99m Tc-HMPAO in Brain SPECT Imaging; Feasibility of Ultra-Utilization of the Cold kit with higher 99mTc activities - A Chromatographic Study.

Abstract

Brain SPECT imaging has become one of the most interesting subjects for most specialists in nuclear medicine because of its ability to present clear and detailed data about the brain anatomy and function, allowing the specialist to obtain the greatest sensitivity and specificity for disease detection (Catafau, 2001). Technetium-99m complex of hexamethyl propylene amineoxime (HMPAO) (which is called also exametazime) is widely used as an efficient brain perfusion agent due to the ability of the primary 99mTc-HMPAO complex to penetrate the intact blood-brain barrier (BBB) which is believed to be attributed to its lipophilic property (Hung, 1994). The primary lipophilic 99mTc-HMPAO complex is inherently unstable and rapidly converts to a secondary less lipophilic complex. Retention in the brain results from the inability of the secondary complex to cross the blood-brain barrier (Ponto, 1990). Thus, we have here one more impurity ligand in addition to the 2 other persisting impurities which are the free and hydrolyzed-reduced technetium. The radiochemical purity of the 99mTc-HMPAO preparation could be recognized by using a combination of three thin layer chromatographic (TLC) systems (Tikofsky, 1993). HMPAO commercial kits are one of the most expensive materials in nuclear medicine, in addition to that; the added Tc-99m activities proposed by the commercial venders can be enough to perform only 2 patient studies per vial which is not cost effective especially in developing countries that obstacles depending on brain SPECT as a routine imaging protocol. To overcome this, there are two postulated solutions; one is to collect a larger number of patients per session which is not practical as it may take several weeks to have a good number of patients especially in small NM facilities, the other solution is to do a cold kit fractionation which is best idea allowing having secured single patient doses at different times; however, this method needs a well trained personnel to perform it. These two solutions are still not fully applicable because of the limited Tc-99m capacity of the HMPAO vial; this motivated us to study the effect of adding up to 200% of the recommended activity to one of the commercial kits which will allow us to know to which activity limit we can have a safe preparation and if we exceeds these limits what will be the produced impurities out of this, and hence, the preparation is modifiable and recoverable or not