def valid_tetrodes(tetrode_ids, tetrode_units):
    """
    Only keep valid tetrodes with neuron units so that there is corresponding spike train data.

    :param tetrode_ids: (list) of tetrode ids in the order of LFP data
    :param tetrode_units: (dict) number of neuron units on each tetrode
    :return: (list) of tetrode ids with neuron units
    """
    return [x for x in tetrode_ids if tetrode_units[x] > 0]