FILTER OUT ANNULAR DATA FROM MATRIX

I have a 501x501 matrix, want annular data 20-150 Km about centre(251,251).. X=1x501; Y=1x501; z=501x501; X=X(:);and so on. made all columnar data X=1*251001; y=1X251001;Z=1x251001;

next found Eucledian Distance= EUCD=(X.*X +Y.*Y).^(0.5);

NOW, Z(EUCD>150)=NaN; Z(EUCD<20)=NaN;

F=[X Y EUCD Z];

F=F(~any(isnan(F),2),:);

tried to plot contourf (X,Y,Z')

getting square plot instead of circular

Pleas ehelp