Progettazione e Realizzazione di Bobine a Radiofrequenza per Risonanza Magnetica a Campo Ultra Alto

Il fenomeno della Risonanza Magnetica Nucleare (NMR) viene sfruttato in diagnostica medica nelle tecniche di imaging (MRI) e spettroscopia (MRS) che permettono di ottenere informazioni anatomiche e funzionali, uniche o complementari a quelle accessibili con altre tecniche diagnostiche. Inoltre, la diagnostica tramite MRI e MRS e completamente non invasiva e, utilizzando radiazioni non ionizzanti, presenta rischi molto ridotti per il paziente. In risonanza magnetica il rapporto segnale/rumore, e quindi la risoluzione spaziale e temporale, dipendono in prima istanza dall’intensita' del campo magnetico statico applicato. La ricerca negli ultimi anni ha portato alla realizzazione di scanner anche per MRI con campi magnetici sempre piu intensi per l’utilizzo sull’uomo. Attualmente il campo a 1.5 Tesla rappresenta lo standard clinico, anche se gli scanner a 3 Tesla iniziano ad essere ampiamente utilizzati, e sono di recente installazione in alcuni centri di ricerca scanner con campo a 7 Tesla. Nel giugno scorso e' stato installato un magnete a 7 Tesla (950-MR scanner, GE Healthcare) presso l’IRCCS Stella Maris di Calambrone (PI), che costituisce un primo centro di ricerca italiano (IMAGO7) per risonanza magnetica a campo ultra alto. Con il crescere dell’intensita' del campo magnetico statico compaiono problemi, sia teorici che pratici, nella progettazione dell’hardware responsabile dell’eccitazione del campione e della ricezione del segnale: le bobine a radiofrequenza (RF). E' necessario sviluppare bobine a radiofrequenza ad hoc per ogni applicazione clinica e di ricerca, adattandole alla morfologia del paziente e scegliendo il modello da utilizzare con le migliori caratteristiche elettromagnetiche in modo da massimizzarne la sensibilita'. Le bobine RF per campo alto e ultra alto presentano numerose problematiche, tra cui la forte disomogeneita' del campo generato ed i possibili eccessi della potenza assorbita localmente dal campione, da cui conseguono rispettivamente prestazioni non ottimali e un possibile rischio per il paziente. A causa della natura prototipale dello scanner, e' necessario realizzare le bobine autonomamente in modo da studiare soluzioni innovative, o adattamenti dei modelli utilizzati a basso campo, in grado di garantire prestazioni ottimali anche a 7 Tesla per tutte le applicazioni di interesse. Per questo motivo e' stato fondato un nuovo laboratorio RF dedicato in particolare al campo ultra alto. La progettazione e la realizzazione di bobine per campo ultra alto rappresenta pertanto un interessante argomento di ricerca, affrontato in questo lavoro di tesi, svolto all’interno del progetto SEVEN dell’INFN, con la collaborazione dell’IRCCS Stella Maris, dell’Universita' dell’Aquila, del Dipartimento di Ingegneria delle Telecomunicazioni e della General Electrics. Nella progettazione di una bobina occorre innanzitutto scegliere se essa dovra' svolgere la sola fase di trasmissione del segnale, la sola fase di ricezione o entrambe. Nel primo caso si utilizza generalmente una bobina di volume, nel secondo una bobina di superficie. E' talvolta necessario utilizzare bobine