Birdcage Coils
Birdcage coil design with two end rings, multiple legs/rungs, and double capacitors (blue dots).
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RF-body coil with birdcage design
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Quadrature head coil with birdcage design
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The birdcage coil is the most commonly used RF-transmit device used in clinical MRI today. Virtually all body coils in cylindrical superconducting scanners are of this design. As shown in the diagram (above left), the birdcage coil consists of two circular conductive loops referred to as end rings connected by an even number of conductive straight elements called rungs or legs. The number of rungs depends on the size of the coil (body coil > head coil) and typically ranges from about 8 to 32. Birdcage coils also contain capacitors between conducting elements variably arranged based on the frequency characteristics desired. In clinical MRI a high-pass configuration is generally used with pairs of capacitors located along the end rings. Together this design approximates a continuous conducting surface.
In transmit operation sinusoidal currents are applied to each rung that are sequentially phase shifted around the coil's periphery. If there are N rungs, the phase shift between the currents in neighboring elements is 360°/N. According to antenna theory whenever the current distribution over a cylindrical surface satisfies sinusoidal angular dependence, a resonant condition exists and a homogeneous magnetic field can be created inside the conductor.
Some insight into the generation of a circularly polarized B1 field can be appreciated by considering the diagram below showing a hypothetical 6-rung birdcage coil. The current in each rung is directed into the page (denoted by ✕ arrow ends). The curved local fields generated around each rung are drawn according to Maxwell's right-hand rule. Each rung is driven by a sinusoidal current, but the peak current of each successive rung is delayed by 360°/6 = 60°. As each rung peaks in turn the central magnetic field is seen to rotate. This is an RF-resonance phenomenon called the first mode, or homogeneous mode of the coil.
In transmit operation sinusoidal currents are applied to each rung that are sequentially phase shifted around the coil's periphery. If there are N rungs, the phase shift between the currents in neighboring elements is 360°/N. According to antenna theory whenever the current distribution over a cylindrical surface satisfies sinusoidal angular dependence, a resonant condition exists and a homogeneous magnetic field can be created inside the conductor.
Some insight into the generation of a circularly polarized B1 field can be appreciated by considering the diagram below showing a hypothetical 6-rung birdcage coil. The current in each rung is directed into the page (denoted by ✕ arrow ends). The curved local fields generated around each rung are drawn according to Maxwell's right-hand rule. Each rung is driven by a sinusoidal current, but the peak current of each successive rung is delayed by 360°/6 = 60°. As each rung peaks in turn the central magnetic field is seen to rotate. This is an RF-resonance phenomenon called the first mode, or homogeneous mode of the coil.
Generation of rotating B1 field (green arrow) by a birdcage coil. Sinusoidal current is applied sequentially to rungs 1-6, each of which generates a local field in directions shown. A phase shift of 360°/6 = 60° between the successive rungs produces a resonantly rotating B1 field.
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Magnetic field map inside a birdcage coil.
(Courtesy of P. Futter, computed with FEKO) |
In addition to serving as RF-transmitters, birdcage coils can also be used as receivers. This mode of dual operation requires additional electronics near the coil, including decoupling circuitry, low noise preamplifiers, and a transmit-receive switch. To be useful as receivers, however, birdcage coils must have their rungs relatively close to the object being imaged. Thus birdcage head and knee coils are often used in both transmit and receive mode since they are near the patient. Birdcage body coils (the standard for nearly all cylindrical scanners) are too far away and are therefore used only for RF-transmission.
References
Hayes EC, Edelstein WA, Schenck JF et al. An efficient highly homogeneous radiofrequency coil for whole-body NMR imaging at 1.5 T (pdf). J Magn Reson 1985;63:622–828.
Gurler N, Ider YZ. FEM based design and simulation tool for MRI birdcage coils including eigenfrequency analysis (pdf). Proceedings of 8th Annual Conference on Multiphysics Simulation and its Applications, Milan, 2012.
Hayes EC, Edelstein WA, Schenck JF et al. An efficient highly homogeneous radiofrequency coil for whole-body NMR imaging at 1.5 T (pdf). J Magn Reson 1985;63:622–828.
Gurler N, Ider YZ. FEM based design and simulation tool for MRI birdcage coils including eigenfrequency analysis (pdf). Proceedings of 8th Annual Conference on Multiphysics Simulation and its Applications, Milan, 2012.
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