Nucpaedia
Nucpaedia
Mechanism 11 of 16

Physical entrapment

No biology at all. The particle is larger than the next vessel, so it stops there. Distribution equals blood flow at the instant of injection, and nothing else.

Because the mechanism is purely mechanical, the safety considerations are mechanical too. Reduce the particle number in pulmonary hypertension and in known right-to-left shunt, where fewer arterioles are available or the particles will bypass the lung entirely and lodge in brain and kidney. Inject supine, so gravity does not tilt the distribution.

The same principle, scaled up, is a treatment. Y-90 microspheres embolise in a tumour's arterial bed and deliver a pure beta dose over a couple of millimetres — which is why the MAA lung shunt fraction must be measured first. If too much of the injectate will reach the lungs, the treatment becomes radiation pneumonitis.

PULMONARY ARTERIAL TREEprecapillaryarterioles~10 µm lumendistal capillary bedparticles 10–90 µmunder 0.1% of the bed occludedthe particle wedges at the first narrowing it cannot passa right-to-left shunt sends particles past the lung to brain and kidneyMAAMAAMAAMAAMAAMAAMAAMAAMAAMAAMAAMAA
MAA particle in transitlodged in an arteriolevessel
A mechanical map of flow. Particles of 10–90 µm pass the larger vessels and wedge in precapillary arterioles — a few hundred thousand of them occlude under 0.1% of the bed, which is why this is safe. Where flow is absent, no particles arrive and the segment is cold. A right-to-left shunt sends particles past the lung to brain and kidney, which is both the hazard and the way the shunt is quantified.

The agents

Tc-99m MAAmacroaggregated albumin

SPECT / planar

Tc-99m · t½ 6.01 h · particles 10–90 µm · 200,000–700,000 per dose

Pulmonary arterial treeprecapillary~10 µm lumendistal bed10–90 µm particles<0.1% of the bed occludedthe particle stops at the first narrowing it cannot passMAAMAAMAAMAAMAAMAAMAAMAAMAAMAAMAAMAA
Handle
Precapillary arterioles and capillaries
Trapping
Lodges on first pass through the lung. Cleared by fragmentation and macrophages over several hours (quoted lung half-lives range from about 2 to 8 h).
Use
Perfusion lung scan, quantitative differential perfusion before lung resection, hepatopulmonary shunt fraction before Y-90, right-to-left shunt quantification, LeVeen shunt patency, peritoneovenous studies.
Pitfall
Reduce the particle number in pulmonary hypertension, right-to-left shunt and pregnancy. Inject supine to avoid a gravitational gradient. Blood drawn back into the syringe clots and gives hot spots. Free pertechnetate gives thyroid and gastric activity; a shunt reveals itself as brain and kidney uptake.

Y-90 microspheresTheraSphere (glass) · SIR-Spheres (resin)

Therapy

Y-90 · t½ 64 h · pure β⁻, mean tissue range 2.5 mm · glass 20–30 µm, resin 20–60 µm

Tumour feeding arterytumourmean tissue range 2.5 mm— pure β⁻, no imaging photonsthe MAA lung shunt fraction must be measured before thisY90Y90Y90Y90
Handle
Tumour arterial bed
Trapping
Embolise in the arterioles feeding the tumour and irradiate a few millimetres of surrounding tissue. Imaged after treatment by bremsstrahlung SPECT, or by PET using the tiny internal pair-production branch.
Use
Radioembolisation of hepatocellular carcinoma and hepatic metastases; radiation segmentectomy and lobectomy.
Pitfall
The MAA lung shunt fraction must be measured first — a high fraction risks radiation pneumonitis. Non-target deposition through unrecognised collaterals causes gastroduodenal ulceration and cholecystitis, so mapping angiography and coil embolisation precede treatment.

Technegas / Tc-99m DTPA aerosolventilation agents

SPECT / planar

Tc-99m · Technegas particles ~30–60 nm carbon-encapsulated · DTPA droplets ~0.5–2 µm

TECHNEGAS · 30–60 nmDTPA AEROSOL · 0.5–2 µmbehaves like a gas —reaches the alveoli evenlydroplets impact in turbulentcentral airflow — hot spots in COPDTGTGTGTGTGTGTGTGDTPADTPADTPADTPADTPADTPA
Handle
Alveolar and airway deposition
Trapping
Technegas behaves like a gas and deposits alveolar, so central hot-spot deposition is minimal. DTPA droplets are large enough to impact in turbulent central airflow.
Use
Ventilation imaging for the V/Q study; multiple projections, unlike single-view xenon.
Pitfall
DTPA aerosol clumps centrally in COPD and in patients who cannot breathe evenly, producing hot spots that mimic disease — Technegas is markedly better here. DTPA clears with a half-time of about 60 minutes, markedly faster in smokers and in ARDS.
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