A surface coil is essentially a loop of conducting material, such as copper tubing. The in-bore solenoidal sending coil is used as the transmitter of RF energy. This type of receiver coil is placed directly on or over the region of interest for increased magnetic sensitivity. The loop may form various shapes and be bent slightly to conform to the imaged body part. Surface coils have a good SNR for tissues adjacent to the coil and because the signal decrease with the distance, an eligibility homogeneity correction will equalize this over the field of view. A rule of thumb for surface coils is that the sensitivity decreases appreciably beyond a distance equal to the diameter of the coil.
The positioning of the coil is an important determinant of performance. As only the region close to the surface coil will contribute to the signal, there is an improvement in the SNR for these regions, compared to the use of receiver coils that surround the appropriate part of the body. These coils are specifically designed for localized body regions, and provide improved signal to noise ratios by limiting the spatial extent of the excitation or reception.

See also the related poll result: '3rd party coils are better than the original manufacturer coils'

A surface coil placed over a region of interest will have an effective selectivity for a volume approximately subtended by the coil circumference and one radius deep from the coil center. Such a coil can be used for simple localization of sites for measurement of chemical shift spectra, especially of phosphorus, and blood flow studies. Some additional spatial selectivity can be achieved with magnetic field gradients.

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.

Imaging coils are radio frequency coils used in magnetic resonance imaging for sending and/or receiving electromagnetic radiation. Several MR imaging coils in different shapes and sizes are necessary for different body parts and to handle individual applications.
For Example:
Birdcage coils provide high homogeneity and good signal to noise ratio (SNR) in brain MRI scans.
Surface coils with small coil diameter and higher SNR enable to image the temporomandibular joints (TMJ).
The implemented body coil allows the scanning of body parts with large field of views.
Micro coils are available to image finger joints.
High performance phased array coils are today state-of-the-art for a wide range of applications from head, spine, knee or shoulder to cardiac MRI.
For different types of coils see Volume Coil, Sense Coil, Array Coil, Surface Coil, and Bird Cage Coil.

See also the related poll result: '3rd party coils are better than the original manufacturer coils'

Quick Overview

A moiré pattern is an interference pattern created for example when two grids are overlaid at an angle, or when they have slightly different mesh sizes. The human visual system creates an imaginary pattern of roughly horizontal dark and light bands, the moiré pattern that appears to be superimposed on the lines.
In MRI, the appearance of moiré fringes can be caused by a variety of reasons e.g., inhomogeneity of the main magnetic field caused by a defect shielding (interference with RF pulses), interferences produced by aliasing, and interferences of echoes from different excitation modes (with different echo times).

Take spin echo-based techniques, or a surface coil. This artifact is often sensitive to shimming or susceptibility gradients.