Radionuclide Cisternography with In-111 DTPA

Radionuclide cisternography injects indium-111 DTPA into the cerebrospinal fluid and images its flow over 24 to 72 hours to answer three questions: is there a CSF leak, is this normal-pressure hydrocephalus, and is the shunt patent. It is one of the few nuclear medicine studies where the physics of a long-lived, medium-energy radionuclide — and the discipline of imaging faint activity days after injection — determine whether the study succeeds or fails.

Introduction

Cerebrospinal fluid is produced in the ventricles, flows out into the subarachnoid space, and is slowly reabsorbed over the cerebral convexities. When that circulation is disrupted — a dural defect that lets CSF leak into the nose or ear, a hydrocephalus that refluxes fluid back into the ventricles, or a surgical shunt that clogs — the consequences can be serious, and the questions are often hard to answer with anatomic imaging alone.

Radionuclide cisternography answers them functionally. A radiotracer is injected into the CSF by lumbar puncture, and a gamma camera follows it as it ascends the spinal canal and circulates over the brain across one to three days.¹ Because the tracer moves with the CSF, its distribution reveals flow that a single CT or MRI snapshot cannot. The study is not high-volume, and it has been partly displaced by high-resolution CT and MRI for some indications, but it remains valuable — and sometimes uniquely so — for confirming and localizing intermittent CSF leaks, characterizing normal-pressure hydrocephalus, and evaluating shunt patency.¹²

What makes cisternography a physics-forward study is the radionuclide. Indium-111 DTPA has a 67-hour half-life and two relatively energetic photopeaks, which is exactly what a multi-day, slow-flow study requires — but those same properties dictate collimator choice, energy-window setup, and dose. This article walks through the physics, the acquisition, the interpretation patterns, and the quality control, from the perspective of the medical physicist who has to make faint activity on a 72-hour image trustworthy. DRPS provides this support as part of its PET/CT and nuclear medicine physics and medical physics consulting services across Florida, Maryland, Virginia, Washington DC, California, and Nevada.

Topic Explanation

What is radionuclide cisternography?

Radionuclide cisternography is the imaging of CSF dynamics using an intrathecally injected radiotracer. After lumbar intrathecal injection — typically through a 22-gauge or smaller needle — the patient lies supine for a period so the tracer can begin its ascent, and images are acquired at intervals: often around 2 to 4 hours, then at 24 hours, and at 48 and 72 hours as needed.³ The tracer normally ascends through the basal cisterns within 2 to 4 hours, appears in the Sylvian and interhemispheric cisterns, and reaches the parasagittal region over the convexities by about 24 hours, with clearance from the basal cisterns.³

The study supports three distinct clinical questions, each with its own pattern:

CSF leak (rhinorrhea or otorrhea): tracer appears at an abnormal site — the nose, an ear, or a surgical bed — often detected with counted pledgets.
Normal-pressure hydrocephalus (NPH): tracer refluxes into and is retained in the lateral ventricles with delayed convexity flow.¹
Shunt patency (radionuclide shuntogram): tracer injected into a shunt reservoir is followed to its distal drainage site.¹

For the collimator and counting hardware this study depends on, see our guides to gamma-camera collimator selection and scintillation well-counter quality control.

Why indium-111 DTPA

Indium-111 pentetate (In-111 DTPA) is the only radiotracer approved by the FDA for intrathecal administration, and it is well suited to the task.⁴ DTPA is a chelate that is not appreciably reabsorbed across the arachnoid membrane, so it remains in the CSF long enough to trace flow over days rather than hours. Indium-111's 67-hour physical half-life is the enabling property: it allows imaging out to 48 and 72 hours, which is necessary because CSF flow is slow and the abnormal patterns (ventricular retention, intermittent leak) may only declare themselves on delayed images. A short-lived tracer would have decayed away before the diagnostic pattern emerged.

The maximum recommended intrathecal dose in an average adult is 18.5 MBq (500 µCi) of indium-111 pentetate, supplied as a no-carrier-added solution.⁴ Only preservative-free, formulations intended and labeled for intrathecal use should ever be injected into the CSF space — a critical safety point in this study.

