Abstract
Contactless thermal imaging generally relies on mid-infrared cameras and fluorescence imaging with temperature-sensitive phosphors. Fluorescent thermometry in the near-infrared (NIR) region is an emerging technique for analysing deep biological tissues but still requires observation depth calibration. We present an NIR fluorescence time-gated imaging (TGI) thermometry technology based on fluorescence lifetime, an intrinsic fluorophore time constant unrelated to observation depth. Fluorophore used is NaYF4 co-doped with Nd3+ and Yb3+ that emits fluorescence at 1000 nm. An agarose gel-based phantom with the fluorophore embedded at a 5-mm depth was covered by sheets of meat to vary the observation depth. The temperature was determined independently from depth by sequences of NIR fluorescence decay images, and the rate of change in the fluorescence lifetime per temperature was almost constant (−0.0092 ~ −0.010 °C−1) at depths ranging from 0 to 1.4 mm of meat, providing non-contact and absolute measurements of temperature in deep biological tissues.
Highlights
Fluorescence imaging has been developed for contactless thermometry applications[8,9,10,11] since temperature-dependent changes in the fluorescence lifetime of materials such as rare-earth-doped ceramics particles[12], carbonous compounds[13], and Cr3+-activated compounds[14] have been reported
We first attempt temperature imaging that is based on NIR-II fluorescence lifetime of rare-earth-doped NaYF4, which depends on the temperature of the surrounding media but not on observation depth
Using an InGaAs camera that is sensitive to NIR-II19, the time-gated imaging (TGI) system captures a pixel-level fluorescence decay curve that is simultaneously converted into fluorescence lifetime
Summary
Fluorescence imaging has been developed for contactless thermometry applications[8,9,10,11] since temperature-dependent changes in the fluorescence lifetime of materials such as rare-earth-doped ceramics particles[12], carbonous compounds[13], and Cr3+-activated compounds[14] have been reported. Recent works reported rare-earth-doped NaYF4 ceramics as NIR-II/III ratiometric nanothermometers[24] for deep tissues[25]; this technique still required an observation depth-dependent calibration. We first attempt temperature imaging that is based on NIR-II fluorescence lifetime of rare-earth-doped NaYF4, which depends on the temperature of the surrounding media but not on observation depth. This material shows NIR-II emission with high efficiency[26] and a relatively long lifetime[27,28]. Temperature imaging of NaYF4: Nd3+, Yb3+ was demonstrated in a mimic of deep biological tissues to investigate the depth dependency of our TGI thermometry for NIR-II fluorescence
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