Carolina Laboratory for Advanced Radiological Imaging and Technology

Let’s look at what happens when we use a 180 pulse instead of a 90. Now, the spin is pointed straight down instead of in the MXY plane. Here is the final move, and it’s a favorite: put a 180° inversion pulse in front of everything. Instead of knocking the longitudinal magnetization down to zero,…

Contrast agents are where all this T1 talk pays off at the workstation. Gadolinium is strongly paramagnetic, and the key idea is simple: it gives nearby water protons a fast new pathway to relax, which shortens their T1 (and, to a lesser degree at usual doses, their T2). As a rate, the effect just adds…

Our original equation quietly assumed a 90° pulse — tip all the magnetization into the transverse plane, read it, repeat. But you can tip by any angle α you like, and gradient-echo imaging usually chooses much less than 90°. Why give up signal on purpose? Because a small flip angle leaves most of the magnetization…

In the spin echo, a 180° pulse rephased the spins and handed us the true T2. What if we leave that pulse out? Then we have a **gradient echo**, and it behaves differently in one important way. Without the 180°, we rephase the spins by briefly reversing a magnetic field gradient instead. That works —…

Here is an inconvenient truth about the `e^(−TE/T2)` term: if you just excite the tissue and watch, the signal fades much faster than the true T2 would predict. The reason is that the main magnetic field is never perfectly uniform. Protons sitting in slightly different local fields precess at slightly different speeds, so they drift…

Notice that TR and TE are numbers *you* choose on the console, but T1 and T2 are not — they are baked into the tissue (and into the field strength). So where do a tissue’s T1 and T2 actually come from? The one-sentence version: **relaxation is most efficient when molecules tumble at roughly the same…

TR controls the recovery of the longitudinal signal. TE controls the opposite process happening to the signal we actually measure — the transverse part. We also adjust TE to control contrast. The instant after excitation, the transverse magnetization `Mxy` (the part we can detect) is at its maximum, and from there it only shrinks. It…

We keep saying “long TR” and “short TR,” but what is TR physically doing? To answer that, we have to look at what the magnetization does *between* pulses. Right after an RF pulse tips the magnetization, the longitudinal component `Mz` (the part lined up with the main magnet) has been knocked down. It then climbs…

In the last post, we made a T1-weighted image by driving the T2 term toward 1. To make a T2-weighted image, we do the mirror image of that trick: this time, we want the **T1** contribution to step out of the way and approach 1. Look again at the T1 part of the equation, `(1…