Key Technical Principles

Decay: why delayed imaging works

The activity remaining at imaging time $t$ follows simple exponential decay:

A (t) = A_{0} e^{- λ t}, λ = \frac{ln 2}{T _{1/2}} = \frac{0.693}{67.2 h}

At the two key delayed time points:

\frac{A ( 48 )}{A _{0}} = e^{- (\frac{0.693}{67.2}) \times 48} = e^{- 0.495} \approx 0.61

\frac{A ( 72 )}{A _{0}} = e^{- (\frac{0.693}{67.2}) \times 72} = e^{- 0.742} \approx 0.48

About 61% of the injected activity is still present at 48 hours and nearly half at 72 hours. Contrast this with technetium-99m (6-hour half-life), which would retain less than 0.5% of its activity at 48 hours — far too little for delayed CSF imaging. This is the quantitative reason In-111 is the workhorse for cisternography: its half-life is matched to the physiology.¹

Photopeaks, collimator, and windows

Indium-111 emits two principal gamma photons, at 171 keV and 245 keV. Both are more energetic than the 140 keV of technetium-99m, and the 245 keV photon in particular will penetrate the thin septa of a low-energy collimator, producing septal-penetration artifacts and degraded resolution. A medium-energy collimator is therefore required to preserve image quality.³ Choosing the wrong collimator is one of the most common and consequential errors in this study — the images look plausible but are blurred by penetration.

Both photopeaks should be used. Imaging with symmetric energy windows (commonly on the order of 20% wide) centered on each of the 171 and 245 keV peaks captures roughly twice the useful counts of a single-window setup, which directly improves the detectability of faint, delayed activity.³ Correct dual-peak energy calibration is a physics QC item that must be verified before the study.

The comparison below summarizes why In-111 DTPA, not a technetium agent, is the standard intrathecal tracer:

Property	In-111 DTPA	Tc-99m DTPA
Physical half-life	67.2 hours	6.0 hours
Principal photons	171 and 245 keV	140 keV
Collimator	Medium-energy	Low-energy high-resolution
Practical delayed-imaging window	Out to 48–72 hours	A few hours only
FDA-approved for intrathecal use	Yes	No (used off-label, short studies)
Typical CSF role	Cisternography, leak, NPH, shuntogram	Rapid shunt or short-interval studies

Detecting a leak by counting pledgets

When the clinical question is a skull-base leak, imaging is combined with pledget counting. Cotton pledgets are placed in the nostrils or external auditory canals, left in place during the study, then removed and counted in a well counter alongside a blood sample drawn at the same time.⁵⁶ The interpretive quantity is the pledget-to-serum count ratio:

R = \frac{counts per gram (pledget)}{counts per mL (serum)}

with both samples counted at the same time (or decay-corrected to a common time). Because tracer reaches the blood only slowly, a pledget that has absorbed leaking CSF will show a ratio well above one, whereas a dry, non-leaking pledget tracks background blood levels. Institutional threshold values vary, so the ratio is interpreted together with the images and clinical context, but the principle is robust: CSF carries the tracer, and a pledget bathed in leaking CSF lights up relative to blood.⁵ Prolonged imaging with repeated pledget placement can capture intermittent leaks that a single time point would miss.⁶ For the counting hardware, well-counter QC is essential — see scintillation well-counter quality control.

Clinical Impact

Cisternography earns its place where anatomic imaging is ambiguous. For CSF leaks, it can confirm that a suspected rhinorrhea is truly CSF and, especially with pledget counting and delayed imaging, capture intermittent leaks that CT cisternography — a single-time-point study — can miss.⁶ For normal-pressure hydrocephalus, the pattern of ventricular reflux with delayed convexity clearance contributes to the diagnostic picture, although it is used adjunctively alongside clinical assessment and MRI rather than as a stand-alone test.¹ For shunt patency, the radionuclide shuntogram gives a direct functional readout of whether tracer moves through the shunt to its distal site — information that hardware imaging alone cannot provide.¹

The honest framing is that cisternography is a niche but genuinely useful study, and its yield depends heavily on execution. A recent comprehensive review of nuclear CSF imaging emphasizes that both cisternograms and shuntograms have well-defined normal and pathologic patterns — and equally well-defined pitfalls, from tracer that never ascends because of a technically poor injection, to septal-penetration artifacts from the wrong collimator, to misread ventricular activity.¹ The difference between a diagnostic study and an uninterpretable one is largely physics and technique. Spontaneous intracranial hypotension — the "low-pressure" counterpart to a frank leak — is another setting where cisternography can show indirect signs such as early bladder activity and soft-tissue tracer accumulation, underscoring that the whole-body distribution, not just the head, carries information.⁷

Practical Optimization Tips

Confirm the tracer and the route

Only preservative-free indium-111 pentetate labeled for intrathecal use may be injected into the CSF. Verifying the correct formulation and route is the single most important safety step in the study.⁴

Use the medium-energy collimator — every time

The 245 keV photon mandates a medium-energy collimator. A low-energy collimator will pass penetrating photons and blur the images. Confirm the collimator on the camera before the first image, and make it a checklist item.³

Peak both photopeaks and verify uniformity

Set and verify symmetric windows on both 171 and 245 keV peaks, and confirm flood-field uniformity for the medium-energy collimator configuration. Faint 72-hour activity is unforgiving of a mispeaked window or a nonuniform detector. See gamma-camera uniformity QC.

