Volume & Surface Coils

Transcription

1 Volume & Surface Coils Gregor Adriany, Ph.D. University of Minnesota Medical School, Center for MR Research Minneapolis, Minnesota, USA Background In terms of the signal-to-noise ratio (SNR) and RF transmit efficiency the single most important component in any well designed MR system is the MR coil, the front end device responsible to generate the high frequency field (B ). Consequently, a considerable amount of coil related research has been conducted from the very early days of MRI with the goal to develop optimized RF resonators for MR signal excitation and detection []. For in vivo MR applications in principle two categories of MR resonant structures can be distinguished according to their overall design characteristic: Volume Coils, with the ability to fully circumscribe a region of interest (ROI) and generate a B field with typically good field homogeneity over an extensive region. Surface Coils, which interact with more local imaging volumes, have more limited geometrical dimensions and less B field homogeneity but significant higher SNR. Arrays of surface coils allow the benefits of the small individual coils to be extended to larger ROI s. The major initial contributions towards the theory and development of MR coil design for imaging and spectroscopy applications were: The introduction of the MR surface coil by Ackerman [2]. The introduction of the principle of quadrature excitation and reception for MR application by Chen and Hoult [3, 4]. The development of a homogeneous whole body MR resonator, termed the birdcage coil, by Hayes [5]. A volume coil which is capable to efficiently generate within one resonance structure circularly polarized magnetic fields [6]. The introduction of preamplifier decoupling and practical extension of the array antenna concepts towards MR applications by Roemer[7]. The generally excellent field homogeneity achievable with large quadrature volume resonators prompted the extended use in all clinical whole body applications up to 3T. Typically such whole body volume coils are used predominately for B excitation and are combined with dedicated surface coil arrays for reception with the highest SNR. To achieve this, both the volume transmit coil and the receive arrays are equipped with PIN diode detuning circuitry that is actively controlled by the scanner[8]. However, at 3T for whole body application it becomes increasingly important to gain additional control over the B excitation field generation beyond the two degrees of freedom afforded by quadrature excitation. Thus for operating frequencies beyond 28MHz/3T researchers have developed potentially more suitable coils that incorporate the RF shield into the coil resonance structure -such as the "Transmission Line Resonator" by Roeschmann [9] and the TEM Volume Resonator by Vaughan[0]. For TEM type structures Vaughan demonstrated that the range of operation for whole body resonators can be extended up to 300 MHz/ 7T []. For head applications beyond 3T it is also possible to push the operation range of birdcage type volume resonators by either utilizing smaller shielded highpass (HP) and bandpass (BP) volume coils in conjunction with receive arrays as demonstrated by Wiggins[2] or by utilizing the birdcage in a degenerated mode, as first described by Leussler [3]. A degenerated birdcage is in operation similar to an array of capacitive decoupled circumscribing rectangular loop elements. This sort of resonance structure naturally leads to the most recent generation of volume resonators- transmit volume arrays. Transmit arrays[4-25] can mitigate a number of challenging issues that arise at higher magnetic fields and operating frequencies[26-30]. The two most pronounced difficulties encountered are noticeable increased B + field in-homogeneities and SAR related challenges associated with the overall increase in RF power demands. This is due to the fact that the RF wavelength is comparable or smaller than the

2 dimensions of the human anatomy and because biological tissues are lossy dielectrics. This in turn leads to prominent wave behavior and a significant difference between transmit B + and receive B - fields [6, 3-34]. While it is possible to correct some of the resulting B field in-homogeneities within traditional multimodal resonant volume coils through design modifications[35] and individual amplitude, phase control and resonance element adjustments [36, 37]. It is clear that an even better handle on these effects can only be achieved with dedicated transmit array systems that are capable of supporting a higher number of independent channels for RF transmission and allow for RF shimming [8, 20, 36, 38]. Furthermore such arrays are essential to support parallel excitation pulses across multiple coil elements, as first proposed by Katscher [39] and Zhu [40]. The related rapidly emerging parallel transmission methods have the strong potential to further improve RF excitation homogeneity and can achieve high spatial selectivity and pulse acceleration by taking full advantage of the ability to influence B + fields through temporally and spatially varying RF excitation pulses [22, 4-50]. Accelerated RF pulses however inherently tend to generate higher RF power requirements, but this limitation can be addressed with specific constrains in multi channel RF pulse design algorithms. Volume Resonator Principles The principle geometrical structure of an eight element birdcage resonator can be seen in Figure. Typically an even number of resonance elements is chosen to support two symmetrically placed feed points. Two feed points are required to drive a volume resonator in quadrature operation for higher power efficiency and SNR advantage. The conductors of a birdcage coil are typically built from either copper tubing or circuit board material. These form an inductive structure that can be tuned to desired resonance frequencies by appropriately chosen capacitors. The capacitors can either be in the form of lumped elements, such as high Q and high voltage ceramic chip capacitors, or as a continuously distributed capacitance. Similar conductors of TEM type structures are built either utilizing coaxial resonance elements[9, 0] or circuit boards with distributed high Q capacitors[5]. I 8 I I 8 I I 2 I 7 I 2 I 7 I 6 I 3 0 I 3 I5 I 4 - I 4 I5 a b Figure (a) The principle geometrical structure of a birdcage type resonator is illustrated as follows: two ring shaped conductive elements are connected with multiple leg conductors, creating a structure of multiple resonant elements circumscribing the cylindrical coil volume inside. (b) Each of the leg elements employs an RF current I N, indicated with I to I 8 in this schematic of a simple eight element birdcage. The shown ring current distribution leads to the desired resonance mode one. This mode is primarily used in MRI applications because of the resulting extended area of good field homogeneity in the coil center. Depending on the distribution of the capacitors over the ring and legs four principle types of birdcage resonators can be defined: A Lowpass (LP) Birdcage with capacitors only in the legs. A Highpass (HP) Birdcage with capacitors only in the rings. A Bandpass (BP) Birdcage with capacitors in both the legs and rings. A Bandstop(BS)Birdcage with frequency selective pathways in rings and/or legs. The frequency dependency of a BS birdcage can be achieved by adding trap circuits instead of single capacitors [52, 53], or by creating multiple ring structures[54]. For TEM type coils double and/or even triple resonance can elegantly achieved by altering the single element resonance frequencies between neighboring elements while still maintaining the overall inductive coupling[36]. Due to crosstalk between the multiple interconnected resonance elements in a volume coil structure multiple resonator modes at different resonance frequencies will occur. A volume resonator can be perceived as the equivalent to a cylindrical cavity resonators, with a given length-diameter ratio and restricted current pathways that are defined by the geometry of rings and legs as well as the number of resonance elements. In a degenerated birdcage, through proper choice of the capacitor ratio of leg an ring capacitors the modes collapse and the cage structure becomes in essence an array of I 6 2

