Signal-to-Noise Ratio (SNR) in MRI - K-space.org

Signal-to-Noise Ratio (SNR) in MRI

Consider an imaging sequence with the following parameters:













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Basics:






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NRO (Nx): Frequency encoding (readout) matrix size Ny: In-plane phase encoding matrix size Nz: Number of slices TR: Repetition time TE: Echo time NEX: Number of excitations (number of averages) FOVx: Field-of-view in the readout direction FOVy: Field-of-view in the phase encoding direction FOVz: Field-of-view in the slice direction ∆z: Slice thickness (= FOVz/Nz for 3D) ∆V: voxel size BW: sampling bandwidth (in frequency encoding encoding direction) Noise ∝ BW Signal ∝ voxel size Averaging is the only mechanism to make the SNR better Phase encoding involves averaging (independent measurements) Observe the inverse relationship between acquisition time and SNR

General SNR Expression: SNR ∝ ∆V ⋅

N PE ⋅ NEX BW

(1)

where,  Ny N PE =  N y ⋅ N z

( Multislice) (3 D)

 FOVx   FOV y ⋅   N  x   Ny

∆V = 

  ⋅ ∆z  

(2) (3)