configurations. This chapter summarizes recent instrumentational developments in
X-ray excited imaging; discusses their attributes, advantages, and drawbacks; and
describes specifications and applications of various contrast agents for XF- and
XLCT. The feasibility of the modalities for future clinical applications will be also
highlighted.
Keywords High atomic number contrast agents, Medical imaging, Molecular
imaging, X-ray fluorescence, X-ray luminescence
1 Introduction
X-ray excitation is a promising mode for the field of medical imaging. The deeppenetrating X-rays can produce signals of functional information of an imaged
object with high resolution and sensitivity. The nature of the generated signals is
of different origins. While we focus in this chapter on fluorescent high energetic
X-rays (XFCT) and optical and near-infrared (NIR) photons (XLCT), other types of
signals, e.g., X-ray acoustic [1], can also be exploited for imaging.
In laboratories, XF is generally used to identify small amounts of chemical
species without affecting the sample. Each element is characterized by spectral
lines that are a unique signature. In XF, fluorescent X-rays emitted by the elements
in the probe are either directly measured by an energy-discriminating solid-state
detector (energy-resolving XF) or separated by wavelength by diffraction in a crystal
(wavelength-dispersive XF). XF is non-destructive, highly sensitive (1–10 ppm),
and used to measure many chemical species simultaneously and quantitatively. It is
capable of determining subtle amounts of elements in biological tissues such as
tumor specimen [2] or lead in bone [3] with monochromatic synchrotron radiation.
The ability to perform XF imaging studies in vivo, during conventional X-ray CT,
will aid tremendously in characterizing active biological processes. Preliminary
work [4–6] has shown that XF can be performed in a tomographic manner using a
CT-like scanning geometry with a conventional poly-energetic X-ray source.
The molecular contrast in XFCT comes from high atomic number (high-Z)
elements which are present either exogenously or endogenously. The conceptual
design of an XFCT imaging system consists of an X-ray source and an energydispersive X-ray detection system. In the physical process of fluorescence, the
exciting X-ray interacts with an inner shell electron (K or L) of the probe which
results in a vacancy that is subsequently filled by an electron from an outer shell as
shown in Fig. 1. The difference in energy between the two states is emitted as a
characteristic X-ray (aka fluorescent X-ray). The detection of these photons and
subsequent image reconstruction will allow the determination of the spatial distribution of molecular probes.
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X-ray excited imaging; discusses their attributes, advantages, and drawbacks; and
describes specifications and applications of various contrast agents for XF- and
XLCT. The feasibility of the modalities for future clinical applications will be also
highlighted.
Keywords High atomic number contrast agents, Medical imaging, Molecular
imaging, X-ray fluorescence, X-ray luminescence
1 Introduction
X-ray excitation is a promising mode for the field of medical imaging. The deeppenetrating X-rays can produce signals of functional information of an imaged
object with high resolution and sensitivity. The nature of the generated signals is
of different origins. While we focus in this chapter on fluorescent high energetic
X-rays (XFCT) and optical and near-infrared (NIR) photons (XLCT), other types of
signals, e.g., X-ray acoustic [1], can also be exploited for imaging.
In laboratories, XF is generally used to identify small amounts of chemical
species without affecting the sample. Each element is characterized by spectral
lines that are a unique signature. In XF, fluorescent X-rays emitted by the elements
in the probe are either directly measured by an energy-discriminating solid-state
detector (energy-resolving XF) or separated by wavelength by diffraction in a crystal
(wavelength-dispersive XF). XF is non-destructive, highly sensitive (1–10 ppm),
and used to measure many chemical species simultaneously and quantitatively. It is
capable of determining subtle amounts of elements in biological tissues such as
tumor specimen [2] or lead in bone [3] with monochromatic synchrotron radiation.
The ability to perform XF imaging studies in vivo, during conventional X-ray CT,
will aid tremendously in characterizing active biological processes. Preliminary
work [4–6] has shown that XF can be performed in a tomographic manner using a
CT-like scanning geometry with a conventional poly-energetic X-ray source.
The molecular contrast in XFCT comes from high atomic number (high-Z)
elements which are present either exogenously or endogenously. The conceptual
design of an XFCT imaging system consists of an X-ray source and an energydispersive X-ray detection system. In the physical process of fluorescence, the
exciting X-ray interacts with an inner shell electron (K or L) of the probe which
results in a vacancy that is subsequently filled by an electron from an outer shell as
shown in Fig. 1. The difference in energy between the two states is emitted as a
characteristic X-ray (aka fluorescent X-ray). The detection of these photons and
subsequent image reconstruction will allow the determination of the spatial distribution of molecular probes.
126
D. Vernekohl and L. Xing