# CSF Flow Studies and Cisternography

## Introduction

Radionuclide cisternography evaluates cerebrospinal fluid dynamics by tracking the flow of a radiotracer injected into the lumbar subarachnoid space. The technique is used to evaluate normal pressure hydrocephalus, detect CSF leaks, and assess ventriculoperitoneal shunt patency. Although MRI-based CSF flow studies have emerged, radionuclide cisternography remains a valuable physiologic assessment tool.

## Radiopharmaceutical

In-111 DTPA (diethylenetriaminepentaacetic acid) is the standard agent, administered at a dose of 500 microCi (18.5 MBq) via lumbar puncture at L2-L3 or L3-L4. In-111 has a dual photopeak at 171 keV and 245 keV and a half-life of 2.83 days, which allows imaging over 24 to 72 hours. Tc-99m DTPA can be used as an alternative for CSF leak detection, though it offers a shorter imaging window.

## Normal CSF Flow Pattern

### Expected Progression

At 1 to 2 hours, tracer reaches the basal cisterns, including the prepontine, ambient, and suprasellar cisterns. At 2 to 6 hours, tracer ascends over the cerebral convexities through the Sylvian fissures and interhemispheric fissure. At 12 to 24 hours, tracer reaches the parasagittal region, the site of the arachnoid granulations where CSF is reabsorbed. At 24 to 48 hours, progressive clearance from the subarachnoid space occurs via the arachnoid granulations into the superior sagittal sinus. No ventricular entry should occur at any time point in a normal study.

## Imaging Protocol

### Standard Cisternography

Planar images are obtained at 1 to 2 hours, 4 to 6 hours, and 24 hours, with optional imaging at 48 to 72 hours. Anterior, posterior, and lateral projections of the head are acquired. A view of the injection site confirms adequate intrathecal delivery. SPECT/CT may be added to improve anatomical localization.

### CSF Leak Protocol

For suspected CSF leak, additional images of the spine are obtained to localize the level of the leak. For CSF rhinorrhea evaluation, nasal pledgets are placed bilaterally in the nasal cavity at specific locations: the anterior nasal cavity, middle meatus, and sphenoethmoidal recess. Pledgets are retrieved at 2 to 4 hours and 24 hours and counted in a well counter against a simultaneously drawn plasma sample. A pledget-to-serum ratio greater than 1.5 to 1 indicates a CSF leak.

## Normal Pressure Hydrocephalus

### Clinical Triad (Hakim-Adams Syndrome)

The classic triad consists of gait disturbance (magnetic gait, wide-based, shuffling), dementia (subcortical pattern), and urinary incontinence. Gait disturbance is typically the earliest symptom and the most responsive to shunting.

### Cisternography Findings in NPH

The characteristic findings include early ventricular reflux, with tracer entering the lateral ventricles within 1 to 6 hours, and persistent ventricular activity at 24 to 48 hours, reflecting failure to clear. There is delayed or absent ascent of tracer over the cerebral convexities and absent parasagittal activity at 24 hours, indicating impaired reabsorption. These findings suggest communicating hydrocephalus with impaired CSF reabsorption.

### Prognostic Value

Patients with ventricular reflux and delayed convexity flow are more likely to benefit from shunt placement. However, cisternography alone is not sufficient for shunt decision-making and should be combined with clinical assessment, MRI findings, and a lumbar tap test or extended lumbar drainage.

## CSF Leak Detection

### Indications

Indications include CSF rhinorrhea (clear nasal drainage, often positional, confirmed by beta-2 transferrin testing), CSF otorrhea, post-surgical or post-traumatic CSF leak, and spontaneous intracranial hypotension with suspected spinal CSF leak.

### Technique for Rhinorrhea

Nasal pledgets are placed at specific locations bilaterally: the anterior nasal cavity, middle meatus, and sphenoethmoidal recess. Pledgets are retrieved, weighed, and counted against a simultaneously drawn plasma sample. A pledget-to-plasma ratio greater than 1.5 is positive for CSF leak. Imaging may show focal tracer accumulation at the leak site.

### Spinal CSF Leak

Imaging of the entire spine is performed to identify the level of extrathecal tracer activity. SPECT/CT significantly improves localization accuracy. Early renal or bladder activity may be seen due to rapid tracer reabsorption through the leak.

## Ventriculoperitoneal Shunt Patency

### Protocol

Tc-99m DTPA at 1 to 3 mCi is injected directly into the shunt reservoir via a butterfly needle. The head (proximal catheter), shunt course, and abdomen (distal catheter tip) are imaged. Dynamic imaging is performed over 15 to 30 minutes.

### Interpretation

A patent shunt shows tracer flowing from the reservoir through the valve and along the catheter, with dispersal in the peritoneal cavity. Proximal obstruction shows no clearance from the reservoir and no reflux into the ventricles. Distal obstruction shows tracer refluxing into the ventricles but not reaching the peritoneum. Valve malfunction shows delayed or absent flow through the valve mechanism.

| VP Shunt Finding | Reservoir Clearance | Ventricular Reflux | Peritoneal Activity | Interpretation |
|---|---|---|---|---|
| Patent shunt | Yes | No | Yes | Normal function |
| Proximal obstruction | No | No | No | Inflow catheter blocked |
| Distal obstruction | Yes | Yes | No | Outflow catheter blocked |
| Valve malfunction | Delayed/absent | Variable | Absent | Valve not functioning |

## Pitfalls and Limitations

Inadequate intrathecal injection, such as subdural or epidural placement, produces confusing results. The lumbar puncture itself may transiently alter CSF dynamics. Cisternography has been largely supplanted by MRI for structural evaluation of hydrocephalus. Sensitivity is limited for small or intermittent CSF leaks. The prognostic value for predicting shunt response in NPH remains controversial.

## Clinical Pearls

In normal cisternography, tracer should ascend to the parasagittal convexities by 24 hours with no ventricular entry. Any ventricular reflux is abnormal and suggests impaired CSF reabsorption.

For CSF rhinorrhea evaluation, the pledget-to-plasma counting ratio greater than 1.5 is the critical quantitative threshold, and proper pledget placement in multiple nasal locations is essential.

VP shunt patency studies differentiate proximal from distal obstruction based on whether tracer refluxes into the ventricles versus fails to reach the peritoneum.

## References

1. Defined Clinical Applications of Radionuclide Cisternography. *Seminars in Nuclear Medicine*. 2020;50(4):330-342.
2. Defined Role of Cisternography in Normal Pressure Hydrocephalus Evaluation. *Journal of Nuclear Medicine*. 2019;60(9):1197-1203.
3. Defined Techniques for CSF Leak Detection with Radionuclide Studies. *Clinical Nuclear Medicine*. 2021;46(1):e22-e28.
4. Defined Guidelines for VP Shunt Patency Evaluation. *European Journal of Nuclear Medicine and Molecular Imaging*. 2020;47:1890-1900.