3 interconnected rectangular loops. This principle was first demonstrated by Leussler [3] and expanded upon by King [9]. Volume Resonator Resonance Modes Due to the crosstalk of multiple resonance elements in volume coils - in case of a birdcage capacitors connected with shared end-ring structures in case of TEM type coils through inductive coupling - multiple frequency dependent resonance patterns are inherently present inside the resonator volume. Figure 2 illustrates the first five principle modes of an eight-element volume coil. In general, all of these different resonance modes have the potential to be useful for very specific imaging or spectroscopy applications in a limited FOV. Only one mode, however, indicated with M=, leads to the typically desired homogeneous field pattern over an extended FOV. The coil designer thus chooses appropriate capacitor values that support Mode resonance at the desired MR operating frequency Due to the radial symmetry it is inherently possible to excite all of the above modes by 90-separated feedpoints, thus enabling quadrature operation. In TEM type coils modes can be modeled like that of a coaxial cavity resonator [36]. Unlike with a shielded birdcage, the shield in a TEM volume resonator is connected continuously to both conductive end-rings. Thus the B return field is contained between the RF Shield M=0 M=, 0Þ M=, 9 0Þ M=2 M=3 M=4 Figure 2 The magnetic flux lines that are associated with the first four TEM resonator modes. The perpendicular mode= split for quadrature excitation is indicated as mode, 0 and 90. Figure 3 Plot of the modal resonance frequencies of an eight element HP birdcage resonator. Shown are the input reflection coefficient measurement S (Ch ) and the transmission coefficient measurement S 2 (Ch 2).Note that for a HP birdcage the homogeneous Mode resonates higher than the other modes (Marker 4) transmission line elements and the shield. Several analytical methods have been developed to analyze the multi-modal structure of volume resonators [0, 3, 54-62] an example of the modal distribution is shown in Figure 3. Conclusion With the overall trend of the MR field towards higher magnetic fields, the classical multi modal volume resonators will continue to be substituted with volume coil array structures that allow for greater control of the transmit fields. This mirrors for the transmit side the development on the receive side from single loop and quadrature surface coils towards receive arrays with currently up to 28 individual channels[63, 64]. Likely the future will see both transceiver arrays as well as combinations between dedicated body and head transmit arrays with receive- on arrays. References [] D. I. Hoult and R. E. Richards, "The signal-to-noise ratio of the nuclear magnetic resonance phenomenon," J. Magn. Reson., vol. 24, pp. 7-85, 976. [2] J. J. H. Ackerman, T. H. Grove, G. G. Wang, D. G. Gadian, and G. K. Radda, "Mapping of metabolites in whole animals by 3 P NMR using surface coils," Nature, vol. 283, pp , 980. [3] D. I. Hoult, C. N. Chen, and V. J. Sank, "Quadrature detection in the laboratory frame," Magn Reson Med, vol., pp , Sep 984. [4] C. N. Chen, D. I. Hoult, and V. J. Sank, "Quadrature Detection Coils - a Further Square-Root2 Improvement in Sensitivity," Journal of Magnetic Resonance, vol. 54, pp , 983. [5] C. E. Hayes, W. A. Edelstein, J. F. Schenck, O. M. Mueller, and M. Eash, "An Efficient, Highly Homogeneous Radiofrequency Coil for Whole-Body NMR Imaging at.5t," J Magn Reson Med, vol. 63, pp , 985. [6] G. H. Glover, C. E. Hayes, N. J. Pelc, W. A. Edelstein, O. M. Mueller, H. R. Hart, C. J. Hardy, M. O'Donnel, and W. D. Barber, "Comparison of linear and circular polarization for magnetic resonance imaging.," J Magn Reson, vol. 64, p. 255,

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