Signal intensity interpretation in MR imaging has a major problem.
Often there is no intuitive approach to signal behavior as signal intensity is a very complicated function of the contrast-determining tissue parameter, proton density, T1 and T2, and the machine parameters TR and TE. For this reason, the terms T1 weighted image, T2 weighted image and proton density weighted image were introduced into clinical MR imaging.
Air and bone produce low-intensity, weaker signals with darker images. Fat and marrow produce high-intensity signals with brighter images.
The signal intensity measured is related to the square of the xy-magnetization, which in a SE pulse sequence is given by
Mxy = Mxy0(1-exp(-TR/T1)) exp(-TE/T2) (1)
where Mxy0 = Mz0 is proportional to the proton or spin density, and corresponds to the z-magnetization present at zero time of the experiment when it is tilted into the xy-plane.

See also T2 Weighted Image and Ernst Angle.

(DE - dual / double echo) Dual echo sequences include images with different weightings and / or echo times and are used to obtain both, proton density and T2 weighted images or in phase and out of phase gradient echo images, simultaneously without increasing the measurement time.

(FISP) A fast imaging sequence, which attempts to combine the signals observed separately in the FADE sequence, generally sensitive about magnetic susceptibility artifacts and imperfections in the gradient waveforms. Confusingly now often used to refer to a refocused FLASH type sequence.
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. FISP 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 (with TR 20-50 ms, flip angle 30-45°) 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 FISP exhibits vastly different contrast to FLASH type sequences. Used for T1 orthopedic imaging, 3D MPR, cardiography and angiography.

(FLASH) A fast sequence producing signals called gradient echo with low flip angles. FLASH sequences are modifications, which incorporate or remove the effects of transverse coherence respectively.
FLASH uses a semi-random spoiler gradient after each echo to spoil the steady state (to destroy any remaining transverse magnetization) by causing a spatially dependent phase shift. The transverse steady state is spoiled but the longitudinal steady state depends on the T1 values and the flip angle. Extremely short TR times are possible, as a result the sequence provides a mechanism for gaining extremely high T1 contrast by imaging with TR times as brief as 20 to 30 msec while retaining reasonable signal levels. It is important to keep the TE as short as possible to suppress susceptibility artifacts.
The T1 contrast depends on the TR as well as on flip angle, with short TE.
Small flip angles and short TR results in proton density, and long TR in T2* weighting.
With large flip angles and short TR result T1 weighted images.

TR and flip angle adjustment:
TR 3000 ms, Flip Angle 90°
TR 1500 ms, Flip Angle 45°
TR 700 ms, Flip Angle 25°
TR 125 ms, Flip Angle 10°

The apparent ability to trade TR against flip angle for purposes of contrast and the variation in SNR as the scan time (TR) is reduced.

See also Gradient Echo Sequence.

(GEMS) This pulse sequence uses a changeable flip angle instead of a 90° pulse and a gradient instead of a RF pulse to rephase the FID.
T2*, T1 weighted and proton density images can be acquired. The flip angle in combination with the TR determines the T1 weighting and the TE controls the amount of dephasing. To minimize T2* the echo time should be short.

See also Gradient Echo Sequence.