Noninvasive hemoglobin sensing and imaging: optical tools for disease diagnosis

Abstract

Significance: Measurement and imaging of hemoglobin oxygenation are used extensively in the detection and diagnosis of disease; however, the applied instruments vary widely in their depth of imaging, spatiotemporal resolution, sensitivity, accuracy, complexity, physical size, and cost. The wide variation in available instrumentation can make it challenging for end users to select the appropriate tools for their application and to understand the relative limitations of different methods.

Aim: We aim to provide a systematic overview of the field of hemoglobin imaging and sensing.

Approach: We reviewed the sensing and imaging methods used to analyze hemoglobin oxygenation, including pulse oximetry, spectral reflectance imaging, diffuse optical imaging, spectroscopic optical coherence tomography, photoacoustic imaging, and diffuse correlation spectroscopy.

Results: We compared and contrasted the ability of different methods to determine hemoglobin biomarkers such as oxygenation while considering factors that influence their practical application.

Conclusions: We highlight key limitations in the current state-of-the-art and make suggestions for routes to advance the clinical use and interpretation of hemoglobin oxygenation information.

Keywords: hemoglobin; imaging; sensing; spectroscopy.

Fig. 1

The optical absorption of hemoglobin and associated variants. Representative spectra are shown for oxygenated, deoxygenated, carbamino, carboxy (visible and NIR^19,20), methemoglobin (visible and NIR^19,20), and sulfhemoglobin.

Fig. 2

Pulse oximetry. Schematic illustration of pulse oximetry in the two different operation modes: (a) transmission and (b) reflection. The detected light is cyclic due to the pulsatile nature of blood in the peripheral vascular system. Both transmission and reflection modes have alternating components (AC) and direct components (DC). In tissue, the transmission and reflection of light vary based on the changes in absorption due to blood volume and oxygenation. That is $\mathbf{R}+\mathbf{A}+\mathbf{T}\approx 1$, when $\mathbf{R}$, $\mathbf{A}$, and $\mathbf{T}$ are the normalized reflection, absorption, and transmission intensities, respectively. For this reason, in reflection pulse oximetry, the peak intensity of light will be off by half a cycle from that of the transmission cycle. Examples of pulse oximeter devices include (c) transmission-based devices widely used in a clinical setting. Reproduced with permission from Ref. . (d) Low-power devices in development that adhere to the skin and use flexible OLED illumination. Reproduced with permission from Ref. . (e) Battery-free pulse oximeters in development that use near field communication for power. Reproduced with permission from Ref. .

Fig. 3

Spectral reflectance imaging. (a) Overview of spectral reflectance imaging methods. Point-scanning spectroscopy can be used to build spectral information using a standard spectrometer. Alternatively, a spectral camera can be used to collect either a limited number of wavelengths (multispectral, typically $<10$ spectral bands) or a more continuous spectrum (hyperspectral). (b) Endoscopy images of the esophagus with (i) RGB imaging and (ii) narrowband imaging, which improves the contrast of the blood vessels. Reproduced with permission from Ref. . (c) Endoscopy of a porcine esophagus to determine tissue viability with 24 spectral bands from 460 to 690 nm (spectral resolution of 10 nm) using a slit hyperspectral imaging and fiber bundle probe and the resulting (i) reconstructed RGB image and (ii) unmixed oxygenation. Reproduced with permission from Ref. . (d) Hypoxia of tumors can be imaged using a liquid crystal tunable filter in conjunction with a CCD; this is demonstrated in mouse tumors; (i) and (iii) light microscopy of tumor vasculature in a dorsal skin window chamber, and the additional information of hemoglobin saturation is shown in (ii) and (iv) illustrating low oxygen saturation of the tumors. Reproduced with permission from Ref. .

Fig. 4

Principles of depth-resolved imaging. (a) In photoacoustic imaging, the absorption of light pulses generates a broadband acoustic wave detected at the tissue surface by an ultrasound transducer. (b) Photoacoustic imaging of oxygenation of the finger in combination with ultrasound to image the veins and arteries. Reproduced with permission from Ref. . (c) In DOI (and DCS techniques), illuminated light is scattered in tissue collected by an offset optical detector at the tissue surface. (d) DOI data acquired from the human finger is processed to quantify oxygenation, hemoglobin concentration, and water. Reproduced with permission from Ref. . (e) In OCT, coherent light illuminates the tissue, and the light that reflects at interfaces is collected and combined with a reference arm, so interference occurs; from this interference, depth-resolved images of the absorption and scattering properties of tissue can be resolved. (f) Oxygen resolved spectroscopic OCT on mice brains illustrating how the fraction of inspired oxygen (${\mathrm{FiO}}_2$) affects the oxygenation of the arteries and veins in the brain. Reproduced with permission from Ref. .

Fig. 5

Tomographic imaging of the human breast for cancer detection (a) PAI of ${\mathrm{sO}}_2$ in breast with infiltrating ductal carcinoma (IDC); S-factor was defined to account for system accuracy and fluence compensation. Reproduced with permission from Ref. . (b) DOI of breast IDC (indicated by the red box) resolves ${\mathrm{sO}}_2$, THb, ${\mathrm{H}}_2\mathrm{O}$, lipid, concentrations of which serve to highlight the tumor. Reproduced with permission from Ref. . (c) DCS of blood flow relative to an ultrasound image of low-grade carcinoma; the tumor is circled in yellow. These images are referenced to positions s1 and s2 to compare the ultrasound, 3D reconstruction, and cross-section. Reproduced with permission from Ref. .

Fig. 6

Hemoglobin imaging of the human brain. (a)–(d) Reflectance spectral images of the brain in an adult undergoing epileptogenic tissue resection (a) reference RGB rendering, (b) change in oxygenated hemoglobin over a single timeframe, (c) change in deoxygenated hemoglobin over a single timeframe, and (d) change in total hemoglobin over a single timeframe. Reproduced with permission from Ref. . (e), (f) DOI of a neonate during a seizure: (e) changes in HbT concentration mapped throughout the onset of a seizure and (f) average changes in Hb, ${\mathrm{HbO}}_2$, and tHb postonset of the seizure. Reproduced with permission from Ref. .