A Low-Pass Trombone Birdcage Coil with Broad Tuning Range

Field: Engineering (Physical Sciences)

Introduction The advantages of the birdcage coil are generally recognized as its ability to produce a large homogenous RF volume with a high SNR and Q factor [1]. In order to study the effects of shield size and configuration on birdcage coil performance, we needed to build birdcage coils at various resonant frequencies with a wide tuning range. The resonance of a standard birdcage coil is typically adjusted for fine tuning at the expense of B1 homogeneity by using one or more adjustable capacitors [2]. Jin's simulations found that tuning a large birdcage at 1.5T with a single capacitor that varied from 60pF to 180pF achieved a tuning range of 5.75MHz, but caused B1 deviation of up to 30% [2]. To overcome this weakness, several methods for symmetric tuning of birdcages and alternative symmetric volume coil designs have been presented [3-8]. These designs were implemented at high fields only, with the exception of Huh's method which achieved balanced tuning over 1MHz at 1.5T with a ring inductively coupled to a highpass birdcage [7]. Dardinzki implemented a coil design tunable by RF shielding at 9.4T [6], but the method is unsuitable for our applications investigating shield configurations [9]. An elegant design implemented at 4.7T and 9.4T was suggested by Xu and Tang, in which the leg inductance in a high-pass birdcage coil was varied by using telescoping tubes and nesting rods [8]. Our group has implemented this birdcage design in 1.0cm and 3.5cm diameter coils for MR microscopy at 4.7T and 1.5T. The results demonstrate a broad tuning range without sacrificing homogeneity. To operate at 1.5T, the design was extended to a low-pass configuration achieving 6MHz tuning range. Methods Brass telescoping tubes and rods (K&S Engineering, Chicago, IL), commonly available from craft stores, were used to construct the coils. The stability of the connection between the tubes and rods was tested using an HP4195A Network Analyzer by observing the lack of noise in the S11plot as the coil was vibrated and stretched. End plates for tube and rod connections were machined in-house, with the number of plates and design varying accordingly for a high pass or low pass coil (see figure 1). To allow leg length adjustment with a shield present, the shield former was milled in two nesting pieces which move with either end of the coil. The shield was made from thin copper, and was affixed to only one side of the shield former. High Q, high voltage, non-magnetic chip capacitors (ATC, Huntington Station, NY) were used to resonate the coils at the center of their length adjustment. A 1cm diameter, 8 leg, high pass trombone coil was constructed for use at 4.7T. In order to examine tuning ranges of the design at various resonant frequencies, and in order to extend the design to typical current clinical fields, two 3.5cm diameter, 16 leg, trombone birdcages were also constructed. One coil was resonated for imaging at 4.7T, and the other for imaging at 1.5T. To resonate a small coil at low field in a high pass configuration requires the use of large capacitance values, degrading the Q of the coil from a manufacturer specification of 700 (for low-pass configuration capacitor values) to 150 (for high-pass configuration capactor values). To avoid complications involved with having eight modes below 64MHz, and in order to avoid degrading the Q of the coil with extremely large, inherently low Q capacitors, a unique low-pass design of the trombone was implemented, in which the low pass capacitance was split at the end of each leg to maintain symmetry during leg length adjusment. The tuning range of the designs as a function of center frequency was examined theoretically, and verified using an HP4195A Network Analyzer. Homogeneity of the design was verified using spin-echo imaging on a 4.7T/33cm GE Omega system in the MRSL and on a GE Signa 1.5T at The M.D. Anderson Cancer Center in Houston, TX. Results It is possible to derive a linear characteristic relationship between frequency of operation and tuning range for a given coil configuration. Consistent with these results, the 3.5cm coil configuration, adjustable from a length of 6.8cm to 8.9cm the coil resonated at 200.24MHz has a tuning range of 19MHz, and the coil resonated at 63.85MHz has a tuning range of 6MHz (see figure 2). This 6MHz tuning range at 1.5T is equivalent to that which is achievable with a high range single tuning capacitor, and is substantially greater than that reported by other symmetric tuning methods. The 1cm birdcage designed for 4.7T, adjustable from a leg length of 2.54cm to 3.93cm, has a 46MHz tuning range around 200.24MHz (see figure 2). Even if this wide tune range were able to be accomplished with a single capacitor, the distortion in the B1 field would be too great for suitable imaging. In anticipation of utilizing the multiple receiver capabilities of most clinical systems and placing multiple similar birdcages nearby, the 16 leg, 64MHz, low pass coil was closely shielded. The coil was fed in quadrature, also an extension to previous work presented on symmetrically tuned birdcages. The coil tuned easily over a 6MHz range. Images of a homogenous saline phantom were obtained at 1.5T. Discussion Symmetric tuning of volume coils has been implemented primarily at high fields, or at low field with a limited tuning range [3-8]. The trombone coil design is characterized by the ease of tuning and presence of coil homogeneity over a large tuning range. Implementing the design at 1.5T required a unique low pass design, and resulted in a coil with a wide tuning range and excellent homogeneity. The symmetry of the tuning mechanism and the broad tuning range allows for future exploration of related ideas, including the possibility of increased isolation of quadrature tuning and exploration of shield geometries.