An Efficient, Highly Hokogeneous Radiofrequency Coil for Whole-Body NMR Imaging at 1.5 T

We have developed radiofrequency coils for high-field head and whole-body imaging which achieve near optimal rf field (Br) homogeneity and signal-to-noise ratio (SNR). The design, based on a lumped element delay line, has a number of advantages. The rf field uniformity is significantly better than that of a saddle coil or slotted tube resonator. The improved Br homogeneity is needed to generate accurate multiecho pulse sequences. Using this coil, we obtain, as expected, the nearly linear increase in signal-to-noise as a function of frequency or static magnetic field I30 (I). The coil’s cylindrical symmetry allows quadrature drive and reception which decreases rf power requirements by a factor of two and increases signal-tonoise by a factor of fi (2). Our head-size coil and body-size coil operate at 64 MHz. Designs for operation at lower frequencies or somewhat higher frequencies are possible. Small scale versions of this coil design should be applicable to conventional NMR spectroscopy. The use of a superconducting solenoidal magnet for whole-body MR imaging requires a transverse rf field within a cylindrical volume. A perfectly homogeneous transverse magnetic field in an infinitely long cylinder can be produced by a surface current which runs along the length of the cylinder and is proportional to sin 8, where 0 is the cylindrical coordinate azimuthal angle. The conventional saddle coil (3) actually approximates the ideal sinusoidal current distribution for six equally spaced values of 0 (0 = 0, 60, 120, 180, 240, 300”). Four conductors carry currents of equal magnitude whereas no conductors are needed at 8 = 0 and 180” because the currents there are zero. To improve the approximation to the ideal current distribution with more conductors requires a means of developing unequal, sinusoidally weighted currents in adjacent conductors. A standing wave in a transmission line generates the required sinusoidal current distribution. Hinshaw and Gauss (4) employ this principle by winding a one-wavelength-long coaxial cable onto a toroidal form. They removed the coaxial shielding from the cable lying on the inner diameter of the toroid. The exposed portions of the center conductor of the cable generate a homogeneous rf magnetic field in the inner bore of the toroid. This structure is limited to lower frequencies by the need to wind a many-turn toroid from a single