def _get_mgz_header(fname):
    if not fname.endswith('.mgz'):
        raise IOError('Filename must end with .mgz')
    header_dtd = [('version', '>i4'), ('dims', '>i4', (4,)),
                  ('type', '>i4'), ('dof', '>i4'), ('goodRASFlag', '>i2'),
                  ('delta', '>f4', (3,)), ('Mdc', '>f4', (3, 3)),
                  ('Pxyz_c', '>f4', (3,))]
    header_dtype = np.dtype(header_dtd)
    with gzip_open(fname, 'rb') as fid:
        hdr_str = fid.read(header_dtype.itemsize)
    header = np.ndarray(shape=(), dtype=header_dtype,
                        buffer=hdr_str)
    # dims
    dims = header['dims'].astype(int)
    dims = dims[:3] if len(dims) == 4 else dims
    # vox2ras_tkr
    delta = header['delta']
    ds = np.array(delta, float)
    ns = np.array(dims * ds) / 2.0
    v2rtkr = np.array([[-ds[0], 0, 0, ns[0]],
                       [0, 0, ds[2], -ns[2]],
                       [0, -ds[1], 0, ns[1]],
                       [0, 0, 0, 1]], dtype=np.float32)
    # ras2vox
    d = np.diag(delta)
    pcrs_c = dims / 2.0
    Mdc = header['Mdc'].T
    pxyz_0 = header['Pxyz_c'] - np.dot(Mdc, np.dot(d, pcrs_c))
    M = np.eye(4, 4)
    M[0:3, 0:3] = np.dot(Mdc, d)
    M[0:3, 3] = pxyz_0.T
    M = linalg.inv(M)
    header = dict(dims=dims, vox2ras_tkr=v2rtkr, ras2vox=M)
    return header
Adapted from nibabel to quickly extract header info
dataset/ETHPy150Open mne-tools/mne-python/mne/source_space.py/_get_mgz_header