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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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SNR and Voxel Size

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You decide to try and increase the SNR by increasing the voxel size. Why might this work? List also any tradeoffs. (Assume bandwidth is kept constant.)

Refer to these equations: SNR calculation and voxel size calculation. You can increase the voxel size by ↑ FOV, ↓ Ny, or ↑ slice thickness. The SNR is increased because there is more signal in each voxel. But isn't there more noise too? The noise is "white" (covering a wide range of frequencies, uncorrelated from measurement to measurement). This means the noise doesn't increase and so the signal-to-noise ratio goes up.

Tradeoffs: more partial volume artefact, decreases in-plane spatial resolution.

Further reading on this topic:
Books: MRI From Picture to Proton p73-74, 168, MRI The Basics p99
Online: MRItutor 1, 2

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