def METHOD_NAME(vectors, sphere_vertices):
    """Find the closest vector on the sphere vertices using a cKDT tree
        sphere_vertices must be normed (or all with equal norm).
    Parameters
    ----------
    vectors : numpy.ndarray (2D)
        Vectors representing the direction (x,y,z) of segments.
    sphere_vertices : numpy.ndarray (2D)
        Vertices of a Dipy sphere object.
    Returns
    -------
    dir_sphere_id : numpy.ndarray (1D)
        Sphere indices of the closest sphere direction for each vector
    """
    sphere_kdtree = cKDTree(sphere_vertices)
    _, dir_sphere_id = sphere_kdtree.query(vectors, k=1, workers=-1)
    return dir_sphere_id