diff --git a/pyeit/eit/fem.py b/pyeit/eit/fem.py
index a58a7a7..a5030c9 100644
--- a/pyeit/eit/fem.py
+++ b/pyeit/eit/fem.py
@@ -238,10 +238,7 @@ def solve_nd(self, ex_mat, perm):
         # 4. solving nodes potential using boundary conditions
         b = self._natural_boundary_nd(ex_mat)
 
-        def f_init(b):
-            return np.dot(r_matrix, b).ravel()
-
-        f = np.array(list(map(f_init, b)))
+        f = np.dot(r_matrix, b[:, None]).T.reshape(b.shape[:-1])
 
         # 5. build Jacobian matrix column wise (element wise)
         #    Je = Re*Ke*Ve = (nex3) * (3x3) * (3x1)