agent is present along the pencil-beam path. Nevertheless, spatial information is
required for attenuation correction [15] and can become important for advanced
scatter removal strategies (see chapter “Scatter and Attenuation Correction”) or other
XFCT imaging modes (see chapter “High-Resolution 2D Imaging”).
The turnkey solution for high sensitivity is a high solid angle coverage as the
fluorescent X-rays are emitted isotropically over the full 4π solid angle. For CZT
detectors, large 4 Â 4 cm
2 crystals are typically grown for efficient gamma- or X-ray
detection, and they reach energy resolutions down to 3% FWHM at 122 keV. In
collaboration with the group from Dr. He [16], we tested the Polaris large CZT
crystal prototype detector with an energy resolution of 2.5% which is about three
times worse than the energy resolution of the spectroscopic CdTe detector used in
our previous experiments [6, 17–19]. The experiment was conducted with both
detectors placed on the same distance to an iodine phantom that was excited with
a 50 kVp X-ray source equipped with an Ag anode. The reconstructed images
provide better contrast and higher sensitivity for the CZT detector. The improved
performance results from the higher detection efficiency and the better angular
coverage which even overcomes lower energy resolution and the X-ray absorbing
shielding of the CZT detector.
In the scenario of lower energy X-rays from L-shell fluorescence of high atomic
number contrast agents like gold or platinum and lower atomic number K-shell
fluorescence from elements, for example, silver and iodine, the XF photons can be
measured with advanced detector technology, e.g., a silicon drift detector (SDD). As
SDDs are made of silicon, their stopping power for high energetic X-rays is meager.
However, in the lower energy regime, their high energy resolution outperforms other
detector materials. For example, our group showed that for L-shell imaging of gold
as the contrast agent, SDD detectors exceed CdTe detectors [17]. In contrast to
K-shell fluorescence of high atomic number contrast agents which is suitable in
principle for whole-body clinical imaging, the lower energy regime of L-shell
fluorescence is restricted to small animal preclinical applications and organ-specific
surface close clinical applications due to the high X-ray attenuation of tissue for low
energetic X-rays. There are two advantages of lower excitation energies. Firstly, at
these energies, synthetic materials exist that can reflect and refract these X-rays
[20]. Specifically, compound poly-capillary X-ray lenses are available either to focus
the X-ray power to a small spot on the surface of the subject or to bend the X-rays for
higher flux X-ray parallel beam [20]. In this design, the X-ray power is used
efficiently compared to conservative pencil-beam collimation and focused on a
narrower area. Moreover, synthetic materials can be used to produce X-rays mirrors
with energy-dependent reflection indexes. Thus, these mirrors can be used to shape
the exciting energy spectrum more precisely than with absorbing filter materials.
The second advantage of choosing lower X-ray excitation energies is that they
can be stimulated from small form factor sources (X-ray energies up to 50 keV). The
target anode in these sources is very close to the exit window or nozzle. Therefore,
the imaged subject can be placed very close to the X-ray source focal spot for high
X-ray exposure. These sources don’t require advanced cooling methods and can be
operated with compressed air or water cooling.
X-Ray Excited Fluorescent Materials for Medical Application
131
required for attenuation correction [15] and can become important for advanced
scatter removal strategies (see chapter “Scatter and Attenuation Correction”) or other
XFCT imaging modes (see chapter “High-Resolution 2D Imaging”).
The turnkey solution for high sensitivity is a high solid angle coverage as the
fluorescent X-rays are emitted isotropically over the full 4π solid angle. For CZT
detectors, large 4 Â 4 cm
2 crystals are typically grown for efficient gamma- or X-ray
detection, and they reach energy resolutions down to 3% FWHM at 122 keV. In
collaboration with the group from Dr. He [16], we tested the Polaris large CZT
crystal prototype detector with an energy resolution of 2.5% which is about three
times worse than the energy resolution of the spectroscopic CdTe detector used in
our previous experiments [6, 17–19]. The experiment was conducted with both
detectors placed on the same distance to an iodine phantom that was excited with
a 50 kVp X-ray source equipped with an Ag anode. The reconstructed images
provide better contrast and higher sensitivity for the CZT detector. The improved
performance results from the higher detection efficiency and the better angular
coverage which even overcomes lower energy resolution and the X-ray absorbing
shielding of the CZT detector.
In the scenario of lower energy X-rays from L-shell fluorescence of high atomic
number contrast agents like gold or platinum and lower atomic number K-shell
fluorescence from elements, for example, silver and iodine, the XF photons can be
measured with advanced detector technology, e.g., a silicon drift detector (SDD). As
SDDs are made of silicon, their stopping power for high energetic X-rays is meager.
However, in the lower energy regime, their high energy resolution outperforms other
detector materials. For example, our group showed that for L-shell imaging of gold
as the contrast agent, SDD detectors exceed CdTe detectors [17]. In contrast to
K-shell fluorescence of high atomic number contrast agents which is suitable in
principle for whole-body clinical imaging, the lower energy regime of L-shell
fluorescence is restricted to small animal preclinical applications and organ-specific
surface close clinical applications due to the high X-ray attenuation of tissue for low
energetic X-rays. There are two advantages of lower excitation energies. Firstly, at
these energies, synthetic materials exist that can reflect and refract these X-rays
[20]. Specifically, compound poly-capillary X-ray lenses are available either to focus
the X-ray power to a small spot on the surface of the subject or to bend the X-rays for
higher flux X-ray parallel beam [20]. In this design, the X-ray power is used
efficiently compared to conservative pencil-beam collimation and focused on a
narrower area. Moreover, synthetic materials can be used to produce X-rays mirrors
with energy-dependent reflection indexes. Thus, these mirrors can be used to shape
the exciting energy spectrum more precisely than with absorbing filter materials.
The second advantage of choosing lower X-ray excitation energies is that they
can be stimulated from small form factor sources (X-ray energies up to 50 keV). The
target anode in these sources is very close to the exit window or nozzle. Therefore,
the imaged subject can be placed very close to the X-ray source focal spot for high
X-ray exposure. These sources don’t require advanced cooling methods and can be
operated with compressed air or water cooling.
X-Ray Excited Fluorescent Materials for Medical Application
131