Sam C. W. Hyde

Depth-resolved holography through turbid media using photorefraction

A technique based on photorefractive holography for imaging objects obscured by a scattering medium is presented. Using ultrashort pulse illumination, depth-resolved whole-field images of three dimensional objects embedded in scattering media have been obtained. Bulk photorefractive crystals and photorefractive multiple quantum-well (MQW) devices have been investigated as the hologram recording element. Images have been obtained through media of up to 16 scattering mean free paths with a system based on bulk rhodium-doped barium titanate (Rh:BaTiO/sub 3/). Using MQW devices, a real-time image acquisition (<0.4 ms) has been demonstrated when imaging through eight scattering mean free paths. The relative merits of photorefractive holography are discussed, including its potential to provide a higher dynamic range of detection than traditional photographic film based or electronic holography. This could be important for in vivo imaging through biological tissue.

Real-time 3-D holographic imaging using photorefractive media including multiple-quantum-well devices

We discuss a real-time coherence gated three-dimensional (3-D) imaging system, based on photorefractive holography with ultrashort pulses, which has been applied to imaging through turbid media with a view to developing biomedical instrumentation. Sub-100-/spl mu/m depth-resolved images of 3-D objects embedded in a scattering medium have been obtained. Using a long integration time in rhodium-doped barium titanate (Rh:BaTiO/sub 4/), an image of a test chart has been obtained through 16 mean-free paths of scattering medium. Real-time depth-resolved imaging through 13 mean free paths of scattering medium has been demonstrated using a fast response time (<0.4 ms) photorefractive multiple quantum well device. This latter system can acquire depth-resolved images direct to video with no requirement for frame grabbing or signal processing. We discuss the tradeoffs and limitations of these photorefractive media for this application.

Whole-field fluorescence lifetime imaging with picosecond resolution using ultrafast 10-kHz solid-state amplifier technology

We report the development of a high temporal resolution whole-field fluorescence lifetime imaging system based on an ultrafast solid-state laser system and a time-gated image intensifier operating at up to 10 kHz. The temporal instrument response is /spl sim/110 ps and we have imaged (environmentally perturbed) differences in fluorescence lifetime as small as 20 ps. Fluorophores exhibiting single- or double-exponential fluorescence decay profiles are routinely imaged and a near real-time update time of 3 s for the fluorescence lifetime map has been demonstrated using a modest personal computer. We also present provisional fluorescence lifetime images of tissue constituents. This fluorescence lifetime imaging technology is applicable to almost any optical instrument configuration and, when coupled with existing all-solid-state diode-pumped ultrafast laser technology, may yield a potentially inexpensive instrument for in vitro and in vivo biomedical imaging.

Two-dimensional fluorescence lifetime imaging for in-vitro and in-vivo application

We report the development of a fluorescence lifetime imaging (FLIM) system based on a time-gated image intensifier and solid-state laser amplifier with a system response of < 100 ps. We have sued this system to image lifetimes as short as 100 ps and to image changes in the environment of a fluorescent phantom, causing lifetime differences less than 10 ps. The versatility of this FLIM system has been demonstrated by measuring both the temporal and spectral profiles of multiple fluorescent samples in a single acquisition. Fluorescence lifetime imaging of tissue constituents has also been carried out and first results suggest that this technique can provide a means of distinguishing between different tissue constituents.

Novel ultrafast tunable solid state lasers for real-world applications including medical imaging

This paper reviews ultrafast Kerr Lens Mode-locked solid- state lasers with particular emphasis on all-solid-state diode-pumped laser technology which has the potential to provide low-cost compact devices for ultrafast instrumentation, particularly for biomedical applications.We have demonstrated the use of ultrafast solid-state lasers for 3D imaging through turbid media using time-gated photorefractive holography, and for fluorescence lifetime imaging.

Time-gated holographic imaging using photorefractive multiple quantum well devices

A depth-resolved imaging system is described for recording three dimensional images of objects embedded in diffuse media. Time-gated holographic imaging, employing photorefractive multiple quantum well devices as the recording media, is used to obtain real-time whole-field depth-resolved two dimensional images. Infra-red radiation has been used which corresponds to the medical imaging window.

Time-gated holographic imaging using photorefractive media

We demonstrate a coherence gating system to image through turbid media by using photorefractive holography with ultrashort pulses. Sub-100 micrometers depth-resolved images of 3D objects embedded in a scattering medium have been obtained. Using a long integration time in Rhodium-doped Barium Titanate, an image of a test chart has been obtained through 16 mean free paths of scattering medium. Real-time depth- resolved imaging through 13 mean free paths of scattering medium has been demonstrated using a fast response time photorefractive multiple quantum well device. We will discuss the trade-offs and limitations of these photorefractive media for this application.

High-resolution whole field fluorescence lifetime imaging of fluorophore distribution and environment

We report the development of a high temporal resolution fluorescence lifetime imaging (FLIM) system suing a time- gated image intensifier to provide whole field FLIM images. The gate width has been optimized to 110 ps, and changes in the environment of a fluorescent phantom, causing lifetime differences of 20 ps, have been detected. Environmental changes of the fluorescent indicator, Lucifer Yellow, have been sensed by measuring changes in its fluorescence lifetime in the presence of the protein albumin. We also present provisional fluorescence lifetime images of tissue constituents.

Two-dimensional fluorescence-lifetime imaging using a 5-kHz/110-ps gated image intensifier

We report the demonstration of a high temporal resolution fluorescence lifetime imaging (FLIM) system using a time- gated image intensifier to provide whole field FLIM images. The gate width has been optimized to 110 ps, and changes in the environment of a fluorescent phantom, causing lifetime differences of 20 ps, have been detected. Environmental changes of the fluorescent indicator, Lucifer Yellow, have been sensed by measuring changes in its fluorescence lifetime when unbound and when bound to the protein albumin.

Depth-resolved holography using photorefractive media

Various techniques, including coherence gating have been employed to discriminate against scattered light when imaging objects in a diffusing medium. We demonstrate a coherence gate using a photorefractive holographic imaging system. Using ultrashort pulse illumination, depth resolved images of three dimensional objects embedded in such a scattering medium have been obtained. We have investigated the use of bulk photorefractive crystals (Rh:BaTiO3) and photorefractive multiple quantum well (MQW) devices as the hologram recording medium. Using a long integration time in bulk Rh:BaTiO3, an image of a test chart has been obtained through up to 16 mean free paths (mfp) of scattering medium. Conversely a fast response time (less than 0.4 ms) has been demonstrated in the MQW device when imaging through 8 mfp of scattering medium.