Postoperatively, patients underwent CT scanning with 0.1 mL of contrast dye (Iomeron, iomeprol, Bracco) to precisely determine the catheter tip position and permit adjustments if needed. Two anesthesiologists independently evaluated the CT images with regards to the insertion level, depth relative to the lamina, and catheter tip location in relation to key anatomical structures. Tip locations were defined as paravertebral or non-paravertebral. Different locations were identified for both the paravertebral and the non-paravertebral spaces.

Paravertebral space locations:

Anterior paravertebral: Between the anterior and posterior borders of the vertebral corpus.

Periforaminal paravertebral: Within 1 cm of the foramen. Posterior paravertebral: From the foramen to the articular process.

Non-paravertebral locations:

Prevertebral: Anterior to the vertebral body. Intramuscular: Posterior to the lamina within the erector spinae muscles. Epidural and Pleural: Standard anatomical definition.

Postoperatively, patients received a CT scan with 0.1 mL of contrast dye (Iomeron, iomeprol, Bracco) to evaluate the catheter tip position. Only patients with a catheter in the paravertebral space received a second bolus of 1 mL contrast dye and 9 mL saline 0.9%. Two anesthesiologists independently evaluated the catheter tip position across the different anatomical areas close to the paravertebral space. Additionally, the contrast distribution was assessed in the anteroposterior and craniocaudal direction of catheters with a paravertebral catheter tip position.

Paravertebral space locations:

Anterior paravertebral: Between the anterior and posterior borders of the vertebral corpus.

Posterior paravertebral: From the foramen to the articular process.

Non-paravertebral locations:

Prevertebral: Anterior to the vertebral body. Intramuscular: Posterior to the lamina within the erector spinae muscles. Epidural, Intercostal and Pleural: Standard anatomical definitions applied.