Molecular magnetic resonance imaging
PhD, J Grimm2,
1 Siemens AG, Medical Solutions, Erlangen,
2 Center for Molecular Imaging Research, MGH and Harvard Medical School,
Boston, United States
Molecular MRI (mMRI) is a special implementation of
Molecular Imaging for the non-invasive visualisation of biological processes at
the cellular and molecular level. More specifically, mMRI comprises the
contrast agent-mediated alteration of tissue relaxation times for the detection
and localisation of molecular disease markers (such as cell surface receptors,
enzymes or signaling molecules), cells (e.g. lymphocytes, stem cells) or
therapeutic drugs (e.g. liposomes, viral particles). MRI yields topographical,
anatomical maps; functional MRI (fMRI) provides rendering of physiologic
functions and magnetic resonance spectroscopy (MRS) reveals the distribution
patterns of some specific metabolites. mMRI provides an additional level of
information at the molecular or cellular level, thus extending MRI further
beyond the anatomical and physiological level. These advances brought by mMRI
are mandatory for MRI to be competitive in the age of molecular medicine. mMRI
is already today increasingly used for research purposes, e.g. to facilitate
the examination of cell migration, angiogenesis, apoptosis or gene expression
in living organisms. In medical diagnostics, mMRI will pave the way toward a
significant improvement in early detection of disease, therapy planning or
monitoring of outcome and will therefore bring significant improvement in the
medical treatment for patients.
In general, Molecular Imaging demands high sensitivity
equipment, capable of quantitative measurements to detect probes that interact
with targets at the pico- or nanomolar level. The challenge to detect such
sparse targets can be exemplified with cell surface receptors, a common target
for molecular imaging. At high expression levels (bigger than 106 per cell) the
receptor concentration is approx. 1015 per ml, i.e. the
concentration is in the micromole range. Many targets, however, are expressed
in even considerably lower concentrations. Therefore the most sensitive
modalities, namely nuclear imaging (PET and SPECT) have always been at the
forefront of Molecular Imaging, and many nuclear probes in clinical use today
are already designed to detect molecular mechanisms (such as FDG, detecting
high glucose metabolism). In recent years however, Molecular Imaging has
commanded attention from beyond the field of nuclear medicine. Further imaging
modalities to be considered for molecular imaging primarily include optical imaging,
MRI and ultrasound. � 2006 Biomedical Imaging and Intervention Journal. All
Keywords: Molecular imaging, MRI, contrast agents
CONTRAST AGENTS FOR mMRI
Clinical MRI scanners offer a spatial resolution of 250 �m
in-plane (small bore experimental systems allow for 50 �m isotropic voxels for in
vivo measurements), unlimited depth penetration along with exceptionally
good soft tissue contrast. The above-mentioned concentration of molecular
imaging targets in the micromolar range is challenging and requires
sophisticated imaging strategies. Improvements in MRI design to reduce the
lower detection limit are possible only to a certain extent; hence biophysical
amplification mechanisms to enhance the signal from the label are necessary.
For MRI, two different classes of contrast agents exist:
agents that influence mainly the signal in T2- (negative contrast agents,
reducing the signal) or in T1-weighted images (positive contrast agents,
increasing the signal). For both classes, methods for signal amplification have
been developed. In general, both take advantage of either very high relaxivity
probes, background reduction (SNR optimisation) via activation of
low-relaxivity probes by the targeted molecular marker (induced changes in
relaxivity) or pronounced tissue accumulation. The latter is possible only with
a very restricted number of highly expressed molecular marker (e.g. fibrin for
Negative Contrast Agents
For T2-contrast agents, the most prominent labels are
iron-oxide nanoparticles (Superparamagnetic Iron Oxide (SPIO), Very Small
Paramagnetic Iron Oxide (VSPIO) or Ultrasmall Superparamagnetic Iron Oxide
(USPIO)). These particles usually consist of a crystalline iron-oxide core,
surrounded by polymer coating, often dextran, polyethyleneglycol or citrate.
The advantage of these preparations is that each particle contains thousands of
iron atoms resulting in very high T2 relaxivities of up to 200 (mMs)-1
, which makes detection of even low concentrations of contrast agents (�mol
to nmol range) possible. Noteworthy, particles smaller than 300 nm also produce
a substantial T1 relaxation.
