TwinTree Insert

06-08 Partial Fourier Imaging

o reduce the scan time, it is possible to omit some of the phase-encoding steps and use the acquired data to estimate the miss­ing data, given that the data set has conju­gate symmetry [⇒ MacFall 1988].

The image can then be reconstructed in the normal manner. Ideally, it would only be necessary to ac­quire half the data set, but imperfections in the magnetic field and ef­fects such as flow lead to phase errors. To compensate for these phase errors, it is nec­essary to collect slightly more than half the data set and then calculate a phase correc­tion. If 70% or more of the raw data is ac­quired, it is not necessary to calculate a phase correction.

If used together with a spin-echo se­quence, only 55% of the phase-encoding steps have to be acquired before one can ac­curately reconstruct the image.

With gradi­ent-echo techniques, a rather larger fraction of the data set is required for accurate re­construction due to the larger phase errors arising from the field in­ho­mo­ge­neity errors in the gradient-echo signal.

In both cases, the final images will have a reduced signal-to-noise ratio compared with images ac­quired using all the phase-encoding steps since the noise in the two halves of the par­tial Fou­rier image will be correlated.

06-08-01 Three-Dimensional Fourier Imaging

In the late 1970s and early 1980s, all origi­nal images acquired at Paul C. Lauterb­ur’s la­bo­ra­to­ry were three dimensional (3D). Other research groups and manufactu­rers in­tro­duc­ed two-dimensional (2D) imaging because such images could be ac­quired fas­ter and easier.

The 3D imaging methods available to­day are all based on 3D-Fourier re­con­struc­t­ion. The 2D spin-warp sequence can be ex­tended to a 3D sequence by applying the RF pulse without a slice gradient, causing the whole sample to be excited. Three-di­­men­si­o­nal imaging usually does not use spin-echo sequences because of the long scan time, but rather gradient-echo (GRE) or rapid spin-echo (RSE/FSE/TSE) se­­quen­ces.

To obtain spatial information in what was previously the slice direction, we have to apply a second phase-encoding gradient. To obtain full spatial encoding of the vo­l­­ume, it is necessary to step the second phase-encoding gradient through its full ran­ge of values for each step of the first phase-encoding gradient.

spaceholder redThe disadvantage of 3DFT imaging is — that unless the repetition time is very short — the scan time will be excessively long.

The main advantage of the 3D technique is that it has a signal-to-noise benefit over 2D techniques (if the voxel size is kept con­stant, the signal-to-noise ratio im­proves by the square root of the number of sli­ces). Other advantages are that the sli­ces are con­tiguous (which is not the case with multi­ple-slice techniques), that any de­sir­ed slice orientation can be reconstructed from the data set, that very thin sli­ces can be ob­tained, and that the slice profile of the 3D set is rectangular.

Among the additional problems with 3D imaging are that data processing re­quire­­ments are greatly increased, that viewing 3D data sets with typically 64-128 images creat­ed during one data acquisition gener­ally requires a separate workstation, and that for each 3D image set we have only one type of contrast.

A compromise between full 3D and 2D imaging is to excite a thick slice (slab), which is then sub-encoded into slices using a 3D sequence (Figure 06-22). This al­lows the 3D region to be accurately defined, but if used with only a small number of phase encodings in the third dimensions, signifi­cant ringing artifacts can occur. The slabs must have a good slice profile; otherwise some of the slices must be used to en­code the edges of the slabs to avoid such ringing arti­facts [⇒ Johnson 1983].

Figure 06-22:
(a) 2D multiple slices; (b) 3D slab, which can be used to create 2D slices; (c) 3D volume, which can be used to calculate slices in any direction of the entire volume.