Magnetomotive photoacoustic/ultrasound imaging has shown superior specificity in visualizing targeted objects at cellular and molecular levels. By detecting magnet-induced displacements, magnetic-particles-targeted objects can be differentiated from background signals insensitive to the magnetic field. Unfortunately, background physiologic motion interferes during measurement, such as cardiac-induced motion and respiration, greatly reducing the robustness of the technique. In this paper, we proposed cyclic magnetomotive imaging with narrowband magnetic excitation. By synchronizing magnetic motion with the excitations, targeted objects moving coherently can be distinguished from background static signals and signals moving incoherently. HeLa cells targeted with magnetic nanoparticle-polymer core-shell particles were used as the targets for an initial test. A linear ultrasound array was interfaced with a commercial scanner to acquire a photoacoustic/ultrasound image sequence (maximum 1000 frames per second) during multi-cycle magnetic excitation (0.5 – 40 Hz frequency range) with an electromagnet. An image mask defined by a threshold on the displacement-coherence map was applied to the original images for background suppression. The results show that contrast was increased by more than 60 dB in an in-vitro experiment with the tagged cells fixed in a polyvinyl-alcohol gel and sandwiched between porcine liver tissues. Using a single sided system, cells injected subcutaneously on the back of a mouse were successfully differentiated from the background, with less than 20 µm coherent magnetic induced displacements isolated from millimetric background breathing motion. These results demonstrate the technique’s motion robustness for highly sensitive and specific diagnosis.
Photoacoustic (PA) imaging has been widely used in molecular imaging to detect diseased cells by targeting them with
nanoparticle-based contrast agents. However, the sensitivity and specificity are easily degraded because contrast agent
signals can be masked by the background. Magnetomotive photoacoustic imaging uses a new type of multifunctional
composite particle combining an optically absorptive gold nanorod core and magnetic nanospheres, which can
potentially accumulate and concentrate targeted cells while simultaneously enhancing their specific contrast compared to
background signals. In this study, HeLa cells molecularly targeted using nanocomposites with folic acid mimicking
targeted rare circulating tumor cells (CTCs) were circulated at a 6 ml/min flow rate for trapping and imaging studies.
Preliminary results show that the cells accumulate rapidly in the presence of an externally applied magnetic field
produced by a dual magnet system. The sensitivity of the current system can reach up to 1 cell/ml in clear water. By
manipulating the trapped cells magnetically, the specificity of detecting cells in highly absorptive ink solution can be
enhanced with 16.98 dB background suppression by applying motion filtering on PA signals to remove unwanted
background signals insensitive to the magnetic field. The results appear promising for future preclinical studies on a
small animal model and ultimate clinical detection of rare CTCs in the vasculature.
Results on magnetically trapping and manipulating micro-scale beads circulating in a flow field mimicking metastatic cancer cells in human peripheral vessels are presented. Composite contrast agents combining magneto-sensitive nanospheres and highly optical absorptive gold nanorods were conjugated to micro-scale polystyrene beads. To efficiently trap the targeted objects in a fast stream, a dual magnet system consisting of two flat magnets to magnetize (polarize) the contrast agent and an array of cone magnets producing a sharp gradient field to trap the magnetized contrast agent was designed and constructed. A water-ink solution with an optical absorption coefficient of 10 cm−1 was used to mimic the optical absorption of blood. Magnetomotive photoacoustic imaging helped visualize bead trapping, dynamic manipulation of trapped beads in a flow field, and the subtraction of stationary background signals insensitive to the magnetic field. The results show that trafficking micro-scale objects can be effectively trapped in a stream with a flow rate up to 12 ml/min and the background can be significantly (greater than 15 dB) suppressed. It makes the proposed method very promising for sensitive detection of rare circulating tumor cells within high flow vessels with a highly absorptive optical background.
