chapter 7: gradient echo imaging methods

Chapter 7: Gradient Echo Imaging Methods

Mark D. Herbst, MD, PhD

Two Main Imaging Methods

• Spin Echo – uses 180 degree refocusing RF pulse to refocus spins and form an echo that is captured and put into k-space

• Gradient Echo – uses gradient pulse to form the echo

• Gradient Echo – abbreviated GE, or GRE for Gradient Recalled Echo, or FFE for Fast Field Echo, or SAGE (small angle GE)

Advantages of GE Methods

• Faster than SE (spin echo) because of short TRs and short TEs

• Shows flowing blood as bright good for MRA• Good for functional MRI• Good for subtle hemorrhage (microhemorrhage

technique or magnetic susceptibility technique)• Can be combined with SE to produce very fast

images (GRASE=gradient and spin echo)

Disadvantages of GE Methods

• Can have too much metal artifact• GE gives T2*-weighted images (“T-two-

star”) instead of T2WI• Need to be aware of fat and water being

in-phase or out-of-phase at interfaces between water and fat

Spin Echo

Gradient Echo

Gradient Echo

Spoiled Gradient Echo

• “Spoiling” refers to the destruction of any T2 contrast in the image, leaving you with a fast way of getting a T1WI using a gradient technique. The “spoiler pulse” completely dephases the spins in the xy plane, removing any evidence of T2 or T2* contrast.

GRASS and Spoiled GRASS

Spin Echo

Effect of Flip Angle on Signal

Effect of Flip Angle on Net Magnetization Vector

Effect of Flip Angle on Longitudinal and Transverse Magnetization

Effect of Flip Angle on Contrast

Effect of TR on M

GRASE

Spin Echo Planar Imaging (EPI)

Gradient Echo Planar Imaging (EPI)