Image acceleration in parallel magnetic resonance imaging by means of metamaterial magnetoinductive lenses

📝 Abstract

Parallel magnetic resonance imaging (MRI) is a technique of image acceleration which takes advantage of the localization of the field of view (FOV) of coils in an array. In this letter we show that metamaterial lenses based on capacitively-loaded rings can provide higher localization of the FOV. Several lens designs are systematically analyzed in order to find the structure providing higher signal-to-noise-ratio. The magnetoinductive (MI) lens is find to be the optimum structure and an experiment is developed to show it. The ability of the fabricated MI lenses to accelerate the image is quantified by means of the parameter known in the MRI community as g-factor.

💡 Analysis

Parallel magnetic resonance imaging (MRI) is a technique of image acceleration which takes advantage of the localization of the field of view (FOV) of coils in an array. In this letter we show that metamaterial lenses based on capacitively-loaded rings can provide higher localization of the FOV. Several lens designs are systematically analyzed in order to find the structure providing higher signal-to-noise-ratio. The magnetoinductive (MI) lens is find to be the optimum structure and an experiment is developed to show it. The ability of the fabricated MI lenses to accelerate the image is quantified by means of the parameter known in the MRI community as g-factor.

📄 Content

arXiv:1112.2276v1 [physics.med-ph] 10 Dec 2011 Image acceleration in parallel magnetic resonance imaging by means of metamaterial magnetoinductive lenses Manuel J. Freire1, Marcos A. Lopez1, Jose M. Algarin1, Felix Breuer2, and Ricardo Marqu´es1 1Department of Electronics and Electromagnetism, Faculty of Physics, University of Seville, Avda. Reina Mercedes s/n, 41012 Sevilla, Spain ∗and 2Research Center Magnetic Resonance Bavaria (MRB), W¨urzburg, Germany (Dated: October 30, 2018) Abstract Parallel magnetic resonance imaging (MRI) is a technique of image acceleration which takes advantage of the localization of the field of view (FOV) of coils in an array. In this letter we show that metamaterial lenses based on capacitively-loaded rings can provide higher localization of the FOV. Several lens designs are systematically analyzed in order to find the structure providing higher signal-to-noise-ratio. The magnetoinductive (MI) lens is find to be the optimum structure and an experiment is developed to show it. The ability of the fabricated MI lenses to accelerate the image is quantified by means of the parameter known in the MRI community as g–factor. ∗Electronic address: freire@us.es 1 One of the most severe limitations of metamaterials for applications is their narrow band response inherent to the resonant nature of the elements that constitute the periodic struc- ture (see [1] and references therein). In Magnetic Resonance Imaging (MRI), MR images are acquired by measuring radiofrequency (RF) magnetic fields in the MHz range inside a relatively narrow bandwidth of a few tens of kilohertz. Therefore, the narrow band response of metamaterials is not a problem for MRI, so that MRI should be then considered one of the most promising field of applications for metamaterials. In addition, since the wavelength associated with RF fields is of the order of the meters, it is possible to use conventional printed circuit techniques to develop quasi-continuous metamaterials with constituent el- ements and periodicities two orders of magnitude smaller than the wavelength. Several works have explored MRI applications of metamaterials [2]–[12] by using different elements to build the periodic structure, such as swiss-rolls [2]–[5], wires [6] and capacitively-loaded rings [7]–[12]. Capacitively-loaded rings have the key advantage over swiss rolls and wires of providing three dimensional (3D) isotropy when they are arranged in a cubic lattice, which is an essential property if the device has to image 3D sources. One of the most striking properties of metamaterials is the ability of a metamaterial slab with relative permittivity ε and relative permeability µ, both equal to −1, to behave as a super-lens with sub-wavelength resolution [13]. In the case of MRI applications, since the frequency of operation is sufficiently low, we are in the realm of the quasi-magnetostatics, and we only need a metamaterial slab with µ = −1 to behave as a super-lens [13]. In pre- vious works, some of the authors showed that a 3D array of capacitively-loaded rings can behave as an effective homogeneous medium with µ = −1 [7]. The authors also explored both theoretically and experimentally the ability of this structure to behave as a super-lens for MRI [8]–[11]. Thus for example, it was shown that in some circumstances, this device can enhance the sensitivity of a single MRI surface coil, as a consequence of its subwavelength focusing properties [8],[9]. As it is well known, the generation of images in MRI is based on the detection of spatial variations in the phase and frequency of the RF waves absorbed and emitted by the nuclear spins of the imaged object. These spatial variations are induced by some static magnetic field gradients and the image acquisition involves many repeated measurements and then signal processing (inverse Fourier transforming) before obtaining an image of a single slice of tissue. Actually, the long acquisition time is the main drawback of MRI in comparison with computerized tomography, and time reduction without degrading 2 the signal-to-noise ratio (SNR) is the main aim of research in the MRI comunity. Image acceleration in MRI is achieved by means of techniques known in general as parallel MRI (pMRI) [15–17]. pMRI works by taking advantage of the spatially sensitive information inherent in a receiving array of multiple surface coils in order to partially replace time- consuming spatial encoding. PILS, SENSE and GRAPPA are examples of these parallel techniques [15–17]. For instance, in the PILS technique [15], it is assumed that each individ- ual coil in the array has a localized sensitivity pattern or field of view (FOV). However, this localization takes place only at distances very close to the array because of the spreading of the magnetic field at farther distances. SENSE and GRAPPA are the commercially avail- able techniques [16, 17]. These techniques are not restricted to linear coil configurations or localized sensitivities. Howe

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