Chapter 6 Lecture Spin Echo Imaging Methods Mark D. Herbst, MD, PhD Two Main Types of MR methods –Spin Echo –uses RF pulse to get an echo –Gradient Echo –uses gradient pulse to get an echo

The spin echo process Net magnetization vector starts at equilibrium, pointing up along the +z direction. 90 degree RF pulse  M now in xy plane Spins in xy plane begin dephasing, and when totally dephased, M= degree refocusing RF pulse is applied Spins in xy plane begin to rephase, and echo forms. This echo is captured and stored in k- space.

The Spin Echo Process

Three basic spin echo methods PD-weighted (proton density) – long TR, short TE T2-weighted – long TR, long TE T1-weighted – short TR, short TE

Causes of Spin Dephasing Spin-spin interactions – energy exchange among the spinning nuclei causes relaxation (dephasing) Magnetic field inhomogeneity – cases spins to dephase, reversed by 180 degree RF refocusing pulse

Multiple Spin Echo Done by using more than one 180 degree RF refocusing pulse Commonly used to get both PDW and T2W images at the same time Used in FSE (fast spin echo, turbo spin echo) to get images faster by filling k- space faster than conventional spin echo (CSE)

Inversion Recovery Gives high level of T1 contrast Used to suppress either water or fat (STIR suppresses fat, FLAIR suppresses water) Starts with a 180 degree RF inversion pulse, placing M in the –z direction Then, wait TI (inversion time) before applying the 90 degree RF pulse that puts M in the xy plane Finally, apply the 180 degree RF refocusing pulse to form the echo for data collection, put it in k-space.

TI: inversion time The time of inversion The time that M is allowed to stay inverted (-z) The time between the initial 180 degree inversion pulse and the 90 degree pulse that puts M in the xy plane.

180 degree pulse

IR pulse diagram (STIR and FLAIR)

STIR – fat at null point

FLAIR-- water at null point