Denoise Flow Phantom Data
The following compares divergence-free wavelet denoising with existing methods on a flow phantom with correct segmentation.
see demo_phantom_seg.m for performances with an incorrect segmentation
Contents
Clear all and check path
close all clc clear if ~exist('dfwavelet_thresh','file') error('Cannot find dfwavelet functions. run setPath!'); end
Load phantom data and set parameters
load 4dFlow_007 % Eddy Current Correction calib = [127-100,128+100;127-40,128+40;32-20,32+20]; [vxEddy,vyEddy,vzEddy] = eddyCorr(vx,vy,vz,calib); vx = vx-vxEddy; vy = vy-vyEddy; vz = vz-vzEddy; % Crop to pipe for faster performance % Comment the following to denoise the entire flow data imMag = imMag(20:250,1:64,17:49); vx = vx(20:250,1:64,17:49); vy = vy(20:250,1:64,17:49); vz = vz(20:250,1:64,17:49); vxEddy = vxEddy(20:250,1:64,17:49); vyEddy = vyEddy(20:250,1:64,17:49); vzEddy = vzEddy(20:250,1:64,17:49); % Set parameters imThresh = 7000; % Image threshold for segmentation imMask = (imMag>imThresh)*1; % Segmentation mask vMag = getVelMag(vx,vy,vz); % Velocity magnitude vMax = 1.0127e+03; % Maximum speed FOV = size(vMag); % Field of view N = FOV(3); % Number of slices (for plotting) ph0 = zeros(FOV); % Reference phase in phase contrast % Plot figure,imshow_flow(imMask,vx,vy,vz,vMax,[1,2,3]),title('Original Flow Field','FontSize',14); figure,imshow_flowmag(imMask,vx,vy,vz,vMax,[1,2,3]),title('Original Velocity Magnitude','FontSize',14); pause(1);


Get Noisy Data
load 4dFlow_007_noisy % Crop to pipe for faster performance % Comment the following to denoise the entire flow data vxN = vxN(20:250,1:64,17:49); vyN = vyN(20:250,1:64,17:49); vzN = vzN(20:250,1:64,17:49); % Eddy Current Correction vxN = vxN-vxEddy; vyN = vyN-vyEddy; vzN = vzN-vzEddy; % Mask flow data with good segmentation [vxN,vyN,vzN] = maskIM(imMask,vxN,vyN,vzN); % Plot figure,imshow_flow(imMask,vxN,vyN,vzN,vMax,[1,2,3]),title('Noisy Flow Field','FontSize',14); figure,imshow_flowmag(imMask,vxN,vyN,vzN,vMax,[1,2,3]),title('Noisy Velocity Magnitude','FontSize',14); % Calculate errors disp('Noisy Flow Error') [vNRMSE_Noise,vMagErr_Noise,angErr_Noise] = calcVelError(imMask,vx,vy,vz,vxN,vyN,vzN); PVNR = 20*log10(1/vNRMSE_Noise); fprintf('PVNR: \t\t\t%.2fdB\nNRMSE: \t\t\t%f\nvMag Error: \t\t%f\nAbsolute Angle Error: \t%f\n\n',PVNR,vNRMSE_Noise,vMagErr_Noise,angErr_Noise); pause(1)
Noisy Flow Error PVNR: 33.44dB NRMSE: 0.021276 vMag Error: 0.012291 Absolute Angle Error: 0.008179


