Abstract
As multiwavelength photoacoustic imaging becomes increasingly widely used to obtain quantitative estimates, the need for validation studies conducted on well-characterized experimental phantoms becomes ever more pressing. One challenge that such studies face is the design of stable, well-characterized phantoms and absorbers with properties in a physiologically realistic range. This paper performs a full experimental characterization of aqueous solutions of copper and nickel sulfate, whose properties make them close to ideal as chromophores in multiwavelength photoacoustic imaging phantoms. Their absorption varies linearly with concentration, and they mix linearly. The concentrations needed to yield absorption values within the physiological range are below the saturation limit. The shape of their absorption spectra makes them useful analogs for oxy- and deoxyhemoglobin. They display long-term photostability (no indication of bleaching) as well as resistance to transient effects (no saturable absorption phenomena), and are therefore suitable for exposure to typical pulsed photoacoustic light sources, even when exposed to the high number of pulses required in scanning photoacoustic imaging systems. In addition, solutions with tissue-realistic, predictable, and stable scattering can be prepared by mixing sulfates and Intralipid, as long as an appropriate emulsifier is used. Finally, the Gruneisen parameter of the sulfates was found to be larger than that of water and increased linearly with concentration.
Highlights
Photoacoustic imaging (PAI) is an emerging modality that can provide high-resolution three-dimensional (3-D) images of soft tissue.[1,2,3] Since different endogenous, exogenous, and genetically encoded chromophores have different absorption spectra, photoacoustic images obtained at multiple wavelengths carry information on the distributions of these chromophores and, information on tissue composition, physiology, and pathology.[1,2,4,5] the use of multiwavelength PAI for detecting, isolating, assessing the relative variation, or fully quantifying chromophore contributions in an accurate and robust manner is still challenging[6,7]
We extend the previous work by providing detailed experimental measurements of the optical and PA properties of aqueous solutions and emulsions of nickel sulfate hexahydrate and copper sulfate pentahydrate—namely, their absorption spectra, absorption linearity, absorption behavior in mixtures of sulfate species, transient and permanent photostability, absolute Grüneisen characteristics, and compatibility with intralipid as a scattering medium—and present the case for them as near-ideal chromophores for use in multiwavelength PAI phantoms
While the principal aim of this paper is to describe the properties of sulfates as phantom chromophores for multiwavelength PAI, this section gives a brief summary of some of the types of phantom architecture into which they might be usefully incorporated
Summary
Photoacoustic imaging (PAI) is an emerging modality that can provide high-resolution three-dimensional (3-D) images of soft tissue.[1,2,3] Since different endogenous, exogenous, and genetically encoded chromophores have different absorption spectra, photoacoustic images obtained at multiple wavelengths carry information on the distributions of these chromophores and, information on tissue composition, physiology, and pathology.[1,2,4,5] the use of multiwavelength PAI for detecting, isolating, assessing the relative variation, or fully quantifying chromophore contributions in an accurate and robust manner is still challenging[6,7] (a field broadly known as quantitative photoacoustic tomography). Despite recent advances on the study of phantom matrix materials for general photoacoustic (PA) system characterization and quality control,[8,9,10,11,12] for multiwavelength and quantitative PAI purposes further efforts are needed, with one of the crucial issues being finding suitable chromophores.[13,14] It is essential for these to be well-characterized, well-behaved, and stable They should exhibit spectral variation and if possible be tissue-realistic, the extent to which this is necessary will depend on the application.[15,16]. We extend the previous work by providing detailed experimental measurements of the optical and PA properties of aqueous solutions and emulsions of nickel sulfate hexahydrate and copper sulfate pentahydrate—namely, their absorption spectra, absorption linearity, absorption behavior in mixtures of sulfate species, transient and permanent photostability, absolute Grüneisen characteristics, and compatibility with intralipid as a scattering medium—and present the case for them as near-ideal chromophores for use in multiwavelength PAI phantoms
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