Abstract:
A generator that allows for a non-fission based method of producing and recovering  99m Tc from neutron-irradiated molybdenum. This generator system is based on the isolation of  99m Tc, as the decay product from a source of  99 Mo labelled molybdenum carbonyl Mo(CO) 6  through a distillation process. The  99m Tc obtained from this distillation is produced with high efficiency and purity in a solvent-free form, which can then be dissolved in water or other solvents to produce a solution at the required specific activity and concentration, as reasonably determined by the operator.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the production of technetium-99m, a radioisotope for diagnostic medical purposes. In particular, the invention provides a method and apparatus for the recovery of technetium-99m from molybdenum 99. 
       BACKGROUND OF THE INVENTION 
       [0002]    The most commonly used radioisotope for diagnostic medical purposes is  99m Tc. The energy of its radiation is ideal for diagnostic imaging, and its half-life of 6 hours is short enough so that relatively large amounts can be used with low probability of radiation injury.  99m Tc is the radioactive daughter of molybdenum-99 ( 99 Mo), which has a half-life of 66 hours. When  99 Mo is produced, the  99m Tc daughter begins to accumulate, but because of its shorter half-life, it reaches an equilibrium level where there will be approximately one disintegration of a  99m Tc atom for every disintegration of a  99 Mo atom. In 24 hours (4 half-lives of the  99m Tc), this equilibrium is very nearly reached. If one has a source of  99 Mo, it is therefore possible to remove an almost equivalent amount of  99m Tc every 24 hours. 
         [0003]    Currently, the recovery of  99m Tc from  99 Mo is by means of an apparatus called a generator, as described in U.S. Pat. Nos. 4,837,110; 4,280,053; 4,020,351. The most prevalent type of generator consists of molybdenum-99 molybdate adsorbed onto an aluminum oxide column; the  99 Mo undergoes nuclear decay to  99m Tc, which is in the chemical form as the pertechnetate. The  99m Tc is obtained from the column by eluting with a sodium chloride solution. As a consequence of the decay of  99 Mo, the generator produces less  99m Tc over time, such that by the third day (72 hours), the generator produces less than half the amount it produced on the day it was first received and put in use. Typically, a generator is used for about one week before it is replaced by a new generator. After a few months, the old generators are discarded when the  99 Mo has decayed to essentially background levels. 
         [0004]    The major source of high specific activity  99 Mo used in these generators is isolated from the fission product mixture obtained from nuclear fission of uranium-235 and has a very high specific activity. When a fissionable atom, such as uranium 235 ( 235 U) undergoes fission in a nuclear reactor, it splits into two fragments known as fission products. In approximately 6% of such fissions, one atom of  99 Mo is formed, which is equivalent to 3% of all fission products. Under these conditions, the most economical and efficient preparation of  99 Mo requires highly enriched uranium (HEU). The use of HEU in this process presents many non-proliferation issues for companies wanting to use this technology. Furthermore, after separation of the  99 Mo for use in technetium generators, the remaining HEU is contaminated with immense amounts of other fission products. The highly active waste generated by this technology presents serious disposal and storage issues for companies wishing to use this technology. 
         [0005]    A feature of the  99 Mo produced in this fission-based process is that it is relatively free of non-radioactive isotopes of molybdenum and can be conveniently adsorbed on a column that is no bigger than a small pencil. As a result, Curie amounts of  99 Mo can be loaded on a generator and shielded in a lead container, which is easily transported to a radiopharmacy for dispensing. 
         [0006]    Alternatively,  99 Mo can be produced in commercial quantities by neutron irradiation of  98 Mo in a nuclear reactor [ref: “Obtaining Mo-99 in the IRT-T research reactor using resonance neutrons”. Ryabchikov, Skuridin, Nesterov, Chibisov, Golovkov;  Nuclear Instruments and Methods in Physics Research , B 213, 364 (2004)]. The production rate in a typical high neutron flux reactor yields desired product in the range of 1 Ci/g to 10 Ci/g specific activity using natural molybdenum metal. This yield is highly unfavourable compared to over 10 4  Ci/g for fission-based production. Even if the separated  98 Mo isotope, which is approximately 24% in natural abundance, is used, the proportion of  99 Mo in this target is very low compared to that obtained as a fission product. As a result of the low specific activity, the size of an aluminum oxide column to accommodate this  99 Mo in a generator would be larger and require greater amounts of shielding. In addition to the shielding and associated handling problems, the volume of sodium chloride solution, time of recovery and efficiency of recovery detracts from the use of this type of non-fission produced technetium generator from an economical and commercial perspective. 
