Significant advantages of circular polarization compared to linear polarization for MRI

 
Strong spin manipulation (cf. polarization/excitation) can be conducted in ultra-low field MRI using circularly polarized electromagnetic radiation and pulse sequences such as the Carl–Purcell–Meiboom–Gill (CPMG) sequence (Shim J.H. et al 2013).
 
A circularly polarized field can be generated by creating a phase difference of 90° in the currents of two identical coils e.g. by adopting an orthogonal coil configuration.
 
Using circular instead of linear polarization of the pulses resolves the problem of the "Bloch–Siegert shift" (cf. "AC-Stark shift").
 
Comparison between linear and circular polarization for magnetic resonance imaging of objects and human subjects indicates according to Glover G.H. et al (1985) that use of circular polarization reduces excitation power up to 50%, impoves signal-to-noise ratio (√2) and reduces significantly artifact intensity.
 
According to Schratter M. et al (1990) it improves signal-to-noise ratio, lowers background noise and therefore ameliorates significantly images sharpness, and moderately the recognition of anatomical details. According to the same authors, additional advantages of circular polarization are shorter measurement times (reduced number of acquisitions) and possibility to use thinner slices.
 
 

Additionally, as mentioned by the study of Mrózek M. et al entitled "Circularly polarized microwaves for magnetic resonance study in the GHz range: application to nitrogen-vacancy in diamonds": 

"In contrast to linear polarization, the circular polarization allows one to eliminate the Bloch-Siegert shift (1), increase the effective strength (2), improve the homogeneity (3) of the field-matter interaction and to address a specific spin state in the case of many-state quantum systems, which is crucial for experiments in the field of quantum information (4,5). For small frequencies this task is easily accomplished by two orthogonal coils driven by two radio-frequency (RF) signals phase shifted by 90°, produced by the, so called, quadrature coils."