Could the small change in lifetime arise from additional multiple scattering of fluorescence/excitation photons by large volume of tumor? I also wonder if anyone could estimate refractive index of cancer tissue and compare it to normal. Probes change lifetimes as much as 10% when refractive index of its media changes by about the same.
Article In Vivo Fluorescence Lifetime Imaging for Monitoring the Eff...
Hi Aleksandr,
While I wouldn't be confident in answering your question with regards to refractive indices, is it possible the increase in fluorescence lifetime is due to a FRET interaction between the probe and tumour being bound? This state of affairs would cause a reduction in the observed fluorescence lifetime. Is this what you are seeing?
Derek.
If you are capturing scattered photons which have gone through a long scattering path, you will see an apparent delay, thus increased lifetime. Light travels 1 foot/ns, scattered photons are known to be delayed as compared to ballistic photons.
@Derek.
Yes, FRET mechanism can explain shorterning of lifetime, but problem here is that for NIR range there are virtually no FRET acceptors in living tissue.
@Artem.
Thanks for helping to clarify! For short lifetimes this effect will be most pronounced. However we typically see shortening of lifetimes. I am curious to know if anyone was able to model effective refractive index of the typical breast cancer tumor?
I am very new to this field, but If I would have to guess, fluorescence anisotropy could potentially affect the lifetimes. If you attach a big molecule to your fluorophore, its rotational correlation time changes and the emitted light remains polarized for a longer time.
For ruling out scattering, you could use a longpass filter in the emission path which would stop the scattering photons from the sample.
Saurabh
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