Figure 1. Locations of 19 catheter tips (circled x) placed by a midline approach. The sections are at the level of the pedicle (Figure 1a), upper intervertebral foramen (Figure 1b), and intervertebral disc and lower portion of the foramen (Figure 1c). In these and other images, anterior is at the top of the image and anatomic left is at the right of the image. (D - dorsal root; IAP - inferior articular process; ISL - interspinous ligament; LF - ligamentum flavum; PLL - posterior longitudinal ligament; SAP - superior articular process; V - ventral root.)

Figure 2. a. CT image after 0.4ml contrast in a 55-year-old woman with an epidural catheter tip in the posterior epidural space (arrow). Multiple air bubbles (black areas) are evident in the spinal canal and along the spinous process.

b. After 4ml further injection, contrast outlines the posterior epidural fat (F) and passes predominantly to the left of the dura (to the right in the image).

c. With an additional 10ml injection, contrast passes out both foramina, especially the left (large straight arrow), and also accumulates anterior to the dura. Air bubbles produce no impediment. Tethering of the posterior dura produces a fold (curved arrow). An epidural tissue membrane directs a layer of contrast (small arrow) along the outer aspect of the dural sac, distinct from the larger posterior collection. In these and all other images, anterior is towards the top of the figure and anatomic right is to the left in the figure.

Figure 3. CT image after 4ml contrast in a 38-year-old woman with a catheter tip between the left ligamentum flavum and posterolateral dura (not seen in this image). Contrast does not pass anterior to the dura, but does accumulate posterolateral to the dura and between the posterior fat and canal wall, as well as out both intervertebral foramina, despite a large air bubble in the left foramen. The arrow indicates posterior passage after solution exits the left foramen.

Figure 4. a. CT image after 0.4ml contrast in a 40-year-old woman with foraminal catheter tip (curved arrow), surrounded by air.

b. After a total of 14ml injection, contrast smoothly encircles the dura without compression, and contrast passes out the intervertebral foramina anteriorly into the psoas muscles (curved arrows). Contrast is also seen passing posteriorly (straight arrow).

Figure 5. a. CT image after 4ml contrast in a 71-year-old woman; contrast accumulates locally in the psoas muscle (arrows). The far lateral tip is at the air bubble but not seen in these views.

b. After an additional 10ml injection, contrast spreads around the dural sac, outlining it without compression. Contrast emerges through the contralateral intervertebral foramen (arrow). Solution stays anterior to the posterior epidural fat (dark triangular area) and does not pass anterior to the dura.

Figure 6. a. CT image in a 48-year-old woman with far lateral catheter tip (curved arrow).

b. After a total of 14ml injection, contrast accumulates anteriorly and posterolaterally in the epidural space, compressing the dura. Two dural folds (straight arrows) are evidence for tethering of the posterior dura. In other views, contrast passes out both foramina.

Figure 7. a. CT image after 4ml contrast in a 57-year-old woman with a posterior midline catheter which passes through the posterior epidural fat (arrows). A combination of air bubbles, fat and catheter have dammed the injectate, preventing its spread into the left epidural posterior space (right in the image). Contrast does pass out the right intervertebral foramen.

b. After an additional 10ml injection, solution has passed anterior to the dura and out both intervertebral foramina, but only slight contrast is seen passing the barrier of air bubbles and posterior epidural fat into the contalateral posterior epidural space (arrow).

Figure 8. CT image after 4ml contrast in a 64-year-old woman with a right foraminal catheter tip (not seen in this image). Contrast passes posteriorly into the paravertebral muscles (solid arrow). The catheter is visible as it crosses the ligamentum flavum (outlined arrow). A collection of solution is partially trapped posteriorly by air and posterior epidural fat where the catheter enters the spinal canal. At other levels, and at this level after further injection, spread of contrast was not impeded. Anterirorly, the solution freely exits the right intervertebral foramen, and is seen at this level lateral to the right pedicle.

Figure 9. CT image after 4ml contrast in a 53-year-old woman with a right foraminal catheter tip (not seen in this image). Contrast layers on the outer aspect of the dura, forming a thin annulus. With further injection, solution passed readily through the intervertebral foramina and posteriorly between the epidural fat and the ligamentum flavum, proving this is an epidural, not subdural, injection. Anesthetic block was normal.

Figure 10. CT image after infusion of 10ml of contrast-containing solution over 1 hour in a 31-year-old woman with a catheter tip against the left pedicle (not seen in this view). There is substantial posterior epidural accumulation between the dura and ligamentum flavum. A loculus of contrast within the dura (arrow) represents a subdural collection.

Figure 11.a. CT image after 0.4ml contrast in a 52-year-old woman with a left foraminal catheter tip (not seen at this level). The catheter is seen just to the (anatomic) left of the spinous process (arrows) and contrast is present in the left posterolateral epidural space.

b. After an additional 14ml injection, contrast is also found anterior to the dura, exiting the right intervertebral foramen, and passing posteriorly along the catheter.