The imag(M) calling sequence computes the element-wise imaginary part of M.  The result, R, is formed as R[i,j] = imag(M[i,j]).

$\mathrm{with}\left(\mathrm{MTM}\right)\:$

$M≔\mathrm{Matrix}\left(2\,3\,'\mathrm{fill}'=-3+\mathrm{I}\right)\:$

$\mathrm{imag}\left(M\right)$

$\left[\begin{array}{ccc}1& 1& 1\\ 1& 1& 1\end{array}\right]$