Getting more early photons with less background: detection rate and signal-to-background improvements in enhanced early photon imaging

With tissue samples less than 1 mm in thickness, optical projection tomography (OPT) has proven to be a very powerful imaging modality that can achieve high spatial resolution images. This high resolution is achieved by collecting the photons with non-significant scattering through 1 mm of tissue, the so-called diffusion limit. But, with samples thicker than 1 mm, scattered photons dominate and the highly resolved images give way to significantly “blurred” images in OPT[1]. However, as increased scattering relates to increased time of travel of the photons so time-domain OPT has been used to only collect the early-arriving photons that have travelled a more direct route through the tissue to reduce detection of scattered photons. Yet very early photons are extremely rare compared to scattered photons. Our recent suggested early photon count rates can be significantly enhanced by running the detector in a “deadtime” regime where the deadtime incurred by early-arriving photons acts as a shutter to later-arriving scattered photons[2]. In this work, we will demonstrate that running in the deadtime also had the unexpected advantage of significantly reducing the number of background photons detected. Proposed approach increases the early photon detection rate by 3-orders-of-magnitude in comparison with conventional approaches in 4-mm thick tissues with 780 nm light while the laser power is far below the level that would significantly damage the tissue. In addition, the signal to background (caused by after pulsing) was improved by 70-fold compared to conventional approaches designed to collect an equal number of early photons.