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Radiofrequency (RF) coils are specialized antennas used in MRI to transmit RF energy into the patient and/or receive the emitted MR signal from tissues. They are critical determinants of image quality, signal-to-noise ratio (SNR), spatial resolution, scan time, and artifact behavior.

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MRI uses RF pulses at the Larmor frequency to excite hydrogen nuclei. RF coils generate and detect these signals.

Examples of coils: (a) head coil, (b) knee coil, (c) shoulder coil, (d) torso coil. Courtesy of Siemens Healthcare.

Principle of Operation

When placed in a static magnetic field ($B_0$), hydrogen nuclei precess at the Larmor frequency:

$$ \omega_0 = \gamma B_0 $$

Where:

• ( $\omega_0$ ) = Larmor frequency
• ( $\gamma$ ) = gyromagnetic ratio
• ( $B_0$ ) = magnetic field strength

RF coils operate at this resonant frequency.

Examples:

• 1.5 T MRI → ~64 MHz
• 3 T MRI → ~128 MHz

High-Field MRI Considerations

At 3T and above:

• B1 inhomogeneity increases
• Dielectric effects become prominent
• RF penetration becomes uneven