Basic Physics of MR Signal and Image Generation - Advance Image Processing in Magnetic Resonance Imaging - page 33

groups of imaging sequences is the k-space sampling method : it can be homogeneous
or not, it can include the partial k-space filling instead of full k-space filling, etc.
In the present section, only a quick description is given: the most recent and
efficient methods and their applications will be described in more detail in the rest of
the topic.
One feature that distinguishes different image acquisition methods is how
quickly one can acquire data for an image and how easily the methods can be
extended to generate higher spatial resolution. The two-dimensional Fourier trans-
form method described in the preceding text is excellent for generating high-
quality, high-spatial resolution images, but it is rather slow because only one line
in k-space is acquired for each TR. The EPI method is well known as a very fast
method that generally has limited spatial resolution. The EPI pulse sequence is
uniquely characterized by a zigzag-like pathway that goes back and forth rapidly
in the frequency direction (k x ) and moves in small steps in the phase direction
(k y ), as schematically shown in Figure 1.17.
As in the two-dimensional Fourier transform method, we apply the Fourier
transform in both frequency and phase directions obtaining the two-dimensional
spatial map of MRI data, which is an MR image. As it can be seen, a single RF

RF

Gz

Gy

Gx

(a)

k y

Last line

k x

First line

(b)

FIGURE 1.17 (a) Timing diagram of EPI sequence and (b) relevant k-space path.