The complex numbers extend the real numbers by including the imaginary unit i (satisfying i2 = - 1). The term 'complex' when used as an adjective means that the field of complex numbers is as the underlying number field considered. In MRI, complex data are used for example in the Fourier transforms.

An imaginary number is a component of a complex number (satisfying i2 = - 1). In MRI, complex data are used for example in the Fourier transforms.

A real number is a component of a complex number. In MRI, complex data are used for example in the Fourier transforms.

(PE) The partial echo technique (also called fractional echo) is used to shorten the minimum echo time. By the acquisition of only a part of k-space data this technique benefits (like all partial Fourier techniques) from the complex conjugate symmetry between the k-space halves (this is called Hermitian symmetry).
The dephasing gradient in the frequency direction is reduced, and the duration of the readout gradient and the data acquisition window are shortened. Partial echo gives a better SNR at a given TE when a smaller FOV or thinner slices are selected, allows a longer sampling time, and a larger water fat shift (WFS, see also bandwidth) due to a lower gradient amplitude. The resolution is not affected. This is often used in gradient echo sequences (e.g. FLASH, Contrast Enhanced Magnetic Resonance Angiography) to reduce the echo time and yields a lower gradient moment. The disadvantage of using a partial echo can be a lower SNR, although this may be partly offset by the reduced echo time.
Also called Fractional Echo, Read Conjugate Symmetry, Single Side View.

See also Partial Fourier Technique and acronyms for 'partial echo' from different manufacturers.

In some cases raw data are displayable. These are typically just the real part of the complex raw data, and have been scaled to emphasize the interesting details. Therefore, they cannot be successfully transformed to reproduce the displayed images, but may help to find artifacts or the reason for bad image quality.