I’ve spent a long time around scanners and re-read this book before helping students. It’s remarkable easy to lose track of the fundamentals, though maybe that’s just me.
(vastly simplified) MRI basically functions on two fundamental mechanisms--"spin echo" and "gradient echo". Spin echo signal is described by T1 and T2. Gradient echo signal is described by T1 and T2*. The difference between T2 and T2* relate to local magnetic properties of the tissue which is called "susceptibility". Blood contains iron so its presence alters T2* and this is exploited clinically. A good example of T2* imaging used clinically is susceptibility-weighted imaging (SWI).
T2* effects increase with higher MRI main field strength. From what I can tell so far these ultra low-field scanners have to rely on spin echoes.
These are different MRI sequences that are weighted differently to produce a specific contrast that show different characteristics of the tissue that is imaged.
I really liked ‚MRI made easy‘ as an introduction to MRI physics. Just google it, it’s a free Book
It sounds like different modes of taking (or interpreting/visualizing?) an MRI.