1
Molecular Imaging: Applying Systems Biology
to Medical Imaging
Charles Xiao Bo Yan

Abstract—Conventional medical imaging technologies are
imperfect because they do not provide information at the genetic
or molecular level. Thanks to research in systems biology, a large
number of potential targets for the development of molecular
imaging were discovered, which makes possible the extraction of
information at the molecular level, such as gene expression or
protein-protein interactions. The three imaging paradigms used
in molecular imaging are MRI, radionuclide imaging and optical
imaging. Direct imaging and indirect imaging are the two major
technics employed. Imaging probes are used in both technics to
provide better image contrast. Molecular imaging enables medical
imaging at high sensitivity and specificity, which improves
diagnostic accuracy and reduces the need for biopsy. It is also a
tool that could accelerate systems biology research and improve
drug development.
Index Terms—Molecular Imaging, Systems Biology, Medical
Imaging, Imaging Probes.
I. INTRODUCTION
M
imaging is an indispensable tool for the
diagnosis, treatment guidance and follow-up of many
diseases nowadays. However, a number of weaknesses
associated with the current medical imaging modalities
(technological methods) make their usefulness limited in many
circumstances. This is mainly due to the fact that most
imaging modalities only provide information at the anatomical
level, which is insufficient for diseases at the genetic or
molecular level. For instance, in cancer diagnosis and followup, it is inappropriate to merely use the physical shape and size
of the tumour for deciding whether it is benign or not and how
to treat it. Instead, its type, distribution, gene expression and
cellular functions are the driving decision parameters for
setting the method of treatment, if it should be treated at all
[1]. Another problem with current imaging technologies is
that their spatial resolution are typically greater than 2 mm
wide, which means that tumours containing fewer than
500,000 cells are likely to pass undetected [1]. This paper
discusses how molecular imaging shows the promise to solve
many problems that exist in medical imaging, including the
ones mentioned above, how research in systems biology has
made possible the development of molecular imaging, and how
molecular imaging will in turn become a very useful tool to
EDICAL
study biological processes from a systems biology perspective.
Systems biology is a research paradigm that studies the
genetic and metabolic pathways as a whole system instead of
studying the individual elements in the system. In systems
biology, more emphasis is put on the interrelationships among
the components rather than the isolated behaviour of each
component.
Through an iterative refinement of the
mathematical model of the system by systematically perturbing
and monitoring its components and by reconciling the
experimentally observed responses with the model’s
predictions, systems biology reveals many relevant data about
the interactions and workings of the pathways that were
unknown before [2].
Molecular imaging could be broadly defined as the in vivo
imaging, characterization, and measurement of biological
processes at the cellular and molecular level [1]. Therefore, it
allows processes such as gene expression and protein-protein
interactions to be imaged. It results from the convergence of
the disciplines of medical imaging and cellular biology.
Research in cellular biology using a systems biology approach
has led to the discovery of a large number of potential targets
for imaging probes (probes are discussed in more detail later),
which plays a vital role in producing the image contrast needed
to obtain information at a cellular or molecular level.
The promises made by molecular imaging are numerous.
Molecular imaging will greatly reduce the need for animal
tissue sampling or human biopsy when studying the
progression of a disease or the follow-up of a gene therapy. It
has the advantage of being non-invasive, and because it could
be repeated many times (unlike the case of biopsy), it provides
both spatial and temporal dimensions to the understanding of
the gene expression of the disease or therapy [3]. The fact that
molecular imaging detects changes at the cellular level makes
it much more sensitive than conventional medical imaging.
This means that cancerous tumours are detected at a much
earlier stage, and combined with image-guided therapy, the
disease could be treated right at the time of recognition [4].
Finally, if advances in systems biology provides better
understanding of the human genome, molecular imaging will
change medical imaging from diagnosis of diseases to the
prediction and prevention of diseases.
