(GRASS) This sequence is very similar to FLASH, except that the spoiler pulse is eliminated. As a result, any transverse magnetization still present at the time of the next RF pulse is incorporated into the steady state. GRASS uses a RF pulse that alternates in sign. Because there is still some remaining transverse magnetization at the time of the RF pulse, a RF pulse of a degree flips the spins less than a degree from the longitudinal axis. With small flip angles, very little longitudinal magnetization is lost and the image contrast becomes almost independent of T1. Using a very short TE eliminates T2* effects, so that the images become proton density weighted. As the flip angle is increased, the contrast becomes increasingly dependent on T1 and T2*. It is in the domain of large flip angles and short TR that GRASS exhibits vastly different contrast to FLASH type sequences.

(SEMS) This pulse sequence is composed of a 90° RF pulse followed by a 180° refocusing pulse. Both RF pulses are applied in the presence of a slice select gradient.
By choosing of different TR and TE, depending on the T1 and T2 values of the tissues, proton density, T1 weighted and T2 weighted images can be acquired.
The inversion recovery option enlarge the RF pulses with a 180° inverting pulse, applied a TI time before the beginning of the pulse sequence in order to manipulate image contrast.
See also Spin Echo Sequence.

(SR) Particular type of partial saturation pulse sequence in which the preceding pulses leave the spins in a state of saturation, so that recovery at the time of the next pulse has taken place from an initial condition of no magnetization. A rare used MRI pulse sequence that generates a predominantly proton density dependent signal, basically employing a 90° RF excitation pulse, with a very long repetition time. With this technique T1 times can be measured faster than with inversion recovery pulse sequences.
This saturation recovery sequence consists of multiple 90° radio frequency (RF) pulses with a short repetition time. A spoiler gradient pulse dephases the longitudinal magnetization that remains after the first 90° radio frequency pulse. A repetition time interval after the application of this spoiling gradient turns an additional 90° pulse the new developed longitudinal magnetization into the transverse plane, followed by recording a gradient echo.

Contrast is the relative difference of signal intensities in two adjacent regions of an image.
Due to the T1 and T2 relaxation properties in magnetic resonance imaging, differentiation between various tissues in the body is possible. Tissue contrast is affected by not only the T1 and T2 values of specific tissues, but also the differences in the magnetic field strength, temperature changes, and many other factors. Good tissue contrast relies on optimal selection of appropriate pulse sequences (spin echo, inversion recovery, gradient echo, turbo sequences and slice profile).
Important pulse sequence parameters are TR (repetition time), TE (time to echo or echo time), TI (time for inversion or inversion time) and flip angle. They are associated with such parameters as proton density and T1 or T2 relaxation times. The values of these parameters are influenced differently by different tissues and by healthy and diseased sections of the same tissue.
For the T1 weighting it is important to select a correct TR or TI. T2 weighted images depend on a correct choice of the TE. Tissues vary in their T1 and T2 times, which are manipulated in MRI by selection of TR, TI, and TE, respectively. Flip angles mainly affect the strength of the signal measured, but also affect the TR/TI/TE parameters.
Conditions necessary to produce different weighted images:
T1 Weighted Image: TR value equal or less than the tissue specific T1 time - TE value less than the tissue specific T2 time.
T2 Weighted Image: TR value much greater than the tissue specific T1 time - TE value greater or equal than the tissue specific T2 time.
Proton Density Weighted Image: TR value much greater than the tissue specific T1 time - TE value less than the tissue specific T2 time.

See also Image Contrast Characteristics, Contrast Reversal, Contrast Resolution, and Contrast to Noise Ratio.

In MRI, an echo is the emission of energy in form of an electromagnetic resonance signal of a nuclei after its excitation. At this point spins are back in phase again and the signal is measured. The desired number of echoes is selectable. Often until eight echoes are permissible for 2D or 3D scans using spin echo, inversion recovery or MIX techniques. Two echoes are permissible for all other techniques. A multi echo imaging sequence is needed for simultaneous measurement of T2 and density weighted images.