Brain Perfusion Agents (HMPAO, ECD)⁹⁹ᵐTc-HMPAO · ⁹⁹ᵐTc-ECD

Quick Answer

Tc-99m-HMPAO and Tc-99m-ECD are lipophilic agents that cross the intact blood–brain barrier and are retained in brain tissue proportional to perfusion at the moment of injection — the "snapshot" property that enables ictal SPECT (inject at seizure onset, image later), brain-death perfusion imaging, and dementia/cerebrovascular perfusion assessment. HMPAO additionally labels leukocytes for infection imaging.

Mechanism

Lipophilic complexes diffuse across the intact BBB and convert to hydrophilic forms trapped intracellularly, so distribution is fixed shortly after injection and reflects perfusion at that instant with negligible later redistribution. Uptake tracks regional cerebral blood flow (gray-matter-weighted).

HMPAO vs ECD

Biodistribution

Gray matter (perfusion-weighted); HMPAO shows some redistribution and requires prompt use; ECD has faster soft-tissue clearance. Non-brain uptake and free pertechnetate appear with poor preparation.

Clinical Indications

• Ictal/interictal SPECT for epilepsy focus localization (fixed uptake = injection-time perfusion; see epilepsy page).
• Brain-death perfusion (hollow-skull / empty-light-bulb sign; ± hot-nose sign).
• Dementia and cerebrovascular perfusion patterns (now largely complemented by FDG-PET).
• HMPAO-labeled WBC imaging for infection (see infection page).

Interpretation Highlights

• Ictal: focal hyperperfusion at the seizure-onset zone (opposite of interictal FDG hypometabolism).
• Brain death: no intracranial parenchymal uptake with preserved scalp/venous activity — confirm tracer was delivered.
• Crossed cerebellar diaschisis: reduced contralateral cerebellar perfusion from a supratentorial lesion — a normal-variant confounder to recognize.

Reporting Checklist

• State agent, injection-to-event latency (ictal), and image timing.
• Describe regional perfusion pattern; account for atrophy and crossed cerebellar diaschisis.
• For brain death, confirm extracranial delivery (valid injection) and absent intracranial perfusion.

Common Pitfalls

• Delayed HMPAO use after reconstitution degrading the study (instability).
• Interpreting perfusion patterns without accounting for atrophy or crossed cerebellar diaschisis.
• Reading brain-death studies without confirming the tracer was actually delivered (failed injection mimics absent perfusion).

Board Pearls

HMPAO and ECD are lipophilic, cross the intact BBB, and are retained in proportion to perfusion at the moment of injection. That snapshot property is exactly what makes ictal SPECT possible — inject at seizure onset, image the patient later once stable, and the tracer still reflects onset-time perfusion.

ECD is more stable after reconstitution and clears soft tissue faster (cleaner images); HMPAO shows some redistribution and must be used promptly — but HMPAO also labels leukocytes for infection imaging, a second job ECD lacks.

Applications span ictal/interictal epilepsy localization, brain-death perfusion (hollow-skull/empty-light-bulb sign — confirm extracranial delivery), and dementia/cerebrovascular perfusion (now largely complemented by FDG-PET). Read against atrophy and recognize crossed cerebellar diaschisis (contralateral cerebellar hypoperfusion from a supratentorial lesion) as a variant, not new disease.

Related Pages

• Related: Epilepsy — seizure focus localization, brain-death perfusion imaging, brain FDG & dementia imaging, infection & inflammation (HMPAO-WBC).
• Pitfalls: Pearls, pitfalls & normal variants.

Figure / Diagram Suggestions

• A BBB trapping mechanism cartoon (lipophilic → trapped hydrophilic).
• An ictal snapshot timeline (inject at onset → image when stable).
• A crossed cerebellar diaschisis teaching image.

Self-Check (Board-Style)