I’ve been reading on the physics of how a Magnetic Resonance Imaging (MRI) works from this link regarding the T1 and T2 relaxation time of protons. So far I understand the concept except what they’re trying to explain in the last 2 paragraphs about spin echo. This is what I got from these last 2 paragraphs, please correct me if I misunderstood something: -The T2 is the time in which the magnetization vector in the XY plane has dropped by 63.2%. - In reality however, there are several external factors (called fixed factors in the link) that make this T2 time differ even if it concerns the same amount of protons in a tissue. Such factors can be that the magnetic field differ in homogeinity between MRI’s, patients having some type of metal in them, etc. Thus, because of those external factors, waiting a T2 time doesn’t necessarily make the magnetization vector in the XY plane drop to 63.2% - To be able to correct for these external factors, there’s a certain technique called the spin echo. Right after an RF pulse is turned off, one waits half a T2 time for the protons to relax/dephase. At ½ T2 time a 180 degrees RF pulse is given so that the magnetization vector of the protons again tips to the direction of the XY plane and rephase again, but now at the other side. The time given for the protons to rephase at the other side of the XY plane is also ½ T2. In that time the protons rephase together again by that 180 degrees RF pulse, creating a magnetization vector at the other side, which is the spin echo. Here are my questions regarding this: 1. Why does a MRI really need to know the exact real T2 time of the protons in a tissue to measure the magnetization vector and form an image? Isn’t it enough just to measure the rate per any other time unit in which a certain amount of protons relax? Since different tissues have different relaxation rates overall, T2 isn’t the only relaxation time that differs among them. 2. How does the spin echo actually determine the exact real T2 time by making the protons rephase together again at the other side of the XY plane? The ½ T2 time itself is also influenced by the external factors in the MRI. What makes waiting a ½ T2 time give the exact magnetization vector decrease for that ½ T2 time, without influence of the fixed external factors, while waiting a full T2 time won’t give the exact 63.2% decrease because of the external fixed factors? Hope someone could enlighten me on this.