Sources of Image Noise
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.