Analisi di un Sistema "Wireless Power Transfer" Transcutaneo per Cuore Artificiale

Si propone un sistema “Wireless Power Transfer” transcutaneo per l’alimentazione di cuori artificiali. Il sistema può essere visto come una rete due porte ed è costituito da due elementi: un loop in trasmissione esterno e un loop in ricezione impiantato sotto cute. Ognuno dei due loop è dotato di un capacitore posto in serie ad essi. I valori di capacità sono stati scelti in modo da massimizzare il parametro |S21| alla frequenza di 13.56MHz. Tale parametro tiene conto di quanta parte della potenza trasmessa alla porta 1 viene effettivamente ricevuta alla porta 2, sotto l’ipotesi di carico adattato. I tessuti umani sono stati modellati come mezzi dispersivi tramite il modello di debye del secondo ordine. Per il progetto e l’analisi del sistema è stato usato il software CST Microwave Studio, che implementa il metodo di analisi agli elementi finiti (FEM). Successivamente è stata analizzata la distribuzione del SAR locale all’ interno dei tessuti e confrontata con i limiti imposti dalla normativa ICNIRP

Notes on Profile Inversion and Closed Form Formulation of Compact GRIN Lenses

This note is an addendum to the paper Profile Inversion and Closed Form Formulation of Compact GRIN Lenses [1] and summarizes a few comments obtained after publication of [1]

Design and Development of Radio Frequency Coils for Sodium Magnetic Resonance Imaging at 7 T

The main goal of this Thesis is the design and development of Radio-Frequency (RF) coils for sodium Magnetic Resonance Imaging (MRI) at Ultra High Field (UHF). The advantage of using UHF MR scanners is due to the possibility to achieve improved Signal-to-Noise-Ratio (SNR) and spatial resolution. These characteristics are fundamental in case of imaging with nuclei different from proton, which provide an intrinsically lower signal because of their lower in-vivo concentration and lower gyromagnetic ratio. Moreover, the overlap between sodium and proton images allows the accurate localization of regions with an anomalous sodium concentration, thanks to the anatomically more detailed proton images. For this reason, in case of imaging with non-proton nuclei, Dual-Tuned (DT) coils are preferable, since they allow the signal acquisition in a fixed spatial orientation of the subject, thus removing the need of patient’s repositioning between two consecutive acquisitions with two different RF coil resonating at the two Larmor frequencies of proton and sodium, respectively. Therefore, with a DT coil, automatically co-registered images can be obtained. The cost to pay is an increase in the design and development complexity with respect to a standard RF coil. In this Thesis, RF coils prototypes for sodium imaging (Larmor frequency ≃ 79 MHz) have been designed and developed for two different applications: human knee and human head imaging. Concerning the knee imaging, both surface coils, suitable for the signal reception, and volume coils, suitable for the sample excitation, have been designed and developed. All surface coils for knee imaging are dual-tuned. The first DT-RF coil prototype has been developed to study and characterize the issues related to the coupling between the two resonant structures, which usually compose a DT coil. New decoupling strategies have been proposed and developed as an alternative to the standard decoupling by using trap circuits, including models based on PIN diodes and Micro-Electro-Mechanical System (MEMS) switches. The volume RF coil for the knee imaging, built to be sensitive to the sodium signal only, has been designed according to the birdcage model. It has been developed to face with potential issues related to sodium volume coil interfacing with the MR system and signal acquisition before starting the construction of DT volume coils. Concerning the head imaging, an imbricated DT-RF coil, consisting of two concentrically placed birdcages, and the related electronic interface needed to connect the coil to the MR system, have been developed. Finally, an unconventional DT volume coil model (four-ring model), consisting of two birdcage-like resonant structures arranged on the same cylindrical surface and tuned at the two frequencies of interest, has been taken into account. The four-ring model has been optimized though electromagnetic simulations, with the main purpose of increasing the magnetic field homogeneity at the proton Larmor frequency at 7 T (≃ 300 MHz), and finally compared with the imbricated model

Profile Inversion and Closed Form Formulation of Compact GRIN Lenses

New formulas for the design of cylindrical Graded-Index (GRIN) lens-antennas with integrated feeder are presented. The possibility of integrating the feeder within the lens makes the system more compact, avoids complex mechanical design and alignment errors. The lens is characterized in analytical form by Geometrical Optics (GO). The expression of the radially varying refractive index is derived by applying the conservation of the momentum inside the lens and imposing parallel rays at the lens output interface. The mathematical procedure to retrieve the refractive index is based on the inversion of a truncated Abel transform. The Poynting vector at the lens aperture is derived by applying the conservation of energy in each elementary ray-tube. By approximating the Poynting vector to a Gaussian function the total efficiency of the lens-antenna is derived in a closed form, allowing for a quick lens design. The proposed formulation has been successfully validated by using ray-tracing and a full-wave simulations. Finally, we present examples of practical design of GRIN lenses by using holes of different shapes in a dielectric ABS/Teflon host media

Double-Tuned Birdcage Radio Frequency Coil for 7 T MRI: Optimization, Construction and Workbench Validation

The aim of the present study is the optimization, construction, and workbench validation of a double-tuned 1H- 23Na volume radio frequency (RF) coil suitable for human head imaging at 7 T, based on the birdcage geometry. The birdcage-like design which is considered is the four-ring model, in which two standard birdcage-like structures with the same diameters are nested along the longitudinal axis. Simulations based on Maxwell’s equations are performed to evaluate the RF magnetic field homogeneity and the RF coil efficiency varying the coil geometrical parameters. The RF magnetic field homogeneity is evaluated both on the transverse (z = 0) and longitudinal (y = 0) planes without performing the impedance matching procedure, so that the RF coil symmetry is not perturbed by the matching network. The RF coil efficiency is instead dependent on the effective coil input RF power, and it is evaluated after matching the coil, so that the reflected power is minimized, assuming that the stimulation power is totally delivered to the RF coil. Considering the simulation results and the target application, the useful RF coil geometrical parameters are fixed. The four-ring model, which showed the best performances, has been built and tested on a workbench, using a cylindrical phantom filled with a 0.05 M saline solution as load. This provides the first example of a four-ring realization intended 1H- 23Na for human head imaging at 7 T