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

3.3 Signal localization
would result in an ideal square slice profile. In practice the frequency range is higher
since on the one hand a perfect sinc pulse is technically not possible and on the other
hand a sharper desired slice profile requires a higher energy of the RF-pulse. Applying
too much energy to a human body per time unit would lead to a warming of the
tissue by more than 1 ◦ C, which can be harmful for living cells. This absorbed energy
amount is called Specific Absorption Rate (SAR) and restricts the slope and the speed
of switching gradients, the energy of the RF-pulses and the sharpness of the slice profile.
The fact that the profiles are not ideal square cause a slice distance of more than
zero or a doubled measurement time with interleaved slice measurement. Elsewhere
partial volume effects would garble the resulting images depending on the sequence.
Figure 3.7 shows the selection of a segment, placed perpendicular in the xy-plane
by a linear gradient G z in z-direction. However, slice-selection in arbitrary direction
and location needs a combination of gradients in three directions
⎧
⎨ G x = G ss sinθ cos φ
G ss = (G x , G y , G z ) = G y = G ss sinθ sin φ
⎩
G z = G ss cosθ
(3.24)
and a RF-pulse
ω hf = ω 0 + γG ss c 0 , ∆ω = γG ss ∆c (3.25)
with the excitation frequency ω hf , bandwidth ∆ω, the center of the segment c 0 and its
thickness ∆c.
A frequency response of all spins situated in the selected segment will be detectable
after an excitation done as described in this section. Obviously further dispartment of
the signal will be necessary to locate a certain volume element’s response within the
activated segment.
3.3.2 Voxel Selection
During the precession period the second step of spatial localization is called frequencyand
phase encoding. Frequency encoding makes the oscillating frequency of the complex
MR-signal dependent on its spatial origin which can be utilized to locate a column of
the activated slice. How to impose different Larmor frequencies on different locations
has already been described in the previous sections. We need to apply another gradient
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