Contrast-enhanced photoacoustic (PA) imaging has been proposed to identify circulating metastatic cancer cells magnetically trapped in the vasculature. However, its sensitivity is limited by the presence of a strong blood-background signal. This technique can be further improved by the significant suppression of blood background. In the phantom study presented here, significant background suppression is demonstrated with magnetomotive photoacoustic imaging. Magnetic particles with a mean diameter of 10 μm were integrated (concentration of 0.05 mg / ml) into an ink-water solution with an optical absorption coefficient of 5 cm − 1 to mimic cells targeted with magnetic nanoparticles and magnetically trapped in the human vasculature. Two mechanically moveable permanent magnets were used to accumulate microparticles in the investigated solution and manipulate them within a thin, 1.6-mm-diameter Teflon tube mimicking a blood vessel. Our results clearly indicate that the undesirable background can be effectively suppressed using the difference of PA images corresponding to different locations of accumulated particles.
Trapping and manipulation of micro-scale objects mimicking metastatic cancer cells in a flow field have been
demonstrated with magnetomotive photoacoustic (mmPA) imaging. Coupled contrast agents combining gold nanorods
(15 nm × 50 nm; absorption peak around 730 nm) with 15 nm diameter magnetic nanospheres were targeted to 10 μm
polystyrene beads recirculating in a 1.6 mm diameter tube mimicking a human peripheral vessel. Targeted objects were
then trapped by an external magnetic field produced by a dual magnet system consisting of two disc magnets separated
by 6 cm to form a polarizing field (0.04 Tesla in the tube region) to magnetize the magnetic contrast agents, and a
custom designed cone magnet array with a high magnetic field gradient (about 0.044 Tesla/mm in the tube region)
producing a strong trapping force to magnetized contrast agents. Results show that polystyrene beads linked to
nanocomposites can be trapped at flow rates up to 12 ml/min. It is shown that unwanted background in a photoacoustic
image can be significantly suppressed by changing the position of the cone magnet array with respect to the tube, thus
creating coherent movement of the trapped objects. This study makes mmPA imaging very promising for differential
visualization of metastatic cells trafficking in the vasculature.
Several imaging modalities such as optical coherence tomography, photothermal, photoacoustic and magnetic resonance imaging, are sensitive to different physical properties (i.e. scattering, absorption and magnetic) that can provide contrast within biological tissues. Usually exogenous agents are designed with specific properties to provide contrast for these imaging methods. In nano-biotechnology there is a need to combine several of these properties into a single contrast agent. This multifunctional contrast agent can then be used by various imaging techniques simultaneously or can be used to develop new imaging modalities. We reported and characterized a multifunctional nanoparticle, made from gold nanoshells, which exhibits scattering, photothermal, photoacoustic, and magnetic properties.
Magnetic nanoparticles (MNPs) have been used extensively ex vivo for cellular and molecular separations. We recently
showed that a coupled nanoparticle combining a superparamagnetic core with a thin, isolated gold shell providing strong
absorption in the near infrared can be used for magnetomotive photoacoustic imaging (mmPA), a new technique in
which magnetic manipulation of the particle during PA imaging greatly enhances molecular contrast specificity. This
particle can also be biologically targeted for in vivo applications, where mmPA imaging provides a spatially localized
readout of magnetic manipulations. As an initial test of potential in vivo molecular assays and integrated molecular
therapeutics using magnetic manipulation of nanoparticles, we present experiments demonstrating PA readout of trapped
magnetic particles in a flow field. An aqueous solution containing a concentration of 0.05-mg/ml 10-μM
superparamagnetic iron oxide particles flowed in a 1.65-mm diameter Zeus PTFE (Teflon) sublite wall tubing at three
velocities of 0.8, 1.5 and 3.0-mm/s. Opposed permanent magnets separated by 40-mm were positioned on both sides of
the tube. As expected, the targeted objects can be magnetically captured and accumulated locally. By translating the
magnets, a dynamic magnetic field (0.1-0.3-T) was alternately generated on the side of the tube closest to one of the
magnets and created a synchronous PA motion from accumulated targeted objects. This synchronized motion can be
used to differentiate the stationary background or other PA sources moving asynchronously with magnetic manipulations
(e.g., moving blood) from targeted cells moving synchronously with the magnetic field. This technology can potentially
provide sensitive molecular assays of cellular targets travelling in the vasculature (e.g., metastatic tumor cells).
