Optical spectroscopy, imaging, and therapy tissue phantoms must have the scattering and absorption properties that are characteristic of human tissues, and over the past few decades, many useful models have been created. In this work, an overview of their composition and properties is outlined, by separating matrix, scattering, and absorbing materials, and discussing the benefits and weaknesses in each category. Matrix materials typically are water, gelatin, agar, polyester or epoxy and polyurethane resin, room-temperature vulcanizing (RTV) silicone, or polyvinyl alcohol gels. The water and hydrogel materials provide a soft medium that is biologically and biochemically compatible with addition of organic molecules, and are optimal for scientific laboratory studies. Polyester, polyurethane, and silicone phantoms are essentially permanent matrix compositions that are suitable for routine calibration and testing of established systems. The most common three choices for scatters have been: (1.) lipid based emulsions, (2.) titanium or aluminum oxide powders, and (3.) polymer microspheres. The choice of absorbers varies widely from hemoglobin and cells for biological simulation, to molecular dyes and ink as less biological but more stable absorbers. This review is an attempt to indicate which sets of phantoms are optimal for specific applications, and provide links to studies that characterize main phantom material properties and recipes.

https://www.spiedigitallibrary.org/journals/journal-of-biomedical-optics/volume-11/issue-4/041102/Review-of-tissue-simulating-phantoms-for-optical-spectroscopy-imaging-and/10.1117/1.2335429.pdf

Review of tissue simulating phantoms for optical spectroscopy, imaging and dosimetry

Frequent determination of glucose concentrations in diabetic patients is an important tool for diabetes management. This requires repetitive lancing and finger bleeding. Use of noninvasive (NI) detection techniques offers several advantages, such as the absence of pain and exposure to sharp objects and biohazard materials, the potential for increased frequency of testing, and hence, tighter control of the glucose concentrations, and the potential for a closed-loop system including a monitor and an insulin pump. These potential advantages have led to considerable interest in the commercialization of NI glucose monitoring devices. Review of the scientific, patent, and commercial literature indicates that the spectroscopic basis for NI determination of glucose is not yet well established, and attempts at commercialization may be several steps ahead of our understanding the origin and characteristics of an in vivo glucose-specific or glucose-related signal. Several technologies have potential for leading to viable measuring devices, but most of the data are based on in vitro experimentation. Because of the technical complexity of in vivo glucose measurements, this review aims at discussing the gap between the established need and current technology limitations.

Spectroscopic and Clinical Aspects of Noninvasive Glucose Measurements

The aim of this article is to critically discuss the technical and clinical aspects of glucose sensors and to briefly review current technical developments. This includes sensors for spot glucose measurements as well as those used for continuous glucose monitoring. Continuous glucose monitoring in particular should supply the diabetic patient with all the information required to optimize insulin therapy and metabolic control. Such systems should also allow hypo- and hyperglycemic episodes to be avoided. During the last 30 years numerous attempts have been made to develop glucose sensors, and new major breakthroughs have been announced repeatedly. However, up until now no glucose sensor has been available that can be used by diabetic patients in daily life conditions. Also one type of glucose sensor, a glucose electrode, recently received approval by the Food and Drug Administration (USA) and is commercially available. Other glucose sensors employing the transdermal, microdialysis or open tissue microperfusion technique are currently under clinical development and may also become available in the near future. The types of glucose sensors referred to so far are not truly non-invasive, but only minimally invasive. They measure glucose concentration in the interstitial fluid of the skin or the subcutis. Non-invasive optical glucose sensors are designed to monitor glucose changes in the skin by directing light through it. They measure the characteristics of the reflected light that are changed as the result of an interaction with glucose. However, none of the attempts with optical glucose sensors have resulted thus far in the development of a sensor that allows monitoring of glucose with sufficient accuracy and precision within the clinically relevant glucose range in daily life conditions. Nevertheless, more minimal-invasive glucose sensors systems will become available for practical use in the near future, whereas it is still uncertain if this can be said for any non-invasive glucose sensor.

Sensors for glucose monitoring: technical and clinical aspects

This note summarizes the characterization of the acoustic properties of four materials intended for the development of tissue, and especially breast tissue, phantoms for the use in photoacoustic and ultrasound imaging. The materials are agar, silicone, polyvinyl alcohol gel (PVA) and polyacrylamide gel (PAA). The acoustical properties, i.e., the speed of sound, impedance and acoustic attenuation, are determined by transmission measurements of sound waves at room temperature under controlled conditions. Although the materials are tested for application such as photoacoustic phantoms, we focus here on the acoustic properties, while the optical properties will be discussed elsewhere. To obtain the acoustic attenuation in a frequency range from 4 MHz to 14 MHz, two ultrasound sources of 5 MHz and 10 MHz core frequencies are used. For preparation, each sample is cast into blocks of three different thicknesses. Agar, PVA and PAA show similar acoustic properties as water. Within silicone polymer, a significantly lower speed of sound and higher acoustical attenuation than in water and human tissue were found. All materials can be cast into arbitrary shapes and are suitable for tissue-mimicking phantoms. Due to its lower speed of sound, silicone is generally less suitable than the other presented materials.

https://iopscience.iop.org/article/10.1088/0031-9155/52/20/N02/pdf

Acoustical properties of selected tissue phantom materials for ultrasound imaging.

A multiplex sensor and a method of use that provides enhanced selectivity and sensitivity. In this method, one or more parameters of the sample are measured by means of multiple spectroscopic techniques, and the interferences from physiological and spectral variables are reduced or eliminated. One embodiment of the present invention comprises a multiplex sensor and a method of use that provides enhanced selectivity and sensitivity. One or more parameters of the sample are measured by means of at least two different spectroscopic techniques, wherein the at least two different spectroscopic techniques are selected from the group consisting of: (a) infrared absorbance, (b) scattering, (c) emission, (d) polarization, and (e) photoacoutics.

Multiplex sensor and method of use

A preliminary study of using a gelatin-based in vivo tissue model to investigate some of the near infrared photoacoustic characteristics that may influence the sensitivity of non-invasive photoacoustic detection of blood glucose in human tissue is described. It is shown that the optical absorption change due to glucose in the near infrared region is small and that the pulsed photoacoustic technique may offer a better detection sensitivity than other conventional optical transmission measurement systems being developed for blood glucose monitoring.

https://iopscience.iop.org/article/10.1088/0031-9155/38/12/014/pdf

Glucose determination by a pulsed photoacoustic technique: an experimental study using a gelatin-based tissue phantom

Pulsed laser photoacoustic spectroscopy has been used as the basis for sensing analytes in aqueous systems. The spectra obtained with the photoacoustic system are significantly different from conventional transmission spectra and, in particular, the spectra of some organic solvents and alcohols show a marked enhancement in feature strength due to the dependence of physical and thermal factors in the photoacoustic generation process. The photoacoustic process has been shown to be almost immune to the effects of optical scattering, which gives this sensing technique advantages over conventional spectroscopic techniques in the potential application to water-pollution monitoring and to measuring blood-glucose concentrations.

G.B. Christison

Papers

Glucose determination by a pulsed photoacoustic technique: an experimental study using a gelatin-based tissue phantom

A laser photoacoustic sensor for analyte detection in aqueous systems

Photoacoustic generation in liquids with low optical absorption