Abstract:
A process and apparatus for producing the  18 F isotope from water enriched with the  18 O isotope using high energy protons from a cyclotron. The apparatus has a cyclotron target cavity that is connected to a fluid loop that contains a water reservoir, pump, and pressure regulator. Water is continuously recirculated through the target cavity to increase reliability. After irradiation long enough to produce a desired amount of  18 F, water in the target loop is diverted through an  18 F extraction device before being returned to the target loop. The returning water may also be purified and additional water added to the target loop as needed to permit continuous irradiation and production of  18 F.

Description:
BACKGROUND 
     1. Technical Field 
     The invention relates to production of an  18 F radioisotope by means of proton irradiation of  18 O enriched water. 
     2. Background 
     The  18 F isotope (hereinafter, F-18 isotope or F-18) has become widely used in nuclear medicine for diagnostic studies using a Positron Emission Tomography (PET) body scanning technique. The F-18 is typically used to label an injectable glucose derivative. Because of its short half-life (109 min), this isotope must be used as soon as possible after production. This makes it impossible to accumulate a sufficient quantity for delayed use. Therefore, work shifts usually start near midnight with production for distant (via automobile) hospitals first, followed by that for nearby hospitals in the very early morning. Any shortage in production has an immediate and direct effect on users. As a result, reliability and predictability of production are extremely important for users as well as suppliers of this isotope. 
     The two main methods of producing F-18 use an  18 O(p,n) 18 F reaction in a cyclotron. Both gaseous oxygen and liquid water enriched with  18 O (hereinafter, O-18) have been used as target materials. However, the gaseous approach is very difficult in practice because the F-18 is very reactive and hard to recover from a gaseous medium. The overwhelming majority of production facilities use water enriched with O-18(H 2 [ 18 O], hereinafter, O-18 water). 
     Using O-18 water is not without problems, also. For production efficiency, it is desirable to use water that is as much enriched as possible. However, 95% enriched O-18 water costs approximately $150 per ml. Also, PET has been gaining greater acceptance and the building of new O-18 water production facilities is lagging behind demand. The cost pressures make conservation and reuse of the O-18 water target material even more important. 
     In a typical system for F-18 production, the target is typically loaded with a pre-determined amount of O-18 water by means of a syringe or pump. The volume of water in the target is about 0.8 ml, but another 1-2 ml is required to fill the lines leading to the target. The water delivery system is then isolated from the target by means of a valve and the target is irradiated. This can be described as a “static” target, meaning that the target material remains in the target throughout the irradiation time. 
     The irradiated water is then removed from the target, typically by means of inert gas pressure, and transported over a delivery line leading outside the cyclotron shielding to a collection vial about 25 feet (8 m) from the target. The F-18 isotope is then separated from the water and processed for production of a radiopharmaceutical agent. 
     A considerable amount of O-18, typically 25-30%, is lost after each run. The O-18 isotope is used up in three ways. First, a very small amount, on the order of nanoliters, is actually converted to F-18. The next most important loss of O-18 is due to a combination of leakage and isotopic exchange with  16 O oxides in the target, transport lines and storage vessels. After one run of an hour or two, the enrichment factor can drop from 95% to 85-90%. This is still high enough to be economical to run a cyclotron, but the amount of contamination is too high, as will be explained below. (As the enrichment factor falls, the irradiation time increases. 80% is a minimum under current economic conditions.) 
     The third loss is due to leakage of target material from the pressurized target and attached tubing which may lead to a reduced water level in the target and, if severe enough, to a catastrophic failure. Target cooling relies on the liquid water material present in the target to function as a heat conductor. A typical 1 ml target must dissipate over 500 W of heat for as long as 2-3 hours. Many target systems are pressurized to as high as 500 psig or higher to improve target thermal stability. In these conditions, containment of a small amount of water becomes a significant technical problem. Loss of a very small amount of target material may have dramatic consequences such as target foil rupture, target body degradation, and loss of target yield. 
