To stimulate the NMR spin system, an RF-coil must produce a time-varying excitation field B1(t) with the following characteristics:
✔ B1(t) must have components that rotate near the resonant frequency (ωo), and
✔ B1(t) must have components perpendicular to the static magnetic field (Bo)
Some insight into the nature and inefficiency of linear polarization can be appreciated by considering the diagram below. Here you can see that a field like B1 (depicted as a big green arrow) oscillating at the resonance frequency (ωo) can be decomposed into two counterrotating subfields (red and blue arrows), each half as large. One of these subfields (red arrow) rotates in the same direction and frequency (ωo) as the spins undergoing NMR; the other subfield (blue arrow) rotates in the opposite direction and frequency (−ωo). Only the subfield associated with red arrow is effective at inducing net nuclear transitions and a change in net nuclear magnetization required for MRI. The other subfield is ineffective for NMR and serves only to deposit unwanted thermal energy into tissue.
In the 1980's linearly polarized transmit-receive RF-coils were the norm, but by the 1990's had become largely replaced by quadrature or circularly polarized (CP) coils. The physical principles underlying circular polarization will be addressed in the next several Q&A's.
Today linearly polarized transmit RF-coils are largely restricted for use in very small animal imaging and spectroscopy applications (such as double resonance cross-polarization or high-power ¹H-decoupling).
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Transmit-receive LP coils are not completely dead for human clinical MR imaging! Some specially designed solenoid coils have been developed for interventional procedures, such as 3D breast biopsy. See, for example, Smith MR, Zhai X, Kurpad KN et al. Excite and receive solenoid radiofrequency coil for MRI-guided breast interventions. Magn Reson Med 2011; 65:1799-1804.
Glover GH, Hayes CE, Pelc NJ, et al. Comparison of linear and circular polarization for magnetic resonance imaging. J Magn Reson 1985; 64:255-270.
Doty FD, Enzmenger G, Kulkarni J, et al. Radiofrequency coil technology for small animal MRI. NMR Biomed 2007; 20: 304–325
What are the function(s) of radiofrequency (RF) coils?
What is the difference between linearly polarized (LP) and a circularly polarized (CP) coils?