Mechanism 15 of 16

Blood-pool confinement

Nothing is targeted. The label is fixed inside a red cell, and the diagnosis comes from where blood pools that it should not — or from counting how the pool changes across the cardiac cycle.

Labelling chemistry decides the study. Stannous ion enters the red cell and reduces pertechnetate inside it, where the technetium binds the beta chain of haemoglobin and is trapped. In vitro labelling reaches about 98% efficiency; the in vivo method around 75–80%, and the difference shows up as free pertechnetate in stomach and thyroid — which on a bleeding study is a false positive waiting to happen.

For GI bleeding, the mechanism dictates the technique. Because the label stays in the circulation for hours, you can image intermittently for up to 90 minutes and catch an intermittent bleed that angiography would miss — but only dynamic imaging localises it. A hot spot on a single delayed image tells you there is blood somewhere in the bowel; it does not tell you where it came from, because blood moves.

labelled red cellextravasated into bowelfree pertechnetate

The finding is movement, not brightness. Labelled cells stay in the vascular compartment. At a bleeding site they spill into the bowel and are then carried along it — antegrade, and often retrograde. That movement, tracked across a dynamic acquisition, is what localises the source. Free pertechnetate from imperfect labelling appears in stomach and thyroid and imitates a bleed.

The agents

Tc-99m labelled red cellsUltraTag (in vitro) · modified in vivo · in vivo

SPECT / planar

Tc-99m · t½ 6.01 h · in vitro labelling ~98% efficient, in vivo ~75–80%

Handle: The beta chain of haemoglobin, inside the red cell
Trapping: Stannous ion enters the cell and reduces pertechnetate there; reduced technetium binds haemoglobin and cannot cross back out.
Use: MUGA / equilibrium radionuclide angiography for LVEF — reproducible to about ±5% and still the standard for cardiotoxicity monitoring. GI bleeding, detecting 0.1–0.4 mL/min against angiography’s 1 mL/min. Cavernous haemangioma: early photopenia with late fill-in on blood-pool SPECT is nearly pathognomonic.
Pitfall: Labelling efficiency is destroyed by heparin, and reduced by doxorubicin, hydralazine, methyldopa, quinidine, prazosin, iodinated contrast and circulating anti-Tc antibodies. Free pertechnetate appears in stomach and thyroid. On a bleeding study, a hot spot seen only on a delayed image cannot be localised — blood moves. For haemangioma, lesions under about 1.5–2 cm are below resolution.

Tc-99m human serum albuminHSA

SPECT / planar

Tc-99m · t½ 6.01 h

Handle: Plasma protein — stays in the vascular compartment
Trapping: Simple intravascular label with no cell handling required.
Use: Plasma volume, some cardiac blood-pool work, peritoneovenous shunt patency, and as a carrier in lymphoscintigraphy.
Pitfall: Leaks from the vascular space faster than labelled red cells, so it is poor for prolonged studies such as GI bleeding.

← 14Hepatobiliary transport 16 →Aggregate binding — amyloid and tau IndexAll 16 mechanisms