     Although 70-75% of the initial O-18 water remains, the biggest effective loss is due to contamination. Any contamination in the liquid water increases the formation of super-heated steam with increased leakage and loss of cooling. Because the consequences are so adverse, the water recovered after only one run in a static target system must be sent back to the supplier for reprocessing to remove contaminants. 
     Existing static target systems do not provide any mechanism to timely detect the critical loss of target material during irradiation. In addition, in a static target it is impossible to monitor the amount of radioactive F-18 being produced with any certainty. The result of a production run may not be known until after its completion, up to several hours after start of production. Given the fact that production and delivery schedules do not allow much flexibility due to the extremely short half-life of the F-18, this uncertainty results in a decrease in reliability and availability of the product. 
     SUMMARY 
     Accordingly, one objective of the invention is to increase the reliability of the production of F-18 from O-18 enriched water irradiated by high energy protons produced by a cyclotron. Further objectives are to increase the efficiency so that the cyclotron can be irradiating O-18 without interruption. Still another objective is to continually reuse O-18 water from which F-18 is periodically extracted. Another objective is to be able add additional new O-18 water as it is lost due to system leakage and the like so that the system can run for an extended period without interruption. 
     These objectives and more are realized with a process that continuously recirculates O-18 enriched water through a target loop that includes a target cavity for a cyclotron that irradiates the target cavity with protons to convert a portion of O-18 to F-18. 
     Longer irradiation without failure is achieved by using a combination of one or more of the following: maintaining a pressure of at least about 250 psig in the target cavity; recirculating the O-18 water through the target cavity at least about once every two minutes; and maintaining an O-18 water volume in the target loop that is at least about ten times the volume of the target cavity, itself. Additional benefit can be obtained by substantially cooling the O-18 water after exiting the target cavity and before reintroduction. 
     Increased efficiency is obtained by periodically recharging the target loop with additional O-18 water without interrupting irradiation and using protons having an energy of about 16 Mev and an intensity of at least about 40 μA on the target cavity. 
     Rather than stop irradiation and loose cyclotron time, F-18 can be extracted from irradiated O-18 water in the target loop by periodically, e.g., every hour or two, briefly diverting the target loop through an F-18 extraction device without interrupting irradiation of the target cavity. 
     Because the amount of O-18 that is converted to F-18 is quite small, e.g., less than 0.1% of the O-18 is converted, after F-18 is extracted, the remaining O-18 water can be purified by solid phase purification devices and reintroduced into the target loop. 
     The aforementioned target loop can be implemented with, in order: an O-18 water reservoir; a pump; a target cavity; and a pressure regulator. The pump must be capable of generating the minimum desirable pressures of 250 psig and, for a typical target loop volume of 10 ml, a flow rate of 2 ml/min. Cooling of the O-18 water may be accomplished with a coil of tubing connected on the output side of the target cavity. 
     The F-18 may be recovered from some types of F-18 extraction devices with an eluant and a gas source for forcing the F-18 eluate into a delivery vial. 
     O-18 water purification devices are preferably connected through a valve to the output of the F-18 extraction device and may reintroduce O-18 water into the target loop by means of a simple check valve. 
     Production efficiency can be further increased by having a source vial with new O-18 water to periodically, without stopping irradiation, recharge the target loop as O-18 water is used up due to leakage and the like. 
     Valves and tubes are provided to controllably connect various elements to perform various functions to carry out the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of apparatus for practicing the invention; 
     FIG. 2 is a graph of the reservoir vial and exchange cartridge radioactivity for two experimental runs; 
     FIG. 3 is a graph of target water conductivity for the same runs as in FIG. 2; and 
