Fujita H, Braum W O, Morich M A
USA Instruments, Inc., Aurora, Ohio 44202, USA.
Magn Reson Med. 2000 Oct;44(4):633-40. doi: 10.1002/1522-2594(200010)44:4<633::aid-mrm18>3.0.co;2-k.
A birdcage coil possesses various intrinsic resonant modes. One of the modes produces sinusoidal current distribution around the rung elements of the birdcage and generates a highly uniform B(1) field in the transverse plane. Another mode is the end-ring resonant mode, in which there are no currents in the rungs of the birdcage, leaving the currents flowing in the top and bottom end-rings. Thus, this mode behaves like a "Helmholtz pair," producing a highly uniform B(1) field along the axis of the birdcage. By combining the sinusoidal resonant mode and the end-ring resonant mode at the same frequency, a quadrature birdcage coil for a vertical B(0) field is made possible. In particular, the coil dimension can be chosen such that the two end-ring loops behave as a Helmholtz pair, assuring the optimized B(1) field uniformity around the coil center region. The coil is well suited for a vertical B(0) MRI system, as both B(1) fields are perpendicular to the vertical B(0) field. To mimic a vertical B(0) field orientation, an existing horizontal bore MRI system was used with the RF coil oriented to simulate a "vertical" B(0) field. This is a report of the successful quadrature implementation of this novel birdcage coil.
鸟笼式线圈具有多种固有谐振模式。其中一种模式在鸟笼的横档元件周围产生正弦电流分布，并在横向平面内产生高度均匀的B(1)场。另一种模式是端环谐振模式，在这种模式下，鸟笼的横档中没有电流，电流只在顶部和底部端环中流动。因此，这种模式的行为类似于一个“亥姆霍兹对”，沿鸟笼轴产生高度均匀的B(1)场。通过在相同频率下将正弦谐振模式和端环谐振模式相结合，就有可能制造出用于垂直B(0)场的正交鸟笼式线圈。特别是，可以选择线圈尺寸，使得两个端环回路表现为一个亥姆霍兹对，确保线圈中心区域周围的B(1)场均匀性达到最佳。该线圈非常适合垂直B(0) MRI系统，因为两个B(1)场都垂直于垂直B(0)场。为了模拟垂直B(0)场方向，使用了现有的水平孔径MRI系统，并将射频线圈定向以模拟“垂直”B(0)场。本文报道了这种新型鸟笼式线圈正交实现的成功案例。
