A small diameter coil obtains a better signal with a higher signal-to-noise ratio than a coil with a large diameter. However its sensitive volume is lower. When several small coils are combined (coil elements in phased array) to record the signal simultaneously and independently, a greater level can be explored.
Thanks to the geometry of the coils and the lack of noise correlation recorded by the different elements in the phased array, the signal obtained will have a better signal-to-noise ratio than that delivered by one large coil.
Employed in this way, acquisition from a coil composed of several elements in phased array will increase the signal-to-noise ratio of the image.
Combination of images from two facing elements
Combination of k-space data from three coils
Each coil element has a limited reception volume, with variable sensitivity depending on the distance from the coil element. The signal received by each coil element thus contains spatial data (position of the coil, reception level, sensitivity level) that can be used to reconstruct the image, as a complement to gradient-induced spatial encoding.
The use of parallel acquisition to reduce the number of phase encoding steps (by utilizing the spatial data linked to the sensitivity profile of the different coil elements), can reduce acquisition time by half or by two thirds for 1.5 T MRI.
Whatever the case, the coil signals can only be combined during signal processing, so it is essential for each coil element to have a separate reception and digital conversion channel.