14.5 CSF Studies: Cisternography, Leak Localization, and Shunt Patency
Key Takeaways
- Radionuclide cisternography uses about 0.5 mCi of preservative-free In-111 DTPA injected intrathecally by a credentialed physician, imaged with a medium-energy collimator and dual windows on the 171 and 245 keV photopeaks.
- Normal cisternography shows activity ascending over the cerebral convexities by about 24 hours with no persistent ventricular reflux; early ventricular reflux that persists at 24–48 hours with delayed convexity clearance is the communicating-hydrocephalus pattern.
- For suspected CSF rhinorrhea or otorrhea, ENT places weighed pledgets before injection; pledgets and a simultaneous serum sample are counted in a well counter, and a pledget-to-serum activity ratio below about 1.3 is normal while roughly 1.5 or greater supports a leak.
- Shunt patency studies inject a small activity of Tc-99m DTPA or pertechnetate into the shunt reservoir under strict aseptic technique, then image proximal reflux and distal flow to distinguish proximal from distal obstruction.
- Only preservative-free, sterile, pyrogen-free formulations may be given intrathecally, and the technologist verifies the product, route, and patient before the physician injects.
14.5 CSF Studies: Cisternography, Leak Localization, and Shunt Patency
Quick Answer: In-111 DTPA (~0.5 mCi), preservative-free, intrathecal, imaged with a medium-energy collimator and dual windows at 171 and 245 keV. Normal = convexity flow by 24 h, no persistent ventricular reflux. Leak = pledget-to-serum ratio ≥ ~1.5. Shunt = inject the reservoir, watch proximal reflux and distal clearance.
Section 14.2 named these studies inside the broader CNS chapter. The blueprint lists cisternogram, CSF leak, and CSF shunt patency as three separate procedures, and each has technique that can invalidate the result.
Why In-111 DTPA
| Property | Consequence |
|---|---|
| Half-life ≈ 2.8 days | Supports imaging out to 24, 48, and 72 hours, which the slow CSF circulation requires |
| Photopeaks 171 and 245 keV | Medium-energy collimator; set dual windows (about ±10% each) and add the counts |
| DTPA chelate | Stays in the CSF space, is not absorbed into brain tissue, and clears by arachnoid granulations into blood |
| Formulation | Must be preservative-free, sterile, and pyrogen-free for intrathecal administration |
The absolute rule: an intrathecal injection is the highest-consequence route in the department. The technologist independently verifies the product name, formulation, activity, expiration, patient identity, and the ordered route before handing the syringe to the physician performing the lumbar puncture. Never substitute a multi-dose vial containing preservative.
Radionuclide Cisternography
Indications: normal-pressure hydrocephalus evaluation, CSF flow dynamics, shunt or reservoir function questions, and CSF leak workups.
| Step | Practice |
|---|---|
| Activity | About 0.5 mCi (≈18.5 MBq) In-111 DTPA intrathecally by lumbar puncture |
| Post-injection care | Patient often lies flat for a defined period per the physician; watch for post-LP headache |
| Imaging times | Commonly 2–4 h, 6 h, 24 h, and often 48 and 72 h |
| Views | Anterior, posterior, and both laterals of the head; add spine views when a spinal leak is suspected |
| Counts/time | Fixed time per view so serial images are comparable |
Pattern Recognition
| Time | Normal | Communicating (normal-pressure) hydrocephalus pattern |
|---|---|---|
| 2–4 h | Activity in basal cisterns | Basal cisterns plus early ventricular reflux |
| 24 h | Activity over the cerebral convexities, ventricles clear | Persistent ventricular activity, delayed or absent convexity flow |
| 48–72 h | Progressive clearance | Continued ventricular retention |
Transient ventricular reflux that clears by 24 hours can be a normal variant — persistence is the abnormal finding.
CSF Leak Localization with Pledgets
Used for suspected rhinorrhea (nasal) or otorrhea (aural) leaks.
| Step | Practice |
|---|---|
| 1. Weigh and label pledgets | ENT places sized pledgets (roughly 1 cm², absorbent) in defined nasal or aural positions; record left/right and site meticulously |
| 2. Inject In-111 DTPA intrathecally | Same activity and technique as cisternography |
| 3. Image | Serial head and, when indicated, spine images at the protocol times |
| 4. Remove pledgets at the protocol interval | Place each in a separate, labeled, pre-weighed counting tube |
| 5. Draw a simultaneous blood sample | Usually about 5 mL into a heparinized tube; separate serum or plasma |
| 6. Count in a NaI well counter | Same geometry, same count time, background corrected |
| 7. Compute the ratio | Activity per gram of pledget divided by activity per gram (or mL) of serum |
| Ratio | Interpretation (laboratory-specific) |
|---|---|
| < ~1.3 | Normal — activity is explained by blood-pool background |
| ≥ ~1.5 | Supports an active CSF leak at that site |
Failure modes: mislabeled or swapped pledgets, pledgets left in the wrong position, blood contamination of the pledget from the placement trauma, drying and weight loss before counting, and forgetting the serum sample entirely — without the denominator there is no result.
Shunt Patency Studies
Question: is a ventriculoperitoneal (or ventriculoatrial/ventriculopleural) shunt flowing, and if not, is the obstruction proximal (ventricular catheter) or distal (peritoneal end)?
| Step | Practice |
|---|---|
| Access | Physician accesses the shunt reservoir with a small needle under strict aseptic technique, often after skin prep and hair clipping |
| Agent | A small activity (roughly 0.5–1 mCi class) of Tc-99m DTPA or pertechnetate, preservative-free |
| Proximal limb | With the distal limb manually occluded, look for reflux into the ventricles — reflux indicates a patent proximal catheter |
| Distal limb | Release the occlusion and acquire dynamic images: prompt flow down the tubing and spillage into the peritoneal cavity indicates a patent distal limb |
| Quantitation | Time–activity curve over the reservoir or ventricles; a prolonged clearance half-time suggests distal obstruction |
| Delayed views | Often at 30 minutes to a few hours to confirm peritoneal distribution |
Findings map: ventricular reflux present + no distal flow → distal obstruction. No ventricular reflux + normal distal flow → proximal obstruction. Neither → total occlusion or a technical access failure. Sluggish everything with a slow curve → partial obstruction.
Radiation and infection safety: this is a sterile CNS procedure. Prepare the dose aseptically, keep the field sterile, use the smallest volume the protocol allows, and document the reservoir accessed.
Comparison Table
| Study | Route and agent | Key measurement |
|---|---|---|
| Cisternography | Intrathecal In-111 DTPA ~0.5 mCi | Convexity flow by 24 h vs persistent ventricular reflux |
| CSF leak | Intrathecal In-111 DTPA + nasal/aural pledgets | Pledget-to-serum ratio (≥ ~1.5 positive) |
| Shunt patency | Reservoir injection of Tc-99m DTPA/pertechnetate | Proximal reflux and distal peritoneal spill; clearance half-time |
Bottom line: verify the route and the formulation, count the pledgets against a serum sample, and describe shunt findings in terms of proximal versus distal limbs.
Which formulation and route requirement applies to In-111 DTPA used for radionuclide cisternography?
Nasal pledgets are removed after In-111 DTPA cisternography and counted with a simultaneous serum sample. Which result best supports an active CSF leak?
During a shunt patency study, Tc-99m DTPA injected into the reservoir refluxes readily into the ventricles when the distal limb is occluded, but after the occlusion is released there is no flow down the distal tubing and no peritoneal activity on delayed images. This pattern indicates: