RF Pulse Shape
The pulses are shaped like truncated sinc functions [ sinc (x) = (sin (x))/x ]. Without truncation sinc(x) → ±∞. A sinc(x) function is used because the Fourier transform of a sinc function is a rectangle. Remember that the time domain and frequency domain are related by the Fourier transform. Thus the shape of the excited tissue will be a rectangle: a "slice". We use a sinc function so that the edges of the slice (between one slice and the next one) can be made as sharp as possible.
Further reading on this topic:
Books: MRI From Picture to Proton p112, 346, Q&A in MRI p74, MRI The Basics p94-95