22 research outputs found
Design and Development of Radio Frequency Coils for Sodium Magnetic Resonance Imaging at 7 T
No full textThe 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
Experimental Setup to Compare Measurements and Numerical Simulations in Magnetic Resonance Imaging RF Dosimetry
No full textScelta intertemporale e autocontrollo in bambini di età prescolare. Presentazione in Simposio: Equità e scelta intertemporale: aspetti evolutivi, evoluzionistici e sociali nella presa di decisione.
No full textNumerical and Workbench Design of 2.35 T Double-Tuned (1H/23Na) Nested RF Birdcage Coils Suitable for Animal Size MRI
No full textThe birdcage Radio Frequency (RF) coil is one of the most used configurations in Magnetic Resonance Imaging (MRI) scanners for the detection of the proton (1H) signal over a large homogeneous volume. More recently, birdcage RF coils have been successfully used also in the field of X-nuclei MRI, where the signal of a second nucleus (e.g. 13C, 23Na, 31P, and many others) needs to be detected with high sensitivity and spatial homogeneity. To this purpose several technical solutions have been adopted to design Double Tuned (DT) volume RF coils, including the recent configuration of the nested birdcage RF coils. One of the main problems in the design of DT RF coils is the decoupling between the 1H and X channels, and a number of solutions have been adopted over the years. In this work, based on numerical and workbench methods, we report the decoupling optimization of DT (1H/23Na) nested RF birdcage coils suitable for 2.35 T MRI scanners encompassing an inner Low-Pass (LP) birdcage used for X-nuclei, an outer High-Pass (HP) birdcage for 1H and an external cylindrical RF shield. We show that a suitable geometrical selection of the two coaxial RF birdcage coils (relative angular orientation, diameters and lengths) and RF shield (diameter, length) allows a significant decoupling optimization. We also provide valuable information about the RF B1+ field homogeneity and efficiency. Our approach was validated both with numerical simulations and workbench testing using DT nested RF coil prototypes
Sei goloso o impaziente? Effetto del tipo di ricompensa in una prova di scelta intertemporale con bambini di età prescolare.
No full textLa capacità di inibire una risposta consolidata è una componente importante delle funzioni esecutive, coinvolte nell’ADHD (Cornoldi et al. 2010). Nella prova di scelta intertemporale, in cui il soggetto deve scegliere tra una ricompensa minore e immediata (SS) ed una maggiore e ritardata (LL) (Mazur, 1988; Moore, Barresi and Thompson, 1998; Thompson, Barresi and Moore, 1997), benché la scelta dell’opzione LL venga considerata un indice di tolleranza al ritardo, tale preferenza potrebbe essere piuttosto una risposta impulsiva verso la quantità.
Abbiamo testato 151 bambini di età prescolare in un compito di scelta intertemporale con 2 vs. 6 pezzi di cibo, low-symbolic tokens (carte raffiguranti 2 vs. 6 pallini) e high-symbolic tokens (carte raffiguranti un topolino e un elefante). I simboli migliorano la capacità inibitoria poiché permettono di distogliere l’attenzione dalle caratteristiche degli stimoli che scatenano risposte impulsive (Sigel 1970; Werner et al., 1963; Mischel et al., 1989). Due sono le ipotesi testate: (i) Tolleranza del ritardo: se la scelta di LL fosse dovuta ad autocontrollo, gli stimoli simbolici, riducendo l’attrattiva della ricompensa, dovrebbero incrementare il numero di scelte per LL, con un maggiore impatto degli high-symbolic tokens rispetto ai low-symbolic tokens (%LL: High-Symb > LowSymb > Food); (ii) Fallimento nell’inibizione: se la scelta di LL fosse dovuta all’incapacità di inibire la preferenza per la quantità, allora gli high-symbolic tokens dovrebbero ridurre la percentuale di scelte di LL, mentre i low-symbolic tokens non dovrebbero produrre alcun effetto, poiché mantengono inalterata la salienza percettiva della ricompensa (%LL: Food ~ LowSymb > HighSymb)
Are you greedy or impatient? Effect of reward type on intertemporal choice in preschool children.
No full textScelta intertemporale e autocontrollo in bambini di eta’ prescolare. Scegliere il futuro: prospettive interdisciplinari sulle scelte intertemporali
No full textDouble-Tuned Birdcage Radio Frequency Coil for 7 T MRI: Optimization, Construction and Workbench Validation
No full textThe 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
False belief understanding and "cool" inhibitory control in 3-and 4-year-old Italian children.
No full textDuring preschool years, major developments occur in both executive function and theory of mind (ToM), and several studies have demonstrated a correlation between these processes. Research on the development of inhibitory control (IC) has distinguished between more cognitive, "cool" aspects of self-control, measured by con?ict tasks, that require inhibiting an habitual response to generate an arbitrary one, and "hot," affective aspects, such as affective decision making, measured by delay tasks, that require inhibition of a prepotent response. The aim of this study was to investigate the relations between 3- and 4-year-olds\u27 performance on a task measuring false belief understanding, the most widely used index of ToM in preschoolers, and three tasks measuring cognitive versus affective aspects of IC. To this end, we tested 101 Italian preschool children in four tasks: (a) the Unexpected Content False Belief task, (b) the Con?ict task (a simpli?ed version of the Day-Night Stroop task), (c) the Delay task, and (d) the Delay Choice task. Children\u27s receptive vocabulary was assessed by the Peabody Picture Vocabulary test. Children\u27s performance in the False Belief task was signi?cantly related only to performance in the Con?ict task, controlling for vocabulary and age. Importantly, children\u27s performance in the Con?ict task did not signi?cantly correlate with their performance in the Delay task or in the Delay Choice task, suggesting that these tasks measure different components of IC. The dissociation between the Con?ict and the Delay tasks may indicate that monitoring and regulating a cool process (as ?exible categorization) may involve different abilities than monitoring and regulating a hot process (not touching an available and highly attractive stimulus or choosing between a smaller immediate option and a larger delayed one). Moreover, our ?ndings support the view that "cool" aspects of IC and ToM are interrelated, extending to an Italian sample of children previous ?ndings on an association between self-control and ToM
