Chapter 6 LectureSpin 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=0.• 180 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