Planar Butler Matrix Technology for 7 Tesla MRI

Introduction Variation of the B1 field distribution at high field causes variation of the flip angles inside the patient’s body which leads to shading in the acquired images. Numerous methods have been proposed to mitigate B1 inhomogeneity. More recently, it was shown that “B1 shimming” can be performed using multiple transmitters [1,2], but utilizing these additional transmit channels effectively is extremely critical. Arrays formed from the orthogonal modes of a Degenerate Birdcage Coil (DBC) have been shown to have beneficial properties [3, 4]. In order to access these modes simultaneously, a Butler Matrix [5] is used to drive the individual rungs of the DBC in linear combinations to form the uniform birdcage mode as well as higher gradient modes. The other advantage of using the modes of a birdcage coil excited by a Butler Matrix is that they form naturally decoupled orthogonal modes that do not require decoupling strategies [6]. The Butler Matrix has also been found to provide reflection coefficients that are insensitive to the load [7]. In this work a novel reduced-size 8×8 high-power Butler Matrix has been designed and fabricated at 300 MHz to excite a coil array in 7 Tesla MRI. Methods and Construction The realization of an 8×8 Butler Matrix, Fig.1 (a), requires a combination of 90○ hybrid couplers (3dB) and fixed phase shifters. Line crossovers of conventional Butler Matrix designs are one of the main drawbacks, since they may add several undesired effects. To overcome this problem, a new layout of the matrix has been designed as shown in Fig.1 (b) which uses ports at the edges as well as at the center of the matrix. To design the 90○ hybrid couplers, Branch-Line (BL) couplers were used, Fig. 2(a), because of their high power and high voltage capability. BL couplers split the RF power equally between their outputs with a relative phase of 90○. Considering the dimensions of the BL coupler, the length of the branch line and series line is generally chosen to be a quarter wavelength at the design frequency. Our realization is based on microstrip technology and has been designed on two different substrates: (1) RO4003 with εr = 3.38, tanδ=0.0027 and a thickness of h=1.524 mm, and (2) RO3010 with higher permittivity of εr = 10.2, tanδ=0.0023 and h=1.28 mm. For the RO4003 substrate, this results in a branch line length of 13.59 cm, a series line length of 13.21 cm, and a total size of 14.7 ×16.2 cm for the BL coupler. To realize an 8×8 Butler Matrix at 300 MHz, constructed from twelve BL couplers, the size of each coupler should be reduced. Therefore, a novel compact size BL coupler using chamfered bends to fold the branches and reduce the total size of the coupler was designed, see Fig 2(b). Using this method, the size of the BL coupler was reduced to 10.7×9 cm. This coupler was implemented in the layout environment of the Agilent ADS software suite and has been optimized, realized (cf. Fig. 3(a)), and tested. If the full Butler Matrix network, shown in Fig 1(b), were to be realized in just one board, the physical dimensions of the Butler Matrix would be 60×65 cm, which is difficult to accommodate in the bore of the MR system. To reduce the total size of the manufactured Butler Matrix, it was split into 6 substrates with cable connections. The output phase shifters and related BL couplers were realized on four separate substrates, with two substrates each stacked above and below the two main substrates (each main substrate includes four BL couplers), as shown in Fig 4 (a). The size of this Butler Matrix is 28(length) × 22(width) × 18 (height) cm. RO3010 substrate with the higher permittivity was considered for the next experiment. The size of individual BL couplers, shown in Fig 3(b), has been reduced to 5.8 × 4cm using this board. The Butler Matrix network was designed (in ADS layout) and realized in one board, Fig 4(b), and its physical dimensions have been reduced to 27cm× 27 cm. Fig. 1:(a) Planar 8×8 Butler Matrix block diagram. (b) Butler Matrix without any crossing