Due to a low concentration and relatively small gyromagnetic ratio of 31 P in human and animal tissue compared to protons ( 27 , 28 ), a dedicated 31 P coil optimized for the 31 P nucleus accompanied by a 1 H coil for the scout imaging and shimming of the region of interest is required. We built a quadrature 31 P coil with dimensions optimized to the heart of lambs aged four to six weeks, and a relatively large 1 H linear coil. The detailed explanation for the construction of the coil is given in the author's previous paper ( 29 ). The 31 P coil consisted of a butterfly loop decoupled with a rectangular loop for a quadrature mode as shown in Figure 3 , by adjusting the overlapping region of the coils. The 1 H RF coil that surrounded the 31 P coil, as shown in the Figure 3 , was positioned such that the field produced by the coil would be perpendicular to that produced by the 31 P butterfly coil. The coupling between the 1 H and 31 P rectangular coil was minimized by choosing the large dimension of the 1 H coil compared to the rectangular coil. The loaded isolation between the 1 H and 31 P butterfly coils measured using vector network analyzer (E5061B ENA Series, Agilent Technologies, Englewood, CO, USA) and 250 mL saline bag phantom at the 1 H frequency was − dB and that at the 31 P frequency was − dB while that between the 1 H and 31 P rectangular coils at the 1 H frequency was − dB and at the 31 P frequency was − dB. Moreover, the 31 P coil affects the 1 H signal to noise ratio (SNR) far more than the 1 H coil affects the 31 P SNR ( 30 ). This is because 31 P coils have relatively high capacitor values compared to the 1 H coil, which provides low impedance at the 1 H frequency ( 31 ). The isolation between the 1 H and 31 P RF coils at 1 H frequency could be further increased by inserting the 1 H traps at the 31 P coils with a little compromise of the 31 P signal. No extra lossy components on the 31 P coil were added so that the performance of the 31 P coil is same as the same coil on the same frame without 1 H coil. The separate coil design for each nucleus allows us to independently match and tune the 1 H and 31 P coils without interference.
(a) Circuit diagram and (b) photograph of 1 H/ 31 P dual tuned RF coils. The 31 P loop consisted of a butterfly loop and an inner rectangular loop decoupled geometrically for a quadrature mode. The outer rectangular loop was 1 H RF coil whose surface area was made large to minimize the coupling with 31 P rectangular loop, and positioned such that it was in quadrature mode with 31 P butterfly loop. Click for larger image Download as PowerPoint slide
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