The goal of the proposed design is to improve the imaging quality in Magnetic Resonance Imaging (MRI) systems by enhancing the magnetic field strength, maintain the uniformity of the field over any required physical length of the RF coil element. The conventional bird-cage coil provides high RF fields but requires increase in the element length in high field systems [2]. The microstrip TEM coil is a foreshortened resonator with the capacitive loading at the ends. Our proposed design uses the metamaterial transmission line resonator for uniform magnetic field [3] and the stepped impedance structure to improve the current density to improve the H-field intensity [4]. As the design is independent of the physical length, it can be used in both head and body coil designs.

The metamaterial transmission line, also known as the composite right and left hand (CRLH) line is aperiodically loaded line with one or more series capacitors and shunt inductors in each periodic cell [2]. The additional series capacitor (CL) and shunt inductor (LL) resonate with the series line inductance (LR) and shunt line capacitance (CR) respectively of the periodic cell. For the zeroth order resonance, both the resonance frequencies are chosen to be identical and equal to the Larmor frequency of the 7T MRI system and the structure becomes independent of the wavelength. Due to the physical constraints at 298 MHz on the elements of RF coil, lumped elements are used. The lines resonating at the zeroth order frequency have a constant surface current density [4]. For repetitive cells, the magnitude of the current density and the near field intensity reduces. To increase the current density and the near field, a stepped impedance structure [4] is used. The stepped impedance lines are the lines with alternate wide and thin conductor lines which provide low and high impedances [6]. When a signal propagates through these lines, the current per unit length of the line remains the same but the current density will be greater in the narrow high impedance section and stronger magnetic near fields are generated in the narrow sections [7]. The CRLH transmission line is embedded in the high impedance section to utilize their combined advantages. The element is open circuited at the end hence its resonance frequency depends only on the shunt resonant circuit [3] and is given by $$ω_{0}=ω_{sh}=1/√(L_{L}C_{R})$$

The full wave simulation results with and without a human head phantom model [5] of the proposed design have been obtained with the HFSS program and compared to the microstrip TEM element results. They show increased amplitude of the near H field in a single unit cell (Fig 2) compared to that of microstrip TEM coil element [1] for the same input power provided (Fig 3). As the number of cells are increased, the field drops substantially as shown in Fig 4. A single unit cell and an element with three cells were printed on a 62 mil Duroid substrate having a dielectric constant of 2.2 as shown in Fig 1. A 5cm long unit cell uses 6.8pF and 39nH lumped series capacitor and shunt inductor respectively along with a distributed spiral inductor. All the single and multi-cell elements are matched to 50Ω with a tunable series capacitor at the input. The field measurement is performed on the fabricated single cell and three cell element using the H-field probes. The results of the 16cm design of the near H-field is comparable to the TEM microstrip coil of same size. The simulation results of the element with 5 cells (27cm long) shows that the near H-field stays uniform but at a lower level for any physical length of the coil which is in fact hard to achieve with microstrip element or the conventional bird-cage coil. To counter this reduction of field in the multi-cell elements, a single cell strategy is adopted for the 16cm element shown in Figure 5. The field distribution obtained is uniform, strong and better than the microstrip element.
The single cell element shows excellent near magnetic field results, unloaded and loaded conditions. However, the multi-cell elements have large decrement of the near field with the phantom loading. As a solution to this we adopted a single cell strategy shown in Fig 5.