     FIG. 4 is a graph of target water pressure for the same runs as in FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic diagram of the apparatus whose component parts will now be described. All of these are used in the field of High Pressure Liquid Chromatography (HPLC) where they are fairly common. Connections between components were made with either {fraction (1/16)} in. (1.6 mm) OD type 316 stainless steel tubing or {fraction (1/16)} in. (1.6 mm) OD, 0.030 in. (0.8 mm) ID polyetheretherketone (PEEK) tubing, as was mechanically convenient. The choice of tubing is believed to be not critical. PEEK compression fittings are used for both types of tubing. 
     The target  11  is the standard “high yield” cyclotron target supplied by General Electric (U.S.) PET Systems AB (Uppsala, Sweden). This target has a silver body with an 0.8 ml target volume behind a 1 cm diameter circular aperture covered with a cobalt alloy Havar™ (Co 42.5%, Cr 20%, Ni 13%, Fe/W/Mo/Mn) foil sealed with a crushed silver o-ring. Using standard components (not illustrated), the target body is cooled by 20 C water and the aperture foil is cooled with 50 psig (340 kPa) room temperature helium gas. 
     Use of PEEK fittings means that the target is electrically insulated from the remainder of the apparatus. Thus, the beam current absorbed by the target material can be measured with an ammeter (not shown) connected between the target  11  and the cyclotron ground. 
     The cyclotron used is a standard one from the target supplier and is not illustrated. It is a model PETtrace™ 2000 negative ion type that accelerates singly negatively charged hydrogen ions. The cyclotron produces a close to Gaussian beam of 16.5 MeV protons with a total beam current of up to 75 μA. As is usual, tungsten collimators are used to center a more uniform beam distribution in the 1 cm diameter target aperture. A carbon foil in the cyclotron beam strips electrons from the negatively charged hydrogen ions to produce protons (positively charged hydrogen ions). 
     The input to the target is supplied with O-18 water by a pump  13  that is in turn connected to a reservoir vial  15  with a capacity of about 5 ml. The pump is a Cole Palmer (Vernon Hills, Ill.) model U-07143-86 single piston type. This pump has a sapphire piston, ruby valve seats, gold-plated stainless steel springs, and type 317 stainless steel housings and fittings. Other wetted parts are made from non-reactive materials such as PEEK. The flow rate is set to about 5 ml/min. 
     A reservoir vial radiation sensor  17  is used to monitor radiation in the vial  15 . This sensor is constructed with a 5 mm NaI scintillation crystal epoxied to a photodiode. (A PMT is not needed.) The assembly is within ½ in. (1.25 cm) of the vial  15 , but a photocurrent amplifier (not illustrated) is located 10 feet (1 m) away to reduce the effects of a neutron flux generated by the irradiated target. 
     The input to the vial 15 comes from a valve V 1  in parallel with an Upchurch (Oak Harbor, Wash.) model CV-3302 liquid check valve  19 . This line is also connected to a Cole Palmer digital conductivity meter  21  having a micro-flow cell consisting of a {fraction (1/16)} in. (1.6 mm) ID glass tube with embedded platinum electrodes. 
     Valve V 1  is a Rheodyne (Rohnert Park, Calif.) model 7000 pneumatically actuated 6-port with two positions, A and B, indicated by the solid and dashed lines, respectively. In position A, 3 pairs of adjacent ports are connected, while in position B, the three other adjacent pairs are connected. As illustrated, one of the ports is sealed off. The pneumatic actuator gas lines are not illustrated. 
     The output of the target  11  goes through a cooling coil  23  that consists of 10 feet (3 m) of loose 2 in. (5 cm) dia coils of {fraction (1/16)} in. (1.6 mm) OD stainless tubing. The cooling coil is essentially suspended in ambient air and provides cooling for water exiting the target  11 . The coil is connected to an Alltech (Deerfield, Ill.) 10 micron stainless steel filter  25  that filters out, e.g., silver particles, that may have been picked up in the target. The filter is connected to an Upchurch model U-469 back pressure regulator  27  adjustable in the range of 250-500 psig (1.7-3.4 MPa). The pressure in the volume after the pump  13  is monitored by an Omega Engineering (Stamford, Conn.) model PX176-500 0-500 psig (0-3.4 MPa) pressure transducer  29 . It is well know that higher pressures in the target volume increases the boiling point allowing higher intensity irradiation. However the present apparatus leaked at 500 psig (3.4 Mpa) and the maximum pressure could not be used. 
     When valve V 1  is in the A position, the pump  13  circulates water through the target loop L 1 . Circulation is at the rate of about 5 ml/min. With a calculated loop volume of about 5 ml added to the reservoir vial  15  volume of 5 ml to yield 10 ml, this means that 2 minutes is required for one round trip. 