DivFree Wavelet with SureShrink and MAD sigma estimation
Here, we use Median Absolute Deviation to estimate noise std and then use SureShrink to find the optimal threshold that minimizes MSE
minSize = 8*ones(1,3); % Smallest wavelet level size % Denoise [vxDFWsm,vyDFWsm,vzDFWsm] = dfwavelet_thresh_SURE_MAD(vxN,vyN,vzN,minSize,res); % Plot figure,imshow_flow(imMask,vxDFWsm,vyDFWsm,vzDFWsm,vMax,[1,2,3]) title('Div Free Wavelet w/ SureShrink (Flow Field)','FontSize',14) figure,imshow_flowmag(imMask,vxDFWsm,vyDFWsm,vzDFWsm,vMax,[1,2,3]) title('Div Free Wavelet w/ SureShrink (Vel Mag)','FontSize',14) % Calculate errors disp('DivFree Wavelet w/ SureShrink and MAD') [vNRMSE_DFWsm,vMagErr_DFWsm,angErr_DFWsm] = calcVelError(imMask,vx,vy,vz,vxDFWsm,vyDFWsm,vzDFWsm); fprintf('NRMSE: \t\t\t%f\nvMag Error: \t\t%f\nAbsolute Angle Error: \t%f\n\n',vNRMSE_DFWsm,vMagErr_DFWsm,angErr_DFWsm); pause(1);
DivFree Wavelet w/ SureShrink and MAD NRMSE: 0.017056 vMag Error: 0.010097 Absolute Angle Error: 0.006121


DivFree Wavelet with SureShrink, MAD and random cycle spinning
To remove the blocking artifacts, we do partial cycle spinning Here we do 2^3=8 random shifts
spins = 2; % Number of cycle spinning per dimension isRandShift = 1; % Use random shift minSize = 8*ones(1,3); % Smallest wavelet level size % Denoise [vxDFWsms,vyDFWsms,vzDFWsms] = dfwavelet_thresh_SURE_MAD_spin(vxN,vyN,vzN,minSize,res,spins,isRandShift); % Plot figure,imshow_flow(imMask,vxDFWsms,vyDFWsms,vzDFWsms,vMax,[1,2,3]) title('Div Free Wavelet w/ SureShrink and Partial Cycle Spinning (Flow Field)','FontSize',14) figure,imshow_flowmag(imMask,vxDFWsms,vyDFWsms,vzDFWsms,vMax,[1,2,3]) title('Div Free Wavelet w/ SureShrink and Partial Cycle Spinning (Vel Mag)','FontSize',14) % Calculate errors disp('DivFree Wavelet w/ SureShrink, MAD and Partial Cycle Spinning') [vNRMSE_DFWsms,vMagErr_DFWsms,angErr_DFWsms] = calcVelError(imMask,vx,vy,vz,vxDFWsms,vyDFWsms,vzDFWsms); fprintf('NRMSE: \t\t\t%f\nvMag Error: \t\t%f\nAbsolute Angle Error: \t%f\n\n',vNRMSE_DFWsms,vMagErr_DFWsms,angErr_DFWsms); pause(1);
DivFree Wavelet w/ SureShrink, MAD and Partial Cycle Spinning NRMSE: 0.015193 vMag Error: 0.009123 Absolute Angle Error: 0.004896


Finite Difference Method
The following implements finite difference method denoising as described in: Song SM, Pelc NJ., et al. JMRI 1993 Noise reduction in three-dimensional phase-contrast MR velocity measurements.
% Denoise [vxFDM,vyFDM,vzFDM] = fdmDenoise(vxN,vyN,vzN,res); % Plot figure,imshow_flow(imMask,vxFDM,vyFDM,vzFDM,vMax,[1,2,3]) title('Div Free FDM','FontSize',14) figure,imshow_flowmag(imMask,vxFDM,vyFDM,vzFDM,vMax,[1,2,3]) title('Div Free FDM','FontSize',14) % Calculate errors disp('Finite Difference Method Error') [vNRMSE_FDM,vMagErr_FDM,angErr_FDM] = calcVelError(imMask,vx,vy,vz,vxFDM,vyFDM,vzFDM); fprintf('NRMSE: \t\t\t%f\nvMag Error: \t\t%f\nAbsolute Angle Error: \t%f\n\n',vNRMSE_FDM,vMagErr_FDM,angErr_FDM); pause(1);
Finite Difference Method Error NRMSE: 0.033693 vMag Error: 0.019750 Absolute Angle Error: 0.020871


