Patent Number: 050376020
Section: summary

The present invention relates to a facility and method for producing radioisotopes having application to Positron Emission Tomography ("PET"). More particularly, the present invention relates to a system utilizing a relatively small, light-weight Radio Frequency Quadrupole ("RFQ") accelerator for accelerating a beam of .sup.3 He.sup.++ ions to an energy level sufficient to produce desired radionuclides when a selected target material is bombarded with the accelerated beam. BACKGROUND OF THE INVENTION PET is a nuclear medicine procedure for imaging and measuring physiologic processes within the body. It depends upon the distribution into the body of a systematically administered radiopharmaceutical labeled with a radioactive isotope ("radioisotope") that decays through the emission of positrons. This is very distinct from other nuclear imaging techniques such as Computed Tomography ("CT") which measures the distribution of electron density, or Magnetic Resonance Imaging ("MRI") which measures the distribution of protons in the body. There are literally hundreds of possible radiopharmaceuticals that find application to neurology, oncology, and cardiology. PET is typically directed to the study of metabolism processes, blood flow, blood pooling, and receptor sites in the brain. In accordance with PET practice, a radiopharmaceutical (sometimes termed the "labeled compound") is injected into or inhaled by a patient after he or she has been positioned properly relative to an adjacent scanner device. It is the function of the scanner device to detect the gamma-rays that are produced when positrons emitted from the radioisotope annihilate with surrounding electrons. For example, a brain metabolism study might involve the injection of a fluorodeoxy-glucose radiopharmaceutical containing .sup.18 F into the blood stream so that it is taken up in the brain at sites of metabolic activity. When an .sup.18 F nucleus decays it emits a positron which, within a distance of a few millimeters, annihilates with an electron producing two oppositely directed 0.511 MeV gamma-rays. Crystal gamma-ray detectors in the scanner device surrounding the patient's head detect the arrival of the gamma-rays and identify the paths on which they traveled, defining the lines along which the annihilation events occurred. Time-of-flight techniques may also be used to locate the position of the events along the lines. Appropriate electronic circuits and a computer system(s) acquire data during the scan and map the distribution of the annihilation events, which coincide with the presence of the radioisotope. Quantitative evaluation of the function under study, as well as an image for display, are produced as a final product of the PET scan. Radioisotopes are presently generated by accelerating protons to an energy of 12 MeV (or deuterons to an energy of 6 MeV) with a cyclotron. This proton/deuteron beam is extracted from the cyclotron and steered to a target material. Automatic chemical processors convert the target material into basic chemical building blocks, called "precursors", needed to make the radiopharmaceuticals of interest. Some state-of-the-art systems produce the final radiopharmaceutical with the aid of a programmed robot to avoid radiation exposure to a radiochemist. The PET scanner, which resembles a CT scanner in physical appearance, along with the cyclotron, targets, and chemical processors form the basic PET system. Unfortunately, the half-life associated with many radioisotopes of interest to PET applications is very short (on the order of minutes), hence it is not possible to manufacture the radiopharmaceuticals at a manufacturing site and transport them to a patient location. Rather, the patient must travel to the site of the PET system where the needed radioisotopes can be produced and used immediately. Because of the sheer size, mass and expense of building and operating just the cyclotron (which is only one element of a PET system), there are relatively few PET facilities available throughout the world. (At present, it is estimated that there are only about 20 PET facilities in the United States, and about 60-70 worldwide.) Only the largest hospitals are able to afford, support and staff such systems. Thus, the benefits of PET remain available to relatively few. What is needed therefore is a PET system that is more affordable and accessible to a larger number of patients and doctors. There are numerous disadvantages of existing low energy cyclotron-based PET systems. For example, some of the radionuclides are produced using a proton beam, while others are produced using a deuteron beam, therefore some beam switching apparatus is required. While such beam switching apparatus is well known in the art, it adds to the complexity and expense of the system. Further, large amounts of power are required for such systems to operate (e.g., the proton/deuteron cyclotron typically requires 100 kW of power to operate). Also, such systems require enriched target materials if the desired radionuclides are to be efficiently produced by the proton/deuteron beam. Such enriched target materials are not readily available, and are costly to produce. Still further, due to the inherent elliptical cross sectional shape of the proton/deuteron beam, the efficient utilization of the beam in a circular target chamber is made more difficult. Moreover, due to the secondary neutrons that are naturally produced from the proton/deuteron irradiation process, thick shields must be built around the target area to confine such neutron radiation. It is not uncommon, for example, for the target chamber of such systems to be surrounded by concrete walls that are a minimum of four feet thick. This shielding, coupled with the mass and weight associated with the other elements of the system, particularly the cyclotron, results in a system that weighs on the order of 300 tons. Such heavy systems can only be installed on a ground or basement floor, thereby severely restricting