Phaseconjugate symmetry techniques use data from the top half of kspace to estimate data in the lower half of kspace. In 2Dspin warp imaging, the kydirection is generally taken to be synonymous with the phaseencoding axis. Hence the generic terminology "phaseconjugate symmetry" is used for this type of toptobottom data synthesis/estimation.
In theory, phaseconjugate symmetry allows one to acquire data using only half the normal number of phaseencoding steps, thus potentially reducing imaging time by as much as 50%. In practice, the time savings is closer to 40%, but this is still a huge benefit widely used in modern MRI protocols.

Phaseconjugate symmetry methods go under a variety of trademarked names according to vendor. Siemens uses the term "Half Fourier", while Cannon uses the abbreviation AFI ("Asymmetric Fourier Imaging"); Philips and Hitachi each call their sequences "Half scan".
GE has long referred to their technique "½NEX", "¾NEX", or "Fractional NEX", depending on the relative ratio of acquired:estimated kspace. The term "½NEX" is somewhat misleading, however, because what has been halved is the total number of phaseencoding steps (Np), not the number of excitations (NEX) per step. Still GE name reminds us that the degree of kspace undersampling is accompanied by a corresponding reduction in imaging time.
GE has long referred to their technique "½NEX", "¾NEX", or "Fractional NEX", depending on the relative ratio of acquired:estimated kspace. The term "½NEX" is somewhat misleading, however, because what has been halved is the total number of phaseencoding steps (Np), not the number of excitations (NEX) per step. Still GE name reminds us that the degree of kspace undersampling is accompanied by a corresponding reduction in imaging time.
Although phaseconjugate symmetry reduces imaging time while preserving spatial resolution, this is accomplished at the expense of the signaltonoise ratio (SNR). For halfFourier imaging, SNR is reduced by a factor of √½ or 30% less than a comparable sequence using the full number of phaseencoding steps.
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References
Feinberg DA, Hale JD, Watts JC et al. Halving MR imaging time by conjugation: demonstration at 3.5 kG. Radiology 1986; 161:527531.
MacFall JR, Pelc NJ, Vavrek RM. Correction of spatially dependent phase shifts for partial Fourier imaging. Magn Reson Imaging 1988; 6:143145.
McGibney G, Smith MR, Nichols ST, Crawley A. Quantitative evaluation of several partial Fourier recconstruction algorithms used in MRI. Magn Reson Med 1993;30:5159
Feinberg DA, Hale JD, Watts JC et al. Halving MR imaging time by conjugation: demonstration at 3.5 kG. Radiology 1986; 161:527531.
MacFall JR, Pelc NJ, Vavrek RM. Correction of spatially dependent phase shifts for partial Fourier imaging. Magn Reson Imaging 1988; 6:143145.
McGibney G, Smith MR, Nichols ST, Crawley A. Quantitative evaluation of several partial Fourier recconstruction algorithms used in MRI. Magn Reson Med 1993;30:5159
Related Questions
What is partial Fourier imaging?
What is partial Fourier imaging?