Cisternography and CSF Leak

3.5.1 Radiopharmaceuticals

[¹¹¹In]pentetate ([¹¹¹In]In-DTPA)

3.5.2 Uptake mechanism / biology of the tracer

Cisternography is a dynamic investigation to evaluate the displacement, patency, and dispersal of inert radiopharmaceuticals introduced directly into the cerebrospinal fluid (CSF) space, into an intrathecal pump mechanism or into a CSF drain.

3.5.3 Indications

• Communicating hydrocephalus and in particular normal-pressure hydrocephalus;
• CSF leakage;
• Evaluation of a drainage system;
• Epidural pump function;
• Assessment of possibility for intrathecal therapy e.g. in CNS lymphomas.

3.5.4 Contra-indications

None.

3.5.5 Clinical performances

The treatment of hydrocephalus focuses on removal of excess CSF. One method is placement of a ventriculo-atrial, ventriculo-peritoneal, or lumbo-peritoneal shunt. Assessment of the patency of such a drainage system is a major clinical problem. If the drain is patent, activity is seen immediately after the administration of the radiopharmaceutical. The activity will drain to the ventricles and heart or to the abdominal cavity. CT scanning and MRI provide insight into the anatomy of the brain, meninges, and the CSF space. A CT myelogram, whereby radio-opaque contrast is injected into the CSF space, allows imaging for leakage. MR myelography, a heavily T2 weighted image, can be used to detect fluid collections such as CSF pooling and enables detection of leakage without the need for a lumbar puncture, contrast agent, or radioactive isotope, though intrathecal gadolinium is usually preferred. These methods appear to be less sensitive than radionuclide cisternography, though no direct comparative studies are known.

3.5.6 Dosimetry

The typical activity is 20 MBq, and 4 MBq for the evaluation of a drainage system (in a maximum volume of 1 mL). The ED due to 20 MBq [¹¹¹In]InDTPA administered to the patient will be 2.4 mSv [3].

Caveat

“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but  it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."

3.5.7 Interpretation criteria/major pitfalls

• If a substantial portion of the administered radiopharmaceutical ends up beside the puncture site or has leaked out of it, that will become clearly visible after half an h. Later, the radioactivity will appear in the bladder.
• Normally the radiopharmaceutical reaches the convexity after approximately 24 h. This process can be delayed as a result of increased volume of the subarachnoid space or delayed resorption.
• Filling of the ventricles indicates an (often pathological) retrograde movement of the radiopharmaceutical. This so-called reflux into the lateral ventricles is transient, e.g. in cerebral atrophy or in young children, but persists in e.g. a normal-pressure hydrocephalus.
• Asymmetrical images may arise from adhesions in the subarachnoid space.
• The reference values with respect to quantities of the radiopharmaceutical found in the nasal plugs, which must be of adequate size and absorbency, are dependent on the technique used and should be determined for each department. In the literature, several techniques are described.
• Drain: The erroneous subcutaneous injection of the radiopharmaceutical can create phantom images that simulate a blockage of the distal drain. By making a vertex recording, this pitfall can be avoided.

3.5.8 Patient preparation

No special preparations. If necessary, shave injection site.

3.5.9 Methods

Acquisition and processing:

• Gamma camera and computer
• Energy: Indium-111 setting, 173 and 247 keV
• Window: 20%
• Collimator: MEAP
• Time: 100.000 counts or 10 min

Upon draining: over a period of 2 min, frames of 10 sec are recorded with a matrix of 64x64. After 30 min static images of 2 min (anterior and lateral) and a vertex image of 10 min can be made.

Computer: Not required for digital gamma camera at matrix 128x128 minimum.

Prior to imaging the skull, check the injection site. In adults, images are taken of the skull from anterior, posterior, left and right-lateral as well as vertex views at 4, 24 and 48 h. In children, the same images are taken at 1, 2, 6 and 24 h.

If there is a possibility of liquorrhea, nasal and/or ear plugs are inserted 4 h after injection. These should remain in situ for 24 h. As the wadding is removed, blood is also taken, after which 0.5 mL plasma is counted along with the wadding and blanks. Measurements of nasal or ear plugs after 24 h are appropriate given the occurrence of intermittent liquorrhea. It is also appropriate, if liquorrhea is suspected, to perform abdominal imaging after 24 h.

The drain patency investigation is done as follows: During administration, the patient lies supine facing the collimator. The easiest way to immobilize the head is to hold it. After disinfection of the skin above the reservoir, the needle is inserted into the reservoir (liquor). When liquor appears through the needle, it demonstrates the patency of the ventricular portion of the shunt. Next, the CSF pressure is measured, and liquor taken for laboratory tests (biochemistry and culture).

A syringe containing [¹¹¹In]In-DTPA is affixed to the needle and immediately after the injection, the recording starts. A series of recordings is made at 10 secs per image. After 1 min, if possible, a manual pump technique is used to promote drainage. The entire investigation takes about 2 min. If no drainage can be demonstrated in the first 2 min (even after pumping), then recordings are again made after 30 min.

At this time a vertex recording is also indicated in order to confirm that the radiopharmaceutical has been administered into the reservoir. Depending on the location of the drain, additional images are taken. If there is a lumbo-peritoneal drain, dynamic acquisition is recommended during the first hour. Another recording at 24 h could be useful.