CSEM Scientific and Technical Report 2008

High-Efficiency X-ray Microscopy and Micro Computer Tomography
J. Nüesch, P. Seitz
The availability of cost-effective X-ray sources with a spot size of a few micrometers as well as high-efficiency solid-state X-ray cameras makes it
possible to realize a combined X-ray microscope and micrometer-resolution Computer Tomography instrument with high sensitivity. The X-ray
voltage range of 20-100 kVp is ideally suited for the 2D and 3D investigation of biological and organic samples with a volume of about 1x1x1 cm 3.
Due to their inhomogeneous composition and absorptive
constituents, most biological tissues with a thickness
exceeding a few millimeters are, in effect, optically opaque.
The method of choice for 2D and 3D investigation of such
objects with micrometer resolution is imaging with electromagnetic
radiation in the soft and medium X-ray region
(photon energies between 1-50 keV). Recent advances in
microfocus X-ray sources and high-sensitivity X-ray detection
with affordable solid-state cameras have made it possible to
realize table-top instruments for non-destructive X-ray imaging
of a large variety of biological and organic samples with an
effective thickness of up to a few centimeters.
The ultimate resolution of such an X-ray microscope is given
by the spot size of the X-ray source, which can be of the order
of one micrometer. This resolution makes it possible, among
other things, to study the behavior of living cells, also in dense
matrices of functional biological tissues.
A completed versatile instrument, shown in Figure 1, can
acquire X-ray images of samples with a volume of about
1x1x1 cm3 . The acceleration voltage of the source can be
varied in the wide range of 20-100 kVp. The informationcontent
of the signals can be maximized with this voltage
depending on the thickness, the density, and the elemental
composition of a sample.
Figure 1: Versatile instrument for X-ray microscopy and Computer
Tomography with micrometer resolution
The instrument makes use of a commercially available
microfocus X-ray tube with a spot size of 5 µm. The geometric
magnification of the X-ray microscope can be varied with the
mechanically adaptable geometry between X-ray spot, sample
and X-ray camera. High-efficiency X-ray detection is achieved
with a 150 µm thick CsI:Tl scintillator crystal glued onto a 4:1
optical taper. Its small end has an optical aperture of 1.0, and
it is directly coupled to a cooled low-noise CCD with an
effective readout noise of about 8 electrons. Employing this
camera, a typical X-ray exposure takes less than 1 second.
The sample is placed on a high-precision computer-controlled
rotary table, as shown in Figure 2. In this way, the necessary
data is acquired for complete 3D reconstruction of the sample
volume, using the techniques of Computer Tomography (CT).
Figure 2: The sample under investigation (here the head of a bee) is
placed on a precision rotary stage close to the radiation spot of the
X-ray source
An example of the complete 3D acquisition and reconstruction
of a biological object is shown in Figure 3, where the object
under study was the brain of a honey-bee.
Figure 3: Volume rendering of a complete 3D micro-CT scan of the
brain of a bee. Voxel size in this example is about 10 micrometers.
Although CSEM has built an X-ray microscope with biological
samples in mind, it has been used, surprisingly, for the
successful inspection of a large variety of metallic, ceramic
and organic components and microsystems, with the aim of
on-line process control, quality inspection or product authentication.
The particular appeal of X-ray inspection for all three
applications lies in its capacity of very rapidly acquiring
contrast-rich images with micrometer resolution of objects
through various opaque packages, blisters, paper wrappers,
rubber sealants, plastic protectors and cardboard boxes.
This work was partly funded by the Canton of the Grisons and
the Principality of Liechtenstein.
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