TwinTree Insert

06-07 The Complete Imaging Experiment

n two-dimensional MR imaging, a slice is excited, for example, by applying a se­lec­­tive RF-pulse in the presence of the z-gradi­ent. Two different methods can be applied.

06-07-01 Frequency-Encoding Only

The remaining two gradients (x and y) are now combined with a resulting gra­dient of a certain strength and spatial direction. The FID is recorded in the pre­sen­ce of this gradient. Then the combined gradient is ro­tated a certain angle. Once again the FID is recorded, and this process continues until enough in­for­ma­tion is col­lect­ed.

Based on the frequency spectra, an im­age can be reconstructed using the back­pro­j­ect­ion method, as depicted in Figure 06-13.

06-07-02 Two-dimensional FT Method

This method is a combination of phase- and frequency-encoding. It is the stan­dard image formation method today.

One gradient, for example the y-gradi­ent, is switched on and the spins are al­­low­ed to dephase in the presence of this gra­dient. After a certain time, the y-gra­dient is switched off and the FID, or alternatively the spin echo, is recorded in the pre­sence of the x-gradient.

Thus, the y-gradient serves as the phase-en­co­ding gradient (often called pre­pa­ra­tion gradient) and the x-gradient is encoding the frequency information (read­­out gradient). The system is re-excited, but with either the duration or, more com­­mon­ly, the strength of the y-gradient changed.

The whole process is repeated n times for a resolution of n pixels in the y-di­rec­­tion, with a different phase-encoding gradi­ent applied for each excitation.

Since the field inhomogeneity effects will be the same for each repetition, they have no effect upon the final image; rather, they constitute a baseline. This is one of the main advantages of the 2DFT technique.

The re­sul­ting 2D raw data matrix is processed using a 2D Fourier-transform to pro­duce a 2D image (Figure 06-20) [⇒ Pykett 1982]. The combination of frequency- and am­plitude-stepped phase-encoding tech­ni­ques is called 2D spin warp imaging [⇒ Edelstein 1980].

Figure 06-20:
2DFT owes more to MR spectroscopy than to CT re­construction algorithms be­cause both am­pli­tu­de and phase informa­tion are acquired to spatially encode the signal.
The first step generates a one-dimen­sional projection, but here a phase-encod­ing gradient is ap­plied just be­fore the origi­nal gradient is turned on. The phase-en­coding gradient is applied in right angles to the origi­nal gradient and its duration or amplit­ude is succes­sively increased. The corresponding points from each projection are Fourier-transformed a second time to ge­ne­ra­te the final image.

Figure 06-21 summarizes an entire 2DFT imaging experiment, in this case us­ing a spin-echo sequence, although one can use nearly any other pulse se­quen­ce applied in clinical MR imaging.

Figure 06-21:
Complete 2DFT SE imaging ex­periment. The proce­dure consists of the se­lection of the 90° and 180° RF pul­ses, a transverse slice through a brain (z-direc­tion), phase-en­cod­ing (y-direction), and fre­quency-en­cod­ing (x-direction).
 The phase-encoding gradients change the phase in the respective row of the trans­verse slice; the frequency-encoding gradi­ents allocate a specific frequency to each column. Combining both phase and fre­­quen­cy infor­mation allows the creation of a grid in which each pixel possesses a distinct com­bi­na­tion of phase and fre­quency codes. The entire procedure is usually repeated 256 times, with changing phase-encoding gradients to produce a 256×256 image.