Evaluation of technetium-99m exametazime stabilised with cobalt chloride as a blood flow tracer in focal cerebral ischaemia.

A protocol has been devised to effectively extend the limited post-reconstitution shelf life of technetium-99m exametazime as a radiopharmaceutical for imaging cerebral blood flow (CBF) distribution. The potential of 99mTc-exametazime stabilised with cobalt chloride for imaging CBF distribution as late as 4 h after reconstitution has been examined in ischaemic and non-ischaemic tissue in halothane-anaesthetised cats. Focal cerebral ischaemia was produced by permanent middle cerebral artery occlusion. The relationship between 99mTc-exametazime uptake and retention and CBF (assessed with [14C]iodoantipyrine 10 min after first radiopharmaceutical administration) was determined in the same tissue section with double label autoradiography. Over the CBF range 0-80 ml 100 g-1 min-1, the uptake of 99mTc-exametazime (quantitatively and topographically) was linearly related to CBF irrespective of whether the 99mTc-labelled tracer was unstabilised (and administered within 10 min of reconstitution) or was stabilised with cobalt chloride (and administered up to 240 min after reconstitution). For levels of CBF in excess of 80 ml 100 g-1 min-1 the excellent topographical relationship between 99mTc-exametazime distribution and CBF is maintained but quantitatively, 99mTc-exametazime underestimates CBF to a similar degree in animals receiving stabilised and unstabilised 99mTc-exametazime. The presence of the stabiliser, cobalt chloride, extends greatly the period over which 99mTc-exametazime can be used after reconstitution to generate images of CBF distribution in normal and ischaemic cerebral tissue.