Tomography has made revolutionary impacts in a number of fields ranging from medical imaging, magnetic resonance imaging to electron microscopy. Conventional tomography reconstructs a 3D object from a set of equally angled 2D projections. Since the set of projections are in polar coordinates and the object in Cartesian coordinates, interpolation has to be used in the reconstruction process, which introduces artifacts in the reconstructed 3D object. In application to biology and medicine, there are two more difficulties: (i) a limited number of projections due to radiation damage to biological specimens and the patients; and (ii) the missing wedge problem (i.e. specimens cannot be tilted beyond ± 70° in cryo-electron microscopy). Here we apply equally-sloped tomography (EST) to significantly alleviate these difficulties and to demonstrate that EST can dramatically reduce the required radiation dose for achieving a desired resolution.

We applied EST to reconstructing frozen-hydrated keyhole limpet hemocyanin molecules, a frozen-hydrated bacterial cell and a single human immunodeficiency virus (HIV). In comparison with traditional weighted back-projection (WBP), the algebraic reconstruction technique (ART) and the simultaneous algebraic reconstruction technique (SART), EST reconstructions exhibited higher contrast, less peripheral noise, more easily detectable molecular boundaries and reduced missing wedge effects. More importantly, EST reconstructions including only two-thirds the original images appeared to have the same resolution as full WBP reconstructions, suggesting that EST can either reduce the dose required to reach a given resolution or allow higher resolutions to be achieved with a given dose.

The development of the tomographic implementation of x-ray phase contrast imaging holds great promise for biological and medical imaging; however, the radiation dose imparted to biological specimens and patients presents a major obstacle in such an implementation. By using an experimental data set from the European Synchrotron Radiation Facility, we show that EST can reduce the radiation dose by 60%, while obtaining the comparable images reconstructed by the conventional WBP at a full dose. This work may hence open the possibility of significantly radiation dose reduction in biological and medical imaging.