         [0007]    Two other types of generator have been proposed for recovering the  99m Tc from irradiated  98 Mo. When the molybdenum is irradiated in the oxide form (MoO 3 ), it is possible to distill the  99m Tc from this at temperatures of 800-900° C. However, the efficiency of the  99m Tc recovery is dependent on the size of the target and is also lower for each successive recovery. For commercially acceptable amounts, the recovery time is excessively long in relation to the half-life of the  99m Tc with yields of less than 50%. 
         [0008]    A second type of generator that has been extensively studied involves solvent extraction of the  99m Tc. If irradiated MoO 3  is dissolved in KOH to form a solution of K 2 MoO 4 , the  99m Tc may be extracted from this with methyl ethyl ketone (MEK) provided the  99m Tc is in the pertechnetate state. Various ways of recovering the  99m Tc have been investigated, including evaporation of the MEK and adsorption on alumina. Various subsequent procedures have been utilized to improve the purity of the product  99m Tc, however, MEK produced  99m Tc often gives poor yields when used for the labelling of radiopharmaceuticals. 
       SUMMARY OF THE INVENTION 
       [0009]    This invention relates to a generator that allows for a non-fission based method of producing and recovering  99m Tc from neutron-irradiated molybdenum. This generator system is based on the isolation of  99m Tc, as the decay product from a source of  99 Mo labelled molybdenum carbonyl Mo(CO) 6  through a distillation process. The  99m Tc obtained from this distillation is produced with high efficiency and purity in a solvent-free form, which can then be dissolved in water or other solvents to produce a solution at the required specific activity and concentration, as reasonably determined by the operator. 
       BRIEF DESCRIPTION OF DRAWING 
       [0010]    In order that the invention can be more clearly understood, a preferred embodiment is described below with reference to the accompanying drawing which is a schematic layout of apparatus used to generate technetium-99m from labelled molybdenum carbonyl. 
     
    
     DESCRIPTION OF THE INVENTION 
       [0011]    The generator system of this invention involves a distillation procedure to enable the separation of  99m Tc from  99 Mo in a closed system with the opportunity to perform multiple recoveries. The recovery time required to isolate the  99m Tc depends on the level of specific activity, but is short compared to the half-life of  99m Tc. 
         [0012]    The operation of the generator depends on the distillation of molybdenum carbonyl Mo(CO) 6  labelled with a high specific activity of  99 Mo hereinafter referred to as labelled molydenum carbonyl. When  99 Mo in this carbonyl compound decays to  99m Tc, the  99m Tc is not volatile and quantitatively remains in the distillation vessel. It may be recovered from this vessel with any aqueous or non-aqueous solvents at the desired concentration, as determined by the operator. It will be understood that aqueous solutions are desirable for intravenous injection into the human or animal body. The distilled Mo(CO) 6  is recovered in a second vessel where a further  99m Tc recovery can be obtained by a subsequent distillation back to the first vessel, after suitable delay in order to allow for the accumulation of the desired amount of  99m Tc as determined by the needs of the operator. 
         [0013]    The production of the labelled Mo(CO) 6  is outside the scope of this invention. In one method, direct irradiation of the Mo(CO) 6  is envisioned, although this is not expected to be the most productive method. When Mo(CO) 6  is irradiated with neutrons in a nuclear reactor, approximately 70% of the  99 Mo produced is retained as  99 Mo(CO) 6 , a phenomenon known as retention. This irradiated molybdenum carbonyl ( 99 Mo(CO) 6 ) along with the unreacted starting material, can be recovered by distillation. By distillation, it is meant “a process that consists of driving gas or vapour from liquids or solids by heating and condensing to liquid products”. The remaining 30% of the  99 Mo escapes from the Mo(CO) 6  by the Szilard-Chalmers process and is non-volatile, but is also associated with other non-volatile products related to the decomposition of Mo(CO) 6 . To minimize the decomposition products, the target Mo(CO) 6  must be cooled. Alternatively, the irradiation time can be shortened compared to that required to reach the saturation levels of  99 Mo. Therefore, the specific activity of the  99 Mo(CO) 6  is reduced by both losses due to decomposition and/or a shortened irradiation time. 
         [0014]    Alternatively and more preferably, very high specific activity of  99 Mo in Mo(CO) 6  can be obtained by direct irradiation of molybdenum metal powder in a nuclear reactor. Subsequent conversion of this irradiated molybdenum to Mo(CO) 6  can be carried out by standard chemical procedures, such as heating the metal to about 225° C. at 200 atmospheric pressure in the presence of carbon monoxide, or other methods as known to those skilled in the art. 
         [0015]    A significant feature of the Mo(CO) 6  system is that once the  99 Mo has decayed to the extent that it is no longer useful in the generator, the residual carbonyl compound can be heated to a temperature above 150° C. to decompose the compound back to molybdenum powder and can be re-irradiated in the nuclear reactor. In this way, separated  98 Mo used as the target material can be recycled. 
