Signal‐to‐noise ratio versus field strength for small surface coils

• Published in: NMR in biomedicine Vol. 37; no. 10; pp. e5168 - n/a
• Main Authors: Pohmann, Rolf, Avdievich, Nikolai I., Scheffler, Klaus
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01-10-2024
• Subjects: Coils; Field strength; Magnetic fields; Noise measurement; Noise sensitivity; signal‐to‐noise ratio; small animal imaging; surface coils; ultrahigh field; Water analysis; Water sampling; signal‐to‐noise ratio; surface coils; small animal imaging; ultrahigh field; field strength
• ISSN: 0952-3480; 1099-1492; 1099-1492
• DOI: 10.1002/nbm.5168

The increasing signal‐to‐noise ratio (SNR) is the main reason to use ultrahigh field MRI. Here, we investigate the dependence of the SNR on the magnetic field strength, especially for small animal applications, where small surface coils are used and coil noise cannot be ignored. Measurements were performed at five field strengths from 3 to 14.1 T, using 2.2‐cm surface coils with an identical coil design for transmit and receive on two water samples with and without salt. SNR was measured in a series of spoiled gradient echo images with varying flip angle and corrected for saturation based on a series of flip angle and T1 measurements. Furthermore, the noise figure of the receive chain was determined and eliminated to remove instrument dependence. Finally, the coil sensitivity was determined based on the principle of reciprocity to obtain a measure for ultimate SNR. Before coil sensitivity correction, the SNR increase in nonconductive samples is highly supralinear with B01.6–2.7, depending on distance to the coil, while in the conductive sample, the growth is smaller, being around linear close to the surface coil and increasing up to a B02.0 dependence when moving away from the coil. After sensitivity correction, the SNR increase is independent of loading with B02.1. This study confirms the supralinear increase of SNR with increasing field strengths. Compared with most human measurements with larger coil sizes, smaller surface coils, as mainly used in animal studies, have a higher contribution of coil noise and thus a different behavior of SNR at high fields.
The dependence of signal‐to‐noise ratio (SNR) on field strength was measured for small surface coils. After correction for measurement‐specific parameters, the SNR grows in a conductive sample between linearly and ~B02.0, depending on distance to the coil, and even more for nonconductive samples with B01.6–2.7. After coil sensitivity correction, the SNR increases independently of loading with B02.1, confirming the supralinear growth of SNR with increasing field strength, even for non‐negligible coil noise contributions.