بیشینه‌سازی snr کویل‌‌ rf حلقه‌ای در فانتوم آب از راه تنظیم هندسی و فراسطح

توزیع مکانی نسبت سیگنال به نوفه (snr) کویل‌ بسامد ‌رادیویی (rf) حلقه‌ای تحت تاثیر شعاع و فاصله آن از فانتوم قرار دارد. همچنین افزودن فراسطح به کویل نیز توزیع snr را تغییر می‌دهد. این پژوهش به بررسی کمیت‌های فاصله کویل از فانتوم و شعاع کویل با هدف دستیابی به بیشینه snr برای سه کویل حلقه‌ای با شعاع‌های 30، 60 و 120 میلی‌متر از جنس مس در داخل فانتوم آب می‌پردازد. همچنین سه فراسطح هم شکل و هم جنس با کویل‌ها برای تغییر مطلوب توزیع مکانی snr کویل‌ها استفاده شده است. snr‌ کویل‌ها با بهره‌گیری از شبیه‌سازی الکترومغناطیسی تمام‌موج با استفاده از نرم‌افزار cst تحلیل شده است. نتایج نشان می‌دهد کویل‌ با شعاع 30 میلی‌متر، در فاصله‌ی کمابیش برابر با نصف شعاع خود از فانتوم، بیشینه snr در نواحی نزدیک به کویل در داخل فانتوم ایجاد می‌کند. کویل‌ با شعاع 60 میلی‌متر در کمترین فاصله از فانتوم، بیشینه snr در نواحی میانی فانتوم بدست می‌آورد. کویل‌ با شعاع 120 میلی‌متر نیز در کمترین فاصله از فانتوم، بیشینه snr در نواحی دورتر از فانتوم تولید می‌کند. افزون بر این، نتایج نشان می‌دهد اگر به هر یک از سه کویل، یک فراسطح برابر با ابعاد هر یک از دو کویل دیگر اضافه شود توزیع snr‌ هر یک از دو کویل دیگر بیشینه با اختلاف 3 درصد قابل ایجاد است. این یافته‌ها پتانسیل بهینه‌سازی طراحی کویل‌های rf و همچنین استفاده از فراسطوح به عنوان جایگزینی مقرون به صرفه برای بهبود کیفیت تصویربرداری را نشان می‌دهند.

maximization of loop rf coil snr in the water phantom through geometric tuning and metasurface

1. introductionsignal-to-noise ratio (snr) is a fundamental parameter critically influencing the image quality in magnetic resonance imaging (mri) systems. higher snr leads to clearer and more detailed images, improving diagnostic accuracy. a key factor in optimizing snr is the design and configuration of radio frequency (rf) coils of mri systems. the size and positioning of rf coils relative to the subject significantly affect the spatial snr distribution. specifically, smaller coils tend to provide higher sensitivity, while larger coils cover broader areas, at the cost of reduced sensitivity, and often, coils close to phantoms provide higher snr. also, recent advancements have introduced metasurfaces—engineered electromagnetic structures capable of reshaping rf fields—as a promising approach to tailor snr distributions. this study systematically investigates the effects of coil size and distance from a water phantom on snr, using three different loop-shaped rf coils. additionally, it explores the potential of three distinct metasurfaces to modify coil snr profiles. by pairing specific coils with suitable metasurfaces, we demonstrate that the characteristic snr distribution of one coil can be effectively replicated, proposing an innovative method to optimize image quality cost-effectively. 2. methodologythe study was conducted through full-wave electromagnetic simulations at the operating frequency of an mri system with a field strength of 1.5 tesla, corresponding to a resonant frequency of 63.87 mhz. three copper-made loop coils with radii of 30 mm, 60 mm, and 120 mm and widths of 10 mm, 10 mm, and 20 mm, respectively, were selected due to their widespread application in mri technology. alongside, three metasurfaces designed with the same physical dimensions and copper material as the coils were included. these metasurfaces were passively connected to the coils and tuned to the operational frequency using lumped capacitors, ensuring optimal electromagnetic resonance and interaction with the rf fields. a water phantom measuring 60 × 60 × 60 mm was designed, featuring an electrical permittivity of 78 and a conductivity of 1.59 s/m. this phantom served as the imaging target to realistically assess coil performance. each coil was positioned at multiple distances from the phantom, and spatial snr distributions were recorded using cst simulation software. the data were analyzed to determine the optimal distance for maximum snr for each coil. subsequently, metasurfaces were integrated with each coil—except those matched in size to the metasurface—to replicate the maximum snr of the corresponding optimal coil configuration. this approach allowed evaluation of the metasurfaces’ ability to compensate for coil size differences and improve image quality. 3. results and discussionit was observed that when the coil is brought closer to the phantom, two things happen: first, the strong magnetic field around the coil enhances the snr in the phantom. second, the snr inside the phantom decreases due to the increased field losses caused by the phantom. therefore, the smallest coil (30 mm radius) achieved its maximum snr when positioned at approximately half its radius distance from the phantom. this maximum snr covered roughly the first third of the phantom volume. for the medium (60 mm radius) and large (120 mm radius) coils, the highest snr values were observed when placed closest to the phantom surface. the medium coil’s snr predominated in the middle third of the phantom, while the large coil’s snr peaked near the farthest third. importantly, when a metasurface with dimensions matching those of the size of the two other coils was added to each coil, the resulting snr distributions could recreate those of the original with a maximum deviation of only 3%. this finding underscores the metasurfaces’ ability to effectively tailor the snr spatial profiles and compensate for coil geometry limitations. metasurfaces present a cost-effective and simpler alternative to fabricating new coils. their construction primarily involves copper patterns and lumped capacitors for tuning, and they can be passively integrated without complex electronics or cable connections. in contrast, manufacturing new coils demands significant resources, including detailed electronic board design and system integration. the passive nature of metasurfaces, along with their minimal tuning requirements, makes them attractive for enhancing mri coil performance, especially in resource-constrained environments. therefore, this study highlights the considerable potential of metasurfaces to improve mri imaging quality while reducing costs. their ability to modify snr distributions enables flexible adaptation to imaging requirements without necessitating extensive hardware changes. this cost-efficiency, coupled with performance benefits suggests promising applications for developing advanced, accessible mri technologies. 4. conclusionthis research investigated optimal coil geometries and positions for imaging a wrist-sized water phantom using three different loop coils within a full-wave electromagnetic simulation environment. the small coil, when placed at a distance of about half its radius from the phantom, attained maximum snr at the near region of the phantom, while the medium and large coils at proximity to the phantom maximized snr on the phantom’s middle and far regions, respectively. additionally, the study demonstrated that by adding the appropriate metasurface to each coil, the snr of the other two coils can be reproduced with a maximum difference of 3%. future research could focus on experimental validation of these simulation results by integrating metasurfaces into mri systems and evaluating their practical impact on image quality in clinical settings. further exploration of metasurface design optimization and their integration with varied coil geometries could advance the development of novel, cost-effective imaging solutions, expanding accessibility and diagnostic precision in magnetic resonance imaging.