multinucleari, in grado di rilevare il segnale del protone e di un altro nucleo, come ad esempio il 31 P, per ottenere alcune importanti informazioni metaboliche. Nella scelta del prototipo occorre inoltre ottimizzare parametri come la distribuzione del campo magnetico, la resistenza della bobina, il rapporto segnale/rumore e la potenza assorbita dal campione. Per modelli semplici sono generalmente sufficienti la teoria elettromagnetica e il lavoro sperimentale: e' necessario pero valutare le prestazioni della bobina tramite simulazioni, per ottenere ad esempio dati non misurabili sperimentalmente nello scanner, come la distribuzione del campo elettrico, e come ausilio nella progettazione di modelli complessi. E' stato utilizzato, per la prima volta nella progettazione di bobine RF, il software commerciale FEKO che implementa il Metodo dei Momenti (MoM) e che presenta il vantaggio di una maggiore rapidita' di esecuzione rispetto ad algoritmi classici come il Metodo delle Differenze Finite. Questo software e' stato validato tramite il confronto con i dati sperimentali su alcuni prototipi di bobina a 1.5 Tesla e tramite il confronto con le simulazioni ottenute con il sofware CST, che implementa il metodo classico delle Differenze Finite. Sono stati progettati e realizzati, nell’ambito del presente lavoro di tesi, diversi modelli di bobine RF per scanner a 7T: una bobina single loop, un phased array, una bobina di volume di tipo TEM e diversi modelli di bobine dual tuned. Il software di simulazione MoM si e' dimostrato affidabile nella progettazione di bobine, in quanto le predizione delle distribuzioni di campo magnetico sono in accordo con quanto previsto dalle simulazioni con il metodo delle differenze finite e con i dati sperimentali. Tale software e stato quindi utilizzato per ottimizzare la geometria del phased array, per valutare la disomogeneita' di campo nella TEM e per la scelta del modello ottimale di dual tuned. In tutti questi casi la simulazione elettromagnetica ha fornito dati non ottenibili tramite la teoria o l’esperimento. Tutti i prototipi realizzati hanno mostrato prestazioni idonee all’utilizzo nello scanner e confrontabili con i modelli analoghi presenti in letteratura, sia per quanto riguarda i parametri della bobina (adattamento di impedenza, disaccoppiamento) che per quanto riguarda le distribuzioni dei campi elettromagnetici, che risultano sempre adatti all’applicazione, ossia privi di zone con forte concentrazione dei campi elettrici, che comporterebbero dei rischi per il paziente. Lo scanner per risonanza magnetica a 7 Tesla installato a Calambrone sara' operativo a partire da fine anno. Sara' allora possibile eseguire i primi test nello scanner e interfacciare e utilizzare le bobine sviluppate in questo lavoro di tesi. Saranno dapprima eseguite misure su fantocci e successivamente anche applicazioni in vivo, coprendo un vasto spettro di possibili campi di ricerca e ricerca clinica, tra cui ad esempio lo studio di nuove tecniche di mappatura del campo elettromagnetico nello scanner, la spettroscopia del 31 P nell’arto inferiore per lo studio di malattie neuromuscolari, sino ad arrivare all’acquisizione di immagini ad altissima risoluzione nel cervello umano

