The flat top is the area at the top of the frequency encoding gradient where the data sampling occurs. By increasing the BW the amount of time the flat top is on is reduced. Shortening the duration of the flat top shortens the echo spacing and also geometric distortions. The area under the frequency gradient determines the FOV, while the flat top determines the number of samples to be collected.

(FA) The flip angle a is used to define the angle of excitation for a field echo pulse sequence. It is the angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction via the application of a RF excitation pulse at the Larmor frequency. It is also referred to as the tip angle, nutation angle or angle of nutation.
The radio frequency power (which is proportional to the square of the amplitude) of the pulse is proportional to a through which the spins are tilted under its influence. Flip angles between 0° and 90° are typically used in gradient echo sequences, 90° and a series of 180° pulses in spin echo sequences and an initial 180° pulse followed by a 90° and a 180° pulse in inversion recovery sequences.

Quick Overview
Please note that there are different common names for this artifact.

Flow effects in MRI produce a range of artifacts, e.g. intravascular signal void by time of flight effects; turbulent dephasing and first echo dephasing, caused by flowing blood.
Through movement of the hydrogen nuclei (e.g. blood flow), there is a location change between the time these nuclei experience a radio frequency pulse and the time the emitted signal is received (because the repetition time is asynchronous with the pulsatile flow).
The blood flow occasionally produces intravascular high signal intensities due to flow related enhancement, even echo rephasing and diastolic pseudogating. The pulsatile laminar flow within vessels often produces a complex multilayered band that usually propagates outside the head in the phase encoded direction. Blood flow artifacts should be considered as a special subgroup of motion artifacts.

Artifacts can be reduced by reduction of phase shifts with flow compensation (gradient moment nulling), suppression of the blood signal with saturation pulses parallel to the slices, synchronization of the imaging sequence with the heart cycle (cardiac triggering) or can be flipped 90° by swapping the phase//frequency encoding directions.

See also Flow Related Enhancement and Flow Effects.

(FT) The Fourier transformation is a mathematical procedure to separate out the frequency components of a signal from its amplitudes as a function of time, or the inverse Fourier transformation (IFT) calculates the time domain from the frequency domain. The FT is used to generate the spectrum from the free induction decay or spin echo in the pulse MR technique and is essential to most MR imaging techniques. The Fourier transformation can be generalized to multiple dimensions, e.g. to relate an image to its corresponding k-space representation, or to include chemical shift information in some chemical shift imaging techniques. Fourier transformation analysis allows spatial information to be reconstructed from the raw data.

Fractional echo (also called asymmetric or partial echo) is used to shorten the echo time in a sequence, by acquiring partial echoes in the frequency direction. The reduction of echo time is possible because if the first part of the echo is not received, the dephasing lobe of the frequency encoding gradient is not to be on for quite as long, and this saves time.

See also Partial Fourier Technique, Read Conjugate Symmetry, Single Side View and acronyms for 'fractional echo' from different manufacturers.