Specific population of cells can be labeled with magnetic
nanoparticles and followed in vivo (cell tracking). Using this method,
the spatial distribution of immuno-competent cells into tumours over time can
be studied as well as the movement of stem cells, neuronal cells or blood stem
cells in vivo . A widely excepted protocol for cell labelling using
SPIO and Polyamines has been published by Frank et al . Key concerns
with cell tracking are viability, differentiability, chromosomal stability of
the labelled cells as well as the stability of label affiliation with the cell.
Similar to PET-imaging, MRI can be used to image gene
expression and to assess the efficiency of gene delivery. Viral or non-viral
gene therapy schemes require targeting of the gene therapy vector to the tissue
or cells of interest. Delivery of a therapeutic gene can be monitored by
imaging of a reporter gene, which is introduced into the gene therapy vector
and expressed together with the therapeutic gene (Figure 1). The reporter gene
can be encoded for the transferrin-receptor (accumulating iron within the cell)
or for tyrosinase (catalysing the synthesis of melanin) . Melanin has a high
iron binding capacity, which results in a high signal intensity of the
henceforth melanin containing cells in T1-weighted images . Compartmentalisation
of iron oxide particles within targeted cells can produce significant signal
amplification as well. Taking advantage of cell uptake processes via
endocytosis or phagocytosis clearly exceeds the concentration that can be
achieved using cell surface receptor targeting alone.
Figure 1 Principle of reporter genes
for gene expression. The reporter gene (red) is co-expressed
with the gene of interest (green), often by using the same
promoter for both genes. The reporter gene can be the HSV
thymidine kinase (phosphorylating radioactive-labeled nucleoside-analog,
which get trapped in the cell (A), a fluorescent protein like
GFP (B), an intracellular target for a marker (C) or a cell
membrane receptor as the transferin-receptor (D).
A method named �magnetic relaxation switch� facilitates the
reversible activation of MR probes (Figure 2). Target-mediated aggregation of
iron-oxide particles to clusters produces a significant change in the
relaxivity, which can be utilised to image enzyme activities (telomerase, endonucleases,
various proteinases) . This cluster-formation leads to an amplification of
the signal. Based on �magnetic relaxation switches�, MRI can also be used as an
in vitro method for high throughput evaluation of biological specimens.
Figure 2 Principle of the magnetic relaxation
switch (see text).
Typical T1-contrast agents are small molecular weight
compounds containing a single Lanthanide chelate as contrast producing element
(e.g. Gadolinium-DTPA). The tissue concentration necessary to image with these
T1-contrast agents on a molecular level is considerably higher than the
required concentration of iron-oxide particles, it has to be in the order of
mMols, since T1 relaxivity values are usually only in the in the 5-80 (mMs)-1
range . Increasing the molecular weight can be used for enhancing the
relaxivity by restricting free rotation of the Gadolinium. This can be achieved
by using polymeric backbones or in vivo binding of small sized contrast
agents to serum proteins . Multilabelled macromolecular T1-contrast agents
for mMRI take advantage of the fact that relaxivity is a linear function of the
number of lanthanide ions per contrast agent (Figure 3) . Various labelling
concepts, including Gadolinium-loaded liposomes or polymeres with thousands of
Gadolinium atoms, have been developed to overcome this limitation in
sensitivity. For instance, the detection of angiogenetic endothelium was
achieved by large Gadolinium-loaded liposomes, targeted to avb3 integrin
receptors via peptides or antibodies . Advantages on the MRI properties may
be counterbalanced by drawbacks in pharmacokinetics and bio-availability. High
molecular weight compounds have a slower diffusion rate, which restricts
delivery to certain tissues such as necrotic tumour centres or the CNS.
Figure 3 Plaque Imaging with Gadofluorine
M (100 µmol/kg bw), 24 h p.i. imaged with Siemens, 1.5
T, IR turbo flash (300/4/150/20°); (a) WHHL rabbit (with
plaques), (b) New Zealand White (without plaques). Asterisk
(*) indicates aorta (courtesy of B. Misselwitz, Schering AG,
Similar to T2 contrast agents, T1-contrast probes can be
activated by different mechanisms, thus increasing the relaxivity of the agent.
