Atlas of Brain perfusion SPECT

Radiolabeled agents for Brain Perfusion SPECT

Technetium-99m ligands

Technetium-99m has a photopeak of 140 KeV and a half-life of 6 hours. It is eluted from a Molybdenum-99 generator, and preparation of radiopharmaceuticals can be performed on-site from unlabeled kits.

1. Technetium-99m-hexamethyl propylamine oxime (Tc-99m-HMPAO, Ceretec, Amersham Ltd., U.K.), now known as Tc-99m-exametazime, is a lipid soluble macrocyclic amine. Brain uptake of the radiotracer is rapid and reaches its maximum within 10 minutes post-injection time. Its first-pass extraction into the brain is less than that of I-123-IMP, and it underestimates rCBF. However, this underestimation can be corrected by accounting for the freely exchangeable component of HMPAO.

The distribution of the radiotracer remains constant for many hours post-injection. Once it crosses the blood brain barrier, 99m-Tc-HMPAO is converted into a hydrophilic compound in the presence of intracellular glutathione and is trapped, with slow blood clearance.

Limitations:

Perfusion defects are not seen as sharply as with I-123-IMP.
Tc-99m-HMPAO may also mask ischemic lesions in subacute stroke because of hyperemia resulting in normal or increased tracer uptake.
The radiopharmaceutical is chemically unstable in vitro by 30 minutes after preparation.

2. Technetium-99m-bicisate (bicisate ethyl cysteinate dimer, Tc-99m-ECD, Neurolite, Dupont-Merck pharmaceutical Co.), another 99m-Tc-labeled ligand, has high initial cerebral extraction and very slow clearance from the brain. Brain uptake is rapid, and peak brain activity compares favorably with that of other brain perfusion agents, reaching 6% of the injected dose by 5 minutes after intravenous injection. Blood clearance is also rapid, resulting in high brain-to-soft-tissue activity ratios early after injection that improve with time.

Tc-99m-ECD is stable for a much longer time than Tc-99m-HMPAO. Thus distribution by central radiopharmacies is possible as well as on-site preparation.

Both Tc-99m-HMPAO and Tc-99m-ECD exhibit less brain extraction than does I-123-IMP, but the more favorable dosimetry permits a substantially higher dose (20 to 30 mCi) and a higher photon flux.

Radiolabeling with Tc-99m onsite followed by mandated quality control measures, such as chromatography, must be performed in an experienced nuclear medicine laboratory. These procedures are standard in all nuclear medicine services and are commonly performed for other organ systems.

Comments and errors to Chandak@dsg.harvard.edu. nformation on images through Basem@bwh.harvard.edu.
Last updated: July 20, 1995.