Most students find that learning the basic physics of MRI is challenging. Even now I notice certain aspects of MRI theory which I thought I understood, but then I find out that my understanding is inadequate. (The latest of which can be read elsewhere on ReviseMRI.com.) A confusion which occurred in my mind when I first learned basic MRI physics was misunderstanding of the spin echo pulse sequence vs. the gradient echo pulse sequence.
Perhaps one way we can help students to understand MRI pulse sequences is to teach gradient echo before we introduce spin echo.
If students learn about spin echo first, it is likely that they will attribute the formation of an echo to the use of the 180° RF refocusing pulse. The frequency encoding gradient sneaks in to diagrams without much thought or attention—it’s just some encoding by varying the Larmor frequency, right? The pre-dephasing lobe of the frequency encoding gradient is understood in terms of getting maximum signal at the centre of k-space, but no more than that.
Then comes gradient echo: take away the 180° pulse, reduce the flip angle, run the whole thing faster, and …we still have an echo! But how can an echo be formed if the thing which was making the echo has been removed? You see the dilemma.
It may be better to ignore spin echo until the gradient echo pulse sequence is well understood. The revision questions on this site follow this pattern.
Gradients, whilst performing useful functions, also cause dephasing, which contrubutes to the reduction of the MRI signal. We use the pre-dephasing lobe of the frequency encoding gradient to cause extra dephasing in the opposite sense. So, when the signal is being measured, the main frequency encoding gradient has to reverse that dephasing first, before it causes it’s own dephasing. Then the largest part of the signal (with least dephasing) is acquired during recording of the central k-space data points. So in gradient echo, the only sense in which the signal “comes back”—like an echo—is in response to the pre-dephasing lobe of the frequency encoding gradient. Apart from this, the signal is there, available, and decaying away with time (according to the T2* time constant). It’s just modified by the gradients used.
The function and effects of the 180° RF refocusing pulse can then be introduced as an extra trick, used to acquire a larger signal if there is time to do so.
First :A great site and good effort..!
January 30th, 2007, at 9:31 pm #Second: Anyone plz tell me the applications and uses of T2* wImg?
[...] Some examples of cool posts on the blog are a short discussion of statistical methods for testing agreement, ruminations on teaching the gradient echo first, and a series of posts called “Use MRI for: …” (like bees, or lying, or mystical experiences…). Plus notable quotes by Erwin Hahn and Edward Purcell. [...]
February 11th, 2007, at 5:29 pm #