pose a challenge for researchers since their optical properties change constantly according to their physiological state. Examination of those substances noninvasively can be achieved by different optical methods with high sensitivity. Our research suggests the application of a novel noninvasive nanophotonics technique, ie, iterative multi-plane optical property extraction (IMOPE) based on reflectance measurements, for tissue viability examination and gold nanorods (GNRs) and blood flow detection. The IMOPE model combines an experimental setup designed for recording light intensity images with the multi-plane iterative Gerchberg-Saxton algorithm for reconstructing the reemitted light phase and calculating its standard deviation (STD). Changes in tissue composition affect its optical properties which results in changes in the light phase that can be measured by its STD. We have demonstrated this new concept of correlating the light phase STD and the optical properties of a substance, using transmission measurements only. This paper presents, for the first time, reflectance based IMOPE tissue viability examination, producing a decrease in the computed STD for older tissues, as well as investigating their organic material absorption capability. Finally, differentiation of the femoral vein from adjacent tissues using GNRs and the detection of their presence within blood circulation and tissues are also presented with high sensitivity (better than computed tomography) to low quantities of GNRs (<3 mg).

Keywords: Gerchberg-Saxton; blood vessel; gold nanorods; optical properties; reflectance; scattering; tissue viability.

Figure 1

A schematic description of the algorithm for reconstructing µ_s′. Notes: After running T iterations of multi-plane G–S algorithm, the estimated phase ${\widehat{\mathrm{\phi }}}_1$ is retrieved. The calculated phase’s STD together with the tissue thickness, Z, produces an estimation for µ_s′ using a look up table (that was built as described above). Abbreviations: G–S algorithm, Gerchberg-Saxton algorithm; STD, standard deviation.

Figure 2

The experimental setup for recording light intensity images. Notes: (A) A schematic image of the setup. (B) An image of the setup and its components in the lab. The camera records images at multiple planes with equal intervals between them. The experimental setup was designed for reflection measurements. The light source is a helium neon (He-Ne) gas laser with λ=632.8 nm, the focal length of the reflection lens is 75 mm; polarizers were added for optical clearing purposes. The sample is set on 3 axis micrometer plates and can be adjusted in the x-y-z directions.

Figure 3

The synthesized PEGylated GNRs. Notes: (A) TEM image of the GNRs. (B) The absorption spectrum of the GNRs, presenting a strong peak at 645 nm. Abbreviations: PEG, polyethylene glycol; GNRs, gold nanorods; TEM, transmission electron microscopy; au, arbitrary units.

Figure 4

The experimental setup during in vivo measurements.

Figure 5

The STD obtained for ex vivo experiments. Notes: The experiments were conducted on two groups: 1) two ears were measured on the same day as sacrifice, ie, day 0; 2) two ears were measured 6 days following sacrifice, ie, day 6. Following control (orange bars) measurements, 5 µL of 2 mM MB was applied to the ears and reflection signals were measured again 24 h later (blue bars). The algorithm parameters were: 8 recorded images (64 iterations) with 1.27 mm distance between them. (*P<0.05, ***P<0.001). Abbreviations: STD, standard deviation; MB, Methylene blue; au, arbitrary units.

Figure 6

Femoral vein and adjacent tissues effect on the light phase using GNRs. Notes: (A) The experimental setup during in vivo experiments. The mouse was laid on a flat surface with its inner thigh facing the laser. It was set on the sample holder on a 3 axis micrometer stage which enables fine-tuning during experiments. The center of the light beam was directed exactly to the femoral vein. (B) The obtained STD in the in vivo measurements. The mouse’s femoral vein (red bars) and tissues in its environment (green) were irradiated following GNRs injection (0.25 h and 24 h postinjection). The algorithm parameters were: 9 recorded images (81 iterations) with 1.27 mm distance between them. (*P<0.05, ***P<0.001). Abbreviations: GNRs, gold nanorods; STD, standard deviation.

Figure 7

CT images of the femoral vein and its environment pre- and post-GNRs injection. Notes: (A) Pre-GNRs injection; (B) 4 h post-GNRs injection; (C) 8 days post-GNRs injection. No differences can be observed between the three images. Abbreviations: CT, computed tomography; GNRs, gold nanorods.