6 research outputs found
An eightâchannel Tx dipole and 20âchannel Rx loop coil array for MRI of the cervical spinal cord at 7 tesla
RĂSUMĂ: The quality of cervical spinal cord images can be improved by the use of tailored radiofrequency (RF) coil solutions for ultrahigh field imaging; however, very few commercial and research 7-T RF coils currently exist for the spinal cord, and in particular, those with parallel transmission (pTx) capabilities. This work presents the design, testing, and validation of a pTx/Rx coil for the human neck and cervical/upper thoracic spinal cord. The pTx portion is composed of eight dipoles to ensure high homogeneity over this large region of the spinal cord. The Rx portion is made up of twenty semiadaptable overlapping loops to produce high signal-to-noise ratio (SNR) across the patient population. The coil housing is designed to facilitate patient positioning and comfort, while also being tight fitting to ensure high sensitivity. We demonstrate RF shimming capabilities to optimize B1+ uniformity, power efficiency, and/or specific absorption rate efficiency. B1+ homogeneity, SNR, and g-factor were evaluated in adult volunteers and demonstrated excellent performance from the occipital lobe down to the T4-T5 level. We compared the proposed coil with two state-of-the-art head and head/neck coils, confirming its superiority in the cervical and upper thoracic regions of the spinal cord. This coil solution therefore provides a convincing platform for producing the high image quality necessary for clinical and research scanning of the upper spinal cord
Development of an Innovative Solution Minimizing RF Field Inhomogeneity and Energy Deposition in Ultra-High Field MRI Applications
Bien quâapportant de nombreux bĂ©nĂ©fices en termes de qualitĂ© dâimage, lâIRM Ă trĂšs hauts champ est particuliĂšrement sensible aux artefacts liĂ©s Ă la rĂ©duction de la longueur dâonde du champ B1+ nĂ©cessaire pour basculer les spins hors de leur position dâĂ©quilibre. Cette longueur dâonde devenant plus petite que la plupart des rĂ©gions du corps imagĂ©, cela induit une inhomogĂ©nĂ©itĂ© spatiale de lâangle de bascule qui, Ă son tour, entraine des variations dâintensitĂ© dans lâimage finale. Afin de rĂ©duire ces inhomogĂ©nĂ©itĂ©s, des antennes Ă©quipĂ©es de plusieurs transmetteurs pouvant ĂȘtre excitĂ©s en parallĂšle deviennent de plus en plus populaires pour lâimagerie Ă trĂšs haut champ, car la possibilitĂ© dâenvoyer diffĂ©rentes impulsions Ă chaque transmetteur offre un meilleur contrĂŽle sur les interfĂ©rences radiofrĂ©quences. Ce procĂ©dĂ© dâhomogĂ©nĂ©isation de champs est appelĂ© B1+ shimming (ou RF shimming) et requiĂšre du matĂ©riel et des logiciels sophistiquĂ©s, ainsi que temps de scan supplĂ©mentaire, ce qui peut ralentir son intĂ©gration dans le milieu clinique.
Dans des rĂ©gions telles que la moelle Ă©piniĂšre, oĂč les tissus prĂ©sentent dâimportantes variations en termes de propriĂ©tĂ©s Ă©lectromagnĂ©tiques, cela devient encore plus difficile dâobtenir un champ B1+ homogĂšne. La profondeur de la moelle Ă©piniĂšre Ă lâintĂ©rieur du corps peut elle aussi limiter la puissance du champ B1+, rĂ©sultant en un faible signal RM dans cette rĂ©gion. De plus, lâimportante variabilitĂ© anatomique de la rĂ©gion vertĂ©brale dâun patient Ă un autre vient elle aussi complexifier la crĂ©ation dâune impulsion qui rĂ©sulterait en un angle de bascule spatialement homogĂšne pour diffĂ©rents sujets.
Lâobjet de cette maĂźtrise consistait Ă dĂ©velopper un logiciel en libre-accĂšs dĂ©diĂ© au B1+ shimming, couvrant les scenarios les plus couramment utilisĂ©s et prenant en compte la dĂ©position dâĂ©nergie dans les tissus afin dâassurer la sĂ©curitĂ© du patient. La comptabilitĂ© avec des outils de segmentation automatique du cerveau et de la moelle Ă©piniĂšre a Ă©galement Ă©tait implĂ©mentĂ©e afin de pouvoir effectuer un B1+ shimming ciblant ces rĂ©gions spĂ©cifiques. Cette implĂ©mentation a Ă©tĂ© intĂ©grĂ©e au projet Shimming-Toolbox, initialement dĂ©veloppĂ© homogĂ©nĂ©iser le champ B0.
Un test in-vivo a par la suite Ă©tĂ© effectuĂ© dans la moelle Ă©piniĂšre Ă 7T et a montrĂ© une amĂ©lioration de lâhomogĂ©nĂ©itĂ© le long de la moelle Ă©piniĂšre aprĂšs B1+ shimming sur des images anatomique GRE et MP2RAGE, ainsi quâune augmentation du signal dans la rĂ©gion thoracique.----------While it offers important image quality benefits, Ultra-High Field MRI is particularlysensitive to RF related artifacts caused by the decreasing wavelength of the B1+ field required to flip the spins. This wavelength being smaller than most imaged body regions, it results in an inhomogeneous flip angle that induces intensity variations and local loss of signal across the image field of view. To reduce these inhomogeneities, multi-transmit coils with parallel transmission capability are becoming increasingly popular in UHF imaging, as they allow one to send different excitation pulses to each Tx element, providing better control of the RF interference pattern. This
homogenization process is called B1+ shimming (or RF shimming) and requires complex hardware and software tools that hamper its clinical use. In regions such as the spinal cord, where surrounding tissues present important variations in terms of electromagnetic properties, it gets even harder to obtain a homogeneous B1+ field. The depth of the spinal cord in the body may also hamper the generation of a sufficiently strong B1+ field in that region, resulting in a low MR signal. Moreover, the important anatomical variability of the spine region across patients further complicates the design of an excitation pulse that would result in robust inter-subject flip angle homogeneity. For these reasons, it is expected that
performing patient-specific B1+ shimming with a focus on the spinal cord could improve the image homogeneity.
The focus of my masterâs project was on the development of an open-source B1+ shimming software solution that covers the most basic shimming scenarios and accounts for energy deposition in tissues, so as to ensure patient safety. Compatibility with brain and spinal cord segmentation
tools was also implemented so that localised B1+ shimming could be performed over specific regions. This B1+ shimming implementation was integrated within the Shimming-Toolbox project, initially developed to homogenize the main static magnetic field B0.
It was then tested in-vivo to perform patient specific B1+ shimming during spinal-cord imaging at 7T and resulted in an improved homogeneity in the spinal-cord on structural GRE and MP2RAGE images, with coefficients of variation reduced by up to 40% and 11% respectively, as well as in a recovered signal in the thoracic spinal cord