Fractional Nex & Echo Imaging

There several imaging techniques that may be used to shorten the image acquisition time without shortening TR. Two techniques, fractional Nex imaging and fractional echo imaging, are presented in this section, while others will be presented in subsequent sections in this chapter. Before fractional Nex or fractional echo imaging can be understood, it is first necessary to examine a relationship between the data in different halves of k-space.

K-space data is made up of complex data representing the M_x and M_y components of magnetization. The complex data in the right half of k-space is the complex conjugate of the data in the left half of k-space. Similarly, the data in the top half of k-space is the complex conjugate of the data in the bottom half of k-space.

Fractional Nex Imaging

Fractional Nex imaging takes advantage of this complex conjugate relationship between the top and bottom halves of k-space to reduce the number of phase encoding steps. In half-Nex imaging, phase encoding steps +8 through -128 of +128 to -128 are recorded. Steps -128 through 0 are generated from the complex conjugate relationship between the halves of k-space. Phase encoding steps -8 through 0 are recorded to assure the center of k-space is at 0 and there is a smooth transition between the halves. Fractional Nex imaging sequences use Nex values between Nex=1 and Nex=1/2. Because fewer data points are collected in fractional Nex imaging, the signal-to-noise ratio becomes poorer as Nex is decreased. The advantage of fractional Nex imaging is that an image can be recorded faster than with Nex=1 but with the same contrast between the tissues as in the Nex=1 case.

Fractional Echo Imaging

Fractional echo imaging is similar to fractional Nex imaging in that the complex conjugate relationship between the left and right halves of k-space is used to shorten the echo time in an imaging sequence. If the entire echo does not need to be recorded, the minimum TE value is decreased and more signal may be achieved. In some instances, shorter acquisitions times may result.