The present invention relates to method of producing a radiolabelled gallium complex using microwave activation. The present invention relates to a method of producing radiolabelled gallium complexes. The complexes could be used as diagnostic agents, e.g. for positron emission tomography (PET) imaging. PET imaging is a tomographic nuclear imaging technique that uses radioactive tracer molecules that emit positrons. When a positron meets an electron, the both are annihilated and the result is a release of energy in form of gamma rays, which are detected by the PET scmmer. By-eploying natural siubstances that are used by the body as tracer molecules, PET does not only provide information about structures in the body but also information about the physiological function of the body or certain areas herein. A common tracer molecule is for instance 2-fluoro-2-deoxy-D-glucose (FDG), which is similar to naturally occurring glucose, with the addition of a 18F-atom. Gamma radiation produced from said positron-emitting fluorine is detected by the PET scanner and shows the metabolism of FDG in certain areas or tissues of the body, e.g. in the brain or the heart. The choice of tracer molecule depends on what is being scanned. Generally, z tracer is chosen that will accumulate in the area of interest, or be selectively taken up by a certain type of tissue, e.g. cancer cells. Scanning consists of either a dynamic series or a static image obtained after an interval during which the radioactive tracer molecule enters the biochemical process of interest The scanner detects the spatial and tamporal distribution of the tracer molecule. PET also is a quantitative imaging method allowing the measurement of regional concentrations of the radioactive tracer molecule. Commonly used radionuclides in PET tracers are 11C, 18F, 15O 13N or 76Br. Recently, new PET tracers were produced that are based on radiolabelled metal complexes comprising a bifunctional chelating agent and a radiometaL Bifiinctional chelating agents are chelating agents that coordinate to a metal ion and are linked to a targeting vector that will bind to a target site in the patient's body. Such a targeting vector may be a peptide that binds to a certain receptor, probably associated with a certain area in the body or with a certain disease. A targeting vector may also be an oligonucleotide specific for e.g. an activated oncogene and thus aimed for tumour localisation. The advantage of such complexes is that the bifimctional chelating PZ0334/FI/11.04.2003 agents may be labelled with a variety of radiometals like, for instance, 68Ga, 2l3 Bi or 86Y. In this way, radiolabelled complexes with special properties may be "tailored" for certain applications. i 68Ga is of special interest for the production of Ga-radiolabelled metal complexes used as tracer molecules in PET imaging. 68Ga is obtained from a 68Ge/68Ga generator, which means that no cyclotron is required. 68Ga decays to 89% by positron emission Of 2.92 MeV and its 68 min half life is sufficient to follow many biochemical processes in vivo without unnecessary radiation. With its oxidation state of+m, 6SGa forms stable complexes with various types of dictating agents and 68Ga tracers have been used for brain, renal, bone, blood pool, lung and tumour imaging. J. Schumacher et al., Cancer Res. 61, 2001, 3712-3717 describe the synthesis of 68Ga-N,N-{2hydroxy-5-(ethylene-ß-carboxy)benzyl]emylenediamine-NJiI'-diacetic acid ("Ga-HBED-CC). 68Ga obtained from a 68Ge/68Ga generator and Ga3+ carrier are reacted with the chelating agent HBED-CC in acetate buffer for 15 min at 95°C. Uncomplexed 68Ga is separated from the complex using a cation exchange column. The overall preparation is reported to take 70 min. A disadvantage of this method is that the overall preparation time of the radiolabelled complex is very long. Due to the addition of "cold" Ga3+ carrier, the specific activity of the reaction is low. Moreover, the radiolabelled complex had to be purified after the complex formation reaction. WO-A-99/56791 discloses the reaction of 68GaCl3 obtained from a 68Ge/68Ga generator with the tetradentate amine trithiolate chelating agent tris(2-mercaptobenzyl)amine (S3N). The complex formation is carried out at room temperature for 10 min. A disadvantage of the method described is that the radiolabelled complex had to be purified by liquid chromatography before it could be used for in vivo studies. A further disadvantage of the method is the relatively long reaction time 6. Ugur et al., Nucl. Med. Biol. 29, 2002,147-157 describe the synthesis of the 68Ga labelled somatostatin analogue DOTA-DPhe'-Tyr'-octreotide (DOTATOC). The compound is prepared by reacting 68GaCb obtained from a '"Ge/^Ga