If pencil-beam excitation is used in a multimodal XFCT/CT system, the
acquisition time of the CT image is longer than for a standalone CT. Nevertheless,
the single line acquisition would lead to reduced scatter contamination in the CT data
and further increase the CT image quality.
Similar to the attenuation and scatter correction in XFCT, the information from
the anatomical CT image can be used to improve the modeling of the scatter and
attenuation properties, required to solve the diffusion equation in XLCT image
reconstruction. However, the absorption coefficient by the CT measurement is not
directly related to the scatter probability and the absorption coefficients of optical or
NIR photons. Therefore, segmentation and material identification/decomposition
need to be performed on the reconstructed CT images to derive the correct prior
knowledge for the XLCT image reconstruction. The additional information obtained
from the CT images will improve the image quality and precision for retrieving the
nanophosphor concentration from the measured luminescent signal.
3 Imaging and Instrumental Specifications
The performance of X-ray excited imaging depends on various parameters (e.g.,
source-subject distance, subject-detector distance, source and detector specifications,
detector location, collimator design, etc.) which need to be optimized for enhanced
sensitivity, spatial resolution, and scanning time. For polychromatic X-ray sources,
beam filtering with various thicknesses of attenuating materials must be adapted for
each distinct contrast element to provide X-ray energy spectra suitable to enhance
emission photon efficiency from the contrast agent. Moreover, the optimal trade-off
between imaging dose, filtration, output, sensitivity, and spatial resolution needs to
be determined. While the majority of these parameters can be optimized from
simulations for different applications, some of them require experimental validation
or deduction. Proof of principle experiments with a single CdTe detector delivered
sensitivities in the range of 2.5–5 mg/ml for K-shell XFCT imaging with gold
nanoparticles [18]. A schematic of the image acquisition chain and photograph of
this setup are shown in Fig. 2.
For higher energetic K-shell fluorescence from, e.g., gold or platinum, the
stopping power of silicon is low, and silicon detectors do not provide sufficient
photon detection efficiency. In this energy range, it is recommended to exploit the
latest advances in room-temperature semiconductor detectors based on the new
generation of pixelated cadmium-zinc-telluride (CZT) crystals to surmount the key
technical hurdles of XFCT imaging. In recent years, technological advances have
enabled manufacturers to grow large CZT crystals with high energy resolutions
(<1% at 100 keV) and to produce pixelated arrays with small pixel sizes for high
spatial resolution. Other essential detector parameters for XFCT imaging are shaping
time, count rate performance, and energy resolution. It has to be emphasized that
spatial resolution is generally not required for pencil-beam XFCT as the detector
only needs to determine the amount of XF and scatter photons in a particular energy
window. Hence, the detector is a veto detector reporting whether or not a contrast
X-Ray Excited Fluorescent Materials for Medical Application
129
acquisition time of the CT image is longer than for a standalone CT. Nevertheless,
the single line acquisition would lead to reduced scatter contamination in the CT data
and further increase the CT image quality.
Similar to the attenuation and scatter correction in XFCT, the information from
the anatomical CT image can be used to improve the modeling of the scatter and
attenuation properties, required to solve the diffusion equation in XLCT image
reconstruction. However, the absorption coefficient by the CT measurement is not
directly related to the scatter probability and the absorption coefficients of optical or
NIR photons. Therefore, segmentation and material identification/decomposition
need to be performed on the reconstructed CT images to derive the correct prior
knowledge for the XLCT image reconstruction. The additional information obtained
from the CT images will improve the image quality and precision for retrieving the
nanophosphor concentration from the measured luminescent signal.
3 Imaging and Instrumental Specifications
The performance of X-ray excited imaging depends on various parameters (e.g.,
source-subject distance, subject-detector distance, source and detector specifications,
detector location, collimator design, etc.) which need to be optimized for enhanced
sensitivity, spatial resolution, and scanning time. For polychromatic X-ray sources,
beam filtering with various thicknesses of attenuating materials must be adapted for
each distinct contrast element to provide X-ray energy spectra suitable to enhance
emission photon efficiency from the contrast agent. Moreover, the optimal trade-off
between imaging dose, filtration, output, sensitivity, and spatial resolution needs to
be determined. While the majority of these parameters can be optimized from
simulations for different applications, some of them require experimental validation
or deduction. Proof of principle experiments with a single CdTe detector delivered
sensitivities in the range of 2.5–5 mg/ml for K-shell XFCT imaging with gold
nanoparticles [18]. A schematic of the image acquisition chain and photograph of
this setup are shown in Fig. 2.
For higher energetic K-shell fluorescence from, e.g., gold or platinum, the
stopping power of silicon is low, and silicon detectors do not provide sufficient
photon detection efficiency. In this energy range, it is recommended to exploit the
latest advances in room-temperature semiconductor detectors based on the new
generation of pixelated cadmium-zinc-telluride (CZT) crystals to surmount the key
technical hurdles of XFCT imaging. In recent years, technological advances have
enabled manufacturers to grow large CZT crystals with high energy resolutions
(<1% at 100 keV) and to produce pixelated arrays with small pixel sizes for high
spatial resolution. Other essential detector parameters for XFCT imaging are shaping
time, count rate performance, and energy resolution. It has to be emphasized that
spatial resolution is generally not required for pencil-beam XFCT as the detector
only needs to determine the amount of XF and scatter photons in a particular energy
window. Hence, the detector is a veto detector reporting whether or not a contrast
X-Ray Excited Fluorescent Materials for Medical Application
129