Ultrasonic (US) imaging is the most common real-time modality, providing multiple dimensional changes in
morphology for clinical practice. Photoacoustic (PA) imaging has demonstrated great promise as a new functional and
molecular imaging tool. However, absorption in background tissue also generates a PA signal and limits the specific
contrast of molecular contrast agents. To increase the linear range of these agents, the background must be suppressed.
Magnetic nanoparticles provide a new possibility to increase contrast by magnetomotive manipulation during imaging.
A multi-functional imaging system integrating US and PA imaging with magnetic manipulation can take advantage of
each single modality by providing anatomical images and molecular function with greater contrast. However, one key
issue for multi-functional imaging is how to spatially combine and temporarily synchronize US and PA imaging with
magnetomotive instrumentation. In this study, we built a simple system to integrate US and PA imaging with
magnetomotive capability. We evaluated this system by measuring the motion of a phantom including magnetic
nanoparticles (MNPs) using US when these particles were subjected to a time-varying magnetic field.
When conjugated with targeting molecules, quantum dots (QD) can be used as powerful cancer diagnostic tools providing the molecular profiles of cancer cases based on common clinical biopsies. Such personalized analyses will enable doctors to treat and manage the patients' diseases more effectively. The unique optical properties (e.g., size-tunable emission, simultaneous excitation, high brightness and photostability) of these nanoparticles make them superior to conventionally popular organic fluorophores1-2. Polymer-encapsulated, antibody-tagged QDs were prepared and used to successfully stain both fixed and live cells as well as clinical formalin-fixed paraffin-embedded (FFPE) tissue sections. In the tissue staining study, QD bioconjugates targeting mutated p53 and early growth response protein (egr-1) were used to examine prostate cancer tissues. The tissue slides were then analyzed with a wavelength-resolved spectrometer to accurately quantify the protein expression levels. In comparison to traditional qualitatively based diagnostic procedures, quantum dot nanotechnology allows for a more quantitative, rigorous and objective analysis of tissue specimens in question. In addition, new developments in imaging instrumentation could automate spectroscopy measurements and data analysis.
Biocompatible semiconductor quantum dot (QD) probes with extended plasma circulating times have been developed for cancer imaging in living animals. The structural design involves encapsulating luminescent QDs with a triblock copolymer, and linking this amphiphilic polymer to multiple poly(ethylene glycol) (PEG) molecules. In vitro histology and in vivo imaging studies indicate that the QD probes can be delivered to tumor sites by enhanced permeation and retention. Using both systemic injection of long-circulating QD probes and subcutaneous injection of QD-tagged microbeads, we have achieved sensitive and multicolor fluorescence imaging of cancer cells under in vivo conditions. These results raise new possibilities for ultrasensitive and multiplexed imaging of molecular targets in vivo.
We have developed a novel optical coding technology for massively parallel and high-throughput analysis of biological molecules. Its unprecedented multiplexing capability is based on the unique optical properties of semiconductor quantum dots (QDs) and the ability to incorporate multicolor QQs into small polymer beads at precisely controlled ratios. The use of 10 intensity levels and 6 colors could theoretically code one million nucleic acid or protein sequences. Imaging and spectroscopic studies indicate that the QD tagged beads are highly uniform and reproducible, yielding bead identification accuracies as high as 99.99 percent under favorable conditions. DNA hybridization results demonstrate that the coding and target signals can be simultaneously read at the single-bead level. This spectral coding technology is expected to open new opportunities in gene expression studies, high-throughput screening, and medical diagnosis.
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