generator with the chelating agent DOTATOC for 15 min at 100°C. A disadvantage of this method is that the reaction mixture had to be heated at relatively high temperatures. The DOTA chelating agent was functionalised with a peptide targeting vector and peptides and proteins are substances, which are known to be sensitive to heat. Thus, with the method described there is a risk that heat sensitive targeting vectors are destroyed during complex formation. A further disadvantage is that the complex had to be purified by HPLC before it could be used for animal studies. US-A-5070346 disclose 68Ga-labelled complexes of the chelating agent tetraethylcyclohexyl-bis-aminoethanethio] (BAT-TECH). The complexes are synthesised by reacting 68GaCl3 obtained from a 68Ge/68Ga generator with BAT-TECH at 7S°C for 15 min and subsequent filtration. The preparation of the complex was accomplished in 40 min. Due to the high reaction temperature; this method would not be suitable for bifunctional chelating agents comprising a heat sensitive targeting vector, for instance a peptide or a protein. A further disadvantage is the long reaction time of the complex formation reaction. In view of the relatively short half-life of 68Ga there is a need for a fast method for the synthesis of 68Ga-labelled complexes, which could be used as tracer molecules for PET imaging. It has now been found that the use of microwave activation substantially improves • the efficiency and reproducibility of the 68Ga-chelating agent complex formation. Due to microwave activation, chemical reaction times could be shortened substantially; i.e. the reaction is completed within 2 min and less. This is a clear improvement as a 10 minutes shortage of the reaction time saves about 10% of the 68Ga activity. Furthermore, microwave activation also leads to fewer side reactions and to an increased radiochemical yield, which is due to increased selectivity. Solutions of 66Ga3+, 67Ga3+ and 68Ga3+ radioisotopes, which have been obtained by cyclotron production or from a generator contain so-called pseudo carriers, i.e. other metal cations like for instance Fe3+, Al3+, Cu2+, Zn2+ and In3+. As these pseudo carriers compete with Ga3+ in the complex formation reaction, it is important to increase the selectivity of the radiolabelling reaction. Hence, microwave activation has a positive effect on radiolabelling with all Ga-radioisotopes, namely with ^ 67Ga and 68Ga. Microwave activation has been used in nucleophilic aromatic radiofluorations with I8F and it was found that comparable or better yields than those reported for thermal treatments were obtained in shorter reaction times (S. Stone-Elander et al, Appl. Rad. Isotopes 44(5), 1993, 889-893). However, the use of microwave activation in Ga-radiolabelling reactions has not been described yet The invention thus provides a method of producing a radiolabelled gallium complex by reacting a Ga3+ radioisotope with a chelating agent characterised in that the reaction is carried out using microwave activation. Suitable Ga3+ radioisotopes according to the invention are 66Ga3+ 67Ga3+ and 68Ga3+, preferably 66Ga3+ and 68Ga3+ and particularly preferably 68Ga3+. 68Ga3+ and Ga3+ are particularly suitable for the production of radiolabelled complexes useful in PET imaging whereas 67Ga3+ is particularly suitable for the production of radiolabelled complexes useful in single photon emission computerised tomography (SPECT). 66Ga3+ is obtainable by cyclotron production by irradiation of elemental zinc targets. To minimise the amounts of 67Ga production, the target thickness is preferably maintained such that the degraded proton energy is above 8 MeV, and irradiation time is kept short, e.g. <4 his. The chemical separation may be achieved using solvent-solvent extraction techniques using isopropyl ether and HC1 as described in L.C. Brown, Int. J. Appl. Radiat. Isot 22, 1971, 710-713. 66Ga has a relatively long half-life of 9.5 h and the most abundant positron emitted has a uniquely high energy of 4.2 MeV. 67Ga3+ is obtainable by cyclotron production and 67CaCl3 obtained by cyclotron production is a commercially available compound. The half-life of 67Ga is 78 h. 68Ga is obtainable from a 68Ge/68Ga generator. Such generators are known in the art and for instance described by C. Loc'h et al, J. Nucl. Med. 21, 1980, 171-173. Generally, 68Ge is loaded onto a column consisting of an organic resin or an inorganic metal oxide like tin dioxide, aluminium dioxide or titanium dioxide. 68Ga is eluted from the column with aqueous HC1, yielding 68GaCl3. 