Confocal microscopy in turbid media

Abstract

We examine the performance of confocal microscopes designed for probing structures embedded in turbid media. A heuristic scheme is described that combines a numerical Monte Carlo simulation of photon transport in a turbid medium with a geometrical ray trace through the confocal optics. To show the effects of multiple scattering on depth discrimination, we compare results from the Monte Carlo simulations and scalar diffraction theory. Experimental results showing the effects of the pinhole diameter and other variables on imaging performance at various optical depths in suspensions of polystyrene microspheres were found to correspond well with the Monte Carlo simulations. The major conclusion of the paper is that the trade-off between signal level and background scattered-light rejection places a fundamental limit on the sectioning capability of the microscope.

Optical parametrically gated microscopy in scattering media

“…It is expected that this decreased pinhole size contributes to the efficient removal of the multiply-scattered light background, as shown by the experimental results presented later. Although the smaller pinhole may give more attenuation of the ballistic signal [7][8][9], this is compensated by the high optical gain of the OPA process. As shown by Fig.…”

“…These diffusing photons lose most of the information about their initial propagating direction (wave vector), which if not effectively rejected, can greatly degrade the imaging resolution. Therefore, the attainable imaging depth and resolution in scattering media rely heavily on the effectiveness of the imaging techniques for the detection of the ballistic photon signal against the multiplyscattering background [7][8][9].…”

“…Currently, for 3-D optical imaging in scattering media, the detection of ballistic photons is achieved by using high-sensitivity photo-detectors along with a variety of optical gating methods, such as time gating [5,10,11], confocal gating [7,9,12], polarization gating [13,14], and coherence (effectively time) gating [15,16], which help to reject the multiplyscattered light background (in addition to the optical sectioning functions of these techniques). In these gating approaches, there exists an intrinsic limitation, i.e., the compromise between signal strength and background removal [7][8][9].…”

“…In these gating approaches, there exists an intrinsic limitation, i.e., the compromise between signal strength and background removal [7][8][9]. For instance, in confocal microscopy, a smaller pinhole rejects more multiply-scattered background light but at the expense of signal loss, whereas a larger pinhole (weaker gate) collect more ballistic photon signals, but these are compromised by a stronger multiply-scattered light background.…”

“…This inevitably leads to a limited imaging depth due to weak signal, as well as a possible limited resolution due to the strong background. The resultant well-known trade-off between resolution and imaging depth affects nearly all of the current ballistic photon imaging techniques, including confocal microscopy, optical coherence tomography (OCT), and timegated imaging in scattering media [7][8][9].…”