       DESCRIPTION AND OPERATION OF THE GENERATOR 
       [0016]    Other features and advantages of the present invention will become apparent from the following description. It should be understood, however, that the detailed description and the examples while indicating preferred embodiments of the invention are given by way of illustration only, since various changes and modifications within the spirit and scope of the claims will become apparent to those skilled in the art from this detailed description. 
         [0017]    The size of the generator depends on the mass of labelled Mo(CO) 6  to be processed. For example, a 5 Curie generator, which might require only 5 grams of the carbonyl can be relatively small. There is, however, no restriction on the magnitude, and a kiloCurie generator is possible. It should be noted that the size of this generator, with the requisite shielding, could be significantly larger than that used for fission produced  99 Mo. However, this generator would be reusable and could be permanently located at a central site for distribution of the recovered  99m Tc to surrounding hospitals. In such a scenario, the irradiated Mo metal or the labelled Mo(CO) 6  would then be delivered to the central site. Shielding required for transportation of either of these irradiated products would be similar in size to that used for currently available commercial generators based on fission products. 
         [0018]    A schematic diagram of one type of generator is shown in  FIG. 1 , with the following features:
       Valves  1  to  7  indicated by reference characters V- 1  to V- 7  are all remotely actuated and are inside radiation protective shielding  20 , whose thickness is determined by the activity of materials used in the generator.   Vessels A and B are identical and may be heated or cooled by surrounding respective envelopes  22 ,  24 , in which appropriate fluids are allowed to flow, such fluids entering and exiting through any combination of valves indicated by reference characters V- 9  to V- 16 , as required. For example, to adjust vessel A to a single temperature, a fluid at that temperature is allowed to enter into the surrounding envelope through valves V- 9  and V- 13 , and exit through valves V- 10  and V- 14 . It is also possible to have two distinct temperatures maintained in the vessel. For example, a hot fluid enters through valve V- 13  and exits through valve V- 14 , while cold fluid is passed through valve V- 9  to valve V- 10 . To more effectively control the temperature in the envelopes  22 ,  24 , an insulated, horizontally oriented partition  28 ,  30  is disposed in each of the envelopes  22 ,  24  respectively, thereby separating a heating zone from a cooling zone.   Vials C and D are sterile vials fitted with septa, which will be used to introduce solvent to recover the  99m Tc from vessels A or B.       
 
         [0022]    The labelled Mo(CO) 6  is introduced into the generator from a supply vessel  25  through valve V- 6 . Vessel A is cooled to at least 0° C. with valves V- 3  and V- 5  closed and valve V- 1  and V- 8  open to a vacuum pump  26 . The temperature in Vessel A may be reduced further as low as 10° C. to reduce the vapor pressure of carbonyl in the vessel. When all Mo(CO) 6  has been collected in vessel A, valves V- 6  and V- 1  are closed. After appropriate time to allow build up of  99m Tc, vessel A is heated to a temperature such as 100° C. or higher sufficient to allow rapid distillation of the carbonyl. Vessel B, with valves V- 4  and V- 7  closed, is evacuated through valve V- 2  to dry the vessel, and is subsequently cooled to at least 0° C. Valve V- 5  is then opened to allow the distillation of the Mo(CO) 6  into vessel B from vessel A. After this distillation, vessel A will contain only the  99m Tc to be recovered. Valve V- 5  and V- 7  are then closed. The  99m Tc is recovered from vessel A by filling vial C with an appropriate volume of solvent, such as outgassed water containing a small amount of H 2 O 2 . The volume selected will be such that when it has extracted the expected amount of  99m Tc, its specific activity will be that required by the user. Vial C is then connected to valve V- 3  by a hypodermic needle extending to the bottom of the vial. Valve V- 3  is opened to allow the solvent in the vial to be taken up into vessel A, after which valve V- 3  is closed. The upper part of vessel A is cooled while the lower part of vessel A is heated to allow the solvent (outgassed water) to reflux in vessel A and collect the  99m Tc at the bottom of the vessel. Such a method of reflux recovery adapted to remove iodine 125 from the interior of a decay chamber in which iodine 125 is formed by decay of Xenon 125 as described in applicant&#39;s U.S. Pat. No. 6,056,929, the disclosure of which is herein incorporated by reference. Air is then allowed to enter vessel A through valve V- 1  and V- 8 , and valve V- 3  is also opened to allow the solvent containing the  99m Tc to flow in to vial C. After the solution of  99m Tc is collected, valve V- 3  is closed and the air and remaining moisture can be pumped out of vessel A through valves V- 1  and V- 8 . After an appropriate time to allow the accumulation of  99m Tc in vessel B, the Mo(CO) 6  is distilled back into vessel A and the recovery of a second yield of  99m Tc can be obtained from vessel B into vial D in a similar manner, as described above.