In vivo studies

Imaging tumor microenvironments

Imaging tumor microenvironment (viable, necrotic and hypoxic region) can provide prognostic information about the risk of developing tumor metastases and regional sensitivity and maybe used potentially for localized dose escalation in radiation therapy. Dynamic contrast enhanced MRI and diffusion MRI provide important perfusion and structural information of tumor and thus may be surrogate of of tumor microenvironment. PET provides unique molecular imaging capability to image target molecules, which bind to specific microenvironments with high sensitivity. Our group aims to correlate perfusion/hypoxia parameters derived from in-vivo MR and PET measurements with tumor sections to establish translational MR methodology to image relevant tumor microenvironments in-vivo.

Quantitative imaging of microvasculature

Acquisitions of dual T1 and T2* contrasts from a single exogenous magnetic resonance (MR) agent is attracttive. So far, main efforts were focused on the synthesis of new nanoparticles.  Dextran-coated SPION were synthesized and characterized to have a core diameter distribution of 5-10 nm (TEM) and a hydrodynamic diameter of 20 nm (DLS).  FLASH image showed negative contrast, while UTE image showed positive vessel/tissue contrast, consistent with steady-state signal enhancement calculations. Furthermore, co-registration of the dual-contrast images over the same anatomical region provided complementary information, since negative FLASH contrast provides better sensitivity for small and thin vessels, and positive UTE contrast reduces false contrast from erroneously darkening signal intensities of FLASH images with SPION. Our simulations and the results of in vitro and in vivo experiments validates the finding that SPION-assisted dual-mode acquisitions of positive and negative contrast improves the accuracy, sensitivity, and availability of contrast enhanced-MR angiography (CE-MRA).

Novel MR acquisitions / contrasts / processing studies

Susceptibility contrast based structural Magnetic Resonance Imaging

Susceptibility induced internal field gradient is a finger-print of underlying structure and thus provide useful structural information . We develope new MR imaging sequence, which can directly map the strength and the orientation of susceptibilty gradient and apply this method to gain insight on the structure of tumor microvessels. For example, highly tortuous tumor micro vasculature is one of the key characteristics of tumor angiogenesis and the ability to obtain any tortuosity index may provide important prognositic information.

Accelerated acquisitions for Dynamic Contrast Enhanced(DCE) MRI and diffusion propagator measurements

Magnetic resonance techniques for flow and motion can almost always benefit from the encoding of additional dimensions and faster acquisitions. We examine the application of compressed sensing for the reconstruction of 1) multi-dimensional propagator measurements for the characterization of anisotropic structures and flow and 2) dynamic contrast imaging (DCE) used to estimate tissue permeability. Reducing the number of samples is the simplest way to acquire multi-dimensional propagator measurements in practical times, and to improve the compromise of frame rate and image resolution in DCE. We show that the application of compressed sensing is an effective way to reconstruct these experiments, where simple functional forms or sparse spectra found in spectroscopy are not available as priors.

Hardware developments

Magnetic Particle Imaging (MPI) of nano-particles

MRI provides superior in-vivo soft tissue contrast and thus produces excellent anatomical images. Although MRI can be used to visualize human body, positron emission tomography (PET) is usually used with radioactive markers to visualize functional processes. Recently introduced magnetic particle imaging has potential to quantitatively obtain spatial distribution of magnetic particles. Our group is pursuing multispectral magnetic particle imager, which enable simultaneous determination of concentration and species of iron oxide nano particles