Background-Free Modulated Photoacoustic and Fluorescent Imaging

Donald VanderLaan , Robert Dickson, Aida Demisse, Md Islam

Summary

Synchronously Amplified Photoacoustic Image Recovery (SAPhIRe) is a novel photoacoustic (PA) imaging technique permitting complete background signal suppression without resorting to spectroscopic methods. The lower fluences, simpler lasers, decreased acquisition time, and substantially reduced energy deposition needed by SAPhIRe make it clinically relevant – all of which have encumbered photoacoustic imaging on its long journey toward usage on humans.

Description

Synchronously Amplified Photoacoustic Image Recovery (SAPhIRe) is a specialized photoacoustic (PA) imaging modality geared toward detection of a particular type of contrast agent. This type of contrast agent, while also detectable via standard PA, offers the unique ability to be turned on/off optically. This ability permits complete background signal suppression without the usage of spectroscopic PA means. The hurdle overcome is substantially decreased acquisition time – spectroscopic PA realistically requires 10+ wavelengths to suppress endogenous agents enough to detect exogenous chromophores. SAPhIRe-based contrast agents require – at maximum – three PA acquisitions. This makes it clinically-relevant.

When encapsulated within a nanoparticle (NP) whose surface has been modified to target specific cell types, permits usage of this unique contrast agent for functional imaging deep within the body. Because background signal is completely suppressed – as no endogenous chromophores possessing this unique property exist within the body – even small concentrations of NPs can be detected deep within tissue. The corollary is that much lower fluences can be utilized than needed for standard PA or spectroscopic PA. This technology has the potential to revolutionize PA, and could greatly expedite its FDA approval and clinical adoption.

The unique ability, mentioned above, is the ability to optically activate, and automatically deactivate, the contrast agent’s optical absorption. This transient absorption persists beyond 10µ, permitting the agent to be imaged photoacoustically in the off-state, and subsequently imaged via PA in the on-state. The difference between these two acquisitions yields only signal from our NPs – background PA contrast is completely eliminated.

The ability of these contrast agents to change their absorption spectrum temporarily is due to a physical phenomenon whereby after optical excitation, instead of fluorescing or quick vibrational relaxation, they undergo intersystem crossing and enter an excited state where relaxation to ground state is forbidden due to it requiring spin-flipping (i.e., it is prohibited by the laws of quantum mechanics). These contrast agents and their long dark-state lifetime have long been known to those in fluorescence – it is only recently that their utility has been extended to deep-tissue imaging with photoacoustics.

Further Reading

• C Richards, J-C Hsiang, and R Dickson. Synchronously amplified fluorescence image recovery (SAFIRe). J Physical Chemistry B. (2010) 114(1):660-5
• Y-C Chen, and R Dickson. Improved fluorescent protein contrast and discrimination by optically controlling dark state lifetimes. J Physical Chemistry Letters. (2017) 8(4):733-6
• D Mahoney, E Owens, C Fan, J-C Hsiang, M Henary, and R Dickson. Tailoring cyanine dark states for improved optically modulated fluorescence recovery. J Physical Chemistry B. (2015) 119(13):4637-43
• J-C Hsiang, A Jablonski, and R Dickson. Optically modulated fluorescence bioimaging: Visualizing obscured fluorophores in high background. Accounts of Chemical Research. (2014) 47(5):1545-54
• C Fan, J Hsiang, R Dickson. Optical modulation and selective recovery of Cy5 fluorescence. ChemPhysChem. (1012) 13:1023-9
• A Jablonski, J-C Hsiang, P Bagchi, N Hull, C Richards, C Fahrni, and R Dickson. Signal discrimination between fluorescent proteins in live cells by long-wavelength optical modulation. J Physical Chemistry Letters. (2012) 3(23):3585-91

References