Researchers have developed a new, focus-free technique for creating chemical
maps using X-ray fluorescence. The approach offers fast, high-resolution
measurements, which could be useful for analyzing chemical composition for a
range of applications in biomedicine, materials science, archeology, art and
industry.
"Our new method combines the well-known techniques of computational ghost
imaging and X-ray fluorescence measurement to create a high-resolution and
efficient way to produce chemical element maps," said research team leader
Sharon Shwartz from Bar Ilan University in Israel. "We expect it will allow
the chemical mapping of larger objects at higher resolutions than is
possible today while also enabling measurement of complex 3D objects."
In Optica, Optica Publishing Group's journal, Shwartz and colleagues
describe their new X-ray computational ghost fluorescence technique. The
approach doesn't require any focusing and reduces the scanning needed, which
significantly shortens measurement time. Also, the fact that it can be tuned
to detect specific elements while being blind to human tissues could enable
new applications such as full-body security scanners that improve privacy.
"Medical imaging, which is performed at X-ray energies where lenses are not
practical, could also benefit from our approach," said Shwartz. "It could be
applied to increase the quality of medical X-ray imaging by boosting tissue
contrast or for reducing the X-ray dose necessary to get useful images."
Seeing beneath the surface
X-ray fluorescence is used to determine the chemical elements within a
sample by measuring fluorescence emitted from a sample after it is excited
by an X-ray source. The data acquired with this nondestructive analytical
technique can be used to create chemical maps that have revealed hidden
layers in famous paintings and are used to inspect critical aerospace parts,
for example.
Chemical element mapping with X-ray fluorescence traditionally involves
focusing the input X-ray beam and then measuring the fluorescence emitted
from the area. A chemical map is constructed by scanning the sample point by
point and recording the fluorescence intensity at each point. However, this
approach is slow because of the scanning required. Also, the spatial
resolution of the measurements is restricted by the capabilities of the
lenses used for focusing.
"These limitations become even more prominent when X-ray energies higher
than 20 keV are used or when trying to acquire 3D information," said
Shwartz. "Although higher X-ray energies could enable chemical mapping of
thicker objects or samples containing dense and heavy elements, it's not
possible to use these higher photon energies due to the limitations of
standard technologies."
Eliminating lenses
The researchers turned to computational ghost imaging to remove some of the
limitations of conventional X-ray fluorescence analysis. This
non-traditional imaging method works by correlating two beams that do not
individually carry any meaningful information about the object. One beam
encodes a random pattern that acts as a reference and never directly probes
the sample while the other beam interacts with the sample.
The researchers modified the ghost imaging approach so that it could be used
to map chemical elements. Although ghost imaging methods typically involve
measuring transmitted radiation, the researchers measured emitted
fluorescence instead.
"Measuring X-ray fluorescence enables us to identify each chemical element
based on its unique emission spectrum," said Shwartz. "By using a detector
that can resolve the energies of the emitted radiation, we can identify the
contribution of each element to the detected radiation."
The random pattern required for ghost imaging is typically created by adding
a known spatial modulation, or variation, to the intensity of the beam used
to irradiate the object. The researchers achieved this by repeating the
fluorescence measurements for different input beam intensity patterns.
Putting it all together
The new X-ray computational ghost fluorescence approach produces two sets of
data for each photon energy—one with the spatial distributions of the input
beam and one with the emitted fluorescence measurements. A computer program
then puts these data together and overlays all the imaging data from the
various photon energies to create a chemical element map of the object.
The researchers used their new method to create a chemical element map of an
object made from iron and cobalt. They showed that using a compressive
sensing algorithm reduced the number of scans by almost a factor of 10
compared to standard scanning-based techniques.
"Since our setup is simple and can provide better performance than today's
approaches, we expect that it will open new possibilities in many
disciplines including, biology, chemistry, art and archeology," said
Shwartz. "Also, it will be straightforward to extend our method to higher
photon energies that are not accessible with present-day methods."
Next, they plan to apply the new methods to 3D chemical mapping and to
demonstrate the applicability of the method for medical imaging.
Reference:
Y. Klein, O. Sefi, H. Schwartz, S. Shwartz, "Chemical element mapping by
X-ray computational ghost fluorescence," Optica, 9(1), 1–8 (2022).
DOI: https://doi.org/10.1364/OPTICA.441682