     The initial source of O-18 water is source vial  31  that is connected to one of the ports of valve V 1 . This vial has a 50 ml capacity. The concentration of the O-18 isotope is not necessarily 100%. Any concentration can be used, but in normal production, at least 80% and preferably higher should be used to reduce irradiation time and the cost of the cyclotron. 
     A Waters (Franklin, Mass.) model SepPak™ QMA cartridge C 1  containing silica derivatized by quaternary ammonia is connected between the valve V 1  and a second valve V 2 . This cartridge can adsorb F-18 ions from water. The F-18 can then be extracted using eluants such as 20-40 mM sodium or potassium carbonate in water or a water/acetonitrile mixture. The amount of F-18 in cartridge C 1  is monitored by the photodiode sensor  33  adjacent to the cartridge. 
     Valve V 2  is also a Rheodyne series 7000 pneumatically actuated 6-port with positions A and B as indicated by the solid and dashed line, respectively. Only half of this valve is used. One side of valve V 2  is connected to an F-18 delivery line  35  constructed from {fraction (1/16)} in. (1.6 mm) OD PEEK tubing stretching about 25 feet (8 m) from the cyclotron target area to an F-18 delivery vial  37 . 
     The other side of valve V 2  is connected to an in-line pair of deionizing cartridges C 2  and C 3  that are connected to the check valve  19 . These are used to remove impurities from the O-18 water, especially in later stages of a production run. Cartridge C 2  is an Alltech (Deerfield, Ill.) MaxiClean™ model SCX (Strong Cation Exchange) cartridge containing 600 mg of polystyrene resin derivatized with sulfonic acid. Cartridge C 3  is a similar model SAX (Strong Anion Exchange) cartridge derivatized with a tetra-alkylammonium compound. Check valve  19  prevents back flow into these cartridges. 
     A third valve V 3  is connected to valve V 1 . This is a model HVP-E 86779 4-port supplied by Alltech. One of these ports is connected to a Hamilton Gastight™ model 1002 2.5 ml syringe pump  39  (supplied by Alltech) with a pneumatically actuated plunger. The pump body is glass while the plunger is made from polytetrafluorethelyne with the trade name Teflon. As shown, the plunger has two extreme positions, all the way in, designated A, and all the way out, designated B. 
     Another port of valve V 3  is connected to a gas check valve  41  that is connected to a remote helium tank  43  via helium line  45 . The tank is filled with Matheson UHP grade 5.5 (i.e. 99.9995% pure) helium. The other port of valve V 3  is connected to an eluant vial  47  containing a suitable eluant solution such as a sodium carbonate solution in water. 
     All components shown inside the dotted lines are mounted on and between two 8 in. (20 cm) wide by 14 in. (36 cm) high by ¼ in. (6 mm) thick aluminum plates separated by 6 in. (15 cm). This is about the same volume used by the standard liquid target filler apparatus supplied by the cyclotron manufacturer. This assembly is placed within 2-3′ (60-90 cm) of the target  11 . In addition to F-18 delivery line  35  and helium line  45 , all other pneumatic actuator and electrical lines are brought outside the cyclotron radiation shield. While it would reduce the number of long lines to bring all components except the target loop L 1  outside the shield, this would require a long line to the O-18 source vial  31  that would increase the possibility of contaminating the O-18 water. 
     The apparatus is operated under control of an IBM PC compatible computer and control system (not illustrated) based on an Omega Engineering (Stamford, Conn.) model CIO DAS 08 I/O board having analog and digital input and digital output ports. The output ports drive local solenoids that, in turn, drive pneumatic actuators located with the apparatus. In order to monitor operation, the computer also stores in memory readings from the pressure, radiation, and conductivity meters. 
     OPERATION 
     As noted above, production of F-18 for medical uses takes place in a work shift just preceding the beginning of a hospital day. Operation of the apparatus illustrated in FIG. 1 can be carried out with a series of runs that would typically last an hour or more. Before a run starts, it is necessary to make sure that the target loop L 1  is filled with O-18 water. Then, a second production sequence of steps would produce F-18, extract the F-18 produced, and deliver it to the external vial  37  for further processing. 
     When the system is first assembled, the first requirement is to fill the target  11  and reservoir vial  15  with O-18 water. This is accomplished by connecting the vial of O-18 water  31  to valve V 1 . The three valves in the system and the syringe pump  39  are sequenced according to the following Table 1. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Fill Target Loop Sequence 
               
