After a bleeding, blood goes through several transitions regarding magnetic properties, intra- and extracellular distribution and content of proteins and water. Oxyhaemoglobin is degraded to deoxyhaemoglobin and further to methaemoglobin, ferritin and haemosiderin. These molecules can be characterized by their magnetic susceptibility effect. Oxyhaemoglobin is diamagnetic with no practical influence on the magnetic field. All degradation products are paramagnetic (deoxyhaemoglobin, methaemoglobin) or even superparamagnetic (ferritin, haemosiderin).

See also Haemoglobin, Blood Oxygenation Level Dependent Contrast.

(GE) An echo signal generated from a free induction decay by means of a bipolar switched magnetic gradient. The echo is produced by reversing the direction of a magnetic field gradient or by applying balanced pulses of magnetic field gradient before and after a refocusing RF pulse so as to cancel out the position dependent phase shifts that have accumulated due to the gradient.
In the latter case, the gradient echo is generally adjusted to be coincident with the RF spin echo. When the RF and gradient echoes are not coincident, the time of the gradient echo is denoted echo time (TE) and the difference in time between the echoes is denoted time difference (TD).
Gradient echo does not refocus the effects of main field inhomogeneity and therefore is generally used with a short echo time. Disadvantages of gradient echo imaging are compromised anatomic details and artifacts in regions with varying susceptibility e.g. between the air-containing sinuses and brain and especially between haemorrhages and normal tissue.

See also Susceptibility Artifact.