Europium (EU) and delivered good optical and infrared photon emission when
excited with X-rays [39, 41]. Following the literature, the majority of XLCT
experiments were conducted with Eu- or Er-doped nanophosphors. Beyond these
experiments also Tb, Cr, and La were used to study combined XLCT and optically
stimulated luminescence imaging [42, 43]. Another efficient luminescent emitter is
Gd 2 O 2 S:Eu (GOSE) which was used in proof-of-concept experiments [44], but it is
not recommended as a biological tracer, due to its toxicity.
6 Comparison to Other Imaging Modalities
In the last decade, much effort has been dedicated to the development of instruments
that can image both anatomical structure and molecular processes noninvasively. In
the existing designs, a molecular imaging modality (e.g., PET, SPECT, optical
fluorescence tomography (OFT)) is combined with X-ray CT or MRI, and both
modalities are working independently (or even sequentially) to provide co-registered
anatomical and molecular images [45–47]. However, these approaches have inherent
limitations. Radionuclide tracer synthesis, for example, requires complex radiochemistry procedures, and commonly radiotracers must be produced on-site. The
imaging needs to be performed within a short time window after radiotracer
synthesis.
Systems combining CT and OFT [47] do not require radionuclides, but labeling a
small molecule of interest (e.g., glucose) with a fluorescent dye without affecting its
function and biodistribution is challenging, and very few fluorescent agents have
found a use in the clinic. Tissue autofluorescence is also intractable and impedes the
sensitive detection of fluorescent molecules. Gold nanoparticles can be excited
optically via the plasmonic resonance effect and emit red-shifted light. Optical
imaging methods are quite popular in mouse imaging studies because of their high
sensitivity. However, the resolution suffers due to diffuse optical scattering. The
typical resolution for optical fluorescence imaging at 1 cm depth is 3 mm [48],
whereas XFCT imaging achieves sub-mm resolution.
The capability for simultaneous imaging of a large panel of elements in XFCT is
an advantage in comparison to PET. In PET, only 511 keV photons can be measured;
meaning only one target molecule at a time. Furthermore, the signal carriers in
XFCT are stable over time, and each molecule can be excited multiple times which is
not possible with radionuclides used in PET and SPECT. As a proof of concept for
the imaging capability to simultaneously measure various elements, an XFCT
acquisition was performed in a first-generation CT geometry, acquiring a single
pencil-beam line integral at a time [6]. It was possible to measure Au, Gd, and Ba
inclusions with a diameter of 5 mm with a single polychromatic and filtered X-ray
beam. The 5 mm diameter pencil beam was produced by an X-ray tube operated at
150 kVp, and the phantom was imaged in the prototype system using 30 translational
136
D. Vernekohl and L. Xing
excited with X-rays [39, 41]. Following the literature, the majority of XLCT
experiments were conducted with Eu- or Er-doped nanophosphors. Beyond these
experiments also Tb, Cr, and La were used to study combined XLCT and optically
stimulated luminescence imaging [42, 43]. Another efficient luminescent emitter is
Gd 2 O 2 S:Eu (GOSE) which was used in proof-of-concept experiments [44], but it is
not recommended as a biological tracer, due to its toxicity.
6 Comparison to Other Imaging Modalities
In the last decade, much effort has been dedicated to the development of instruments
that can image both anatomical structure and molecular processes noninvasively. In
the existing designs, a molecular imaging modality (e.g., PET, SPECT, optical
fluorescence tomography (OFT)) is combined with X-ray CT or MRI, and both
modalities are working independently (or even sequentially) to provide co-registered
anatomical and molecular images [45–47]. However, these approaches have inherent
limitations. Radionuclide tracer synthesis, for example, requires complex radiochemistry procedures, and commonly radiotracers must be produced on-site. The
imaging needs to be performed within a short time window after radiotracer
synthesis.
Systems combining CT and OFT [47] do not require radionuclides, but labeling a
small molecule of interest (e.g., glucose) with a fluorescent dye without affecting its
function and biodistribution is challenging, and very few fluorescent agents have
found a use in the clinic. Tissue autofluorescence is also intractable and impedes the
sensitive detection of fluorescent molecules. Gold nanoparticles can be excited
optically via the plasmonic resonance effect and emit red-shifted light. Optical
imaging methods are quite popular in mouse imaging studies because of their high
sensitivity. However, the resolution suffers due to diffuse optical scattering. The
typical resolution for optical fluorescence imaging at 1 cm depth is 3 mm [48],
whereas XFCT imaging achieves sub-mm resolution.
The capability for simultaneous imaging of a large panel of elements in XFCT is
an advantage in comparison to PET. In PET, only 511 keV photons can be measured;
meaning only one target molecule at a time. Furthermore, the signal carriers in
XFCT are stable over time, and each molecule can be excited multiple times which is
not possible with radionuclides used in PET and SPECT. As a proof of concept for
the imaging capability to simultaneously measure various elements, an XFCT
acquisition was performed in a first-generation CT geometry, acquiring a single
pencil-beam line integral at a time [6]. It was possible to measure Au, Gd, and Ba
inclusions with a diameter of 5 mm with a single polychromatic and filtered X-ray
beam. The 5 mm diameter pencil beam was produced by an X-ray tube operated at
150 kVp, and the phantom was imaged in the prototype system using 30 translational
136
D. Vernekohl and L. Xing