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II. MOLECULAR IMAGING MODALITIES
A. Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is based on the
phenomenon of nuclear magnetic resonance. The nuclei of
atoms that have an uneven number of nucleons have a nuclear
spin and a magnetic moment. When placed in an external
magnetic field, the magnetic moments of each atom will align
themselves with the external field. If a pulse at radio
frequency is applied to the atoms, their equilibrium will be
disturbed. Once the pulse is removed, the atoms will fall back
to their equilibrium state and in doing so, emit a signal whose
frequency is equal to the atoms’ characteristic resonance
frequency. Because each element has its own resonance
frequency, and because the amplitude of the signal received is
proportional to the number of atoms with the same resonance
frequency, the signal received could be used to construct a
image of the composition of the target. In molecular imaging,
paramagnetic material is typically used to enhance the signal
and provide a good contrast between the anatomical image and
the molecular image.
B. Radionuclide Imaging
Radionuclide imaging encompasses all modalities that are
based on the detection of photons emitted by radioactive
atoms. The most commonly used ones are positron emission
tomography (PET) and single photon emission computed
tomography (SPECT). In positron imaging, positrons are
emitted from nuclei of proton-rich isotopes, which travels up
to a few millimetres and eventually interact with electrons.
Annihilation occurs, and the mass of the electron and positron
is converted into two photons at gamma ray frequency. The
photons travel outward from the site of annihilation at ~180°
to one another. In SPECT, only a single photon at gamma ray
frequency is emitted from the decay of the radioactive isotope.
Scintillation crystals such as bismuth germanate or lutetium
oxyorthosilicate are used to capture the gamma ray photons
[5]. In molecular imaging, radionuclide tracers are used to
provide the image contrast. Fig. 1 shows a diagram explaining
the basic concepts of PET and SPECT.
C. Optical Imaging
Optical imaging makes use of fluorescent or bioluminescent
proteins to create internal biological light. For conciseness,
only the basic concept of bioluminescent imaging (BLI) is
presented here. To illustrate, consider the example of
luciferase, which is a class of enzymes that emit light
(bioluminescence) in the presence of oxygen and a substrate.
The light from these enzyme reactions typically has very broad
emission spectra that frequently extend beyond 600 nm. The
red components of the emission spectra are the most useful for
imaging because of the relatively low absorption by tissues at
these wavelengths. Hemoglobin is the primary absorber of
light in vivo and the hemoglobin absorption spectrum
significantly decreases above 600 nm, and the absorption due
to water content begins to rise at 900 nm. This defines the
spectral window for optically-based imaging modalities.
Detectors based on charge coupled device (CCD) cameras are
used for BLI [6]. In molecular imaging, the bioluminescent
proteins are usually produced within the cell, which will be
discussed in more detail in the next section.
III. MOLECULAR IMAGING TECHNICS
Fig. 1.
Schematic illustrating single photon emission computed
tomography (SPECT) and positron emission tomography (PET). (a) In
SPECT, a single photon is produced as the isotope decays and this single
photon must be detected through rotating detectors using a collimator. (b) In
PET, annihilation eventually occurs with the positron and electron to
produce two high energy (511 keV) gamma rays at ~180° that are detected
using a circular ring of detectors [5].
A. Direct Imaging
The molecular imaging technics could be mainly divided
into two categories, direct imaging and indirect imaging. Of
the two categories, indirect imaging is more widely used for
reasons that will be explained later. In both cases, imaging
probes are employed to aid in the targeting of specific cell
features or gene expressions. Imaging probes are often
molecules that are designed to bind to specific target
molecules such as receptors, mRNAs or intracellular proteins.
In some circumstances, the imaging probes could even be
proteins produced within the cells through the expression of a
transduced gene (genetic material that has been transferred
over). The labeling of the imaging probes allows them to be
detected and imaged and the labeling type depends on the
imaging modality used. For radionuclide imaging, the probes
are radiolabeled (tagged with a radioactive tracer). For
magnetic resonance imaging, paramagnetic tracers are used to
tag the probes. In the case of optical imaging, the probes are
usually fluorescent or bioluminescent proteins that are encoded
by transduced gene [6].
