Basic Mechanism & Process for Technegas Production

The starting material is Tc-99m as standard sodium pertechnetate for injection.

The Technegas generator is essentially a miniature high temperature furnace in which the heating element is also the source of graphite vapour which ultimately coats the Technetium metal. The heating element is made from 100% pure spectroscopic graphite whose electrical and mechanical specifications match the requirements of the machine. It is a 6mm square section rod 50mm long, machined to form a crucible in the centre section such that it can hold a liquid volume of 0.14mL. This hollowed and thinned section also provides the high resistive portion of the rod which becomes the hottest section when electric current is passed through it. The rod is held under spring tension between two high current electrodes. In the present commercial machine, the entire assembly is mounted as a drawer section sliding into the lower chamber of a 6L vessel, and electrically powered from an automatic process-controller.

The crucible is first filled with liquid Sodium Pertechnetate in normal physiological saline, which in most instances contains enough activity [ 260-370MBq or 7-10mCi ] for a single patient administration. The drawer section is then closed and the automatic process takes over, gently blowing pure argon gas over the top of the crucible while warming it to 70°C. This is known as the "simmer" cycle, and takes 6 minutes, during which time the liquid in the crucible dries out and the whole chamber is purged with pure argon, replacing all the original air and water vapour.

At this time the patient is usually prepared and rehearsed with the single use plastic breathing set containing a special filter to capture exhaled Technegas, and is positioned for a posterior view on a gamma camera preferably in the supine position.

The process controller alerts the user that the machine is ready to produce Technegas. At this time, if the initial load of activity is deemed to be inadequate, the process may be "cancelled" the drawer opened and another 0.14mL of solution added to the crucible. The simmer process is then repeated. This stage of the operation may be performed as often as necessary. The use of an external "oven" is often employed in busy departments with less than optimum concentration of Pertechnetate solution to simplify and speed up the process, but at least one 6 minute simmer time is necessary to produce the 100% argon atmosphere necessary to generate pure Technegas.

A much more efficient "concentrator" device is under trial at present. This can extract over 90% of the activity from up to 10mL eluent volume directly into the crucible in about 10 minutes, allowing very dilute solutions of the starting Pertechnetate to be used.

Traces of oxygen as low as 0.1% will produce at least some PerTechnegas, the level rising rapidly to near 100% above 0.5% oxygen.

At the conclusion of this preparation phase, the machine is activated via the control panel "start" button, and the crucible temperature rises to 2550°C by resistively heating it with about 4.5kW of power within 0.75s and holds that value ±50°C for 15s through a feedback servo from an optical sensor before switching off. This fills the 6L chamber with Technegas ready for immediate use via inhalation by the patient.

For operational convenience, the machine allows a 10 minute window in which the Technegas may be administered to the patient, and this is generally accomplished in 2-3 breaths, depending on many variables related mainly to the technique of administration.