DivFree Radial Basis Function
The following implements divergence-free radial basis function denoising as described in: Busch J, Kozerke S., et al. MRM 2012 Construction of divergence-free velocity fields from cine 3D phase-contrast flow measurements.
radius = 4; % Radius of kernel nIter = 20; % Number of iterations for lsqr % Plot during iterations, if on, does gradient descent instead of lsqr doplot = 0; % Denoise % Takes a while to run [vxRBF,vyRBF,vzRBF] = rbfDenoise(vxN,vyN,vzN,imMask,radius,res,nIter,doplot); % Plot figure,imshow_flow(imMask,vxRBF,vyRBF,vzRBF,vMax,[1,2,3]), title('Div Free RBF','FontSize',14) figure,imshow_flowmag(imMask,vxRBF,vyRBF,vzRBF,vMax,[1,2,3]), title('Div Free RBF','FontSize',14) % Calculate errors disp('Radial Basis Function Error') [vNRMSE_RBF,vMagErr_RBF,angErr_RBF] = calcVelError(imMask,vx,vy,vz,vxRBF,vyRBF,vzRBF); fprintf('NRMSE: \t\t\t%f\nvMag Error: \t\t%f\nAbsolute Angle Error: \t%f\n\n',vNRMSE_RBF,vMagErr_RBF,angErr_RBF); pause(1);
RBF Denoising... done Radial Basis Function Error NRMSE: 0.017041 vMag Error: 0.011503 Absolute Angle Error: 0.008135


Plot all
Plotting results from all methods.
figure,imshow_flowmag(... cat(2,imMask,imMask,imMask,imMask,imMask,imMask),... cat(2,vx,vxN,vxDFWsm,vxDFWsms,vxFDM,vxRBF),... cat(2,vy,vyN,vyDFWsm,vyDFWsms,vyFDM,vyRBF),... cat(2,vz,vzN,vzDFWsm,vzDFWsms,vzFDM,vzRBF),... vMax,[1,2,3]); title('Original,Noisy,DFW,DFW/spin,FDM and RBF','FontSize',14) pause(1)

Visualize DivFree Wavelet Coefficients
% Wavelet coefficients for original minSize = 8*ones(1,3); [vx,vy,vz] = maskIM(imMask,vx,vy,vz); [wcdf1,wcdf2,wcn,numLevels,wcSizes] = dfwavelet_forward(vx,vy,vz,minSize,res); wcdf1_tile = wcTile(wcdf1,numLevels,wcSizes); wcdf2_tile = wcTile(wcdf2,numLevels,wcSizes); wcn_tile = wcTile(wcn,numLevels,wcSizes); % Wavelet coefficients for noisy [wcdf1N,wcdf2N,wcnN,numLevels,wcSizes] = dfwavelet_forward(vxN,vyN,vzN,minSize,res); wcdf1N_tile = wcTile(wcdf1N,numLevels,wcSizes); wcdf2N_tile = wcTile(wcdf2N,numLevels,wcSizes); wcnN_tile = wcTile(wcnN,numLevels,wcSizes); % Wavelet coefficeints for div free wavelet w/ cycle spinning [wcdf1sms,wcdf2sms,wcnsms,numLevels,wcSizes] = dfwavelet_forward(vxDFWsms,vyDFWsms,vzDFWsms,minSize,res); wcdf1sms_tile = wcTile(wcdf1sms,numLevels,wcSizes); wcdf2sms_tile = wcTile(wcdf2sms,numLevels,wcSizes); wcnsms_tile = wcTile(wcnsms,numLevels,wcSizes); % Plot figure,imshow3f(abs(cat(2,wcn_tile,wcnN_tile,wcnsms_tile)),[1,2,3],round(N/4)),title('Non-divfree wavelet subbands for Original, Noisy and DFW','FontSize',14);
