A method to create an optimum current distribution and homogeneous B1 field for elliptical birdcage coils

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Abstract
For a birdcage coil with elliptical cross‐section, a sinusoidal current pattern does not provide a homogeneous B1 field. A simple theory was developed to create an optimized current distribution for elliptical birdcage coils. This optimized current pattern can create a perfectly homogeneous B1 field inside any elliptical shape. To verify the theory, a 16‐element high‐pass elliptical birdcage coil was built inside a circular RF shield. The current was optimized by using the inductance characteristics of the coil components to calculate the endring capacitances. The B1 field was theoretically calculated and experimentally mapped for the optimized elliptical birdcage coil and a nonoptimized coil. The results demonstrate that by optimizing the current distribution, a very homogeneous B1 field is produced. This method can be directly applied in design and construction of elliptical birdcage coils for imaging of the naturally occurring elliptical cross‐sectional geometries in the human body.