             
          
           
               
                 Step 
                 V1 
                 V2 
                 V3 
                 Syringe 
                 Typical Time (s) 
               
               
                   
               
             
          
           
               
                 1. 
                 Start 
                 A 
                 A 
                 A 
                 A (in) 
                 — 
               
               
                 2. 
                 Fill Syringe 
                 B 
                 A 
                 B 
                 B (out) 
                 5 
               
               
                 3. 
                 Switch Valves 
                 A 
                 B 
                 B 
                 B (out) 
                 5 
               
               
                 4. 
                 Add Water 
                 A 
                 B 
                 B 
                 A (in) 
                 10  
               
               
                 5. 
                 Purge Cartridges 
                 A 
                 B 
                 A 
                 A (in) 
                 5 
               
               
                 6. 
                 Reset Valves 
                 A 
                 A 
                 A 
                 A (in) 
                 2 
               
               
                   
               
             
          
         
       
     
     In the fill syringe step, O-18 vial  31  is connected through valves V 1  and V 2  to syringe  39 . Then, when the syringe plunger is pulled out, O-18 water is pulled from the vial into the syringe. 
     In the switch valves step, the syringe is connected through valve V 1  to cartridge C 1  and through valve V 2  to cartridges C 2  and C 3 . In the add water step, the plunger of syringe  39  is pushed in and O-18 water is forced through the cartridges C 1 , C 2 , and C 3  and check valve  19  into the reservoir vial  15 . The volume and stroke of syringe  39  was adjusted to produce an injection of about 0.75 ml. The volume of the cartridges and connecting lines is about 1-2 ml. 
     This particular arrangement means that the initial charge of reservoir vial  15  as well as any subsequent recharges with O-18 water will be purified by the ion exchange cartridges C 2  and C 3 . 
     In the purge cartridges step, Valve V 3  connects the 50 psig (340 kPa) helium supply  43  via valve V 1  to cartridge C 1  and via valve V 2  to cartridges C 2 , and C 3 . This purges the cartridges and forces any remaining water into reservoir vial  15 . In the reset valves step, valve V 2  is returned to the A position disconnecting cartridge C 1  from cartridges C 2  and C 3 , in preparation for either a repeat of the fill target sequence or the production sequence. 
     When a system is first assembled, the fill target sequence is repeated about 15 times to fill the loop L 1 , containing the target  11  and reservoir vial  15 , with total of 10 ml of water. In the beginning of a work shift, the fill target sequence is repeated as necessary until reservoir vial  15  contains about 5 ml of water. After completion of the fill target sequences at the beginning of a work shift, the pump  13  and the cyclotron are turned on and left on for the remainder of the shift. Next is a production sequence of steps as listed in Table 2. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Production Sequence: 
               