Effects of the load size on the maximum local SAR at 7T

In this study we investigated the effects of the load size on the maximum local SAR at 7T. Specifically, we resorted to: i) 3D full wave numerical electromagnetic simulations for analyzing a surface loop loaded with anatomic human calves models; ii) 2D analytical approach for analyzing a volume resonator loaded with homogeneous cylindrical phantoms having average tissue dielectric properties. In both cases we noticed that the maximum local SAR decreases with decreasing load size: this holds true if the RF magnetic fields (B1+) for the different load sizes are scaled so to achieve the same slice average value of 1ìT

New insights on the systematics of echinoids belonging to the family Spatangidae Gray 1825 using a combined approach based on morphology, morphometry, and genetics.

Spatangoids are probably the least resolved group within echinoids, with known topological incongruencies between phylogenies derived from molecular (very scarce) and morphological data. The present work, based on the analysis of 270 specimens of Spatangidae (Echinoidea, Spatangoida) trawled in the Sardinian seas (Western Mediterranean), allowed us to verify the constancy of some characters that we consider to be diagnostic at the genus level —such as the path of the subanal fasciole and the relationship between labrum and adjacent ambulacral plates —and to distinguish two distinct forms within the studied material. Based on morphological characters, morphometrics, and molecular analyses (sequencing of two mitochondrial markers: cytochrome c oxidase subunit1 (COI) and 16S), most of the individuals were classified as morphotype A and attributed to the species Spatangus purpureus, the most common spatangoid in the Mediterranean Sea, while a few corresponded to a different morphotype (B), genetically close to the species Spatangus raschi. Preliminary morphological analyses seemed to indicate that morphotype B specimens from Sardinia are slightly different from S. raschi and from Spatangus subinermis individuals, the second species of the family known to occur in the Mediterranean Sea. On the basis of morpho-structural observations and molecular analyses, comparing Mediterranean living forms with species from other areas (Central Eastern Atlantic, North Sea and neighboring basins, South African Sea, Philippines and Indonesian Archipelago, New Zealand, and Hawaiian Islands), the clear distinction of S. purpureus from several other species classified as Spatangus was confirmed. Based on the morphological and genetic differences, we propose to maintain the genus Spatangus including in it only the type species S. purpureus among the living species and to establish the new genus Propespatagus nov. gen. to include several other species previously classified as Spatangus. The clear distinction among different genera was also detected in fossil forms of Spatangus, Propespatagus nov. gen., and Sardospatangus (†) from the European Oligo-Miocene sedimentary rocks of Germany; the Miocene of Ukraine, Italy, and Spatangoids are probably the least resolved group within echinoids, with known topological incongruencies between phylogenies derived from molecular (very scarce) and morphological data. The present work, based on the analysis of 270 specimens of Spatangidae (Echinoidea, Spatangoida) trawled in the Sardinian seas (Western Mediterranean), allowed us to verify the constancy of some characters that we consider to be diagnostic at the genus level —such as the path of the subanal fasciole and the relationship between labrum and adjacent ambulacral plates —and to distinguish two distinct forms within the studied material. Based on morphological characters, morphometrics, and molecular analyses (sequencing of two mitochondrial markers: cytochrome c oxidase subunit1 (COI) and 16S), most of the individuals were classified as morphotype A and attributed to the species Spatangus purpureus, the most common spatangoid in the Mediterranean Sea, while a few corresponded to a different morphotype (B), genetically close to the species Spatangus raschi. Preliminary morphological analyses seemed to indicate that morphotype B specimens from Sardinia are slightly different from S. raschi and from Spatangus subinermis individuals, the second species of the family known to occur in the Mediterranean Sea. On the basis of morpho-structural observations and molecular analyses, comparing Mediterranean living forms with species from other areas (Central Eastern Atlantic, North Sea and neighboring basins, South African Sea, Philippines and Indonesian Archipelago, New Zealand, and Hawaiian Islands), the clear distinction of S. purpureus from several other species classified as Spatangus was confirmed. Based on the morphological and genetic differences, we propose to maintain the genus Spatangus including in it only the type species S. purpureus among the living species and to establish the new genus Propespatagus nov. gen. to include several other species previously classified as Spatangus. The clear distinction among different genera was also detected in fossil forms of Spatangus, Propespatagus nov. gen., and Sardospatangus (†) from the European Oligo-Miocene sedimentary rocks of Germany; the Miocene of Ukraine, Italy, and Spatangoids are probably the least resolved group within echinoids, with known topological incongruencies between phylogenies derived from molecular (very scarce) and morphological data. The present work, based on the analysis of 270 specimens of Spatangidae (Echinoidea, Spatangoida) trawled in the Sardinian seas (Western Mediterranean), allowed us to verify the constancy of some characters that we consider to be diagnostic at the genus level —such as the path of the subanal fasciole and the relationship between labrum and adjacent ambulacral plates —and to distinguish two distinct forms within the studied material. Based on morphological characters, morphometrics, and molecular analyses (sequencing of two mitochondrial markers: cytochrome c oxidase subunit1 (COI) and 16S), most of the individuals were classified as morphotype A and attributed to the species Spatangus purpureus, the most common spatangoid in the Mediterranean Sea, while a few corresponded to a different morphotype (B), genetically close to the species Spatangus raschi. Preliminary morphological analyses seemed to indicate that morphotype B specimens from Sardinia are slightly different from S. raschi and from Spatangus subinermis individuals, the second species of the family known to occur in the Mediterranean Sea. On the basis of morpho-structural observations and molecular analyses, comparing Mediterranean living forms with species from other areas (Central Eastern Atlantic, North Sea and neighboring basins, South African Sea, Philippines and Indonesian Archipelago, New Zealand, and Hawaiian Islands), the clear distinction of S. purpureus from several other species classified as Spatangus was confirmed. Based on the morphological and genetic differences, we propose to maintain the genus Spatangus including in it only the type species S. purpureus among the living species and to establish the new genus Propespatagus nov. gen. to include several other species previously classified as Spatangus. The clear distinction among different genera was also detected in fossil forms of Spatangus, Propespatagus nov. gen., and Sardospatangus (†) from the European Oligo-Miocene sedimentary rocks of Germany; the Miocene of Ukraine, Italy, and North Africa; the Plio-Pleistocene of Italy; and the Mio-Pliocene of Florida (USA). The new data can help in addressing taxonomic ambiguities within echinoids, as well as in improving species identification, and hence biodiversity assessments in the Mediterranean region

