MRI Birdcage

MRI Birdcage"/>

MRI Birdcage

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A conventional birdcage coil that operates at 64 MHz (1.5 T) is simulated using XFdtd Release 7 and the ability to analyze the B₁ field due to the loading of a human head is demonstrated. The coil geometry is taken from the paper “Effects of End-Ring/Shield Configuration on Homogeneity and Signal-to-Noise Ratio in a Birdcage-Type Coil Loaded With a Human Head” [1]. 

As described in the paper, the coil is a 16-rung, low-pass birdcage coil with a 27 cm diameter, 22 cm length, and a shield diameter of 34 cm, shown in Figure 1. This example will consider only the conventional configuration from the paper since the other configurations are readily constructed with slight modifications to this basic design. The model is meshed with a 2 mm base cell size and Remcom’s XACT Accurate Cell Technology is enabled on all parts of the coil. Figure 2 shows the XACT representation of the FDTD mesh for a portion of the coil. A 2 mm cell size is chosen because a high resolution human body model will be loaded into the coil.                  

Figure 1CAD representation of the conventional birdcage coil.               Figure 2XACT mesh representation for a portion of the coil
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At each rung of the birdcage, a gap exists for a circuit component. Following the convention of the paper, a phased voltage source is added to each rung with the phase matching the angular position of the rung in the geometry. Sensors are set to save the steady-state B₁ fields in two planes of the geometry and the simulation is executed with a 64 MHz sine wave as the source signal. 

Following the first simulation, with an unloaded coil, the |B₁⁺| and B fields are displayed as shown in Figures 3 and 4 respectively. For all figures displaying fields, the input power to the coil has been adjusted to 1 W and the scale bar has been set to better visualize the fields in the center of the plane.

                                   Figure 3  |B₁⁺| on the axial plane in the unloaded coil.                                 Figure 4 B on the sagittal plane in the unloaded coil.

A 2 mm model of the Visible Human Head, extracted by Varipose, is then used to load the birdcage, as shown in the 3D view of Figure 5. Axial and sagittal planar views of the mesh can be found in Figures 6 and 7.

Figure 5 Coil loaded with human head.   Figure 6 2 mm mesh representation on the axial plane of the loaded coil.Figure 7 2 mm mesh representation on the sagittal plane of the loaded coil.

Following a second simulation, with a loaded coil, the |B₁⁺| and B fields through the loaded coil are shown in Figure 8 and 9 respectively. As expected, the introduction of the human head disrupts the |B₁⁺| fields. The paper performs a similar analysis for three other end-ring/shield configurations and finds the conventional configuration provides the most homogeneity for an unloaded coil, but the least homogeneity when loaded.

Figure 8 |B₁⁺| on the axial plane of the loaded coil.                               Figure 9B on the sagittal plane of the loaded coil.

Reference

1. Wanzhan Liu, Christopher M. Collins, Pamela J. Delp, and Michael B. Smith.  “Effects of End-Ring/Shield Configuration on Homogeneity and Signal-to-Noise Ratio in a Birdcage-Type Coil Loaded With a Human Head.”Magnetic Resonance in Medicine, no. 51, pp. 217-221, 2004.

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