There are various mechanism to achieve different contrasts, or in another word, to alter the way different brain tissues look like in an MRI image. You might have the option to highlight structures, such as the tumor or blood vessel; you can also have the liberty to absolutely erase the tissues from an image (if you are interested, check out inversion recovery images). All these contrasts are achieved through manipulating the flip-angle (α), echo time (TE), and repetition time (TR) for determining the turn on and off of the magnetic fields (of course, there can also be the inversion time for the inversion recovery images, and maybe even some other parameters).
To make a simple demonstration of how it works, I decided to use a relatively simple, but widely used MRI method, called the gradient-echo images. This is also used in the "mind-reading MRI". You can predict the image contrast based on the signal equation below:.
First, let's take a look at the case when TR and TE are fixed, what will happen if we alter the flip-angle.
Now to make the picture prettier, let's throw in another parameter, the echo time (TE) as an additional variable, and still keep the TR unchanged (for our demonstration, TE<TR).
One last note, the only thing we didn't alter is TR, which is often related to how long the MRI time could be... As a rule of thumb, if you want to keep a relatively constant image contrast (mostly for the case of T1 images), your flip-angle should increase/decrease with your TR, and this scaling factor is proportional to the square root of (TR-new/TR-old).
The image below shows a T2-weighted MRI that provides probably more information for diagnostic purposes.