Machine imperfection-based artifacts manifest themselves due to the fact that the odd
k-space lines are acquired in a different direction than the even
k-space lines. Slight differences in timing result in shifts of the
echo in the acquisition window. By the shift theorem, such shifts in the time domain data then produce linear
phase differences in the
frequency domain data.
Without correction, such
phase differences in every
second line produce striped ghosts with a shift of half the
field of view, so-called Nyquist ghosts. Shifts in the applied
magnetic field can also produce similar (but constant in
amplitude) ghosts.
This
artifact is commonly seen in an EPI image and can arise from both,
hardware and sample imperfections.
A further source of machine-based
artifact arises from the need to acquire the signal as quickly as possible. For this reason the EPI signal is often acquired during times when the gradients are being switched. Such
sampling effectively means that the
k-space sampling is not uniform, resulting in ringing artifacts in the image.
Such artifacts can be minimized by careful setup of the
spectrometer and/or correction of the data. For this reasons reference data are often collected, either as a separate scan or embedded in the imaging data.
The non-uniform
sampling can be removed by knowing the form of the
gradient switching. It is possible to regrid the data onto a uniform
k-space grid.
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