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.
We present a magnetic resonance (MR)-guided near-infrared dynamic contrast enhanced diffuse optical tomography (DCE-DOT) system for characterization of tumors using an optical contrast agent (ICG) and a MR contrast agent [Gd-diethylenetriaminepentaacetic acid (DTPA)] in a rat model. Both ICG and Gd-DTPA are injected and monitored simultaneously using a combined MRI-DOT system, resulting in accurate co-registration between two imaging modalities. Fisher rats bearing R3230 breast tumor are imaged using this hybrid system. For the first time, enhancement kinetics of the exogenous contrast ICG is recovered from the DCE-DOT data using MR anatomical a priori information. As tumors grow, they undergo necrosis and the tissue transforms from viable to necrotic. The results show that the physiological changes between viable and necrotic tissue can be differentiated more accurately based on the ICG enhancement kinetics when MR anatomical information is utilized.
Dynamic contrast enhanced MRI (DCE-MRI) has been proven to be the most sensitive modality in detecting breast
lesions. Currently available MR contrast agent, Gd-DTPA, is a low molecular weight extracellular agent and can
diffuse freely from the vascular space into interstitial space. Due to this reason, DCE-MRI has low sensitivity in
differentiating benign and malignant tumors. Meanwhile, diffuse optical tomography (DOT) can be used to provide
enhancement kinetics of an FDA approved optical contrast agent, ICG, which behaves like a large molecular weight
optical agent due to its binding to albumin. The enhancement kinetics of ICG may have a potential to distinguish
between the malignant and benign tumors and hence improve the specificity. Our group has developed a high speed
hybrid MRI-DOT system. The DOT is a fully automated, MR-compatible, multi-frequency and multi-spectral
imaging system. Fischer-344 rats bearing subcutaneous R3230 tumor are injected simultaneously with Gd-DTPA
(0.1nmol/kg) and IC-Green (2.5mg/kg). The enhancement kinetics of both contrast agents are recorded
simultaneously with this hybrid MRI-DOT system and evaluated for different tumors.
Breast cancer is a significant cause of mortality and morbidity among women with early diagnosis being vital to
successful treatment. Diffuse Optical Tomography (DOT) is an emerging medical imaging modality that provides
information that is complementary to current screening modalities such as MRI and mammography, and may improve
the specificity in determining cancer malignancy. Using high-resolution anatomic images as a priori information
improves the accuracy of DOT. Measurements are presented characterizing the performance of our system. Preliminary
data is also shown illustrating the use of a priori MRI data in phantom studies.ä
Diffuse Optical Tomography (DOT) is a new and promising medical imaging modality which uses near-infrared light to
probe tissue properties. Using multiple wavelengths of light can provide important information about tissue metabolism
and cancer malignancy. Unfortunately, in most DOT acquisition schemes, acquiring data for each wavelength has a
multiplicative effect on the overall imaging time. In this paper, we evaluate a new multiple wavelength laser module
(Praevium Research Inc.) with 12 laser diodes all coupled to a single output fiber. When used in conjunction with a
cooled spectrometer, it allows simultaneous multi-wavelength data acquisition and hence, higher temporal resolution.
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