Master's Thesis - Studierstube Augmented Reality Project - Graz ...

3.3 Signal localization
1. A gradient field modulates B 0 , so that only one slice can be excited by a certain
RF-pulse with a certain bandwidth.
2. Two other gradients modulate the resulting signal, so that the lines of an excited
slice are phase encoded and the contained frequencies can be discriminative split
up.
Section 3.3.1 and 3.3.2 describes this mechanism more precisely.
3.3.1 Slice Selection
Modern medical imaging requires the formation of an arbitrary cross section through
the body. In MRI such a slice or section 1 can be spatially selective excited by using
a linear gradient field augmenting the field B 0 and a ’shaped’ RF-pulse of a certain
bandwidth. Both the slope of the gradient and the bandwidth of the RF-pulse will
influence the thickness of the selected slice and its shape. The lateral profile of a slice is
for small flip angles approximately the Fourier-transformed of the RF-pulse itself. The
slope of the used gradient affects how many spins will be addressable and how much of
the volume will be excited by the RF-pulse. Figure 3.5 shows the relationship between
slope and bandwidth and their effects on the segment’s profile and dilatation.
Figure 3.5: The slice-profile depends on the RF-excitation pulse and the slope of the
slice selection gradient.
Direct transcription of the former assumption would require using a sinc pulse with
a finite frequency bandwidth of ω hf = ω 0 . The Fourier transformation of such a pulse
1 ’slice’ and ’section’ refers to the same in the following context.
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