¹⁹F-MRI of inhaled perfluoropropane for quantitative imaging of pulmonary ventilation

PhD ThesisMRI of exogenous imaging agents offers a safely repeatable modality to assess regional pulmonary ventilation. A small number of studies have validated the safety and potential utility of 19F imaging of inhaled thermally polarised perfluoropropane. However, the relative scarcity of signal in restrictive breath hold length acquisition times inhibits translation of this technique to clinical application. This work presents methods used to maximise the attainable image quality of inhaled perfluoropropane. Novel quantitative measures of ventilation and perfusion have been investigated and discussed. A preliminary healthy volunteer study was conducted to verify the efficacy of the imaging technique and to assess perfluoropropane wash-in and wash-out rates. Quantitative assessment of the suitability of four RF coil designs was performed, comparing power efficiency with loading and signal homogeneity within the sensitive volume of each coil. The 3D spoiled gradient echo sequence was simulated, accounting for the power performance of the chosen birdcage coil design, for calculation of acquisition parameter values required to achieve the highest SNR in a fixed acquisition period for 19F-MRI of inhaled perfluoropropane. Studies on resolution phantoms and healthy volunteers assessed the performance of the optimised imaging protocol, in combination with a compressed sensing technique that permitted up to three-fold acceleration. Two novel lung-representative phantoms were fabricated and used to investigate the behaviour of the MR properties of inhaled perfluoropropane with changing structural and magnetic environments. Finally, a method for lengthening the T2* of inhaled perfluoropropane by susceptibility matching the alveolar tissue to the inhaled gas by intravenous injection of a highly paramagnetic contrast agent is presented. Initial development work was conducted in phantoms and rodents before translation to healthy volunteers. This technique offers the potential to concurrently acquire images reflecting both pulmonary ventilation and perfusion

HF-VHF帯における生体等価ファントムを用いた電磁波と人体相互作用の検証

研究科: 千葉大学大学院工学研究科学位:千大院工博甲第工185号博士(工学)千葉大

RF Studies for Ultrahigh Field MRI RF Coils and Arrays

Over the past few decades, different research groups have worked on different facets of Ultra-High Field (UHF) Magnetic Resonance Imaging (MRI); these developments culminated with the FDA approval of the first clinical 7 Tesla (T) MR scanner, Siemens MAGNETOM Terra in late-2017. MRI is still the preferred non-invasive multi-modal imaging technique for visualization of structural and functional correlates in-vivo and clinical diagnosis. Key issues with UHF MRI are in homogeneities in electric and magnetic fields as the size of imaged object becomes comparable with or larger than the radiofrequency (RF) wavelength. This inherent electromagnetic field inhomogeneity and elevated RF power deposition associated with UHF human imaging can have detrimental effects on the quality and safety in high field MRI. To address these challenges, the research work presented in this study 1) evaluated different cylindrical loop receive (Rx) array geometry to establish their effect on the transmit (Tx) coil RF fields. 2) performed detailed analysis, phantom and in-vivo, comparing the performance of the Tic Tac Toe (TTT) coil with a 16-element Transverse Electromagnetic (TEM) coil using multiple anatomical head models and in-vivo. The abovementioned areas of research included: Rx geometry model extraction from CAD models, and development of multiple anatomically detailed models and evaluation of MR coils simulations using full wave Maxwell's equations. Furthermore, an important part of the thesis work was bench marking of transmit coil performance for efficient and safe use in-vivo. The transmit arrays were tested for reproducibility, reliability and safe usage across multiple studies. Finite Difference Time Domain simulations of the Tx and composite of five head models were used to optimize parameters, to obtain homogenous whole brain excitation with low RF absorption or specific absorption rate (SAR)

Brain and Human Body Modeling

This open access book describes modern applications of computational human modeling with specific emphasis in the areas of neurology and neuroelectromagnetics, depression and cancer treatments, radio-frequency studies and wireless communications. Special consideration is also given to the use of human modeling to the computational assessment of relevant regulatory and safety requirements. Readers working on applications that may expose human subjects to electromagnetic radiation will benefit from this book’s coverage of the latest developments in computational modelling and human phantom development to assess a given technology’s safety and efficacy in a timely manner. Describes construction and application of computational human models including anatomically detailed and subject specific models; Explains new practices in computational human modeling for neuroelectromagnetics, electromagnetic safety, and exposure evaluations; Includes a survey of modern applications for which computational human models are critical; Describes cellular-level interactions between the human body and electromagnetic fields

