Imaging Electrical Properties Using MRI and In Vivo Applications

University of Minnesota Ph.D. dissertation. November 2015. Major: Biomedical Engineering. Advisor: Bin He. 1 computer file (PDF); viii, 137 pages.Electrical properties, namely conductivity and permittivity, describe the interaction of materials with the surrounding electromagnetic field. The electrical properties of biological tissue are associated with many fundamental aspects of tissue, such as cellular and molecular structure, ion concentration, cell membrane permeability, etc. Electrical properties of tissue in vivo can be used as biomarkers to characterize cancerous tissue or provide useful information in applications involving tissue and electromagnetic field. Among many related electrical-property imaging technologies, electrical properties tomography (EPT) is a promising one that noninvasively extracts the in vivo electrical properties with high spatial resolution based on measured B1 field using magnetic resonance imaging (MRI). In this thesis, advanced EPT methods have been developed to improve the imaging quality of conventional EPT. First of all, a multi-channel EPT framework was introduced to release its dependency on a B1 phase assumption and expand its application under high field strength. Secondly, a gradient-based EPT (gEPT) approach was proposed and implemented, showing enhanced robustness against effect of measurement noise and improved performance near tissue boundaries. Using gEPT, high resolution in vivo electrical-property images of healthy human brain were obtained, and an imaging system for rat tumor models was also developed. As a result of malignancy, increased conductivity was captured in tumors using the in vivo animal imaging system. Thirdly, based on EPT theory, quantitative water proton density imaging was proposed using measured B1 field information, provide a new way for estimating water content in tissue for diagnostic and research purpose

Medical Imaging of Microrobots: Toward In Vivo Applications

Medical microrobots (MRs) have been demonstrated for a variety of non-invasive biomedical applications, such as tissue engineering, drug delivery, and assisted fertilization, among others. However, most of these demonstrations have been carried out in in vitro settings and under optical microscopy, being significantly different from the clinical practice. Thus, medical imaging techniques are required for localizing and tracking such tiny therapeutic machines when used in medical-relevant applications. This review aims at analyzing the state of the art of microrobots imaging by critically discussing the potentialities and limitations of the techniques employed in this field. Moreover, the physics and the working principle behind each analyzed imaging strategy, the spatiotemporal resolution, and the penetration depth are thoroughly discussed. The paper deals with the suitability of each imaging technique for tracking single or swarms of MRs and discusses the scenarios where contrast or imaging agent's inclusion is required, either to absorb, emit, or reflect a determined physical signal detected by an external system. Finally, the review highlights the existing challenges and perspective solutions which could be promising for future in vivo applications

Quantitative MR Imaging of the Electric Properties and Local SAR based on Improved RF Transmit Field Mapping

This work presents three new quantitative methods for magnetic resonance imaging. A method for simultaneous mapping of B1 and T1 (MTM) is developed and validated. Electric Properties Tomography (EPT), a method for quantitative imaging of dielectric properties of tissue, is presented. Based on EPT, separate (phase-based) conductivity and (amplitude-based) permittivity measurements are introduced. Finally, a B1-based method for patient-specific local SAR measurements is presented

Quantitative MR Imaging of the Electric Properties and Local SAR based on Improved RF Transmit Field Mapping

This work presents three new quantitative methods for magnetic resonance imaging. A method for simultaneous mapping of B1 and T1 (MTM) is developed and validated. Electric Properties Tomography (EPT), a method for quantitative imaging of dielectric properties of tissue, is presented. Based on EPT, separate (phase-based) conductivity and (amplitude-based) permittivity measurements are introduced. Finally, a B1-based method for patient-specific local SAR measurements is presented

Electrical properties tomography: a methodological review

Electrical properties tomography (EPT) is an imaging method that uses a magnetic resonance (MR) system to non-invasively determine the spatial distribution of the conductivity and permittivity of the imaged object. This manuscript starts by providing clear definitions about the data required for, and acquired in, EPT, followed by comprehensively formulating the physical equations underlying a large number of analytical EPT techniques. This thorough mathematical overview of EPT harmonizes several EPT techniques in a single type of formulation and gives insight into how they act on the data and what their data requirements are. Furthermore, the review describes machine learning-based algorithms. Matlab code of several differential and iterative integral methods is available upon request.Imaging- and therapeutic targets in neoplastic and musculoskeletal inflammatory diseas

Imaging of Magnetic Nanoparticles using Magnetoelectric Sensors

Imaging of magnetic nanoparticles offers a variety of promising medical applications for therapeutics and diagnostics. Using magnetic nanoparticles as tracer material for imaging allows for the non-invasive detection of spatial distributions of nanoparticles that can give information about diseases or can be used in preventive medicine. Imaging biodistributions of magnetically labeled cells offers applicability for tissue engineering, as a means to monitor cell growth within artificial scaffolds non-destructively. In the presented work, the capabilities of an imaging system for magnetic nanoparticles via magnetoelectric sensors are investigated. The investigated technique, called Magnetic Particle Mapping, is based on the detection of the nonlinear magnetic response of magnetic nanoparticles. A resonant magnetoelectric sensor is used for frequency selective measurements of the nanoparticles magnetic response. Extensive modeling was performed that enabled proper imaging of magnetic nanoparticle distributions. Fundamental limitations of the imaging system were derived to describe resolution in correspondence to signal-to-noise ratios. Incorporation of additional parameters in the imaging system for the data analysis resulted in an algorithm for a more robust reconstruction of spatial particle distributions, increasing its imaging capabilities. Experimental investigations of the imaging system show the capabilities for imaging of cell densities using magnetically labeled cells. Furthermore, resolution limitations were investigated and differentiation of different particle types in imaging was shown, referred to as ”colored” imaging. The imaging of biodistributions of magnetically labeled cells thus enable exciting perspectives on further research and possible applications in tissue engineering