Assessing the effectiveness of artificial intelligence (AI) in prioritising CT head interpretation: study protocol for a stepped-wedge cluster randomised trial (ACCEPT-AI) von Vimalesvaran, K, Robert, D, Kumar, S, Kumar, A, Narbone, M, Dharmadhikari, R, Harrison, M, Ather, S, Novak, A, Grzeda, M, Gooch, J, Woznitza, N, Hall, M, Shuaib, H, Lowe, DJ