Instrument Design and Image Reconstruction For A Laboratory X-Ray Microtomograph

Abstract

X-ray microscopy has received considerable attention over the years, since it has the potential of producing high-resolution images of thick specimens in air. We are developing an x-ray microtomograph for three-dimensional imaging of small biological specimens. The instrument, shown in Figure 1, has much in common with projection x-ray microscopes of decades past, but incorporates several technological advances of recent years to partially overcome the limitations of the older instruments. The most important of these are the use of a planar solid-state detector and the provision for volume reconstruction. We describe the design for a relatively low-cost instrument intended for 3-D imaging of biological specimens up to ten cubic millimeters in size.The x-ray source for the microtomograph consists of a modified SEM. The electron beam, in spot mode and focused to about ten nanometers, impinges on a thin foil target to produce an emergent, low-intensity cone beam of characteristic and Bremsstrahlung x-rays. The foil resides in close proximity to an optional aluminum filter and a thin beryllium window which terminates the evacuated electron column. The specimen is mounted on a precision rotating shaft within two millimeters of the target foil. A two-dimensional detector is placed ten to forty centimeters from the sample, providing direct projection magnifications of up to 200 times. Two-dimensional projection views are collected at each of many angular orientations as the sample is rotated through 360 degrees. Cone beam backprojection algorithms are then applied to reconstruct a threedimensional data set.