Plan the time points and the pledgets in advance

Decide the imaging schedule (for example, ~3, 24, 48, and 72 hours) and, for leak studies, the pledget placement and counting logistics before the injection. Coordinate the blood draw to match pledget removal so the count ratio is valid.⁵

Common pitfalls to avoid

Wrong collimator. A low-energy collimator on an In-111 study is the classic technical failure.³
Single-window imaging. Ignoring the 245 keV peak throws away counts the delayed images cannot spare.³
Injecting the wrong formulation. Non-intrathecal or preservative-containing product must never enter the CSF.⁴
Stopping too early. The diagnostic pattern (ventricular retention, intermittent leak) may only appear at 48–72 hours.
Neglecting well-counter QC. An uncalibrated counter makes the pledget-to-serum ratio meaningless.

Regulatory Considerations

Radionuclide cisternography is a medical use of byproduct material governed by 10 CFR Part 35 (or an Agreement State equivalent), and the tracer must be used in accordance with its FDA-approved intrathecal labeling. As a diagnostic administration, it does not require a written directive (which applies to therapeutic administrations), but it must be performed under an authorized user and the facility's radiation safety program.

Key frameworks to reference:

10 CFR Part 35 — Medical Use of Byproduct Material, which governs authorized users, dosage determination and recording, and the radiation safety program for diagnostic radiopharmaceuticals.⁸ See our guide to determination and recording of dosages under 35.63 for the general dosage framework.
FDA prescribing information for indium In-111 pentetate, which specifies the intrathecal indication, the maximum recommended activity, and the labeled route — the controlling document for how the tracer may be administered.⁴
10 CFR Part 20 — Standards for Protection Against Radiation, including occupational and public dose limits and ALARA, which frame the modest but real dose of an intrathecal, long-half-life tracer.⁸
ICRP dose-coefficient publications, used to estimate absorbed and effective dose from the intrathecal administration for informed consent and optimization.

Agreement States administer their own equivalent medical-use programs. Of the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are NRC Agreement States, while Washington, DC and Delaware are regulated directly by the NRC for radioactive material. The dosage-determination, recordkeeping, and radiation-safety requirements a facility must meet come from its license and the applicable authority. This connects to broader nuclear medicine dosimetry practice — see radiopharmaceutical dosimetry and ICRP 128.

Frequently Asked Questions (FAQs)

What is radionuclide cisternography?

Radionuclide cisternography is a nuclear medicine study in which a radiotracer is injected into the cerebrospinal fluid (CSF) space by lumbar puncture, and a gamma camera images the tracer as it circulates over 24 to 72 hours. It is used to evaluate CSF leaks, normal-pressure hydrocephalus, and — as a related shuntogram study — the patency of CSF shunts. The long imaging window follows the slow physiologic flow of CSF from the spinal canal up over the brain.

Why is indium-111 DTPA used for cisternography?

Indium-111 DTPA is the only radiotracer approved by the FDA for intrathecal administration, and its 67-hour half-life allows delayed imaging out to 48 and 72 hours, which is essential because CSF flow is slow. DTPA is not appreciably reabsorbed across the arachnoid, so it stays in the CSF long enough to trace the flow pattern. Its dual photon emissions are well matched to a gamma camera with a medium-energy collimator.

What activity of In-111 DTPA is used and what is the radiation dose?

The maximum recommended intrathecal dose in an average adult is 18.5 MBq (500 µCi) of indium-111 pentetate. Because the tracer is injected into the CSF and indium-111 has a relatively long half-life, the study delivers a higher radiation dose per unit activity than many routine nuclear medicine exams, with the highest dose to structures adjacent to the CSF space. The small administered activity keeps the absolute dose modest, and technique should follow ALARA.

What collimator and energy windows are used for In-111 cisternography?

Indium-111 emits photons at 171 keV and 245 keV, so a medium-energy collimator is required to stop septal penetration that a low-energy collimator would allow. Both photopeaks are typically imaged with symmetric energy windows (commonly about 20% wide) centered on each peak, which improves count sensitivity compared with using a single window.

