One major advantage of using gold nanoparticles is the possibility of tuning their absorption peak by modifying their surface plasma resonance. They are proven to be a promising multi-functional platform that can be used for many imaging and therapeutic applications. As a true multi-modality imaging technique, Photo-Magnetic Imaging (PMI) has a great potential to monitor the distribution of gold nanoparticles non-invasively with MR resolution. With a simple addon of a continuous wave laser to an MRI system, PMI uses the laser induced temperature increase, measured by MR Thermometry (MRT), to provide tissue optical absorption maps at MR resolution. PMI utilizes a Finite Element Method (FEM) based algorithm to solve the combined diffusion and bio-heat equations. This system of combined equations models the photon distribution in the tissue and heat generation due to the absorption of the light and consequent heat diffusion. The key characteristic of PMI is that its spatial resolution is preserved at any depth as long as the temperature change within the imaged medium is detectable by MRT. Agar phantoms containing gold nanoparticles are used to validate the ability of PMI in monitoring their distribution. To make PMI suitable for diagnostic purposes, the laser powers has been kept under the American National Standard Institute maximum skin exposure limits in this study.
We present experimental results that validate our imaging technique termed photomagnetic imaging (PMI). PMI illuminates the medium under investigation with a near-infrared light and measures the induced temperature increase using magnetic resonance imaging. A multiphysics solver combining light and heat propagation is used to model spatiotemporal distribution of temperature increase. Furthermore, a dedicated PMI reconstruction algorithm has been developed to reveal high-resolution optical absorption maps from temperature measurements. Being able to perform measurements at any point within the medium, PMI overcomes the limitations of conventional diffuse optical imaging. We present experimental results obtained on agarose phantoms mimicking biological tissue with inclusions having either different sizes or absorption contrasts, located at various depths. The reconstructed images show that PMI can successfully resolve these inclusions with high resolution and recover their absorption coefficient with high-quantitative accuracy. Even a 1-mm inclusion located 6-mm deep is recovered successfully and its absorption coefficient is underestimated by only 32%. The improved PMI system presented here successfully operates under the maximum skin exposure limits defined by the American National Standards Institute, which opens up the exciting possibility of its future clinical use for diagnostic purposes.
Multi-modality imaging leverages the competitive advantage of different imaging systems to improve the overall resolution
and quantitative accuracy. Our new technique, Photo-Magnetic Imaging (PMI) is one of these true multi-modality imaging
approaches, which can provide quantitative optical absorption map at MRI spatial resolution. PMI uses laser light to
illuminate tissue and elevate its temperature while utilizing MR thermometry to measure the laser-induced temperature
variation with high spatial resolution. The high-resolution temperature maps are later converted to tissue absorption maps by
a finite element based inverse solver that is based on modeling of photon migration and heat diffusion in tissue. Previously,
we have demonstrated the feasibility of PMI with phantom studies. Recently, we have managed to reduce the laser power
under ANSI limit for maximum skin exposure therefore, we have well positioned PMI for in vivo imaging. Currently we are
expanding our system by adding multi-wavelength imaging capability. This will allow us not only to resolve spatial
distribution of tissue chromophores but also exogenous contrast agents. Although we test PMIs feasibility with animal
studies, our future goal is to use PMI for breast cancer imaging due to its high translational potential.
We introduce an entirely new technique, termed Photo-Magnetic Imaging (PMI), which overcomes the limitation of pure optical imaging and provides optical absorption at MRI spatial resolution. PMI uses laser light to heat the medium under investigation and employs MR thermometry for the determination of spatially resolved optical absorption in the probed medium. A FEM-based PMI forward solver has been developed by modeling photon migration and heat diffusion in tissue to compare simulation results with measured MRI maps. We have successfully performed PMI using 2.5 cm diameter agar phantom with two low optical absorption contrast (x 4) inclusions under the ANSI limit. Currently, we are developing the PMI inverse solver and undertaking further phantom and in vivo experiments.
Since diffuse optical tomography (DOT) is a low spatial resolution modality, it is desirable to validate its quantitative accuracy with another well-established imaging modality, such as magnetic resonance imaging (MRI). In this work, we have used a polymer based bi-functional MRI-optical contrast agent (Gd-DTPA-polylysine-IR800) in collaboration with GE Global Research. This multi-modality contrast agent provided not only co-localization but also the same kinetics, to cross-validate two imaging modalities. Bi-functional agents are injected to the rats and pharmacokinetics at the bladder are recovered using both optical and MR imaging. DOT results are validated using MRI results as "gold standard"