Slice-Select Rephasing

Drawings of the slice-select gradient often have a little downward lobe at the end. What is this for?  

To keep pulse sequence diagrams as easy to understand as possible, I have taken a few liberties by simplifying some of the gradient shapes in my drawings. One of my most common simplifications has been to draw gradients as perfect rectangles. A rectangular gradient shape implies that a gradient can change from zero to a positive value instantaneously, which, of course is impossible to achieve in practice. Thus all gradients should be drawn as trapezoids (or some similar shape) having a more gradual rate of rise and fall.

A second gradient simplification (the subject of the question above) refers to the fact that in many references you will see a smaller downward projecting lobe in the slice-select gradient immediately after the RF-pulse. This is called the slice-rephasing lobe. This helps correct for the phase dispersion of transverse magnetization that occurs concomitant with application of the main slice-select gradient. Without this rephasing lobe, intravoxel phase dispersion would occur with resulting signal loss.

Advanced Discussion (show/hide)»

In spin-echo sequences the slice-rephasing lobe is sometimes time-shifted to occur immediately before the 180º-pulse, being combined with the left element of a crusher pair surrounding the 180º pulse.

It should be noted that spatially selective refocusing pulses do not require rephasing lobes because the transverse phase accumulated during the first half of the pulse is cancelled by the second half.

---

References
     Hutchinson JMS, Sutherland RJ, Mallard JR. NMR imaging: Image recovery under magnetic fields with large non-uniformities. J Phys E 1978;11:217-221

---

Related Questions
     How does frequency-encoding let you stimulate a given slice?

---

←  Previous Question

Next Question  →

↑ Complete List of Questions ↑