             
          
           
               
                 Step 
                 V1 
                 V2 
                 V3 
                 Syringe 
                 Typical Time (s) 
               
               
                   
               
             
          
           
               
                 1. 
                 Irradiation 
                 A 
                 A 
                 B 
                 A (in) 
                 300 and up 
               
               
                 2. 
                 Extraction 
                 B 
                 B 
                 A 
                 A (in) 
                 360 
               
               
                 3. 
                 Purge 
                 A 
                 B 
                 A 
                 A (in) 
                  20 
               
               
                 4. 
                 Fill Syringe 
                 A 
                 B 
                 A 
                 B (out) 
                  10 
               
               
                 5. 
                 Prepare to Deliver 
                 A 
                 A 
                 B 
                 B (out) 
                  2 
               
               
                 6. 
                 Elute F-18 
                 A 
                 A 
                 B 
                 A (in) 
                  15 
               
               
                 7. 
                 Deliver F-18 
                 A 
                 A 
                 A 
                 A (in) 
                 240 
               
               
                 8. 
                 Reset valves 
                 A 
                 A 
                 B 
                 A (in) 
                  1 
               
               
                   
               
             
          
         
       
     
     During the Irradiation step, the cyclotron is turned on and the target  11  is irradiated. With valve V 1  in the A position, pump  13  is running and circulates water through the target loop L 1 . Check valve  19  blocks circulation back into the cartridges C 2  and C 3 . Back-pressure regulator  27  maintains the pressure at some level between 250-500 psig (1.7-3.4 MPa). Pressure monitor  29 , that is upstream of the 10-micron filter  14 , signals the control system if an over or under-pressure occurs. The conductivity monitor  21  signals the control system if the conductivity is too high, indicating excessive contamination. During irradiation, the amount of F-18 created is monitored by the reservoir vial radiation sensor  17  and associated circuitry. 
     With valve V 3  in the B position, the helium supply pressurizes the eluant vial  47 , but has no other effect. With valve V 2  and the syringe  39  in the A position, there is no flow through the cartridge C 1 . 
     After a desired amount of F-18 has accumulated in the target, it is extracted. Valves V 1  and V 2  are switched to the B position breaking the loop L 1  at valve V 1  and forming a loop through the cartridges C 1 , C 2 , and C 3 . QMA cartridge C 1  retains F-18 while deionizing cartridges C 2  and C 3  remove impurities from the water. After 360 sec about 85%90% of the F-18 has been absorbed on the cartridge. 
     The F-18 level in the QMA cartridge C 1  is monitored by the photodiode  33  and the conductivity of the water is monitored by the photodiode  17 . 
     In the purge step, as much O-18 water as possible is removed from the QMA cartridge C 1 . Valve V 1  is switched to the A position connecting the cartridge through valve V 3  to the helium source  43  and reestablishing the target loop L 1 . The helium gas pressure pushes water from the QMA cartridges through the deionizing cartridges C 2  and C 3  and past the check valve  19  into vial  15 . 
     The next four steps deliver F-18 to the delivery vial  37 . With valve V 3  in the A position, the syringe  39  is connected to the eluant vial  47 . Pulling the plunger out fills the syringe with about 0.75 ml of eluant. This takes about 10 seconds. Then, valve V 2  is switched to the A position and valve V 3  to the B position. This connects the syringe  39  to the QMA cartridge C 1  and from there to the delivery vial  37 . In the elute step, the plunger of the syringe  39  is pushed in over about a 15 second period. This forces eluant solution into the QMA cartridge C 1 . 
     Next, in the delivery step, valve C 3  is switched to the A position so that the helium source  43  is connected to the QMA cartridge C 1 . The helium gas pressure forces the F-18 containing eluate into the delivery tube  35  and to the delivery vial  37 . This takes about 240 seconds. 
     The recovery steps, starting with filling the syringe  39  and ending with delivery, are then repeated to accomplish complete removal of F-18 from the cartridge C 1 . About 85% of F-18 produced in the target  11  is removed from the QMA cartridge C 1  after two extractions. This estimate is based on known target production efficiency as compared to the amount of F-18 delivered into the receiving vial  37 . 
     A fraction of the remaining 15% of F-18 will be recovered in a subsequent production sequence depending on the length of the next run compared to the 109 minute F-18 half-life. 
     At the conclusion, valve V 3  is switched back to position B to begin another production sequence or left in position A if the target loop L 1  needs replenishing with water using the Fill Target sequence. 
     Four Working Examples 
     Four consecutive trial runs were made without shutting down the system using the same set of cartridges. Two sets of beam current amounts and irradiation times were used. The concentration of O-18 in the starting water was only 80% (because of the expense of higher concentrations). The eluant was 40 mM sodium carbonate solution in water. A Capintec (Ramsey, N.J.) 7BT dose calibrator was used to measure the amount of recovered F-18 after each run. The results appear in Table 3. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Four Trial Runs: 
               
