An inert noble gas administered by inhalation for lung ventilation scintigraphy. The single low-energy photopeak (81 keV) is detectable by gamma camera but is suboptimal (below ideal 140 keV range), leading to lower image quality than Tc-99m aerosol alternatives.
V/Q imaging protocol with Xe-133:
- Ventilation performed first (before Tc-99m perfusion), in a single posterior projection
- Three phases:
- Wash-in: first breath → shows central airways
- Equilibrium: rebreathing for 3–5 min → shows regional ventilation distribution
- Wash-out: breathing room air → air trapping in obstructed regions causes retained Xe-133
- Must be performed in a negative pressure room with scavenging system — Xe-133 is exhaled and must be captured (radioactive gas hazard)
- Posterior-only acquisition limits comparison with Tc-99m perfusion images (multiple views)
Comparison with Tc-99m aerosol (DTPA):
- Tc-99m aerosol allows multiple projections and does not require gas scavenging
- However, aerosol deposition is affected by central airway disease (central hotspots)
Xe-133 vs Krypton-81m (Kr-81m): Kr-81m (t½ 13 s, 190 keV) is preferred in some European centers for superior image quality and continuous inhalation during perfusion imaging — but requires an on-site Rb-81/Kr-81m generator.