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Philips Research - Press Backgrounder
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Magnetic Particle Imaging
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Personalized medicine will require a complete patient-specific picture of the functional processes associated with disease. Accurate quantification of physiological processes is therefore an important requirement for imaging in research, clinical diagnosis and personalized therapy. Functional imaging modalities such as PET and SPECT already provide a great deal of information on physiological processes in the human body and significantly enhance the anatomical information revealed by modalities such as ultrasound, CT and MR imaging.
In the case of suspected cardiovascular disease,
however, the quantitative information ideally
required for diagnosis and therapy selection also
includes measurements such as coronary blood flow,
myocardial perfusion (blood supply to the heart
muscle) and heart ejection fraction (a measure of
the pumping function of the heart). Philips has now
developed Magnetic Particle Imaging (MPI) – a new
imaging technology that generates anatomical and
functional images of the heart from which such
information can be extracted in a quantitative
manner, as has been demonstrated in a pre-clinical
study. The results of the pre-clinical study were
published in volume 54, issue 5 of Physics in
Medicine and Biology (2009).
MPI uses the magnetic properties of injected
iron-oxide nanoparticles to measure the
concentration of these nanoparticles in the blood.
Because the human body contains no naturally
occurring magnetic materials visible to MPI, there
is no background signal. After injection, the
nanoparticles therefore appear as bright signals in
the images, the intensity of which allows the
nanoparticle concentrations to be calculated. By
combining high spatial resolution with short image
acquisition times (as short as 1/50th of a second),
MPI can capture dynamic concentration changes as the
nanoparticles are swept along in the blood stream.
This could ultimately allow MPI scanners to perform
a wide range of functional cardiovascular
measurements in a single scan. These could include
measurements of coronary blood supply, myocardial
perfusion, and the heart’s ejection fraction, wall
motion and flow speeds.
The technology has been used in a pre-clinical study
to generate unprecedented real-time 3D images of
arterial blood flow and volumetric heart motion in a
mouse. The Magnetic Particle Imaging demonstrated in
this pre-clinical study utilized an iron-oxide based
contrast agent called Resovist*, which is already
used in the EU as a contrast agent for MR scans.
The experimental system successfully imaged the
real-time transit of an injected Resovist bolus
through the heart chambers and major blood vessels
of the mouse. The Resovist concentrations used were
comparable to those currently approved for clinical
use. Movement of the bolus was captured at a frame
rate of 46 volumes per second, allowing the
generation of smooth video sequences. In addition,
the acquired MPI images correlated accurately with
anatomical images obtained using MR (Magnetic
Resonance) imaging, demonstrating the excellent
spatial accuracy of MPI for in-vivo imaging. The
achieved true image resolution was approximately 1.5
mm along one axis and 3 mm along the other two axes,
while the voxel size was approximately (0.6mm)3. The
differences in axial resolution are due to the
configuration of magnetic fields used to excite the
magnetic nanoparticles.
Given the fact that a mouse heart is only 5 mm in
size and beats 240 times per minute, these results
represent a major step forward in taking MPI from a
theoretical concept to an imaging tool to help
improve diagnosis and therapy planning, not only for
cardiovascular diseases but also for other diseases
including cancer.
Basic principles
In its simplest form, MPI works as follows: the
iron-oxide nanoparticles are superparamagnetic,
which makes them responsive to external magnetic
fields. As a result, even a weak oscillating
magnetic field generated by the MPI scanner will
cause them to magnetize. While doing so, they emit a
small but detectable electromagnetic signal that can
be picked up by a receiving antenna. It is this
signal that is measured by the scanner. On its own,
however, this signal would only detect the presence
of magnetic nanoparticles in the imaging area, not
their exact location within that area.
Their location is determined as follows: Firstly,
virtually all of the imaging area is saturated with
a strong static magnetic field that forces the
nanoparticles to magnetize in a fixed direction,
thus rendering these particles silent to MPI. Then a
single point within the imaging area is created
where the static magnetic field strength falls to
zero (a so-called ‘Field Free Point’). At this
point, the nanoparticles remain free to oscillate in
response to the applied oscillating magnetic field.
The amplitude of the signal picked up by the
receiving antenna is then a measure of the
nanoparticle concentration only at the Field Free
Point and nowhere else. To create the entire image,
all that needs to be done is to move the Field Free
Point until every point in the imaging area has been
scanned.
* Resovist is an iron-oxide based MRI contrast
agent produced by Bayer Schering Pharma AG.
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