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These questions are concerned MRI hardware and quality control of images.

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Sources of Image Noise

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What are the two largest sources of noise (in images) in MRI?

Electronic noise generated in the receiver circuit and the patient (Brownian motion of ions in bodily fluids).

A modern scanner can keep the pre-amplifier noise down to well below 1 dB, and the use of local receive coils serves to increase SNR because (naturally) the signals received in the region of the surface coil have a higher amplitude. In quadrature coil design, the two receivers are at 90° to one another, enabling the distinguishing of the real and imaginary components of the received signal, increasing the SNR by √2. RF shielding keeps out anomalous external signals, and so they are not considered here.

Further reading on this topic:
Books: MRI From Picture to Proton p66, Q&A in MRI p63-65, MRI The Basics p28
Online: Basics of MRI

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