The spin echo (SE) sequence is the most commonly used pulse sequence in clinical imaging. The sequence comprises two radiofrequency pulses - the 90 degree pulse that creates the detectable magnetisation and the 180 degree rephasing pulse at TE/2 – signal reading at TE . With each repetition, a k-space line is filled, thanks to a different phase encoding. The 180° rephasing pulse compensates for the constant field heterogeneities to obtain an echo that is weighted in T2 and not in T2*.The selection of TE and TR determines resulting image contrast. In T1-weighted images, tissues that have short T1 relaxation times (such as fat) present as bright signal. Tissues with long T1 relaxation times (such as cysts, cerebrospinal fluid and edema) show as dark signal. In T2-weighted images, tissues that have long T2 relaxation times (such as fluids) appear bright.
RF - This line represents the timing of the radio frequency (RF) pulse. This is an electromagnetic wave used excite the protons. The 900 pulse flip the magnetization vectors into the horizontal plane ( usually denoted the xy plane). The 1800 pulse is generated to refocus (or rephase) the magnetization vectors to form the spin echo.
Echo - This line represents the MR signal
Gz - This is slice selection gradient. It is turned on whenever an RF pulse is fired.
Gy - This is the phase encoding gradient. It is applied for each TR at a different value
Gx - This is the frequency encoding gradient
No comments:
Post a Comment