Ultra-High Field Magnetic Resonance for human applications. A novel splittable design of degenerate birdcage with integrated Tx/Rx switches and Butler matrix.

In UHFMRI the use of high static magnetic fields (e.g. 7T), can provide a substantial increase in the SNR and a consequent reduction in scan time or improved spatial resolution, due to the increased polarization of the sample. However, with increasing intensity of the B0 field the frequency of the B1 RF field increases proportionally, reducing the RF wavelength to dimensions comparable to those of the sample (~0.1 m). This shortening of the wavelength in tissue leads to a number of challenges, especially for relatively large coils necessary to image the head or the body, such as inhomogeneities of B1 transmit field and flip angle, decreased power efficiency, as well as increased SAR. In general, these problems are mitigated using multi-channel RF coils, by means of careful RF coil design on the hardware side, and software-related techniques such as RF shimming and pTx on the software side. In both cases, the use of a multi-channel transmit coil is required. The aim of this thesis is the development of a multi-channel RF coil for UHFMRI applications on the human limbs. Several critical issues to be overcome are shown. The chosen design is a splittable, 8-channel degenerate birdcage with dedicated Tx/Rx switches fed by a Butler matrix. All this hardware components are developed in the framework of this thesis. The performance of this coil is demonstrated by simulation, workbench measurements and scanner measurements. An UHFMRI scanner operating at 7 Tesla is currently available at the IMAGO7 Foundation (Pisa) which promotes the research in this field and allows for in-magnet coil tests and measurements. After a brief introduction about the physics of MRI and its application to imaging, the hardware of the system will be discussed, with main focus on RF coils. Next, the design of the three main components of this coil is discussed together with workbench and scanner results. Finally, the measurements carried out with the fully assembled prototype are presented, proving its good performance in terms of efficiency, B1+ homogeneity and SNR. The first in vivo images of the knee are also shown

Evaluation of 3D radio-frequency electromagnetic fields for any matching and coupling conditions by the use of basis functions

A procedure for evaluating radio-frequency electromagnetic fields in anatomical human models for any matching and coupling conditions is introduced. The procedure resorts to the extraction of basis functions: such basis functions, which represent the fields produced by each individual port without any residual coupling, are derived through an algebraic procedure which uses the S parameter matrix and the fields calculated in one (only) full-wave simulation. The basis functions are then used as building-blocks for calculating the fields for any other S parameter matrix. The proposed approach can be used both for volume coil driven in quadrature and for parallel transmission configuration

Local SAR in adults and children at 7T MR: Realistic estimation by the using of simulations

The evaluation of the local Specific Absorption Rate (SAR) is a major concern in ultra high field (UHF) Magnetic Resonance (MR) systems. In fact, at UHF, the energy deposition due to the radio-frequency (RF) field increases and its distribution inside the subject becomes extremely inhomogeneous and subject dependent. Local SAR measurements are not available in present MR systems; thus, electromagnetic simulations must be performed for RF fields and SAR analysis. In this study we resort to 3D full wave numerical electromagnetic simulations for investigating the dependence of the local SAR at 7 T with respect to the subject size in two different scenarios: i) surface coil loaded by adults and children calves; ii) volume coil loaded by adults and children heads. Concerning the first scenario, it has been found that the maximum local SAR decreases with decreasing load size: this holds true if the RF magnetic fields (B-1(+)) for the different load sizes are scaled so to achieve the same B-1(+) slice average value

Analytical theory, circuit and numerical simulations to design a splittable degenerate birdcage for MSK applications