68Ga3+ is particularly preferred in the method according to the invention as its production does not require a cyclotron and its 68 min half-life is sufficient to follow many biochemical processes in vivo by PET imaging without long radiation. Preferred chelating agents for use in the method of the invention are those which present the Ga3+ radioisotopes in a physiologically tolerable form. Further preferred chelating agents are those that form complexes with Ga3+ radioisotopes that are stable for the time needed for diagnostic investigations using the radiolabelled complexes. Suitable chelating agents are, for instance, polyaminopolyacid chelating agents like DTPA, EDTA, fcTPA-BMA, DOA3, DOTA, HP-DOA3, TMT or DPDP. Those chelating agents are well known for radiopharmaceuticals and radiodiagnosticals. Their use and synthesis are described in, for example, US-A-4647447, US-A-5362 475, US-A-5534241, US-A-5358704, US-A-5198208, US-A-4963344, EP-A-230893, EP-A-130934, EP-A-606683, EP-A-438206, EP-A-434345, WO-A-97/00087, WO-A-96/40274, WO-A-96/30377, WO-A-96/28420, WO-A- 96/16678, WO-A-96/11023, WO-A-95/32741, WO-A-95/27705, WO-A-95/26754, WO-A-95/28967, WO-A-95/28392, WO-A-95/24225, WO-A-95/17920, WO-A-95/15319, WO-A-95/09848, WO-A-94/27644, WO-A-94/22368, WO-A-94/08624, WO-A-93/16375, WO-A-93/06868, WO-A-92/11232, WO-A-92/09884, WO-A-92/08707, WO-A-91/15467, WO-A-91/10669, WO-A-91/10645, WO-A-91/07191, WO-A-91/05762, WO-A-90/12050, WO-A-90/03804, WO-A-89/00052, WO-A-89/00557, WO-A-88/01178, WO-A-86/02841 and WO-A-86/02005. Suitable chelating agents include macrocyclic chelating agents .e.g. porphvrin-like molecules and pentaaza-macrocycles as described by Zhang et al., Inorg. Chem. 37(5), 1998,956-963, phthalocyanines, crown ethers^ e.g. nitrogen crown ethers such as the sepulchrates, cryptates etc., hemin (protoporphyrin IX chloride), heme and chelating agents having a square-planar symmetry. Macrocyclic chelating agents are preferably used in the method of the invention. In a preferred embodiment, these macrocyclic chelating agents comprise at least one hard donor atom such as oxygen and/or nitrogen like in polyaza- and polyoxomacrocycles. Preferred examples of polyazamacrocyclic chelating agents include DOTA, TRITA, TETA and HETA with DOTA being particularly preferred. Particularly preferred macrocyclic chelating agents comprise functional groups such as carboxyl groups or amine groups which are not essential for coordinating to Ga3+ and thus may be used to couple other molecules, e.g. targeting vectors, to the chelating agent. Examples of such macrocyclic chelating agents comprising functional groups are DOTA, TRTTA or HETA. In a further preferred embodiment, bifunctional chelating agents are used in the method according to the invention. ""Bifunctional chelating agent" in the context of the invention means chelating agents that are linked to a targeting vector. Suitable targeting vectors for bifunctional chelating agents useful in the method according to the invention are chemical or biological moieties, which bind to target sites in a patient's body, when the radiolabelled gallium .complexes comprising said targeting vectors have been administered to the patient's body. Suitable targeting vectors for bifunctional chelating agents useful in the method according to the invention are proteins, glycoproteins, lipoproteins, polypeptides like antibodies or antibody fragments, glycopolypeptides, lipopolypeptides, peptides, like ROD binding peptides, glycopeptides, lipopeptides, carbohydrates, nucleic acids e.g. DNA, RNA, oligonucleotides like antisense oligonucleotides or a part, a fragment, a derivative or a complex of the aforesaid compounds, or any other chemical compound of interest like relatively small organic molecules, particularly small organic molecules of less than 2000 Da. In a particularly preferred embodiment, macrocyclic bifunctional chelating agents are used in the method according to the invention. Preferred macrocyclic bifunctional chelating agents comprise DOTA, TRITA or HETA linked to a targeting vector, preferably to a targeting vector selected from the group consisting of proteins, glycoproteins, lipoproteins, polypeptides, glycopolypeptides, lipopolypeptides, peptides, glycopeptides, lipopeptides carbohydrates, nucleic acids, oligonucleotides or a part, a fragment, a derivative or a complex of the aforesaid compounds and small organic molecules; particularly preferably to a targeting vector selected from the group consisting of peptides and oligonucleotides. The targeting vector can be linked to the chelating agent via a linker group or via a spacer molecule. Examples of linker groups are disulfides, ester or amides, examples of spacer molecules are chain-like molecules, e.g. lysin or hexylamine or short peptide-based spacers. In a preferred embodiment, the linkage between the targeting vector and the chelating agent part