The direct imaging approach relies on a proportional
relationship between the concentration of the imaging probes
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and the level of gene expression in the target cells or tissues
[7]. An example of direct imaging would be the use of
RASONs, or radiolabeled antisense oligonucleotides probes,
that have been developed to directly image endogenous (within
the cell) gene expression at the transcriptional level
(replication of DNA into mRNA). RASON sequences can be
made complementary to a small segment of target mRNA or
DNA, and could potentially target any specific mRNA or DNA
sequence. In this context, imaging specific mRNAs with
RASONs produces “direct” images of specific moleculargenetic events [3]. Another example of direct imaging is the
imaging of a tumour by using an engineered transferrin
receptor that is transduced and expressed in a tumour cell line,
which becomes detectable when a paramagnetic transferrin
ligand imaging probe is injected [1].
B. Indirect Imaging
Most indirect molecular imaging paradigms involve a
reporter gene coupled to a specific gene under study. Each
reporter gene has a complementary reporter probe. Imaging
the level of reporter gene product activity through probe
accumulation provides indirect information that reflects the
level of expression of the gene under study [3]. In other
words, the expression of the reporter gene, which is being
imaged, infers the expression of the coupled gene under study.
Fig. 2 illustrates the general paradigm used in indirect
imaging. In this case, the gene under study is the promoter
gene. It is coupled with a reporter gene that encodes an
enzyme capable of trapping its complementary reporter probe
within the cell so as to amplify the signal emitted by the
reporter probe. The reporter could very well encode a
receptor or a biased carrier protein, which will serve just as
Fig. 2. Schematic illustrating an indirect imaging approach. A reporter
gene is introduced into the cell, which is driven by a promoter of choice.
Transcription of the imaging reporter gene with subsequent translation of the
mRNA leads to an enzyme. This enzyme can selectively trap an imaging
reporter probe. The imaging reporter probe will not be trapped in those cells
in which there is no expression of the imaging reporter gene. Note that it is
also possible for the imaging reporter gene to encode for an intracellular
and/or cell surface receptor. This receptor would then bind the imaging
reporter probe (a ligand). Levels of the trapped probe can be related to levels
of imaging reporter gene expression in either approach. [5]
well the function of accumulating imaging probes for signal
amplification.
There has been many reports of successful application of
indirect imaging. For example, Blasberg et al. mentioned that
his group has been able to image the transcriptional regulation
of the p53 gene [3]. In this case, a retroviral vector (Cisp53/TKeGFP) was generated, where the gene under study is
the Cis-p53 and the reporter gene is TKeGFP, which encodes
enhanced green fluorescent protein (GFP) (hence the imaging
modality is optical). DNA damage-induced upregulation
(increased response due to stimulus) of p53 transcriptional
activity was demonstrated and correlated with the expression
of p53-dependent downstream genes. In another instance,
indirect imaging was used to monitor the effect of gene
therapy in breast cancers in animal experiments [5]. Her2/neu, the gene linked to the overexpression and
chemotherapy-resistance of breast cancer, was suppressed by a
transcriptional adenovirus regulator. The imaging of the gene
therapy to verify the location, magnitude and duration of the
gene expression is very useful to aid in the optimization of the
therapy [5].
C. Signal Amplification Methods
Signal amplification methods for molecular imaging are
necessary to increase the signal strengths of the imaging
probes. This is because imaging probes are often injected at
minimal dosage and do not provide enough signal strength
when present in low concentration. Some concepts of signal
amplification have already been alluded to previously. Three
molecular imaging signal amplification methods are currently
used: trapping, gene expression and activation. Trapping refers
to the process of confining imaging probes within the cell or
on the cell surface by means of one-way receptors, one-way
carrier proteins, or enzymatic reactions that convert the
probes. The accumulation of imaging probes over time
enhances the signal strength emitted by the probes. In gene
expression method, imaging probes can be inducibly
expressed, such as by the genetic expression of a green
fluorescent protein (GFP) gene, which results in thousands of
GFP copies per cell. The activation method is used where
imaging probes may be in a dormant phase, and switched on in
response to interaction with a target molecule, enzyme, or
receptor. In practice, multiple methods could be used in
conjunction. For example, a dormant gene of a one-way
membrane carrier protein can be turned-on in a target cell
(such as a tumor), producing mRNA coding for the protein.