             
          
           
               
                   
                   
                   
                 Irradiation 
                 Recovered 
               
               
                   
                 Run #: 
                 Beam Current (uA) 
                 Time (min) 
                 F-18 (mCi) 
               
               
                   
                   
               
               
                   
                 1 
                 20 
                  5 
                  98 
               
               
                   
                 2 
                 20 
                  5 
                  91 
               
               
                   
                 3 
                 40 
                 126 
                 2240 
               
               
                   
                 4 
                 40 
                 104 
                 2730 
               
               
                   
                   
               
             
          
         
       
     
     Runs 1 and 2 are too short to produce useful amounts of F-18, but were truncated to check system operation. In principal, the F-18 from many short runs can be combined, but this produces a very dilute solution of F-18. Therefore, a continuous run that delivers 2-4 Ci is preferred. 
     The higher amount of F 18  delivered in run 4, despite a shorter irradiation time, is due to activity remaining in the target loop L 1 , including the target  11  and the reservoir vial  15 , after run 3. There also were two extraction steps performed in run 4 as compared to one extraction step in run 3 which leads to a more complete extraction of the isotope. Further, it is not unusual with prior art static systems for recoveries to vary by 5-10% between otherwise identical runs. 
     For runs 3 and 4, FIG. 2 shows the radioactivity in the reservoir vial  15  and QMA cartridge C 1  as determined by sensors  17  and  33 , respectively, as a function of time, T, in hours and minutes. The output of these two sensors were scaled to approximate the recovered F-18. The only steps that are long enough to see on this scale of hours and minutes are irradiation, extraction and delivery. 
     At the beginning of run 3, radioactivity in the reservoir vial  15 , indicated by the solid trace, builds up approximately exponentially because the irradiation time is comparable to F-18 &#39;s 1 hour and 49 minute half-life. At approximately 2:28, extraction starts and the amount of F-18 in the reservoir vial  15  drops rapidly with a corresponding increase in the QMA cartridge C 1  indicated by the dotted line. The irradiation continues during the extraction step which is why F-18 amount is still rising when the elution step starts at approximately 2:38. This leaves some F-18, some of which is produced during extraction of run 3, in the reservoir vial  15  at the start of irradiation run 4. 
     Although not visible in the graphs because the Fill Target Loop Sequence takes less than 30 seconds, at the end of run 3, the target loop L 1  was recharged with approximately 1.5 ml of O-18 water. This particular target was used for these experiments because it leaked too much to be used in a normal static target production run. The 1.5 ml added was an estimate based on a prior leak test without irradiation. The basic requirement is that the target  11  not run dry. This is fulfilled, if the out take tube of the reservoir vial  15  is always submerged. This is not difficult because, through experience, an estimate can be made of target loop water losses and the Fill Target Loop Sequence can be performed at any time as needed. 
     At approximately 4:19, at the end of radiation run 4, F-18 is extracted, but with greater apparent efficiency than after run 3. This is followed by a short delivery step and then a second extraction step ending just before the graph. What would have been Run 5 was terminated because cyclotron time allocated to the experiments ran out. It is believed that runs could have continued until the O-18 water in the source vial  31  ran out. 
