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.

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.

(MEDIC) MEDIC is a heavily T2* weighted spoiled gradient echo sequence with multiple echoes. MEDIC uses a series of identically phase encoded gradient echoes, sampled per line in k-space. Unipolar frequency encoding gradients are used to achieve flow compensation and to avoid off resonance effects. For each echo the magnitude images are reconstructed and postprocessed by using a sum of squares algorithm to improve the signal to noise ratio. The increased receiver bandwidth reduces the T2* effects and impairment of the spatial resolution.
The multi echo data image combination sequence is potentially useful in imaging of cartilage in joints.

(ISIS) Image selected in vivo spectroscopy is used as a localization sequence to provide complete gradient controlled three-dimensional localization with a reduced number of sequence cycles, e.g. for in vivo 31P spectroscopy. The ISIS method generates three 180° pulses prior to a 90° pulse, after which the free induction decay is recorded. Specific 180° pulses (slice-selective) are combined and the FID's added or subtracted to generate a spectrum.
An advantage of the ISIS method is that the magnetization (before the final 90° pulse) is predominantly along the z-axis and so T2 effects are relatively small. This explains the value of this technique for 31P data acquisition, because some phosphorus metabolites (e.g. ATP) have short T2 values.
A disadvantage is that eight acquisitions are required to accomplish the spatial localization, therefore the sequence cannot be used for localized shimming. Another problem, because any variation between these data collections (for example, due to movement) will degrade these applications, can be solved by incorporating outer volume suppression techniques such as OSIRIS (modified ISIS).

The Dixon technique is a MRI method used for fat suppression and/or fat quantification. The difference in magnetic resonance frequencies between fat and water-bound protons allows the separation of water and fat images based on the chemical shift effect.
This imaging technique is named after Dixon, who published in 1984 the basic idea to use phase differences to calculate water and fat components in postprocessing. Dixon's method relies on acquiring an image when fat and water are 'in phase', and another in 'opposed phase' (out of phase). These images are then added together to get water-only images, and subtracted to get fat-only images. Therefore, this sequence type can deliver up to 4 contrasts in one measurement: in phase, opposed phase, water and fat images. An additional benefit of Dixon imaging is that source images and fat images are also available to the diagnosing physician.
The original two point Dixon sequence (number of points means the number of images acquired at different TE) had limited possibilities to optimize the echo time, spatial resolution, slice thickness, and scan time; but Dixon based fat suppression can be very effective in areas of high magnetic susceptibility, where other techniques fail. This insensitivity to magnetic field inhomogeneity and the possibility of direct image-based water and fat quantification have currently generated high research interests and improvements to the basic method (three point Dixon).
The combination of Dixon with gradient echo sequences allows for example liver imaging with 4 image types in one breath hold. With Dixon TSE/FSE an excellent fat suppression with high resolution can be achieved, particularly useful in imaging of the extremities.
For low bandwidth imaging, chemical shift correction of fat images can be made before recombination with water images to produce images free of chemical shift displacement artifacts. The need to acquire more echoes lengthens the minimum scan time, but the lack of fat saturation pulses extends the maximum slice coverage resulting in comparable scan time.