Design and Construction of a Highly Sensitive Coil for MRI of the Spinal Cord

RÉSUMÉ Un grand nombre de pathologies (sclérose en plaque, lésions, etc.) peuvent toucher la moelle épinière, des techniques non invasives de diagnostic tel que l’imagerie par résonance magnétique (IRM) sont généralement utilisées pour les dépister. Les antennes commerciales pour l’IRM sont conçues pour accommoder une large population, mais elles ne sont pas optimisées pour le rapport signal sur bruit (S/B). L'objectif principal de ce mémoire a été de concevoir et de construire une antenne radiofréquence (RF) en réseau phasé avec six bobines en réception pour l’IRM de la moelle épinière cervicale chez des sujets humains. La configuration optimale de l’antenne avec six canaux a été déterminée à l'aide de simulations électromagnétiques pour modéliser l’antenne en réception. L’antenne a été conçue et construite pour s’ajuster au plus près du sujet humain tout en étant compatible avec l'interface du scanner IRM. Les performances de l’antenne ont été évaluées sur le banc à l'aide d'un fantôme, ainsi que dans l'IRM sur des sujets humains. Les résultats montrent une amélioration moyenne du rapport S/B par un facteur 2 par rapport à l’antenne commerciale. Cette amélioration permet d’avoir une haute résolution qui facilite la représentation des fins détails comme les petites lésions présentes dans la sclérose en plaques. De plus, la géométrie optimisée de l’antenne permet d'utiliser de hauts facteurs d'accélération (par exemple 3), réduisant considérablement le temps d'acquisition. Pour conclure, l’antenne en réseau phasé avec six bobines pourrait servir à l'imagerie anatomique de haute résolution (0,3 mm dans le plan), l'IRM fonctionnelle (IRMf), IRM de diffusion et dans études par spectroscopie pour caractériser le métabolisme des tissus présents dans la moelle épinière et les sections inférieures du cerveau.----------ABSTRACT Spinal cord injuries affect a large number of people, therefore a non-invasive technique such as magnetic resonance imaging (MRI) can be used for diagnosis purposes. While current commercial coils are designed to fit a diverse population, they are not optimized for signal-to-noise ratio (SNR). The major objective of this thesis was to design and construct a six-channel radio-frequency (RF) receive-only phased array coil for MRI of the cervical spinal cord in human subjects. The optimal configuration of the six-channel coil array was determined using electromagnetic simulations framework for modelling the array behavior in the receiving mode. The design and construction of the coil array were focused on offering a tight fit of the human subject, while being compatible with the scanner interface. The RF coil performances were evaluated at the bench using a phantom. Furthermore, it was validated in the MRI on human subjects. The results show an average improvement in SNR by a factor of two compared to the commercial coil. This enhancement enables higher resolution and therefore better depiction of small pathologies such as small lesions in multiple sclerosis. Moreover, the optimized geometry of the RF coil enables the use of aggressive acceleration factors (e.g., 3), which reduces significantly the acquisition time. In conclusion, the six-channel coil array could be used in high resolution anatomical imaging (0.3mm in-plane), functional MRI (fMRI), diffusion tensor imaging (DTI) and spectroscopy studies for characterizing the metabolism of different tissues present in the spinal cord and lower brain sections

Radiotherapy immobilization mask molding through the use of 3D-printed head models

PURPOSE: To evaluate the feasibility of a workflow free of a simulation appointment using three-dimensional-printed heads and custom immobilization devices. MATERIALS AND METHODS: Simulation computed tomography scans of 11 patients who received radiotherapy for brain tumors were used to create three-dimensional printable models of the patients' heads and neck rests. The models were three-dimensional-printed using fused deposition modeling and reassembled. Then, thermoplastic immobilization masks were molded onto them. These setups were then computed tomography-scanned and compared against the volumes from the original patient computed tomography-scans. Following translational +/- rotational coregistrations of the volumes from three-dimensional-printed models and the patients, the similarities and accuracies of the setups were evaluated using Dice similarity coefficients, Hausdorff distances, differences in centroid positions, and angular deviations. Potential dosimetric differences secondary to inaccuracies in the rotational positioning of patients were calculated. RESULTS: Mean angular deviation of the 3D-printout from the original volume for the Pitch, Yaw, and Roll were 1.1 degrees (standard deviation = 0.77 degrees ), 0.59 degrees (standard deviation = 0.41 degrees ), and 0.79 degrees (standard deviation = 0.86 degrees ), respectively. Following translational + rotational shifts, the mean Dice similarity coefficients of the three-dimensional-printed and original volumes was 0.985 (standard deviation = 0.002) while the mean Hausdorff distance was 0.9 mm (standard error of the mean: 0.1 mm). The mean centroid vector displacement was 0.5 mm (standard deviation: 0.3 mm). Compared to plans that were coregistered using translational + rotational shifts, the D95 of the brain from three-dimensional-printed heads adjusted for TR shifts only differed by -0.1% (standard deviation = 0.2%). CONCLUSIONS: Patient head volumes and positions at simulation computed tomography scans can be accurately reproduced using three-dimensional-printed models, which can be used to mold radiotherapy immobilization masks onto. This strategy, if applied on diagnostic computed tomography scans, may allow symptomatic and frail patients to avoid a computed tomography-simulation and mask molding session in preparation for palliative whole brain radiotherapy

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

Electrical design of the coil.

<p><b>(a)</b> Schematic design with actual component values. The interfacing circuit of channel <b><i>Q</i></b> and the tuning capacitor of channel <b><i>I</i></b> (1-23p) are shown. The bold lines represent copper traces conforming the resonator. <b>(b)</b> PCB design with zoomed panel showing one parallel plate capacitor in transparency (black circle). The blue and red regions represent the copper sections on the inner and outer surfaces, respectively. <b>(c)</b> Picture of the resonator showing interfacing circuits (<b><i>Q</i></b> and <b><i>I</i></b>) and cable traps. <b>(d)</b> PCB design of the shield (one of four sections). The blue and red traces represent the gaps on the inner and outer copper surfaces, respectively. <b>(e)</b> Picture showing outer surface of the shield. The arrows show copper regions created to improve RF isolation.</p

Phantom acquisitions at the isocenter.

<p><b>(a, b)</b> SNR maps, computed from the Gradient Echo Multi Slice (GEMS) images of the mineral oil phantom, showing better uniformity and reduced SNR with the proposed coil. The small white circles show the areas with maximum and minimum pixel intensities. The subregions of interest (10x10pixels) used to calculate the uniformity were selected inside these circles. <b>(c, d)</b> SNR maps calculated from Fast Spin Echo Multiple Slice images of the water/NaCl/sugar phantom, showing increased SNR (41.8%) with the proposed coil in a selected circular ROI. The numbers in white are the corresponding average SNR computed inside these ROIs.</p