diff --git a/MATLAB/filter_spinwalk.m b/MATLAB/filter_spinwalk.m
new file mode 100644
index 0000000..d9bd2dd
--- /dev/null
+++ b/MATLAB/filter_spinwalk.m
@@ -0,0 +1,28 @@
+function m1_t = filter_spinwalk(m1, xyz1, tissue_type, mask, fov)
+%
+% m1    : 3 * spins
+% xyz1  : 3 * spins
+% tissue_type : array of tissue types
+% mask  : 
+% fov   : in mm
+%
+
+m1   = squeeze(m1);
+xyz1 = squeeze(xyz1);
+
+if size(m1,1) ~= 3
+    error('m1 must be of size 3*n')
+end
+
+if size(m1) ~= size(xyz1)
+    error('size mismatch between m1 and xyz1')
+end
+
+sz      = size(mask);
+scale   = sz(:) ./ fov(:);
+I123    = floor(xyz1 .* scale) + 1;
+
+ind     = sub2ind(sz, I123(1,:,:,:), I123(2,:,:,:), I123(3,:,:,:));
+% ind_t   = mask(ind) == tissue_type;
+ind_t   = ismember(mask(ind), tissue_type);
+m1_t    = m1(:,ind_t);