Lipophilic diffusion, then chemical conversion
A neutral lipophilic complex crosses the blood–brain barrier in proportion to flow, and is then chemically altered inside the cell into something polar that cannot cross back. The picture freezes perfusion at the instant of injection.
That freezing is the clinically decisive property, and it is unique in the department. HMPAO uptake is complete within one to two minutes of injection, and after a small early back-diffusion about 85% stays put for hours — so the tracer can be injected at the bedside during a seizure and the patient imaged after it ends. No other modality gives you an ictal snapshot.
The two agents differ in the enzyme that does the trapping, and it matters. HMPAO is converted by glutathione, ECD by intracellular esterases. In subacute stroke with luxury perfusion, HMPAO can show hyperperfusion while ECD — needing intact esterase activity in damaged tissue — shows a defect. That discordance is a real finding, not an artefact.
The agents
Tc-99m HMPAOexametazime · Ceretec
SPECT / planarTc-99m · t½ 6.01 h · 140 keV γ
- Handle
- Intracellular glutathione
- Trapping
- Crosses the BBB as a neutral lipophilic complex, then is converted to a hydrophilic secondary complex and trapped. Uptake is complete within 1–2 minutes; about 15% back-diffuses over the next 10–15 minutes and the remaining ~85% holds for hours.
- Use
- Brain perfusion SPECT: ictal and interictal epilepsy localisation, dementia patterns, brain death confirmation. Also the label for white cell studies.
- Pitfall
- Unstable after reconstitution — use within 30 minutes unless a stabilised formulation is used. Free pertechnetate from poor labelling degrades the study. Some retrograde diffusion means high flow is underestimated, flattening the dynamic range.
Tc-99m ECDbicisate · Neurolite
SPECT / planarTc-99m · t½ 6.01 h · 140 keV γ
- Handle
- Intracellular esterases
- Trapping
- Crosses the BBB, then is hydrolysed to a polar mono-acid and trapped.
- Use
- Brain perfusion SPECT; often preferred for image quality.
- Pitfall
- More stable in vitro and faster blood and soft-tissue clearance, giving cleaner images. But it can underestimate flow in subacute stroke and luxury perfusion, where HMPAO shows hyperperfusion — know which agent your department uses before interpreting.