Limitations of quantitative photoacoustic measurements of blood oxygenation in small vessels

Abstract

We investigate the feasibility of obtaining accurate quantitative information, such as local blood oxygenation level (sO2), with a spatial resolution of about 50 microm from spectral photoacoustic (PA) measurements. The optical wavelength dependence of the peak values of the PA signals is utilized to obtain the local blood oxygenation level. In our in vitro experimental models, the PA signal amplitude is found to be linearly proportional to the blood optical absorption coefficient when using ultrasonic transducers with central frequencies high enough such that the ultrasonic wavelengths are shorter than the light penetration depth into the blood vessels. For an optical wavelength in the 578-596 nm region, with a transducer central frequency that is above 25 MHz, the sensitivity and accuracy of sO2 inversion is shown to be better than 4%. The effect of the transducer focal position on the accuracy of quantifying blood oxygenation is found to be negligible. In vivo oxygenation measurements of rat skin microvasculature yield results consistent with those from in vitro studies, although factors specific to in vivo measurements, such as the spectral dependence of tissue optical attenuation, dramatically affect the accuracy of sO2 quantification in vivo.

Quantitative spectroscopic photoacoustic imaging: a review

“…This question has been tackled for some specific scenarios. [11][12][13] Finally, photoacoustic techniques that have no imaging aspect are not covered here, so "gas phase" PA spectroscopy using a PA cell, which is a mature field in its own right, 14 is not described.…”

“…This implies that a relative measurement of the absorption coefficient, Kμ a;blood ðλÞ, will be sufficient to determine sO 2 as long as the multiplicative factor K does not depend on wavelength. 12 To use the amplitudes of PA signals as relative measurements of μ a , it is necessary to find a scenario in which the constant K, which for PA will be K ¼ΓΦ, is independent of wavelength. This is not true in general, in fact it is rarely true, and assuming it to be so 64 is likely to result in significant errors in sO 2 estimates.…”

“…2 and, in certain cases, it may be found that this function is approximately linear, which allows W −1 to be obtained trivially. This approach was taken by Sivaramakrishnan et al 12,71 for the case of a cylindrical absorber (simulating a blood vessel). They considered a uniformly absorbing cylinder of radius a illuminated from all sides equally (i.e., within the diffusive regime), and a bandlimited detector perpendicular to the vessel axis with a central frequency of f c ¼ c 0 ∕λ c , where λ c is the associated center wavelength.…”

“…The same experiment with a 10 MHz transducer (μ a λ c ≈ 3, λ c < a) failed to recover the spectrum correctly. 12 The fact that this method reduces the nonlinear inversion W −1 to a linear problem is appealing. However, its main limitation lies in the fact that it is based on a model of a single absorber in a purely scattering (nonabsorbing) background medium, which is not representative of real tissue that will contain additional vessels, capillaries, extravascular blood, lipids, water, and potentially other absorbing molecules such as melanin.…”

Realtime photoacoustic microscopy in vivo with a 30-MHz ultrasound array transducer

“…More recently, Maslov et al [10] and Zhang et al [11] have demonstrated exquisite images of subcutaneous microvasculature using dark-field confocal photoacoustic microscopy (PAM), a raster-scanning-based reflection geometry imaging system. Unique to photoacoustic methods is the ability to image blood oxygen saturation as well as concentration of total hemoglobin [11][12][13]. Emerging photoacoustic methods are enabling molecular imaging in small animals, including imaging of gene expression [14] and cell receptors [15].…”