The degenerate birdcage is not a common design for ultra-high field transmit array due to the technical difficulties in its construction, such as the interdependence of tuning and degeneracy on the value of capacitors. We present here a combination of an analytical theory, circuit simulations and numerical simulations to be used for an efficient design and construction of the degenerate birdcage at 7T. We demonstrate satisfactory performance in terms of decoupling, B1+ homogeneity and B1+ efficiency on the workbench and with scanner measurements on phantoms and human volunteers

SAR prediction in adults and children by combining measured B1+ maps and simulations at 7.0 Tesla

Purpose: To predict local and global specific absorption rate (SAR) in individual subjects. Materials and Methods: SAR was simulated for a head volume coil for two imaging sequences: axial T1-weighted "zero" time-of-echo (ZTE) sequence, sagittal T2-weighted fluid attenuated inversion recovery (FLAIR). Two head models (one adult, one child) were simulated inside the coil. For 19 adults and 27 children, measured B1+ maps were acquired, and global (head) SAR estimated by the system was recorded. We performed t-test between the B1+ in models and human subjects. The B1+ maps of individual subjects were used to scale the SAR simulated on the models, to predict local and global (head) SAR. A phantom experiment was performed to validate SAR prediction, using a fiberoptic temperature probe to measure the temperature rise due to ZTE scanning. Results: The normalized B1+ standard deviation in subjects was not significantly different from that of the models (P > 0.68 and P > 0.54). The rise in temperature generated in the phantom by ZTE was 0.3°C; from the heat equation it followed that the temperature-based measured SAR was 2.74 W/kg, while the predicted value was 3.1 W/kg. Conclusion: For ZTE and FLAIR, limits on maximum local and global SAR were met in all subjects, both adults and children. To enhance safety in adults and children with 7.0 Tesla MR systems, we suggest the possibility of using SAR prediction

Magnetic resonance imaging at 7 Tesla with dedicated radiofrequency coils :Application to cervical cord and knee

Magnetic Resonance (MR) Imaging is a valuable tool in the diagnosis and monitoring of various musculoskeletal pathologies. New Ultra-High Field (UHF) 7 T MRI systems, with their enhanced Signal-to- Noise Ratio, may offer increased image quality in terms of spatial resolution and/or shorter scanning time compared to lower field systems. However, these benefits can be difficult to obtain because of increased radio-frequency (RF) inhomogeneity, increased Specific Absorption Rate (SAR) and the relative lack of specialized and commercially available RF coils compared to lower field systems. This study reports the feasibility of imaging in bones and cartilages at UHF with a 7 T MR scanner available at the IMAGO7 Foundation (Pisa, Italy). Dedicated radio-frequency coils for proton imaging have been designed, developed, optimized for different anatomical regions and validated in vivo, and are now ready for clinical research studies. The performance of the RF coil prototypes in targeting different anatomical regions are also demonstrated, obtaining images of the neck (the cervical cord) and of the knee (trabecular bone and cartilages)

A Degenerate Birdcage with Integrated Tx/Rx Switches and Butler Matrix for the Human Limbs at 7 T

The theoretically known degeneracy condition of the band-pass birdcage coil has rarely been exploited in transmit coil designs. We have created an eight-channel degenerate birdcage for the human limbs at 7 T, with dedicated Tx/Rx switches and a Butler matrix. The coil can be split into two half cylinders, as required for its application to patients with limited mobility. The design of the coil, the Butler matrix, and Tx/Rx switches relied on a combination of analytical, circuital, and numerical simulations. The birdcage theory was extended to the degenerate case. The theoretical and practical aspects of the design and construction of the coil are presented. The performance of the coil was demonstrated by simulations, workbench, and scanner measurements. The fully assembled prototype presents good performance in terms of efficiency, B1 homogeneity, and signal-to-noise ratio, despite the asymmetry introduced by the splittable design. The first in vivo images of the knee are also shown. A novel RF coil design consisting of an eight-channel splittable degenerate birdcage has been developed, and it is now available for 7 T MRI applications of the human lower limbs, including high-resolution imaging of the knee cartilages and of the patellar trabecular structure