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
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.
Effect of Flip Angle on Longitudinal and Transverse Magnetization