Wolf phase tomography (WPT) of transparent structures using partially coherent illumination

In 1969, Emil Wolf proposed diffraction tomography using coherent holographic imaging to extract 3D information from transparent, inhomogeneous objects. In the same era, the Wolf equations were first used to describe the propagation correlations associated with partially coherent fields. Combining these two concepts, we present Wolf phase tomography (WPT), which is a method for performing diffraction tomography using partially coherent fields. WPT reconstruction works directly in the space–time domain, without the need for Fourier transformation, and decouples the refractive index (RI) distribution from the thickness of the sample. We demonstrate the WPT principle using the data acquired by a quantitative-phase-imaging method that upgrades an existing phase-contrast microscope by introducing controlled phase shifts between the incident and scattered fields. The illumination field in WPT is partially spatially coherent (emerging from a ring-shaped pupil function) and of low temporal coherence (white light), and as such, it is well suited for the Wolf equations. From three intensity measurements corresponding to different phase-contrast frames, the 3D RI distribution is obtained immediately by computing the Laplacian and second time derivative of the measured complex correlation function. We validate WPT with measurements of standard samples (microbeads), spermatozoa, and live neural cultures. The high throughput and simplicity of this method enables the study of 3D, dynamic events in living cells across the entire multiwell plate, with an RI sensitivity on the order of 10 −5 . A scheme for determining the three-dimensional refractive index distribution of transparent structures could prove useful for monitoring morphological changes in living cells. Xi Chen and coworkers from the University of Illinois in the USA say that their technique, called Wolf Phase Tomography (WPT), has the advantages of simple sample preparation, high throughput and also high sensitivity (refractive index variations on the order of 10 -5 can be detected). The team analyzed several samples using WPT including a suspension of 2-µm polystyrene microspheres in immersion oil, sperm cells and neurons. The approach makes use of an adapted phase-contrast microscope and partially coherent illumination (a ring-shaped white light source). Refractive index mapping of biological cells and tissue is useful for the study of cancer and cellular transport and mitosis.