Free diffusion — nothing is trapped
These agents are inert and freely diffusible. They equilibrate with tissue and then leave, so a static image says nothing — the entire diagnostic content is in the shape of the time–activity curve.
It is worth keeping this family in view precisely because it is the exception. Every other mechanism on this page exists to defeat diffusion; here nothing does, and the consequences are instructive. O-15 water has an extraction fraction of 1.0 at every flow rate, which is why it remains the reference standard for absolute blood flow and why no retention-based agent can match it.
On a V/Q study, xenon washout is the most sensitive sign of obstructive airways disease — but only if you acquire it before the perfusion agent, because 81 keV photons cannot be read through 140 keV Tc-99m activity. That ordering rule is a direct consequence of having no trapping step to hold the tracer while you image something else.
The agents
O-15 water—
PETO-15 · t½ 122 s · on-site cyclotron mandatory
- Handle
- None — freely diffusible
- Trapping
- Extraction fraction ~1.0 at all flow rates, so tissue activity is a direct function of perfusion with no correction for a transport or trapping step.
- Use
- The reference standard for absolute myocardial and cerebral blood flow; activation studies historically.
- Pitfall
- No interpretable static image — blood-pool subtraction and kinetic modelling are obligatory. The two-minute half-life confines it to research and specialist quantitative centres.
Xe-133—
SPECT / planarXe-133 · t½ 5.2 d · 81 keV γ · inhaled gas
- Handle
- None — freely diffusible gas
- Trapping
- Wash-in, equilibrium and washout phases. Retention on the washout images means air trapping.
- Use
- Ventilation half of the V/Q study; delayed washout is the most sensitive sign of obstructive disease.
- Pitfall
- Must be acquired before perfusion — 81 keV cannot be imaged through 140 keV Tc-99m. Usually a single posterior view, requiring patient cooperation and a negative-pressure room with a trap. Being fat-soluble, it can appear over the liver in hepatic steatosis.