Abstract
In magnetic resonance imaging (MRI), several studies have demonstrated that the metamaterial-based structures can effectively improve the sensitivity, and thus the signal-to-noise ratio (SNR), of receiving radio-frequency (RF) coils. However, the use of metamaterials for this type of the MRI application is often limited due to the bulkiness of the metamaterial structure at RF wavelengths and a lack of frequency tunability of the final design. In this work, we propose a planar compact sub-wavelength (<; λ/50) spiral resonator to increase the sensitivity of a receive coil with frequency tunability for the 1.5 T MRI. Its double-layered spiral design with a cavity embedded in the substrate between the two spirals allows water deposition for frequency tuning. At the resonance frequency of 64 MHz, the spiral resonator shows a 24% improvement in terms of the B 1 - field at the depth of 30mm into a load experimentally. Even at a penetration depth as much as 60 mm (deep brain in the case of head imaging), an enhancement of 9% was observed. Moreover, the magnetic field enhancement comes with a decrease (10%) in specific absorption rate (SAR). In terms of tuning, by controlling the water level in the cavity, the proposed spiral resonator shows a wide tuning range of 35MHz, centered around 64 MHz, with high tunability sensitivity (2.4-0.75 MHz/ml or 15-4.8 MHz/mm), which is due to the fact that the tuning cavity is located between the two spirals, where the fields are highly confined.
Highlights
Magnetic resonance imaging (MRI) is one of the most widely used imaging techniques in modern healthcare for disease diagnosis as well as fundamental research [1]
In this work, motivated by metamaterial-based resonators, we propose a compact highly sub-wavelength (< λ/50) water-tunable spiral resonator working with a receive coil for B1 field enhancement for a 1.5 T MRI scanner
We have successfully demonstrated the frequency tunability of a highly sub-wavelength spiral resonator designed for a receive loop coil for 1.5 T MRI
Summary
Magnetic resonance imaging (MRI) is one of the most widely used imaging techniques in modern healthcare for disease diagnosis as well as fundamental research [1] It provides advantages over other imaging techniques such as computed tomography (CT) and X-ray in terms of a high soft tissue contrast and the absence of ionizing radiation. Increasing the B0 field (into the ultra-high field range > 4 T) has several technological as well as safety challenges They include the considerably increased hardware complexity and cost, interference patterns formation and a reduced penetration depth of RF coils due to a shorter wavelength in tissues, which leads to a decreased coil sensitivity [4], increased patients’ discomfort due to a more pronounced magnetohydrodynamic effect [7], [8], and safety concerns such as increased RF exposure and potential tissue heating due to local peaks of inhomogeneous B1-fields. There is a need to improve SNR in an MRI scanner without increasing B0 field [9]
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