Variations in the value of TR and TE have an important effect on the control of image contrast characteristics.
Short values of TR (less than e.g. 1000 ms) and TE (less than e.g. 25 ms) are common in images exhibiting T1 contrast.
Long values of TR (greater than e.g. 1500 ms) and TE (greater than e.g.60 ms) are common in images exhibiting T2 contrast.
Middle TR values (e.g. from 1000 to 1500 ms) and middle TE values (e.g. from 25 to 60 ms) are common for density weighted contrast.
The values are depending on the field strength. TR is also a major factor in total scan time.

(TR) The amount of time that exists between successive pulse sequences applied to the same slice.
It is delineated by initiating the first RF pulse of the sequence then repeating the same RF pulse at a time t. Variations in the value of TR have an important effect on the control of image contrast characteristics. TR is also a major factor in total scan time.

See also Scan Time and Image Contrast Characteristics.

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.

The quality of magnetic resonance imaging is particularly dependent on image resolution (matrix, field of view, slice thickness), contrast (TE, TR), signal to noise ratio (bandwidth, signal averaging) and lack of artifacts. These MRI parameters are affected by the field homogeneity, the field strength, the coil, the pulse sequence type and imaging techniques like parallel imaging.

See also Image Contrast Characteristics.

An image produced by controlling the selection of scan parameters to minimize the effects of T1 and T2, resulting in an image dependent primarily on the density of protons in the imaging volume. Proton density contrast is a quantitative summary of the number of protons per unit tissue. The higher the number of protons in a given unit of tissue, the greater the transverse component of magnetization, and the brighter the signal on the proton density contrast image. Conversely the lower the number of protons in a given unit of tissue, the less the transverse magnetization and the darker the signal on the proton density image. Also called (Rho) ρ-weighted.

See also Density Weighted Imaging and Image Contrast Characteristics.