of radiolabelled gallium complex is as such that the targeting vector can interact with its target in the body without being blocked or hindered by the presence of the radiolabelled gallium complex. Microwave activation according to the invention is suitably carried out by using a microwave oven, preferably by using a monomodal microwave oven as. Suitably microwave activation is parried out at 80 to 120 W, preferably at 90 to 110 W, particularly preferably at about 100 W. Suitable microwave activation times range from 20 s to 2 min, preferably from 30 s to 90 s, particularly preferably from 45 s to 60s. A temperature control of the reaction is advisable when temperature sensitive chelating agents, like for instance bifunctional chelating agents comprising peptides or-proteins. as targeting vectors, are employed in the method according to the invention. Duration of the microwave activation should be adjusted in such a way, that the temperature of the reaction mixture does not lead to the decomposition of the chelating agent and/or the targeting vector. If chelating agents used in the method according to the invention comprise peptides or proteins, higher temperatures applied for a shorter time are generally more favourable than lower temperatures applied for a longer time period. Microwave activation can be carried out continuously or in several microwave activation cycles during the course of the reaction. In a preferred embodiment, the invention provides a method of producing a 68Ga radiolabelled PET imaging tracer by reacting 68Ga3+ with a macrocyclic bifunctional chelating agent comprising hard donor atoms, characterised in that the reaction is carried out using microwave activation. In a particularly preferred embodiment of the method described in the last preceding paragraph, the microwave activation is carried out from 30 s to 90 s at 90 to 110 W. If 68Ga3+ is used in the method according to the invention, the 68Ga3+ is preferably obtained by contacting the eluate form a 68Ga3+ generator with an onion exchanger and eluting 68Ga3+ from said anion exchanger. In a preferred embodiment, the anion exchanger is an anion exchanger comprising HCO3 as counterfoils. The use of anion exchangers to treat 68Ga eluate obtained from a 68Ge/68Ga generator is described by J. Schubmacher et al. Int. I. appl. Radiat. Isotopes 32,1981,31-36. A Bio-Rad AG 1x8 anion exchanger was used for treating the 4.5 N HC168Ge/68eluate obtained from a 68Ge/68Ga generator in order to decrease the amount of MGe present in the eluate. - It has now been found that the use of anion exchangers comprising HCO3 as counterions is particularly suitable for the purification and concentration of the generator eluate. Not only the amount of 68Ge present in the eluate could be reduced but also the amount of so-called pseudo carriers, i.e. other metal cations like Fe3+, Al3+, Cu2+, Zn2+ and In3+, that are eluted together with the 68Ga3+ from the generator. As these pseudo carriers compete with 68Ga3+ in the subsequent complex formation reaction, it is especially favourable to reduce the amount of those cations as much as possible before the labelling reaction. A further advantage of the anion-exchange purification step is that the concentration of 68Ga3+ which is in the picomolar to nanomolar range after the elution, can be increased up to a nanomolar to micromolar level. Hence, it is possible to reduce the amount of chelating agent in a subsequent complex formation reaction, which considerably increases the specific radioactivity. This result is important for the production of 68Ga-radiolabelled PET tracers that comprise a bifunctional chelating agent; i.e. a chelating agent linked to a targeting vector, as the increase in specific radioactivity enables the reduction in amount of such tracers when used in a patient. Hence, another preferred embodiment of the method according to the invention is a method of producing a 68Ga- radiolabelled complex by reacting 68Ga3+ with a chelating agent using microwave activation, wherein the 68Ga3+ is obtained by contacting the eluate form a ''68Ge/68 generator with an anion exchanger, prefer-ably with an anion exchanger comprising HCCV as counterions, and eluting 68Ga3+ from said anion exchanger. generators are known in the art, see for instance C. Loc'h et al, J. Nucl. Med. 21, 1980, 171-173 or J. Schuhmacher et al. Int. J. appl. Radiat. Isotopes 32, 19181, 31-36. 68Ge may be obtained by cyclotron production by irradiation of for instance Ga2(SO4)3 with 20 MeV protons. It is also commercially available, e.g. as MGe in 0.5 M HC1. Generally, 68Ge is loaded onto a column consisting of organic resin or an inorganic metal oxide like tin dioxide, aluminium dioxide or titanium dioxide.