This target-mediated increase of relaxivity leads to an increased contrast in
the presence of the target. An enzyme-mediated polymerisation (e.g. bymyeloperoxidases
up-regulated in vulnerable plaques) of paramagnetic substrates into oligomers
represents one possible approach. In contrast to the small monomeric substrates
these polymers have a larger hydrodynamic diameter and slower rotational rate
of paramagnetic metal chelates. This results in a higher longitudinal
relaxivity, and since the oligomeres are eliminated much slower than the
monomers all signal will come after a certain time only from the oligomere,
retained at the side of the target's presence . Another method utilises
conformational changes in the chelates, allowing access of bulk water to the
inner sphere of the chelated lanthanide upon cleavage of a protective group by
an enzyme or binding of calcium to the chelate. Monitoring of gene expression
with T1 contrast agents can be achieved with a carbohydrate-modified Gadolinium
chelate to image the activity of the enzyme �-galactosidase as marker gene. The
enzyme cleaves the carbohydrate residue from the chelate, inducing an increase
in relaxivity by allowing access of water to the gadolinium .
Chemical-exchange saturation transfer (CEST) agents are
another emerging class of negative MRI contrast agents, which facilitate an
activatable contrast. The concept of CEST can be utilised to obtain highly
amplified targeted agents. The principle is based on the fact that many
molecules in the body exchange protons with bulk water through the selective RF
irradiation of exchanging protons. After irradiating a millimolar pool of
metabolite the energy is transferred to a nearby pool of water, producing a
strong increase in enhancement by means of chemical exchange. With CEST and its
newer development paraCEST specific metabolites can be detected. Using this
method, the concentration of metabolites such as glucose can be evaluated
non-invasively in vivo in all organs. It furthermore allows for the
measurement of the pH or the temperature. Multiparametric mMRI might become
feasible as CEST agents with different absorption frequencies of the exchanging
protons can be designed. The sensitivity of CEST can be improved further by
incorporating larger number of exchangeable protons into a (polymeric) CEST
agent. This has been accomplished by LIPOCEST agents, liposomes containing a
paramagnetic shift reagent for water protons in their aqueous inner cavity
Finally an �old friend� in MRI is attracting increasing
attention in the course of Molecular Imaging: Fluorine-19. Xeno-labels, i.e.
elements with low physiological concentration in situ but appropriate
gyro-magnetic properties proved to provide superb traceability for mMRI.
To date molecular imaging is increasingly used in laboratory
studies. High resolution MRI is well suited to screen mouse models for tumours
and other abnormalities and can be applied to non-invasively follow up new
experimental treatment strategies. While dedicated animal systems are
available, clinical scanners are also very capable of obtaining high-resolution
images. To further speed up translational research, a small animal MR scanner
based on a clinical console has been introduced just recently (ClinScan, a
joint Bruker and Siemens development). This system comes with an adapted
clinical user interface and supports therefore most of the features and sequences
used on clinical scanners. Standardisation of the system ensures easy protocol
transfer from and to clinical scanners. Applications being developed for humans
on clinical scanners will be available on ClinScan. On the other hand, any
application being developed for animals can be easily transferred to human
diagnostics. In addition, adaptations for clinical scanner are underway for
experimental molecular imaging. These include, but are not limited to,
implementing dedicated small animal protocols, (quantitative) analysis tools or
dedicated small animal RF coils. In conclusion, MRI is increasingly suited to
carry molecular imaging from animal models into clinical practice.
When will molecular imaging enter clinical medicine? Hot
(hyper-intense) spots, which add information on the molecular mechanisms
underlying a disease could guide radiologist and would be highly beneficial.
Hyper-intense hot spots can be caused by highly specific accumulation of paramagnetic
contrast agents. Most macromolecular Gadolinium-chelates are in an early
pre-clinical research stages and the time needed to reach clinical
applicability is considerable. Furthermore, toxicity concerns due to the
prolonged retention of heavily Gadolinium-loaded molecules within the organism
must be disproved. Superparamagnetic contrast agents can give easily detectable
positive signal in combination with imaging techniques such as off resonance
Superparamagnetic iron-oxide based contrast agents are
easily detectable on T2-weighted sequences, even better so on T2*-weighted
images due to susceptibility effects. Iron-oxide particles currently in
clinical use can be utilised for cellular imaging of phagocyting cells, if combined
with peptides, helping to traverse the membrane; this applicability can be even
expanded to any other cell type. Iron-oxide particles can furthermore be
functionalised with a wide variety of biological active
molecules such as targeting moieties of peptides, antibody
(derivatives), nucleic acids or aptamers to increase specific binding.