Brain and Human Body Modeling

This open access book describes modern applications of computational human modeling with specific emphasis in the areas of neurology and neuroelectromagnetics, depression and cancer treatments, radio-frequency studies and wireless communications. Special consideration is also given to the use of human modeling to the computational assessment of relevant regulatory and safety requirements. Readers working on applications that may expose human subjects to electromagnetic radiation will benefit from this book’s coverage of the latest developments in computational modelling and human phantom development to assess a given technology’s safety and efficacy in a timely manner. Describes construction and application of computational human models including anatomically detailed and subject specific models; Explains new practices in computational human modeling for neuroelectromagnetics, electromagnetic safety, and exposure evaluations; Includes a survey of modern applications for which computational human models are critical; Describes cellular-level interactions between the human body and electromagnetic fields

Design and Simulation of Coils for High Field Magnetic Resonance Imaging and Spectroscopy

The growing availability of high-field magnetic resonance (MR) scanners has reignited interest in the in vivo investigation of metabolics in the body. In particular, multinuclear MR spectroscopy (MRS) data reveal physiological details inaccessible to typical proton (1H) scans. Carbon-13 (13C) MRS studies draw considerable appeal owing to the enhanced chemical shift range of metabolites that may be interrogated to elucidate disease metabolism and progression. To achieve the theoretical signal-to-noise (SNR) gains at high B0 fields, however, J-coupling from 1H-13C chemical bonds must be mitigated by transmitting radiofrequency (RF) proton-decoupling pulses. This irradiated RF power is substantial and intensifies with increased decoupling bandwidth as well as B0 field strength. The preferred 13C MRS experiment, applying broadband proton decoupling, thus presents considerable challenges at 7 T. Localized tissue heating is a paramount concern for all high-field studies, with strict Specific Absorption Rate (SAR) limits in place to ensure patient safety. Transmit coils must operate within these power guidelines without sacrificing image and spectral quality. Consequently, RF coils transmitting proton-decoupling pulses must be expressly designed for power efficiency as well as B1 field homogeneity. This dissertation presents innovations in high-field RF coil development that collectively improved the homogeneity, efficiency, and safety of high field 13C MRS. A review of electromagnetic (EM) theory guided a full-wave modeling study of coplanar shielding geometries to delineate design parameters for coil transmit efficiency. Next, a novel RF coil technique for achieving B1 homogeneity, dubbed forced current excitation (FCE), was examined and a coplanar-shielded FCE coil was implemented for proton decoupling of the breast at 7 T. To perform a series of simulation studies gauging SAR in the prone breast, software was developed to fuse a suite of anatomically-derived heterogeneous breast phantoms, spanning the standard four tissue density classifications, with existing whole-body voxel models. The effects of tissue density on SAR were presented and guidance for simulating the worst-case scenario was outlined. Finally, for improving capabilities of multinuclear coils during proton coil transmit, a high-power trap circuit was designed and tested, ultimately enabling isolation of 13C coil elements during broadband proton decoupling pulses. Together, this work advanced the hardware capabilities of high-field multinuclear spectroscopy with immediate applicability for performing broadband proton-decoupled 13C MRS in the breast at 7 T

In vivo knee cartilage quality assessment by direct quantification of glycosaminoglycans through chemical exchange saturation transfer (gagCEST)

Introduction: Non-invasive imaging of human articular cartilage is an important tool in understanding, and thus allowing the development of effective treatments for, osteoarthritis; a disease responsible of impaired movement of millions of otherwise healthy individuals. Glycosaminoglycans (GAG) have been attributed with important biomechanical properties, vital for the function of the articular cartilage. By selectively irradiating hydroxyl protons at the GAG molecule and studying the saturation effect as it is transferred to water through chemical exchange, the GAG concentration can be inferred. This is termed glycosaminoglycan measurement through chemical exchange saturation transfer – gagCEST. This method would ideally directly measure the concentration of GAG in articular cartilage and increase the knowledge about osteoarthritis pathology. The aim of this thesis is to give the reader an understanding of the gagCEST method, and to evaluate a specific gagCEST work-in-progress (WIP) package. Materials and methods: A WIP package for gagCEST measurements was evaluated on a Magnetom Trio 3 Tesla MRI system (Siemens AG, Erlangen, Germany), compatible with a 15 channel transmit/receive knee-coil. Phantoms with variable amount of GAG and agarose were measured in order to investigate the function of the system. Freeze dried samples of human femoral cartilage were supplied by the orthopedics department in order to optimize the measurement. Finally, several volunteers were scanned to test the in vivo applicability of the sequence at its current stage. Evaluation of measurements were done both using the raw data to create a CEST-spectrum in MatLab, as well as relying on the pixel-wise calculation of the CEST-image produced by the WIP programme. Results: The gagCEST measurements proved to be filled with pitfalls. Phantom measurements were complicated by magnetic field inhomogeneity and incomplete gradient spoiling, grossly disturbing the CEST spectra. Though heterogeneity correction is applied, phantoms with lower T2 relaxation must be produced to get rid of residual magnetization after spoiling. Despite this, some measurements were performed yielding a noticeable CEST effect when considering mean value of several pixels. In vivo measurements were first disturbed by fat chemical shift, but proved to produce images with CEST signal in cartilage. However, the evaluation of in vivo images was difficult, e.g., the CEST signal ratio between cartilage and meniscus was one order of magnitude away from the theoretically expected result. Discussion: The gagCEST WIP sequence proved to provide image volumes in six minutes, displaying high signal in cartilage on healthy volunteers. However, the method requires additional extensive investigation. Phantom measurements are required to create standards and optimize the saturation sequence, for which low T2 and long T1 phantoms must be constructed. Moreover, in vivo measurements must be further scrutinized as magnetic field heterogeneity will quickly ruin a measurement; the cartilage of interest is tightly packed between bone and the meniscus. It is also important to realize that the gagCEST method does not in fact measure GAG, but rather labile hydroxyl protons, and there exist a possibility that other molecules will exhibit similar chemical shifts. Though this is not a physical but biochemical consideration it should be kept in mind when analyzing tissue and “gagCEST” signal. Finally, several authors claim the technique will be feasible at 7 T due to longer T1 and larger chemical shift of GAG hydroxyls, however, it is evident from phantom tests that at least relative measurements are possible at 3 T

