Infrared backscattering for metabolic detection (NIRS – Diffuse Reflectance Spectroscopy)

Name

Description

From an optical point of view, biological tissue can be considered as an absorbing matrix in which a high number of inhomogeneities, which act as light scatterers, are present. When near-infrared (NIR) light is injected into the tissue, scattering provides a way to recover part of the injected light as “diffuse reflectance” from the injection surface.

In the NIR region, the optical absorption is strongly dominated by haemoglobin, the blood constituent that carries oxygen. Smaller contributions to absorption are given by other chromophores, whose absorption spectrum may give information on tissue oxygen consumption. Oxygen metabolism is most certainly a crucial problem in many fields of medical activity. Sport medicine, rehabilitative cardiology, intensive care, anaesthesia, tumour detection, rely on some form of metabolic oxygen analysis. Using a spectral analysis of the backscattered light (in the medical literature, referred to as “NIRS” or “Diffuse Reflectance Spectroscopy”), the key issue of real-time non-invasive evaluation of blood content and oxygenation in a non-stationary condition, such as upon exercise or during respiratory manoeuvres, can be approached by non-invasive optical measurements.

Schemes and typical results

A first line of research relates to the implementation of a line of backscattering instruments optimised for direct clinical use. In the recent past, for example, one of the St Andrews experts has developed a high-performance wearable backscattering spectrometer, on the size scale of a cigarette packet, dedicated to sports, high-altitude medicine and to rehabilitative cardiology. A simplified block diagram is reported in Fig. 1, and a sports medicine example is reported in Figure 2.

On the opposite side of complexity and sensitivity, the tiny variation in near-infrared backscattering deriving from oxygen consumption by a few brain cortex cells, during their normal signalling activity, has recently been detected. In particular, highly sensitive apparatus connected to a miniaturised in-vivo probe has been built, and used during surgery to detect optical backscattering directly from the brain cortex. correlated to external peripheral stimulation. For the first time, signals deriving from individual stimuli have been detected. A representative signal is reported in Figure 3.

In the microfabrication facilities available in St Andrews, microphotonic devices specifically dedicated to in-vivo backscattering measurements are under development.

Finally, as a side activity, we perform steady-state simulation of optical transmission and backscattering in complex biological structures, to the level of complexity of a full human head as derived from MRI scanning.

Figure 1: Simplified functional block diagram of a portable two-channel backscattering unit. The power supplies, the interface logic between microprocessor and peripherals and the debug / diagnostics hardware are not indicated.

Figure 2: Backscattering measurement of deoxyhaemoglobin (cHb), oxyhaemoglobin (cHbO2), total haemoglobin (c) and oxygen saturation (S) measured on a large muscle during an incremental load exercise on a cyclette (A) and subsequent tourniquet-induced venous occlusion (B).

Figure 3: Change in the optical backscattering of the brain cortex area associated with the hind paw of a rat, as a response of a single stimulus (electrical pulse, dotted line) applied to the paw.

Special features and limitations

The advantages of the technique are

• Sensitivity to local haemodynamics
• Simplicity
• Non-invasiveness
• Patient-friendliness
• Potentially low cost

The limitation of the technique is

• Low spatial resolution

Publications

What equipment do you use?

Custom equipment, built in-house.

What analytes do you measure?

Mainly, oxy- and deoxyhaemoglobin, and cytochrome aa3 oxydase.

What physical or chemical substances do you use?

As implemented in St Andrews, the technique is label-free, though in the literature indocyanine green has been used to enhance contrast

Expert(s)

Fixed keywords

Free keywords

Images