Four-dimensional fluorescence microscopy-which records 3D image information as a function of time-provides an unbiased way of tracking dynamic behavior of subcellular components in living samples and capturing key events in complex macromolecular processes. Unfortunately, the combination of phototoxicity and photobleaching can severely limit the density or duration of sampling, thereby limiting the biological information that can be obtained. Although widefield microscopy provides a very light-efficient way of imaging, obtaining high-quality reconstructions requires deconvolution to remove optical aberrations. Unfortunately, most deconvolution methods perform very poorly at low signal-to-noise ratios, thereby requiring moderate photon doses to obtain acceptable resolution. We present a unique deconvolution method that combines an entropy-based regularization function with kernels that can exploit general spatial characteristics of the fluorescence image to push the required dose to extreme low levels, resulting in an enabling technology for high-resolution in vivo biological imaging.4D microscopy | low dose microscopy | noise-suppressing regularization T he study of dynamic processes is an important facet of cell biology research. Fluorescently tagged proteins combined with four-dimensional fluorescence microscopy, which records 3D image information as a function of time, provide a powerful framework for studying the dynamics of molecular processes in vivo. One of the most crucial challenges in 4D fluorescence microscopy is to ensure that normal biological function is not significantly perturbed as a result of the high doses of illumination (phototoxicity) incurred during 4D imaging. Recent work indicates that the maximal photon dose that avoids biological perturbation is 100-to 1,000-fold lower than that typically used for in vivo imaging (1). Dose limitations are even more challenging, given the desire to densely sample in time or to record over extended periods, especially in the context of analyzing multiple subcellular components via multiwavelength imaging.Under normal imaging conditions, widefield microscopy combined with image restoration using deconvolution methods provides an excellent modality for multiwavelength 4D imaging as it makes very efficient use of the illuminating photons. However, its effectiveness, in particular its ability to resolve subcellular detail sufficiently in the presence of noise, is limited by the performance of the deconvolution method. Such limitations can seriously degrade image quality at the low signal levels required for unperturbed in vivo imaging. The noise behavior of the deconvolution algorithm is determined by the efficiency of the noise stabilization term, known as the regularization functional. In particular, the functional's ability to discriminate the noise-related high frequencies from weak high frequencies in the signal ultimately determines the final resolution of the deconvolution. Currently used noise-stabilization techniques are largely based on ad hoc form...