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» Pulse Sequences

How do we change image contrast? What are the effects of parameter changes? This section explores image contrast and image acquisition methods.

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Turbo Gradient Echo

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How does Turbo Gradient Echo differ from conventional Gradient Echo?

The need to wait for "full" recovery of longitudinal magnetisation (Mz) before each iteration through a conventional gradient echo pulse sequence (with different phase encoding each time) increases the overall imaging time. Instead of waiting for "full" recovery, it is possible to simply start the next pulse sequence with the Mz which is available at the time, and make sure there is no Mxy component of magnetisation by applying a "spoiler" magnetic field gradient. (Remember, all our static magnetic field gradients, whilst performing a useful function also cause dephasing. This time the dephasing will help us scan faster.) This is turbo gradient echo readout (TGE, aka TurboFLASH, TFE and FastSPGR).

A Turbo Gradient Echo pulse sequence, in which the TR is made very short, and a spoiler gradient before the start of each iteration through the pulse sequence ensures Mxy is zero. A "very short TR" may be just a few milliseconds. The RF excitation pulse must be small. A 180° degree preparation pulse may also be used to provide T1-weighting.

Since Mz is not allowed to recovery properly between each signal measurement, the MRI signals which are measured are of lower amplitude (all other things being equal), and may change as Mz approaches a steady value (when the amount of recovery of Mz equals the amount lost during each excitation pulse). But this is an acceptable tradeoff in many imaging situations, where speed is paramount (e.g. imaging a beating heart, for example).

The Turbo Gradient Echo pulse sequence affords a significant decease in image acquisition time, whilst sacrificing little image quality. The ability to acquire all the MRI signals needed for an image very quickly allows us to effectively "freeze" the heart motion (image acquisition time <200ms is short compared to the cardiac cycle period).

Further reading on this topic:
Books: Q&A in MRI p120
Online: MR-TIP

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