Time-dependent electromagnetic fields are significantly attenuated by conducting media (including the human body); the skin depth gives a measure of the average depth of penetration of the RF field. A high power frequency tunable RF source can be rapidly switched on and off. This produces a large RF field perpendicular to the magnetic field. This RF field is focused by the body coil. The RF source and coils must be tunable in both frequency and impedance to 'match the impedance' of the patient's body.

The skin depth may be a limiting factor in MR imaging at very high frequencies (high magnetic fields). The skin depth also affects the Q of the coils.

Use of multiple RF pulses with an inhomogeneous RF field to enable acquiring data from only selected regions within the field. This provides a 'one-dimensional' localization along isocontours of the B1 field.

(DRESS) Depth resolved surface spectroscopy is a localization method that employ gradients to select the region from which spectra are acquired.

(DEPTH) A sequence with multiple RF pulses to enable acquiring data from only selected regions.
See Depth Pulses, Volume Selective Excitation, Depth Resolved Spectroscopy.

MRI coils with a small diameter obtain a higher signal to noise ratio (SNR) than coils with a large diameter. A surface coil with a small diameter can be used to improve the resolution because the area of interest is around the optimal signal depth. The field of view of a (superficial) surface coil is half the diameter of the coil. A disadvantage is a lower sensitive volume of the coil. By combining several coils with small diameters (phased array coil) to record the signal simultaneously and independently, the SNR level improves considerably.