Microwave Imaging to Improve Breast Cancer Diagnosis

Breast cancer is the most prevalent type of cancer worldwide. The correct diagnosis of Axillary Lymph Nodes (ALNs) is important for an accurate staging of breast cancer. The performance of current imaging modalities for both breast cancer detection and staging is still unsatisfactory. Microwave Imaging (MWI) has been studied to aid breast cancer diagnosis. This thesis addresses several novel aspects of the development of air-operated MWI systems for both breast cancer detection and staging. Firstly, refraction effects in air-operated setups are evaluated to understand whether refraction calculation should be included in image reconstruction algorithms. Then, the research completed towards the development of a MWI system to detect the ALNs is presented. Anthropomorphic numerical phantoms of the axillary region are created, and the dielectric properties of ALNs are estimated from Magnetic Resonance Imaging exams. The first pre-clinical MWI setup tailored to detect ALNs is numerically and experimentally tested. To complement MWI results, the feasibility of using machine learning algorithms to classify healthy and metastasised ALNs using microwave signals is analysed. Finally, an additional study towards breast cancer detection is presented by proposing a prototype which uses a focal system to focus the energy into the breast and decrease the coupling between antennas. The results show refraction calculation may be neglected in low to moderate permittivity media. Moreover, MWI has the potential as an imaging technique to assess ALN diagnosis as estimation of dielectric properties indicate there is sufficient contrast between healthy and metastasised ALNs, and the imaging results obtained in this thesis are promising for ALN detection. The performance of classification models shows these models may potentially give complementary information to imaging results. The proposed breast imaging prototype also shows promising results for breast cancer detection

Design of a dedicated circular coil for Magnetic Resonance Spectroscopy studies in small phantoms and animal acquisition with a 3 Tesla Magnetic Resonance clinical scanner

Abstract Introduction: Magnetic Resonance Spectroscopy (MRS) is a very powerful tool to explore the tissue components, by allowing a selective identification of molecules and molecular distribution mapping. Due to intrinsic Signal-to-Noise Ratio limitations (SNR), MRS in small phantoms and animals with a clinical scanner requires the design and development of dedicated radiofrequency (RF) coils, a task of fundamental importance. In this article, the authors describe the simulation, design, and application of a 1H transmit/receive circular coil suitable for MRS studies in small phantoms and small animal models with a clinical 3T scanner. In particular, the circular coil could be an improvement in animal experiments for tumor studies in which the lesions are localized in specific areas. Material and methods: The magnetic field pattern was calculated using the Biot–Savart law and the inductance was evaluated with analytical calculations. Finally, the coil sensitivity was measured with the perturbing sphere method. Successively, a prototype of the coil was built and tested on the workbench and by the acquisition of MRS data. Results: In this work, we demonstrate the design trade-offs for successfully developing a dedicated coil for MRS experiments in small phantoms and animals with a clinical scanner. The coil designed in the study offers the potential for obtaining MRS data with a high SNR and good spectral resolution. Conclusions: The paper provides details of the design, modelling, and construction of a dedicated circular coil, which represents a low cost and easy to build answer for MRS experiments in small samples with a clinical scanner