Therefore a wide variety of imaging agents for mMRI is emerging and can
facilitate steps towards clinical mMRI. Translational research with iron-oxide
particles will take advantage of a growing basis of installed high field MRI
scanners, since the lower detection limit decreases with increasing field
DUAL MODALITY DEVICES FOR MOLECULAR IMAGING
Besides being a powerful tool for the acquisition of
molecular information, MRI is capable to add anatomical or functional
information (fMRI, diffusion, perfusion) to molecular images acquired with
other modalities such as PET, SPECT or optical imaging. Combinations of CT and
PET are increasingly used in clinical diagnostics. But there are some unique advantages
for MR PET. Isocentric and simultaneous measurements are possible, no
re-positioning of the patient, MRI can provide navigator technique and may give
attenuation correction for PET, there is a better soft tissue contrast, and
lower radiation exposure enables follow up studies. The ongoing development in
imaging software and computing hardware facilitates co-registration and image
fusion of physically separated units without delay. But repositioning of the
patient and time interval between the scans makes fusion of separately obtained
images difficult and inherently imprecise. Integrated devices would clearly be
preferable even for economic reasons. Simon Cherry pioneered MR PET
Just recently the first fully integrated experimental set-up
(based on semiconductor detectors) for MR PET has been presented by Siemens
(Figure 4). Academic sites build a MR optical device. Even dual probes for
optical and MR imaging have been developed  and triple modality probes are
Figure 4 Mr-PET integrated solution.
PET is acquired with a ring inserted into the magnet –
simultaneous acquisition is possible.
THE BIG PICTURE
Molecular Imaging is one out of three pillars of molecular
medicine, i.e. the translation of basic molecular biology into medicine. The
other two are in vitro diagnostics (IVD) and knowledge driven healthcare
(Figure 5). To determine genetic predisposition, in vitro genomic or
proteomic screening procedures will be used, such as DNA-chip technologies or
mass spectroscopy. IT tools will be mandatory as complex molecular information,
be it in vivo or in vitro diagnostic, has to be integrated by IT
and has to be supplemented by knowledge bases to leverage it into clinical
applications. This means diagnostic data has to be converted into meaningful
Figure 5 Molecular Imaging is one out
of three pillars of molecular medicine.
The financial feasibility and medical practicability of
molecular medicine including comprehensive diagnostics are debatable. The trial
and error method will certainly continue to be the most reasonable approach for
cost-effective therapies without side effects. However, this is not the case
with highly potential therapy regimes (i.e. therapies with a potential for
serious side effects), cost-intensive (molecular) treatment schemata (such as
cell- or gene therapy) or therapies of chronic diseases. In such instances, a
rational therapy selection based on comprehensive diagnostic data is a decisive
factor for efficient and cost sensitive patient care. One of the main cost
drivers in medicine is inappropriate treatment over a prolonged time. From a
medical point of view, many arising molecular therapy concepts are based on
individualised drugs. These treatments must be tailored to the individual
biochemical set-up or disease stage of each respective patient with the support
of diagnostic data. Equipped with these patient-specific data, a therapy regime
is selected, taking into account the different molecular defects for each
disease as well as the particular clinical history and condition of a patient.
An example of how mMRI could contribute to this scope is
radiation treatment planning by semi-automated lymph nodal cancer staging using
a nanoparticle-enhanced lymphotropic mMRI. Preclinical data proves that mMRI
demonstrate great promise for improving quality of diagnosis in general for
oncologic- (tumour angiogenesis imaging), cardiovascular- (vulnerable plaque
imaging) and neurological (Alzheimer�s plaque imaging) diseases.
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|Received 7 August 2005; received in revised version 12 October 2005, accepted 21 December 2005
Correspondence: Siemens Medical Solutions MRET MI, Allee am R�thelheim 4, 91054 Erlangen, Germany. Tel.: +49 9131 843057; Fax: +49 9131 843884; E-mail: email@example.com (Arne Hengerer).
Please cite as: Hengerer A, Grimm J,
Molecular magnetic resonance imaging, Biomed Imaging Interv J 2006; 2(2):e8