The needle is smaller in size than that used during a conventional epidural approach. The procedure is performed with the patient lying on their belly using fluoroscopic (real-time x-ray) guidance, which helps to prevent damage to the nerve root. A radiopaque dye is injected to enhance the fluoroscopic images and to confirm that the needle is properly placed (See Figure 2). This technique allows the glucocorticoid medicine to be placed closer to the irritated nerve root than using conventional interlaminar epidural approach. The exposure to radiation is minimal.
Spinal infusion involves placing a tiny catheter into the spinal sac or epidural space. These spaces around the spinal sac are separated only by a thin membrane called the dura mater which includes a layer called the arachnoid membrane. Spinal or subarachnoid infusions place medicine inside the spinal sac (inside the dura mater) where the medicine can spread out in the cerebral spinal fluid (CSF) and affect wide areas of pain. Epidural ( above the dura ) medicines flow into the epidural space where the fluid spread is much more restricted by the epidural contents (epidural fat and blood vessels). Where the catheter is placed, and the kind of medicine infused depends on the patient, the type of pain, the duration of infusion, and many other factors.
Kenalog in blood - Derby et al. "Size and aggregation of corticosteroids used for epidural injections"