超高速磁振造影技術之研究：渦電流自動補償系統之設計與應用

Echo planar imaging (EPI) is a unique imaging method that can collect an MR image from a
single free induction decay signal (FID). This rapid imaging technique is especially useful in the
applications, which is performed MRI scanning repeatedly, such as function MRI (fMRI),
diffusion weight MRI (DWI). During EPI acquisition, large field gradients have to be generated
and switched quickly at the rate of about 1 kHz. In this situation, eddy currents are induced on the
innermost conducting layer of the magnet significantly. It makes imaging geometric distortions
that will consist of N/2 ghost, shearing, or shift. In order to compensate this eddy currents effects,
the gradient waveform needs a preemphasis treatment. The preemphasis adjustment requires
manual operation, needs considerable skill, and could be difficult to achieve. In contrast,
automated preemphasis adjustments can be faster and more reliable, and can be applied without
operator intervention.
We present a method for optimizing gradient preemphasis, suitable for fast fMRI sequences on a
3T scanner, by fully automated numerical control instead of manual adjustments. Downhill
simplex method and k-space trajectory measurements are combined into an automated procedure of
preemphasis adjustment that is simple to implement, and is found to be reliable and easy to use. The
computations require no specific models relating actual gradient waveforms to preemphasis
parameters. Expectably, it is very useful to MRI study and clinic diagnosis that applies this
technique to fast MR imaging, such as EPI and DW-EPI.