Gene expression produces thousands of membrane-bound
carrier proteins, which allows for the internalization and
trapping of extracellular imaging probes [1].
D. Direct vs. Indirect Imaging
Compared to indirect imaging, the advantage of direct
imaging is that highly specific images can be obtained and that
the delivery of the imaging probes does not require the
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transduction of a coupled reporter gene. However, because the
imaging probe is target specific, a drawback of the direct
imaging approach is that a specific probe needs to be
developed for each molecular target. The development and
validation of both the sensitivity and specificity of the probe
could become as resource consuming as the development of a
new drug [3]. On the other hand, for indirect imaging, once a
reporter-gene and reporter probe pair has been fully tested and
validated, it could potentially be coupled to any gene and
image the expression of the gene under study indirectly. This
is the main reason for which indirect imaging has the potential
to be more widely adopted, especially in animal studies.
However, indirect imaging applications to human patients are
currently limited due to the necessity of transducing target
tissue with the reporter gene. Each new imaging probe and
each new vector of coupled genes requires extensive and timeconsuming safety testing prior to government approval for
human administration [3].
IV. APPLICATIONS OF MOLECULAR IMAGING
A. Medical Imaging
As suggested previously, molecular imaging greatly
enhances the usefulness of medical imaging thanks to its
higher sensitivity and specificity. In cancer diagnosis, the
higher sensitivity of molecular imaging lowers the tumour
detection threshold and its specificity to receptor status and
gene expression reduces the need for invasive evaluation using
biopsy [1]. In addition, gene therapy effectiveness could be
immediately evaluated and followed on an ongoing basis.
Similarly, tracking of tumour cells will allow the metastasis
(spreading) of cancerous tumours to be closely monitored.
Finally, image-guided therapy will expand thanks to the better
detection of smaller regions of disease [1].
B. Systems Biology Research
Molecular imaging is also a valuable tool for research in
systems biology, because it offers the possibility to monitor in
vivo the detailed location, magnitude, and time variation of
gene expression or protein-protein interactions with high
sensitivity in animals and humans [5]. Studies of weak
promoters, post-transcriptional regulation of gene expression,
protein-protein interactions and many others have been
reported [3].
C. Drug Development
In drug development, molecular imaging is used to study in
vivo pharmacodynamics (effect of drug on living organism),
concentrating on looking at how drugs act and what are the
downstream effects of the drugs on tumours and normal tissue.
It is also possible to study in vivo pharmacokinetics (whether
the drugs are actually reaching the tumours) and look at drug
uptake and retention. This is especially important now for
more targeted therapies where the plasma drug concentration
does not necessarily reflect tumour drug concentration [8].
Another area of application is pre-Phase I, where molecules
are injected at one-thousandth of the therapeutic starting dose.
Using molecular imaging one can actually screen the chemical
compounds, look at the upregulated (stimulated) genes in
normal tissues, before testing on human subject. This improves
the quality of decision making, allows decisions to be made at
a much earlier stage and reduce the cost of development [8].
V. CONCLUSION
Molecular imaging greatly improves the conventional
medical imaging modalities thanks to its high sensitivity and
specificity. However, its research and development is still at
its infancy and although it holds great promises, its accuracy is
still being questioned by many researchers today and its
application hasn’t found its way into clinical practice on
human patients. Nevertheless, the potential it holds for
improvements in medical technologies of the twenty first
century is too great to be ignored and its development will for
sure raise new questions about the appropriateness of current
practice.
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