     FIG. 3 shows the target loop L 1  water conductivity over the same time period as in FIG.  2 . This increases with time due to the buildup of various ionic species produced mainly by target corrosion and decreases due to the SAX and SCX cartridges C 2  and C 3  during the extraction step. (Note that, F-18 does not contribute to conductivity changes because it is not present in chemically significant quantities.) The fact that the conductivity returns back to low levels after isotope extraction demonstrates the possibility of indefinite reuse of target material contained in the loop L 1  and reservoir  15 . 
     FIG. 4 shows the pressure in the target  11  over the same time period as in FIG.  2 . It is held relatively constant by the pressure regulator with an increase when the target loop is diverted through the cartridges during extraction steps. 
     Alternative Approaches 
     The above example of operation and system description are provided to illustrate one of many ways to accomplish the recirculation and extraction. A variety of similar components may be used with equal success. For example, any high-pressure piston pump designed for HPLC or similar application and equipped with inert piston and check valves can be used to pump liquid. Similarly, a variety of valve designs are available that could be used to substitute Hamilton and Rheodyne valves provided that they utilize inert materials and are capable of sustaining the required pressure and are compatible with water. 
     Plumbing of the system can be substituted with all stainless steel or plastic material. Appropriate materials can be used to replace PEEK or type 316 stainless steel. Additional cooling of water removed from the target by means of a heat exchanger may be beneficial. Additional pressure, radioactivity and temperature sensors could provide better feedback and monitoring. 
     It may be beneficial to use increased water flow rates to provide better mixing inside the target and to achieve better heat dissipation. With higher water flow rates and additional cooling it may be possible to significantly increase beam current deposited into the target, thus increasing the isotope production rate. Thus, a recirculating target design has the potential to significantly increase production of the isotope. 
     The single syringe was a convenient device for transferring O-18 water and eluant. However, with a different valve arrangement, two syringes could be used or different fluid transfer devices substituted. For example, gas pressure could be used to force fluids out of containers. 
     A wide variety of commercially available cartridges designed for solid phase extraction and ion exchange can be used to substitute for QMA, SAX or SCX cartridges. Additional cartridges and filters can be installed as necessary to remove other potentially harmful impurities, such as a type C-18 cartridge to remove organic materials. Additionally, a sterilizing filter can be incorporated in a purification loop to remove microbial contamination, if necessary. 
     Various solutions can be used to remove extracted F-18 isotope from the QMA cartridge to accommodate requirements of the chemical processing that follow isotope production, as long as these solutions have sufficient ion strength to equilibrate the QMA cartridge and displace fluoride ion. For example, a solution of a tetraalkylammonium base or salt such as tetrabutyl ammonium carbonate or potassium carbonate in an equimolar mixture with a polycyclic aminopolyether such as 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8,8,8]hexacosane can be used to provide increased reactivity of F-18 fluoride in following nucleophilic substitution reactions. Such a solution can be used directly in the synthesis of some useful radiopharmaceutical agents such as [F 18 ] 2-Deoxy-2-Fluoro-D-glucose, thus eliminating one step from the synthesis procedure and increasing yield and reducing synthesis time. 
     Lastly, the invention is not limited to using the particular target and cyclotron employed for the trial runs. Equivalents from other manufacturers should require only minor changes in apparatus. 
     It should therefore be clear that the detailed description of one working embodiment does not prevent inclusion of other equivalent embodiments within the purview of the invention that is defined by the following claims. 
     Applicant do not wish to avail themselves of 35 U.S.C. §112, ¶ 6 unless the phrase “means for’ explicitly appears in a claim, as in claims 20 and 21 as originally filed.