11.2 CT Myelography & Postmyelogram Imaging

Why This Topic Matters

"Postmyelogram" is called out by name as one of only four spine leaf items in the ARRT CT content outline, and it connects directly to a Patient Care blueprint item candidates often underweight: the intrathecal administration route. Because myelography combines a fluoroscopic procedure, a specific contrast-agent restriction, and a CT acquisition, it is a natural place for the exam to write cross-domain scenario questions that test contrast safety alongside spine anatomy.

Core Terms and Rules

• Myelography: fluoroscopically guided imaging of the subarachnoid space (the cerebrospinal-fluid-filled space surrounding the spinal cord and nerve roots, bounded by the thecal sac) after a lumbar puncture (LP) delivers contrast directly into that space.
• Only water-soluble, nonionic, low-osmolar iodinated contrast agents specifically labeled for intrathecal use (for example, iohexol formulated for myelography) may be injected intrathecally. Ionic contrast agents are contraindicated intrathecally — they are neurotoxic in the subarachnoid space and can precipitate seizures or arachnoiditis. This is a hard safety rule, not a preference, and a general-purpose IV formulation is not automatically the correct choice for an intrathecal injection.
• Postmyelogram CT: the cross-sectional CT acquisition performed after the fluoroscopic myelogram, while intrathecal contrast still outlines the thecal sac and nerve root sleeves. It is frequently ordered specifically because it shows nerve-root-level detail that plain fluoroscopic myelography's two-dimensional images cannot.
• Indications: CT myelography is the go-to alternative when MRI is contraindicated or nondiagnostic — implanted hardware causing severe MRI artifact, certain pacemakers or neurostimulators that are not MRI-conditional, severe claustrophobia, or prior spinal instrumentation. Clinical questions addressed include nerve root compression, arachnoiditis, dural tears/CSF leak, and syrinx characterization.
• Positioning and timing: after intrathecal injection, patient positioning (slight Trendelenburg, prone, or lateral decubitus) is used to move contrast to the spinal level of clinical interest, and the scan should be obtained promptly — intrathecal contrast dilutes into the CSF over time, so a delayed scan produces progressively lower attenuation in the thecal sac and reduced conspicuity of the nerve roots. This scan/prep-delay relationship is one of the blueprint's explicit Contrast Media focus themes applied to every procedure in this chapter.

Table: Myelogram vs. Standard IV-Contrast CT

Procedure Flow and Postprocedure Care

The full myelogram-to-CT workflow typically runs in one appointment: the radiologist accesses the subarachnoid space with a spinal needle under fluoroscopic guidance (most often at the L3-L4 or L4-L5 interspace, below the level where the spinal cord ends in most adults), confirms free flow of clear CSF, injects a small, carefully calculated volume of the intrathecal-labeled contrast, then obtains fluoroscopic spot images before the patient is transferred directly to the CT scanner. Because the contrast column is fluid and gravity-dependent, the technologist's positioning choices immediately before the CT acquisition directly determine which spinal level ends up best opacified — a lateral decubitus tilt or reverse Trendelenburg can shift contrast away from a level of interest just as easily as it can shift contrast toward one, so positioning instructions from the supervising radiologist should be followed precisely rather than assumed.

After the CT acquisition, postprocedure care mirrors general lumbar-puncture aftercare: patients are typically monitored for a period, encouraged to stay well hydrated, and watched for a post-dural-puncture ("spinal") headache, which results from persistent CSF leak at the puncture site and classically worsens when upright and improves when lying flat. This complication, along with proper documentation of the injection site, contrast volume, and agent used, belongs to the same postprocedure-care and documentation focus theme tested elsewhere in Patient Care.

Exam Scenario

A patient with an MRI-incompatible spinal cord stimulator has progressive lower-extremity radiculopathy. MRI is contraindicated, so the ordering physician requests CT myelography. After fluoroscopic intrathecal injection of a nonionic, water-soluble, intrathecal-labeled agent at the L3-L4 interspace, the technologist repositions the patient and moves promptly to the CT scanner. The correct next technical step is to acquire thin-section images through the level of interest and reconstruct both bone- and soft-tissue-algorithm image sets with sagittal and coronal reformats — mirroring standard spine protocol from the previous section. The only difference from a routine noncontrast spine CT is the intrathecal contrast column outlining the thecal sac and nerve root sleeves.

Common Traps

• Confusing intrathecal with intravenous or intra-articular routes: myelography contrast never passes through an IV line, and it uses a different injection site and technique than the joint (arthrogram) injections covered in the next section.
• Assuming any nonionic contrast is safe intrathecally: only agents specifically labeled and concentrated for intrathecal/myelographic use are appropriate.
• Forgetting the time sensitivity: delaying the CT acquisition after intrathecal injection reduces contrast concentration in the thecal sac and can produce a nondiagnostic study.
• Overlooking post-procedure care: patients should be monitored for post-dural-puncture headache and positioned/hydrated per department protocol after the lumbar puncture.