The T2 images’ difference was because of the movement of molecules, which control the correlation of time. Also, since air has no protons and proton density, it has no signals in T1 and T2 weighted. The brightest sign seen comes from the CSF and the eye’s aqueous humor, where molecules move around unrestricted. Bone tissue, on the other hand, has the least mobility and is labeled MR invisible. Compared to the white matter, the gray matter is brighter because it has less structure and so increased molecular mobility. Muscle tissue has a shorter T2 and is darker in a T2 weighted image. Lastly, the T2 idea of a fat, which appeared a bit bright, is usually around 50msec, and since it is a large and slow molecule, it should produce a dark image.
. 3)  . What is a fid, and why do you think it has been given this name?
FID measures the decaying magnetization vector and is free because it occurs when there is no motion disturbance. When the magnetization vector rotates in a perpendicular plane to the B1 direction of the receiver coil, voltage is induced. Since there are no photons absorbed or emitted, it doesn’t measure emitted or transmitted radiation. The energy that is produced through excitation is lost because of the transverse and longitudinal relaxation.
1. . Describe in your own words the meaning of the term “phase coherence”. Discuss how phase coherence is lost. . Can lose phase coherence be recovered – if so, under what circumstances and how?
.Phase coherence occurs in the transverse plane and is described as “the degree to which the individual magnetization vectors align in the same direction”. . The net magnetization or the sum of these vectors produces MR signals while processing within the RF receiver coil. The degree of vector alignment is reduced during dephasing, the signal is lost, and the net magnetization decays. If two vectors are aligned in the same direction within the transverse plane, then they are in-phase. After a very short 90-degree pulse, all the vectors will be in-phase due to perfect alignment. These vectors will lose coherence and will diphase due to varying angular precession rates caused by spin-spin relaxation and field homogeneity effects. If the spin vectors are produced from nuclei with different chemical shifts, they will diphase because of the different precession rates. This dephasing can be recovered in the same way as T2 and T2 dephasing.  .