Signal and Noise in Magnetic Resonance Imaging

Signal-to-noise ratio and contrast-to-noise ratio 

At least two kinds of noise arise in MR imaging random and systematic. Random noise (a ) arises from receiver electronics (e.g., Johnson noise), digitization, and from the sample. As the receiver bandwidth increases, noise also increases as the square root of 

For two areas in an image to be distinguishable, a good CNR value is needed. If the signal from a region A is Sa and that of its neighbor B is 5b, then 

One of the most significant developments that has spurred activity in the area of MRM is the commercial availability of high field (e.g., 14.1 and 17.6 T) wide bore magnets. Assuming that the main source of noise is the receiver coil, SNR is proportional to the 7/4th power of the Bq field. Therefore, imaging with high fields is preferable. 

Perhaps the most basic sample characteristic contributing to contrast is the variation in proton density across the sample. Voxel intensity is directly proportional to proton concentration, all other factors being, thus proton concentration differences between voxels give rise to the well-known spin density contrast. Magnetization transfer (MT) contrast is finding increasing application in the clinical realm, but thus far is little used in MRM.  

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