Question

Can you schematically explain;

1.)Spin Echo Imaging Methods

2.) Gradient Echo Imaging Methods

Answer 1

SPIN ECHO IMAGING METHOD

The spin echo imaging sequences can be easily implemented and the parameters can be easily varied. Due to difference in T1, T2 tissue or proton density contrast , the MR contrast varies.
A simple form can be explained that consists of RF pulse series of 90 and 180.
All the individual spins are brought in phase because of the magnetization caused by the 90 RF pulse. DEphasing occurs because of T2 effect(i.e, imhomogeneities in the magnetic field). The true T2 effect of spin-spin interaction also causes dephasing.
To negate the spin, 180 RF pulse is applied after some time such that spins that were precessing more rapidly are now rotating behind the spins that were precessing more slowly. This will create a rephasing in the spin after the same time gap and spin echo is again maintained. This time period of coming back to phase again is called as Echo Time.

The below image shows the 3 steps involved :

GRADIENT ECHO IMAGING METHOD

The gradient echo imaging is using the apired gradient applications to dephase and subsequently rephase the spins.
Here, the only repasing that takes place is a reversla of the intentional dephasing provided by the gradients under control of the pulse sequence.
In gradient-echo imaging, after the phase-encoding gradient is switched off, the frequency encoding gradient is reversed as shown in the figure below. Spins that initially experienced an increased field due to the phase encoding will process at a higher frequency. With the reversal of the magnitude of the gradient, the spins that were processing faster are slowed down and the slower spins precess faster thereby resulting in rephasing of the spins in a manner similar to the 180ºc refocusing pulse in spin-echo imaging. Thus, while the 180º RF pulse of spin echo refocuses the dephased spins, a negative gradient lobe in gradient-echo imaging initially causes a phase dispersion of spins that are refocused by reversing the gradient.

Gradient-echo imaging techniques are preferred in many applications where the most rapid imaging is required. The use of a gradient reversal to dephase and then rephase spins has several additional positive consequences. Because only a single RF pulse is applied, the echo can be acquired more quickly thereby signifi cantly reducing echo time. In addition, if low fl ip angles are used, the repetition time can be reduced. These reductions in TR and TE are what enable gradient-echo techniques to acquire images more rapidly than spin-echo techniques. Moreover,since lower fl ip angles are frequently used with gradient-echo imaging, less energy is deposited than using the 90/180º RF pulses of spin-echo imaging.
Therefore, specifi c absorption rates (SAR), which is defi ned as the RF power absorbed per unit mass of an object, can be reduced using gradient-echo imaging relative to spin echo.