What is the procedure for a CT (Computed Tomography) myelogram?

CT Myelogram Procedure

A CT myelogram is a two-step diagnostic imaging procedure that involves injecting contrast material into the spinal canal followed by CT scanning to evaluate the spinal cord, nerve roots, and surrounding structures. This procedure is particularly valuable when MRI is contraindicated or provides insufficient information 1.

Procedure Steps

Pre-procedure Preparation

• Ensure patient is well-hydrated prior to the procedure 2
• Maintain normal diet up to 2 hours before procedure 2
• Assess for contraindications:
  • Anticoagulation therapy
  • Known allergy to contrast media
  • Local infection at puncture site
  • Increased intracranial pressure
• For patients with history of seizures who are not on anticonvulsant therapy, premedication with barbiturates or phenytoin should be considered 2

Step 1: Myelography (Contrast Administration)

1. Patient positioning: Usually in lateral decubitus or prone position

2. Lumbar puncture:

   • Typically performed between L3-L4 interspace (if pathology is suspected at this level, the interspace immediately above or below may be selected) 2
   • Lateral cervical puncture may also be used for cervical myelograms 2
   • Procedure must be performed under strict sterile conditions 2

3. Contrast administration:

   • Adult dosing:
     • Lumbar/thoracic myelogram: 10-15 mL of 200 mgI/mL solution 2
     • Cervical myelogram (via lumbar injection): 10-15 mL of 200 mgI/mL or 10 mL of 300 mgI/mL 2
     • Cervical myelogram (via lateral cervical injection): 10 mL of 200 mgI/mL 2
   • Injection must be performed slowly over 1-2 minutes to avoid excessive mixing with CSF and premature cephalad dispersion 2
   • Use minimum dose and concentration required for satisfactory contrast 2

4. Patient manipulation:

   • Move patient slowly and only as necessary 2
   • To maintain contrast as a bolus, move medium to target area very slowly under fluoroscopic control 2
   • Keep patient's head elevated above highest level of spine during all positioning 2
   • Do not lower head of table more than 15° during thoraco-cervical procedures 2
   • For patients with excessive lordosis, consider lateral position for injection 2

Step 2: CT Scanning

1. CT scanning is performed after contrast administration to obtain detailed cross-sectional images
2. CT myelography provides high-resolution images of:
   • Spinal canal and thecal sac patency 1
   • Subarticular recesses and neural foramina 1
   • Regions of neuroforaminal and spinal canal narrowing 1

Post-procedure Care

1. Raise head of stretcher to at least 30° before moving patient 2
2. Move patient slowly while maintaining head-up position 2
3. Maintain patient in 30-45° head-up position for 12-24 hours 2
4. Advise patient to remain still in bed, in head-up position for the first 24 hours 2
5. Encourage oral fluids and diet as tolerated 2
6. Monitor for potential complications:
   • Headache
   • Nausea/vomiting
   • Seizures
   • Rare complications like cerebral edema 3

Clinical Applications

CT myelography is particularly valuable in:

• Patients with contraindications to MRI 1
• Cases with significant hardware artifacts on MRI 1
• Preoperative planning for spinal surgery 1
• Assessment of position-dependent cord compression 1
• Evaluation of CSF leaks 4
• Detailed assessment of nerve root compression 1

Important Considerations

• An interval of at least 48 hours should be allowed before repeat examination; 5-7 days is recommended whenever possible 2
• CT myelography following conventional myelography should be deferred for at least four hours to reduce the degree of contrast 2
• Phenothiazine class antinauseants should not be administered to treat post-procedural nausea or vomiting 2
• While MRI is generally preferred for initial imaging of suspected spinal pathology, CT myelography provides complementary information and may be superior in certain scenarios 1
• The procedure has the disadvantage of requiring lumbar puncture for intrathecal contrast injection 1
• Interobserver variability in interpretation can be significant, potentially affecting diagnostic accuracy 5

CT myelography remains an important diagnostic tool in specific clinical scenarios despite the widespread availability of MRI, offering excellent visualization of the spinal canal contents and surrounding structures.