(HS) A method in which approximately one half of the acquisition matrix in the phase encoding direction is acquired. Half scan is possible because of symmetry in acquired data. Since negative values of phase encoded measurements are identical to corresponding positive values, only a little over half (more than 62.5%) of a scan actually needs to be acquired to replicate an entire scan. This results in a reduction in scan time at the expense of signal to noise ratio. The time reduction can be nearly a factor of two, but full resolution is maintained.
Half scan can be used when scan times are long, the signal to noise ratio is not critical and where full spatial resolution is required. Half scan is particularly appropriate for scans with a large field of view and relatively thick slices; and, in 3D scans with many slices. In some fast scanning techniques the use of Half scan enables a shorter TE thus improving contrast. For this reason, the Half scan parameter is located in the contrast menu.

More information about scan time reduction; see also partial fourier technique.

See Partial Fourier Imaging.

(HFI) This technique is sometimes referred to as 'half NEX' imaging or 'PCS' (phase conjugate symmetry).

See Partial Fourier Imaging.

(FWHM) A commonly used measure of the width at half the maximum value of peaked functions such as spectral lines or slice profiles and important measure of the quality of an imaging device and its spatial resolution. As the name states, the FWHM is measured by identifying the points on the signal curve, which are half the maximum value. The horizontal distance between these two points is called the FWHM. For a spectral line, this will be proportional to 1/T2.

(HASTE) A pulse sequence with data acquisition after an initial preparation pulse for contrast enhancement with the use of a very long echo train (Single shot TSE), whereat each echo is individually phase encoded. This technique is a heavily T2 weighted, high speed sequence with partial Fourier technique, a great sensitivity for fluid detection and a fast acquisition time of about 1 sec per slice. This advantage makes it possible for using breath-hold with excellent motionless MRI, e.g. used for liver and lung imaging.

See also Segmented HASTE.