A pulse sequence is a preselected set of defined RF and gradient pulses, usually repeated many times during a scan, wherein the time interval between pulses and the amplitude and shape of the gradient waveforms will control NMR signal reception and affect the characteristics of the MR images. Pulse sequences are computer programs that control all hardware aspects of the MRI measurement process.
Usual to describe pulse sequences, is to list the repetition time (TR), the echo time (TE), if using inversion recovery, the inversion time (TI) with all times given in milliseconds, and in case of a gradient echo sequence, the flip angle. For example, 3000/30/1000 would indicate an inversion recovery pulse sequence with TR of 3000 msec., TE of 30 msec., and TI of 1000 msec.
Specific pulse sequence weightings are dependent on the field strength, the manufacturer and the pathology.

See also Interpulse Times.

Burst pulse sequences are fast imaging sequences capable of image acquisition in less than 100 ms. Basically a train of low flip angle pulses generates a long train of echoes. The complete sequence is performed with the application of a constant read gradient. Phase encoding may be implemented using short phase encoding gradients between echoes.
The advantage of this sequence type is that it is less demanding on gradient speed than other fast techniques (e.g. echo planar imaging EPI) and it produces images, which are substantially free of susceptibility artifacts.
The disadvantage is that the technique is less sensitive than competing methods.

In standard SE MR imaging, each image line measured at each echo after the excitation 90° pulse is assigned to a different image, hence resulting in a multi echo pulse sequence.

The schematic figures of a pulse sequence timing diagram illustrate the steps of basic hardware activity that are incorporated into a pulse sequence. Time during sequence execution is indicated along the horizontal axes. Each line belongs to a different hardware component. One line is needed for the radio frequency transmitter and also one for each gradient (G_s = slice selection gradient x, G_f = phase encoding gradient y, G_f = frequency encoding gradient z, also called readout gradient).
In picture 1, a timing diagram for a 2D pulse sequence is shown.
Slice selection and signal detection are repeated in duration, relative timing and amplitude, each time the sequence is repeated. A single phase encoding component is present each time the sequence is executed.
Additional lines are added for ADC (Analog to Digital Converter) and sampling. A gradient pulse is shown as a deviation above or below the horizontal line. Simultaneous component activities such as the RF pulse and slice selection gradient are indicated as a non-zero deviation from both lines at the same horizontal position. Simple deviations from zero show constant amplitude gradient pulse. Gradient amplitudes that change during the measurement, e.g. phase encoding are represented as hatched regions.

The second picture shows a timing diagram for a 3D pulse sequence.
Volume excitation and signal detection are repeated in duration, relative timing and amplitude, each time the sequence is repeated. Two phase encoding components are present, one in the phase encoding direction and the other in slice selection direction (irrespectively incremented in amplitude) in each time the sequence is executed. A description of the comparison of hardware activity between different pulse sequences.

Pulse sequences, designed to be insensitive to flow, e.g. at every even echo, a spin echo sequence is not flow sensitive. Velocity compensation is achieved by using gradients, which are either symmetrical around a 180° pulse and switched on twice as is the case for motion compensated spin echo pulse sequences, or two antisymmetrical gradient lobes without 180° pulse, which is the way to produce a velocity compensated gradient echo pulse sequence.
The signal of the second echo (and all other even echoes) is independent of the velocity of the object. Thus, velocity-based motion effects stemming from the entire voxel or from spins within a voxel (intravoxel incoherent motion) are suppressed with such pulse sequences.
If higher order motion is relevant, as it may be in turbulent jets across valves, acceleration and jerk effects can also be compensated for by the use of appropriate combinations of gradient- and radio frequency pulses.
With the increasingly stronger gradients, echo times in MR systems can be shortened to the point at which effects other than velocity effects hardly ever become relevant.