Biological tissues analysis by XRF microtomography
Introduction
Since tomographic techniques were developed, X-ray transmission tomography has been used in noninvasive testing for investigating the internal structure of samples (Braz et al., 1999) and with the advent of intense synchrotron radiation sources; the resolution of the tomography was improved into μm regime (Lopes et al., 2003). Although the transmission tomography provides information about the distribution of the attenuation coefficients, the technique cannot give any information about the distribution of trace elements within the samples.
Other complementary tomographic techniques have been developed based on the detection of the scattered (Barroso et al., 1998) and fluorescence photons (Pereira et al., 2007) in order to get some properties that also depend upon the distribution of individual elements within the sample. The X-ray fluorescence (XRF) associated with tomographic techniques can supply important information about a sample's chemical properties and produce high contrast. X-ray transmission tomography is not able to give this information.
X-ray fluorescence tomography is based on detection of photons from the fluorescence emission from the elements in the sample. These photons are acquired by an energy dispersive detector, placed at 90° to the incident beam direction. A value in a projection is obtained by measuring the fluorescence radiation emitted by all pixels along the beam. The sample is translated and another value is measured in the projection. These steps are repeated until the whole sample pass through the beam, completing the projection. The sample is rotated and another projection is measured. The projections are measured until the sample rotated 180°.
The data acquisition of the X-ray fluorescence microtomography results in a bi-dimensional raw data called sinogram. A sinogram consists of the line scans that were taken for every rotational angle. For each emission line of the fluorescence spectrum there is a separate sinogram. In general, a sinogram plots the property measured by the setup over the direction perpendicular to the beam and the angle of rotation. The tomogram which is a two-dimensional slice across the sample can be reconstructed from the sinogram by an appropriate algorithm.
The increase of the mortality rate in Brazil due to cancer was a decisive factor in the choice of the investigated samples. The choice also reflects the world tendency to find diagnostic techniques for cancer and other diseases. The fluorescence mapping of iron, copper and zinc can be very important in diagnostics, because the biochemistry of these elements suggests that these metals may play an important role in carcinogenesis. However, the evidence linking iron, copper and zinc to cancer is far from conclusive and further research is needed (Wu et al., 2004). Using X-ray fluorescence tomography the elemental map can be obtained without sample preparation.
This paper presents a development of a setup to study X-ray fluorescence microtomography of biological samples at the X-ray fluorescence beam line of the Brazilian Synchrotron Light Laboratory Facility (LNLS), Campinas, Brazil.
Section snippets
Experiment
This work presents the development of a system to study X-ray fluorescence microtomography at the X-ray fluorescence facility (D09B-XRF) at the Brazilian Synchrotron Light Laboratory (LNLS), Campinas, Brazil. A quasi-monochromatic beam produced by a multilayer monochromator at 12 keV, ΔE/E=0.03 collimated to a 200 μm×200 μm area with a set of slits, was used to sample excitation. The crystal monochromator is made of W-C and it has 75 layer pairs.
The intensity of the incident beam was monitored
Results and discussion
The result for the reference sample made of polyethylene is shown in Fig. 2.
The image in Fig. 2 shows in the left side the X-ray transmission microtomography, and in the middle side and in the right side, the image shows the XRF microtomography of copper without absorption corrections and with absorption corrections, respectively. It can be observed analyzing the images that while the X-ray transmission tomography shows the polyethylene matrix and the internal cylinder with copper solution, the
Conclusion
The reference sample shows the viability of fluorescence microtomography and confirms that this technique can be used to complement other techniques for sample characterization. Analyzing these images, it is verified that it is very important to use the algorithm with absorption corrections to get the corrected value of concentration.
Analyzing the XRF microtomography of the breast tissue samples, it was verified that it was possible to visualize the elemental distribution of iron, copper and
Acknowledgments
This work was partially supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) and the Laboratório Nacional de Luz Síncrotron (LNLS).
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