How does cisternography detect a CSF leak?

For a suspected skull-base leak (rhinorrhea or otorrhea), cotton pledgets are placed in the nostrils or ear canals before or during the study. After imaging, the pledgets are removed and counted in a well counter and compared with a blood sample drawn at the same time. A pledget-to-serum count ratio well above one indicates that CSF — carrying tracer — is leaking onto the pledget. Imaging can also directly show tracer appearing at the leak site.

What does cisternography show in normal-pressure hydrocephalus?

In normal-pressure hydrocephalus, tracer refluxes into the lateral ventricles and is retained there, with delayed or absent flow over the cerebral convexities on the 24-, 48-, and 72-hour images. The normal pattern is ascent over the convexities to the parasagittal region by 24 hours with clearance from the basal cisterns. The persistence of ventricular activity is the hallmark abnormal finding, though it is interpreted alongside clinical and MRI information.

How is a radionuclide shuntogram different from cisternography?

A shuntogram evaluates whether a CSF shunt is patent. A small activity of radiotracer is injected into the shunt reservoir under sterile technique, and imaging follows the tracer through the shunt tubing to the distal drainage site, typically the peritoneum. Delayed or absent distal flow suggests obstruction. It uses the same imaging principles as cisternography but targets the shunt hardware rather than the native CSF pathways.

What quality control matters most for cisternography?

Because the study spans days and low count rates, gamma-camera uniformity, correct medium-energy collimator selection, accurate dual-window peaking on both indium-111 photopeaks, and well-counter quality control for pledget counting all matter. A qualified medical physicist should verify camera performance, energy calibration, and counting-system QC so that faint or delayed activity is not lost in the background.

Key Takeaways

Cisternography images CSF dynamics with intrathecal In-111 DTPA over 24 to 72 hours, answering questions about leaks, NPH, and shunt patency that anatomic imaging cannot fully resolve.¹
In-111's 67.2-hour half-life is the enabling physics — about 61% of activity remains at 48 hours and 48% at 72 hours, making delayed imaging feasible where technetium agents cannot follow.⁴
The 245 keV photopeak mandates a medium-energy collimator, and both 171 and 245 keV peaks should be imaged with symmetric windows to preserve faint-count sensitivity.³
Only FDA-labeled, preservative-free In-111 pentetate may be injected intrathecally, at a maximum recommended 18.5 MBq (500 µCi).⁴
CSF leaks are quantified by the pledget-to-serum count ratio, which requires well-counter QC and a matched blood sample.⁵⁶
Execution decides the study — collimator, energy windows, uniformity, timing, and counting QC are the difference between a diagnostic result and an uninterpretable one.¹

Conclusion

Radionuclide cisternography is a small-volume study that punches above its weight when it is done right — and fails quietly when it is not. Its whole rationale rests on physics: a radionuclide long-lived enough to follow CSF for three days, energetic enough to require a medium-energy collimator, and imaged faintly enough that uniformity, dual-window peaking, and counting QC actually matter. The clinical questions — leak, NPH, shunt patency — are answered by patterns that only emerge on delayed images and only survive careful technique. For the medical physicist and the nuclear medicine team, the takeaway is that this study rewards discipline: verify the tracer and route, use the right collimator every time, peak both photopeaks, image late enough, and keep the well counter in QC. Get those right, and cisternography still earns its place in the workup.

How DRPS Can Help

Diagnostic Radiation Physics Services helps nuclear medicine departments perform cisternography and shuntogram studies reliably. This includes gamma-camera acceptance and annual testing, medium-energy collimator and dual-window energy verification, uniformity and count-rate QC, well-counter calibration for pledget counting, dose estimation and optimization, and integration into the department's PET/CT and nuclear medicine physics program with radiation safety officer and medical physics consulting support.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware.

A niche study still deserves a rigorous physics foundation — that is what turns faint counts on a 72-hour image into a confident diagnosis.

Related Resources

References

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Grantham VV, Blakley B, Winn J. Technical review and considerations for a cerebrospinal fluid leakage study. J Nucl Med Technol. 2006;34(1):48-51. PubMed
U.S. Food and Drug Administration / GE Healthcare. Indium In-111 Pentetate (Pentetate Indium Disodium In 111) — Prescribing Information. DailyMed, National Library of Medicine. dailymed.nlm.nih.gov
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U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material, and 10 CFR Part 20: Standards for Protection Against Radiation. ecfr.gov