The net magnetization vector (NMV) is spun into the transverse plane after the 90° rf excitation pulse is applied. The recovery of T1 takes place when nuclei lose energy, and NMV starts realigning with B0 leading to the recovery of longitudinal magnetization. Fats usually have a short T1 time because their T1 revitalization period is rapid. The T1 recovery time for water is, however, much longer. Therefore, in order to obtain the utmost difference between water and fat, the TR period selected should be short since, at this period, no tissue shall have fully recovered its longitudinal magnetization. Only the signal coming from ensuing rf pulses can partial saturation affect. However, after a given period of time, the longitudinal magnetization of the fat vector shall have increased as compared to that of water. Hence the next rf pulse shall have additional transverse magnetization, which will formulate a higher signal in the coil receiver causing the fat to emerge bright on a T1image that is weighted. On the other hand, the water vector shall have recovered modest longitudinal magnetization. This means that less transverse magnetization will be available after the rf pulse, and it will appear dark on a T1 image that is weighted.
Magnetic fields of nuclei interaction usually cause the decay of T2 after 90° rf excitation pulse, which lead to loss of transverse magnetization. The rates at which T2 weight crumble vary between fat and water and other isolating tissues. There is a rapid exchange of energy in the hydrogen of fat, making the T2 time short for the fats. This course is not so much efficient in the water, making the T2 time for water to be longer. TE regulates the sum total of T2 decay that took place prior to the received signal. To be able to acquire the maximum difference that exists between fat and water, TE should be long enough to give room for all the tissues to decay. TE should be protracted to be able to obtain T2 as it will give room for fat to decay, and only a small quantity of transverse magnetization will exist. This will stimulate the signal in the receiver coil, which will turn T2 weighted image dark. On the other hand, a large amount of transverse magnetization will stick with water which will generate a large amount of signal in the receiver coil, which will then turn the water bright.
Proton density weighting
Proton density can also be referred to as spin density since it depicts the total number of protons present in a sample of tissue. Those tissues that have little proton substance, such as cortical air and bone, normally generate a little signal, while on the other hand, tissues with high proton substance, such as the brain, normally produce transverse magnetization of high degree and generates a high signal. T1 and T2 effects must be decreased in order to accentuate proton density. Double echo is the sequence by which T2 and proton density can be obtained. The first echo usually has short TE and a long TR which result in weighing of proton density. The second echo, on the other hand, has got long TE and a long TR.