64635 / 64636 4 levels

We are constantly being denied for multiple units of 64635 and 64636. The procedure is bilateral and 4 lumbar levels. I have been coding it 64635 -50 then 64636 -50 on one line then 64636 -50 -59 on the second line. Does anyone have any info to offer on several lumbar levels billed and if I am doing it incorrectly? Thanks!