imaging. Commonly the choice is driven by trade-offs between sensitivity and
acquisition time and often restricted by the XFCT hardware. Another way to localize
XF emitters is the spatial selective X-ray excitation which can be combined with
spatially resolving, energy-discriminating X-ray detectors. Akin to MRI, it is possible to partially encode the X-ray excitation to some of the spatial dimensions by
collimation on the source side and to collimate the detectors to measure the
remaining dimensions. Compared to the sinogram format provided by a rotating
gantry, here the XFCT can directly be measured in image space, without the need for
any image reconstruction (triple-collimation geometry).
7.1 Pencil-Beam XFCT
In the general concept of XFCT imaging, the excitation is limited to a line by
creating a pencil beam. The X-ray pencil beam is commonly generated by collimating the X-ray photons emitted by a standard clinical X-ray source. For lower
energies (<30 keV), it is also possible to focus X-rays as described in chapter
“High-Resolution 2D Imaging”. The collimation is usually realized with masks of
high atomic number materials like lead or tungsten which block the excess X-rays
emitted by the anode of the X-ray source. With this form of excitation, the emission
of XF X-rays from the contrast agents is mainly limited to the line of excitation. Of
course, it is possible that scatter or background X-rays excite target agents outside of
the line of excitation, but this process is highly suppressed as scattered photons
already lost too much energy to stimulate K-shell or L-shell fluorescence, respectively. However, it is recommended to utilize filtration for the polychromatic X-ray
beam as the scatter background magnitude and dose to the imaged object is highly
dependent on the shape of the energy spectra of the X-ray beam. The beam filtration
cuts off the lower energy photons which generate dose to the object but are not
energetic enough to produce fluorescent X-rays. It also attenuates higher energy
X-rays, but in general this only increases the acquisition time slightly. The acceleration voltage of the X-ray source is also a parameter that influences the beam energy
spectrum. Here, the higher energy part of the spectrum is shaped. It is recommended
to produce fewer X-rays with excess energies above the K-shell excitation. These
higher energetic X-rays increase the probability for scatter photons in the energy
range of the XF signal. Furthermore, the scattered X-rays have enough energy to
create XF signals outside of the line of excitation. For optimized performance, the
source voltage and the filtration must be tuned for each contrast agent individually.
While there is an optimal configuration for each contrast agent, it was shown that
multiplexed imaging of multiple contrast agents with a single filter and tube voltage
configuration is possible [18]. In the study, the filter consisted of 1 mm Pb, 1.4 mm
Cu, and 7.3 mm Al optimized for the imaging of platinum, gadolinium, and iodine
simultaneously.
For K-shell fluorescence experiments with high emission energy elements, e.g.,
gold or platinum, the molecular sensitivity can be increased with a spectro-spatial
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D. Vernekohl and L. Xing
acquisition time and often restricted by the XFCT hardware. Another way to localize
XF emitters is the spatial selective X-ray excitation which can be combined with
spatially resolving, energy-discriminating X-ray detectors. Akin to MRI, it is possible to partially encode the X-ray excitation to some of the spatial dimensions by
collimation on the source side and to collimate the detectors to measure the
remaining dimensions. Compared to the sinogram format provided by a rotating
gantry, here the XFCT can directly be measured in image space, without the need for
any image reconstruction (triple-collimation geometry).
7.1 Pencil-Beam XFCT
In the general concept of XFCT imaging, the excitation is limited to a line by
creating a pencil beam. The X-ray pencil beam is commonly generated by collimating the X-ray photons emitted by a standard clinical X-ray source. For lower
energies (<30 keV), it is also possible to focus X-rays as described in chapter
“High-Resolution 2D Imaging”. The collimation is usually realized with masks of
high atomic number materials like lead or tungsten which block the excess X-rays
emitted by the anode of the X-ray source. With this form of excitation, the emission
of XF X-rays from the contrast agents is mainly limited to the line of excitation. Of
course, it is possible that scatter or background X-rays excite target agents outside of
the line of excitation, but this process is highly suppressed as scattered photons
already lost too much energy to stimulate K-shell or L-shell fluorescence, respectively. However, it is recommended to utilize filtration for the polychromatic X-ray
beam as the scatter background magnitude and dose to the imaged object is highly
dependent on the shape of the energy spectra of the X-ray beam. The beam filtration
cuts off the lower energy photons which generate dose to the object but are not
energetic enough to produce fluorescent X-rays. It also attenuates higher energy
X-rays, but in general this only increases the acquisition time slightly. The acceleration voltage of the X-ray source is also a parameter that influences the beam energy
spectrum. Here, the higher energy part of the spectrum is shaped. It is recommended
to produce fewer X-rays with excess energies above the K-shell excitation. These
higher energetic X-rays increase the probability for scatter photons in the energy
range of the XF signal. Furthermore, the scattered X-rays have enough energy to
create XF signals outside of the line of excitation. For optimized performance, the
source voltage and the filtration must be tuned for each contrast agent individually.
While there is an optimal configuration for each contrast agent, it was shown that
multiplexed imaging of multiple contrast agents with a single filter and tube voltage
configuration is possible [18]. In the study, the filter consisted of 1 mm Pb, 1.4 mm
Cu, and 7.3 mm Al optimized for the imaging of platinum, gadolinium, and iodine
simultaneously.
For K-shell fluorescence experiments with high emission energy elements, e.g.,
gold or platinum, the molecular sensitivity can be increased with a spectro-spatial
138
D. Vernekohl and L. Xing