Sample size calculation
We used the Power Analysis and Sample Size (PASS) software program, version 15.0.5 for Windows 2017, to calculate the sample size. Primary data were obtained from a pilot study conducted in Mansoura University Hospital, which included 12 patients. The primary outcome was the number of vertebral levels reached by the dye. Participants were assigned to either the 20 ml group or the 30 ml group. The number of levels reached was 5.6 ± 1.02 for the 20 ml group and 6.8 ± 1.17 for the 30 ml group, with a difference of 1.2 levels. To achieve 95% power in the proposed study, using a two-sided two-sample unequal-variance t-test with a significance level of 5%, a sample size of 23 patients in each group was required. Assuming a dropout rate of 10%, the number of patients in each group was set at 25.