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» Image Creation

How is the MRI image created? This section explores slice selection, spatial encoding and the FFT.

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Partial Flip Angle

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A shorter repetition time (which shortens overall image acquisition time) is allowed if the flip angle is smaller. Why is this?

The T2* time constant decreases.

Smaller flip angles are quicker to apply.

Less time is required for "full" recovery of Mz.

INCORRECT. The T2* time is not changed by RF pulses or magnetic field gradients.

Try again.

INCORRECT. Smaller flip angles can be quicker to apply, but this does not significantly affect the repetition time (TR) of the pulse sequence. How does the flip angle affect the TR?

Try again.

Partial flip angle. The rotation of net magnetisation into the xy-plane need not be 90°. Less time is required for the recovery of Mz.

CORRECT. We can reduce the amount of time it takes to get an image in a large number of ways in MRI. However, the most simple is to reduce the amount of magnetisation which we rotate into the xy-plane.

Partial rotation of the net magnetisation vector produces a smaller signal; less magnetisation is precessing in the xy plane to start with. But there is usually enough for a good signal measurement. Since Mz has a headstart on it's way back to thermal equilibrium (M0), we don't need to wait for as long before we can rotate the net magnetisation again and make another signal measurement (with a different phase encoding, of course).

Partial rotation of the net magnetisation is described by a lower angle of rotation (e.g. a 15° RF pulse).

Further reading on this topic:
Books: Q&A in MRI p103, MRI From Picture to Proton p30, MRI The Basics p244
Online: MR-TIP

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