Abstract
Magnetic resonance imaging (MRI) at ultra-high fields (UHF), such as 7 T, provides an enhanced signal-to-noise ratio and has led to unprecedented high-resolution anatomic images and brain activation maps. Although a variety of radio frequency (RF) coil architectures have been developed for imaging at UHF conditions, they usually are specialized for small volumes of interests (VoI). So far, whole-body coil resonators are not available for commercial UHF human whole-body MRI systems. The goal of the present study was the development and validation of a transmit and receive system for large VoIs that operates at a 7 T human whole-body MRI system. A Metamaterial Ring Antenna System (MRAS) consisting of several ring antennas was developed, since it allows for the imaging of extended VoIs. Furthermore, the MRAS not only requires lower intensities of the irradiated RF energy, but also provides a more confined and focused injection of excitation energy on selected body parts. The MRAS consisted of several antennas with 50 cm inner diameter, 10 cm width and 0.5 cm depth. The position of the rings was freely adjustable. Conformal resonant right-/left-handed metamaterial was used for each ring antenna with two quadrature feeding ports for RF power. The system was successfully implemented and demonstrated with both a silicone oil and a water-NaCl-isopropanol phantom as well as in vivo by acquiring whole-body images of a crab-eating macaque. The potential for future neuroimaging applications was demonstrated by the acquired high-resolution anatomic images of the macaque’s head. Phantom and in vivo measurements of crab-eating macaques provided high-resolution images with large VoIs up to 40 cm in xy-direction and 45 cm in z-direction. The results of this work demonstrate the feasibility of the MRAS system for UHF MRI as proof of principle. The MRAS shows a substantial potential for MR imaging of larger volumes at 7 T UHF. This new technique may provide new diagnostic potential in spatially extended pathologies such as searching for spread-out tumor metastases or monitoring systemic inflammatory processes.
Highlights
Human whole-body magnetic resonance imaging (MRI) systems with a static magnetic flux density field (B0 field) of either 1.5 T or 3 T are the workhorses of routine clinical applications [1,2,3,4,5]
In MRI performed at magnetic flux densities of up to 3 T, usually a body coil irradiates the required excitation energy (B1+ field) into the object, whereas dedicated, smaller radio frequency (RF) coils are used for detection of the MR signals, which are transformed into the well-known MR images
Than we show that conformal resonant right-/left-handed (CRLH) metamaterial ring antennas are appropriate for the construction and operation of a whole-body imaging system run in an ultra-high field (UHF) MRI system
Summary
Human whole-body magnetic resonance imaging (MRI) systems with a static magnetic flux density field (B0 field) of either 1.5 T or 3 T are the workhorses of routine clinical applications [1,2,3,4,5]. A trend towards performing MRI at higher magnetic flux densities (B0) is observed, since higher B0 fields lead to both an improved signal-to-noise ratio (SNR) and an enhanced spatial resolution, providing for a more sensitive detection. These features of ultra-high field (UHF) MRI are especially exploited in functional MRI studies, where the brain activity is investigated. In MRI performed at magnetic flux densities of up to 3 T, usually a body coil irradiates the required excitation energy (B1+ field) into the object, whereas dedicated, smaller radio frequency (RF) coils are used for detection of the MR signals, which are transformed into the well-known MR images
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