Development of a New Multi-Channel MRI Coil Optimized for Brain Studies in Human Neonates

RÉSUMÉ Plusieurs événements et conditions indésirables causent des lésions cérébrales chez les nouveau-nés qui peuvent conduire plus tard à des troubles neurodéveloppementaux. Des études d'imagerie rapides, non invasive et de haute qualité sont nécessaires pour initier un traitement neuroprotecteur précoce et minimiser les effets néfastes sur ces patients. L'imagerie par résonance magnétique (IRM) est une méthode de choix pour détecter ces lésions et évaluer le développement du cerveau in vivo. Les systèmes d'IRM comprennent des antennes spécifiques qui permettent d’interagir avec l'objet étudié au moyen de signaux radiofréquences (RF). Ces antennes jouent un rôle important sur la qualité d'image résultante et donc sur notre capacité à détecter des pathologies subtiles. Plus les antennes sont proches du tissu à imager, meilleure est la qualité d’image. Le but de ce travail était de développer une nouvelle antenne de réception IRM qui peut s'adapter physiquement à la taille de la tête des nouveau-nés dans la gamme de prématurés de 24 semaines à des bébés de 1.5 mois. L'antenne est constituée de treize éléments répartis de manière sphérique, fixés individuellement à un soufflet en plastique compressible, qui peuvent se déplacer de manière indépendante dans des directions radiales et axiales. Un système pneumatique les rétracte au moyen d'un vide, en maximisant l'espace à l'intérieur de l'antenne pour faciliter le placement du sujet. Le vide est ensuite libéré pour permettre l'expansion du soufflet et le mouvement des éléments vers le centre de l'antenne jusqu'à ce qu'ils s'adaptent physiquement à la forme de la tête. La simulation électromagnétique a aidé le processus de conception, révélant la faisabilité de l'idée proposée. Un découplage efficace à l’aide de préamplificateurs a garanti les niveaux requis de découplage global entre les canaux de l’antenne. La validation a été effectuée sur le banc d’essai et sur une IRM 3T en utilisant différents fantômes en forme de tête. Les résultats démontrent une augmentation moyenne de rapport signal-à-bruit (SNR) de jusqu'à 68% dans la région de la tête et 122% dans la région du cortex, par rapport à une antenne commerciale de tête à 32 canaux. La distribution du SNR est stable pour toutes les tailles de fantômes utilisés. En conclusion, une antenne de réception a été conçue, modélisée puis construite. Cette antenne est adaptable avec contrôle pneumatique, ce qui a permis un SNR plus élevée par rapport à une antenne de tête commerciale à 32 canaux utilisée normalement dans la pratique clinique.----------ABSTRACT Several adverse events and conditions cause brain injury in neonates that can later lead to neurodevelopmental disabilities. Fast, non-invasive and high-quality image studies are required to initiate early neuroprotective treatment and minimize adverse effects on these patients. Magnetic Resonance Imaging (MRI) is a vital method to detect these injuries and assess brain development in vivo. The MRI systems include specific types of antennas, commonly known as radiofrequency (RF) coils, to interact with the object under study by means of RF signals. These coils play a strong role on the resulting image quality and hence on our ability to detect subtle pathologies. The closer the coils are to the scanned tissue, the better the image quality. The purpose of this work was to develop a new MRI RF receiver array coil that can physically adapt to infant head sizes from 24-week premature to 1.5-month-old. The coil is made of thirteen spherically distributed elements, individually attached to compressible plastic bellows, that can independently move in radial and axial directions. A pneumatic system retracts them by means of vacuum, maximizing the space inside the coil to facilitate the placement of the subject. The vacuum is afterward liberated to allow the expansion of the bellows and the movement of the elements toward the coil center until they physically adapt to the head shape. Electromagnetic simulation assisted the design process, revealing the feasibility of the proposed idea. A strong preamplifier decoupling guaranteed the required levels of overall decoupling among the coil elements. The validation was performed on the bench and on a 3T scanner using different head-shaped phantoms. The results show up to up to 68% in the head region and 122% in the cortex region, compared to a 32-channel commercial head coil. A stable SNR distribution through the complete size range was also obtained for all the used phantoms. In conclusion, an MRI receiver coil was designed, modeled, and built. The coil is adaptable with pneumatic control, which allowed a higher SNR compared to a commercial 32-channel head coil used normally in clinical practice. The risks associated with mechanical pressure on the head of newborns are non-existent (use of negative versus positive pressure) and the head motion is restricted. In addition, the method has potential applications to other age groups and body parts

An eight‐channel Tx dipole and 20‐channel Rx loop coil array for MRI of the cervical spinal cord at 7 tesla

RÉSUMÉ: 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

Design and construction of an optimized transmit/receive hybrid birdcage resonator to improve full body images of medium-sized animals in 7T scanner.

The purpose of this work was to develop an optimized transmit/receive birdcage coil to extend the possibilities of a 7T preclinical MRI system to conduct improved full body imaging in medium-sized animals, such as large New Zealand rabbits. The coil was designed by combining calculation and electromagnetic simulation tools. The construction was based on precise mechanical design and careful building practice. A 16-leg, 20 cm long, 16 cm inner diameter, shielded quadrature hybrid structure was selected. Coil parameters were assessed on the bench and images were acquired on phantoms and rabbits. The results were compared to simulations and data obtained with an available commercial coil. An inexpensive assembly with an increase of 2 cm in useful inner diameter and 50 Ω matching with larger animals was achieved. A reduction in radiofrequency (RF) power demand of 31.8%, an improvement in image uniformity of 18.5 percentage points and an increase in signal-to-noise ratio of up to 42.2% were revealed by phantom image acquisitions, which was confirmed by in vivo studies. In conclusion, the proposed coil extended the possibilities of a preclinical 7T system as it improved image studies in relatively large animals by reducing the RF power demand, and increasing image uniformity and signal-to-noise ratio. Shorter scans and time under anesthesia or reduced RF exposure, resulting in better images and lower animal health risk during in vivo experiments, were achieved

Rabbit acquisitions.

<p><b>(a, b)</b> Axial TrueFISP slices of different rabbits showing the improved uniformity achieved with the proposed coil. <b>(c, d)</b> Coronal SNR maps with selected rectangular regions of interest and corresponding mean values. The higher increase in SNR is visible at the ends of the coil.</p