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AddnoiseContent-type: text/html addnoiseSection: User Commands (1)Index Return to Main Contents NAMEaddnoise - Adds noise to diffusion MRI measurements.SYNOPSISaddnoise -sigma <noise std> [-noisetype <type>]DESCRIPTIONReads diffusion MRI data and adds noise with standard deviation specified with the -sigma option. By default the noise is Rician with the specified value of sigma, but can be Gaussian by specifying -noisetype gaussian.The -sigma <sig> option specifies the standard deviation of the noise. With the Gaussian noise model, addnoise adds a Gaussian-distributed random number with mean zero and standard deviation <sig> to each measurement in the input data set. If you choose Rician noise, it adds real and imaginary random numbers each Gaussian distributed with zero mean and std <sig>, then takes the modulus, ie S = |S*+N1 + i N2| where S is the noisy signal, S* is the noise free signal, and N1 and N2 are the random numbers. The signal to noise ratio (SNR) is S*/<sig>. That value varies with the diffusion weighting, so often we say that the SNR of a whole data set is S*(0)/<sig>, where S*(0) is the b=0, ie unweighted, signal. If using addnoise with synthetic data from datasynth and the data comes from a compartment or tensor model, S*(0) is usually 1; if it comes from a Monte Carlo simulation, it is the number of walkers in the simulation. You need to adjust <sig> accordingly to give the SNR you want. EXAMPLESAdd Rician noise to synthetic data:datasynth -testfunc 1 -schemefile A.scheme -voxels 10 | addnoise -sigma 0.0625 The above is very similar to: datasynth -testfunc 1 -schemefile A.scheme -voxels 10 -snr 16 only addnoise uses a different random number generator. Add Gaussian noise instead: datasynth -testfunc 1 -schemefile A.scheme -voxels 10 | addnoise -sigma 0.0625 -noisetype Gaussian OPTIONS
AUTHORSDaniel Alexander <camino@cs.ucl.ac.uk>SEE ALSOdatasynth(1)BUGS
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