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要旨:
Purpose/Introduction: Recent publications[1],[2] report a high
capability of a multi-coil setup to generate equivalent linear fields. Hence, the spatial encoding which is performed by scanner’s built-in linear gradient, can be accomplished with a multi-coil setup and therefore can be used for imaging in parallel with shimming[3]. The
accuracy of the linear field produced by multi-coil is the benchmark key factor. Increasing the number of individual coils brings more degrees of freedom and a better generation of linear fields with the cost of a more complex setup to fabricate and maintain. Here, it is demonstrated how optimization of coil position results in generating
superior linear fields with a limited number of coils.
Subjects and Methods: Recent reports[1],[2] have used 48 and 84 coils which were arranged with a layout of 6*8 and 6*14 respectively. For comparison, we simulated a local multi-coil setup with 16, 24, 32, 48 and 84 circular shaped coils. All coils are placed on a cylinder with a diameter/length of 360/300 mm which is large enough to house an RF coil. We used optimization-based search of coil positions that result in three linear orthogonal fields for the FOV of 200*200*200 mm around the isocenter. Simulation started from arranging individual coils in a regular fashion on the cylinder surface. The magnetic field for each coil was calculated using the Biot–Savart law with no constraint for the floating current in the coils. The optimization
was performed using the fmincon function in MATLAB.
Results: As an initial position for the optimization, we placed the coils in a symmetric configuration such that they cover the whole cylinder surface. Figure 1 shows the arrangement of 16 individual coils before and after optimization to produce the linear field. Figure 2 demonstrates a comparison between the ideal linear fields, the generated linear field from 16 symmetrically positioned coils and the generated linear fields from 16 position-optimized coils. Figure 3 illustrates how position optimization can improve the quality of the linear fields generated by a multi-coil setup. As a cost function, we
used cross-correlation between optimized field and the ideal linear field and also the l2-norm of their difference.
Discussion/Conclusion: According to Fig. 3, position optimized arrangement for 16, 24 and 32 coils can bring the same or even better quality compared to the symmetric arrangement for 32, 48 and 84 coils respectively. This proves the importance of the optimal coil configuration to acquire high fidelity linear field with less coils.
