Phase-conjugate Symmetry
How does phase-conjugate symmetry work? Why is it used?
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Phase-conjugate symmetry techniques use data from the top half of k-space to estimate data in the lower half of k-space. In 2D-spin warp imaging, the ky-direction is generally taken to be synonymous with the phase-encoding axis. Hence the generic terminology "phase-conjugate symmetry" is used for this type of top-to-bottom data synthesis/estimation.
In theory, phase-conjugate symmetry allows one to acquire data using only half the normal number of phase-encoding 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.
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Phase-conjugate symmetry. About half of k-space is sampled by reducing the number of phase-encoding steps. The other half of k-space is synthesized/reconstructed.
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Phase-conjugate symmetry methods go under a variety of trademarked names according to vendor. Siemens uses the term "Half Fourier", while Canon uses the abbreviation AFI ("Asymmetric Fourier Imaging"); Philips and Hitachi each call their sequences "Half scan".
GE has long referred to their technique "½-NEX", "¾-NEX", or "Fractional NEX", depending on the relative ratio of acquired:estimated k-space. The term "½-NEX" is somewhat misleading, however, because what has been halved is the total number of phase-encoding steps (Np), not the number of excitations (NEX) per step. Still GE name reminds us that the degree of k-space undersampling is accompanied by a corresponding reduction in imaging time.
GE has long referred to their technique "½-NEX", "¾-NEX", or "Fractional NEX", depending on the relative ratio of acquired:estimated k-space. The term "½-NEX" is somewhat misleading, however, because what has been halved is the total number of phase-encoding steps (Np), not the number of excitations (NEX) per step. Still GE name reminds us that the degree of k-space undersampling is accompanied by a corresponding reduction in imaging time.
Although phase-conjugate symmetry reduces imaging time while preserving spatial resolution, this is accomplished at the expense of the signal-to-noise ratio (SNR). For half-Fourier imaging, SNR is reduced by a factor of √½ or 30% less than a comparable sequence using the full number of phase-encoding steps.
References
Feinberg DA, Hale JD, Watts JC et al. Halving MR imaging time by conjugation: demonstration at 3.5 kG. Radiology 1986; 161:527-531.
MacFall JR, Pelc NJ, Vavrek RM. Correction of spatially dependent phase shifts for partial Fourier imaging. Magn Reson Imaging 1988; 6:143-145.
McGibney G, Smith MR, Nichols ST, Crawley A. Quantitative evaluation of several partial Fourier recconstruction algorithms used in MRI. Magn Reson Med 1993;30:51-59
Feinberg DA, Hale JD, Watts JC et al. Halving MR imaging time by conjugation: demonstration at 3.5 kG. Radiology 1986; 161:527-531.
MacFall JR, Pelc NJ, Vavrek RM. Correction of spatially dependent phase shifts for partial Fourier imaging. Magn Reson Imaging 1988; 6:143-145.
McGibney G, Smith MR, Nichols ST, Crawley A. Quantitative evaluation of several partial Fourier recconstruction algorithms used in MRI. Magn Reson Med 1993;30:51-59
Related Questions
What is partial Fourier imaging?
What is partial Fourier imaging?