Backfolding always occurs due to wrong phase encoding caused by objects outside the planned FOV. Phase encoding gradients are scaled for the field of view only. Tissues outside the FOV do not get properly phase encoded relative to their actual position and 'wraps' into the opposite side of the image.
The Backfolding artifact projects image contents which fall outside the imaging FOV back into the image; the back folded information thus reappearing on the other side of the image. In fact, information along the phase encoding direction can be viewed as projected onto a cylindrical screen with a circumference corresponding to the linear field of view dimension in the phase encoding direction.
Burst pulse sequences are fast imaging sequences capable of image acquisition in less than 100 ms.
Basically a train of low flip angle pulses generates a long train of echoes. The complete sequence is performed with the application of a constant read gradient. Phase encoding may be implemented using short phase encoding gradients between echoes.
The advantage of this sequence type is that it is less demanding on gradient speed than other fast techniques (e.g. echo planar imaging EPI) and it produces images, which are substantially free of susceptibility artifacts.
The disadvantage is that the technique is less sensitive than competing methods.
Cartesian sampling is used to refer to data collection with a fixed value of the phase encoding gradient. In 2D Fourier imaging with common Cartesian sampling of k-spacesensitivity encoding by means of a receiver array enables to reduce the number of Fourier encoding steps. This is achieved by increasing the distance of sampling positions in k-space while maintaining the maximum k-values.
The Cartesian coordinates are obtained from the polar coordinates by the operations
x = r sin f
y = r cos f
using the trigonometric functions sine and cosine.