those facilities where a cyclotron-based PET system could be installed. All of the above factors combine to make the proton/deuteron cyclotron-based PET systems very expensive to build, operate and maintain. As has been indicated, such expense disadvantageously limits the number of PET systems that are built and operated, thereby making the cyclotron-based PET systems generally inaccessible and/or unavailable to many patients, hospitals and doctors. What is needed, therefore, is a radioisotope production system which can produce sufficient quantities of all of the radioisotopes of interest (.sup.18 F, .sup.11 C, .sup.15 O, .sup.13 N) and minimize some or all of the disadvantages discussed above for existing systems. The present invention advantageously addresses this need. SUMMARY OF THE INVENTION The present invention is directed to a relatively inexpensive PET system that is easy to operate and maintain, and that produces all four of the radionuclides of interest to PET applications. Significantly, the system described herein does not require a cyclotron to generate a proton/deuteron beam. Rather, the PET system of the present invention makes use of a readily available ion source to produce a .sup.3 He.sup.++ beam that is accelerated to around 8 MeV using a Radio Frequency Quadrupole ("RFQ") accelerator. This accelerated .sup.3 He.sup.++ beam is then directed to a conventional, non-enriched target material(s) whereat the four primary radionuclides of interest to PET systems, .sup.18 F, .sup.13 N, .sup.15 O, and .sup.11 C, are efficiently produced. Advantageously, the RFQ accelerator is a small, light-weight device and requires significantly less operating power than does the cyclotron. The RFQ advantageously accelerates ions to a prescribed velocity. The RFQ is thus ideal for accelerating multiply charged ions with masses greater than a single proton mass. This characteristic of the RFQ, in combination with the benefits of using .sup.3 He.sup.++ , rather than protons or deuterons as described below, renders use of a .sup.3 He RFQ as an advantageous and novel technique for producing radioisotopes for PET. Further, the neutron-poor nature of the reaction resulting from a .sup.3 He.sup.++ bombardment of the target material significantly reduces the amount of shielding that is required around the target chamber. Moreover, the generally circular cross section of the .sup.3 He.sup.++ beam allows it to interact with the conventional circular cross-section target material in a more efficient manner than is possible with the elliptical cross-sectional shaped proton/deuteron beam of the cyclotron-based system of the prior art. The reduced shielding requirements, coupled with the small RFQ accelerator and the relatively low power requirements thereof, as well as the efficient use of the target material, makes possible a PET system that not only efficiently generates the needed radionuclides for PET applications, but that also is small, light-weight, affordable, and possibly transportable. Hence, the system can either be readily installed in or possibly transported to the hospitals and other medical facilities where it is needed, thereby making the benefits of PET available to a much larger segment of the world's population. The present invention may thus be summarized as a system for producing radionuclides for use with PET is provided, the system including: a source of ions for producing a .sup.3 He.sup.++ beam at a low energy; a radio frequency quadrupole (RFQ) accelerator for accelerating the low energy .sup.3 He.sup.++ beam to a high energy, and a target system. The target system includes at least one target compound selected to produce at least one desired radionuclide when it is irradiated by the accelerated .sup.3 He.sup.++. beam. This desired radionuclide(s) is then combined, in conventional manner, to produce appropriate precursors which can produce any one of the hundreds of possible radiopharmaceuticals that are used in PET or related applications. Further, the present invention may be characterized as a radioisotope production facility for producing radioisotopes for use with PET. Such a facility includes: RFQ accelerator means for producing a high energy beam of .sup.3 He.sup.++ ions; and means for irradiating a selected target material with the high energy .sup.3 He.sup.++ beam; the target material being selected to produce at least one desired radioisotope when irradiated by the high energy .sup.3 He.sup.++ beam. Still further, the present invention encompasses a method for producing a radiopharmaceutical suitable for use with a PET system. This method comprises the steps of: (a) accelerating a beam of .sup.3 He.sup.++ ions using a RFQ accelerator to a high energy level, e.g., at least 8 MeV; (b) irradiating a target compound with the accelerated .sup.3 He.sup.++ beam to produce at least one desired radionuclide; (c) processing the radionuclide obtained in step (b) to produce a desired precursor containing the radionuclide; and (d) preparing a suitable radiopharmaceutical from the precursor. It is a feature of the present invention to provide a PET system that is small and light weight, thereby allowing the system to be transportable. Another feature of the present invention is to provide such a system that operates on roughly 1/5 of the operating power required by the cyclotron-based PET systems of the prior art. A further feature of the invention is to provide a PET system that occupies only about 1/3 of the floor space that is occupied by the cyclotron-based PET systems of the prior art, and that weighs only about 1/10 of what such prior art cyclotron-based systems typically weigh. Yet another feature of the invention is that the single beam used therein, can be readily and inexpensively generated from a commercial source of ions. A further feature of the invention provides a system as above-described that is very simple to operate, typically requiring the operation of only a few push-buttons, thereby requiring minimal training for its operation. This feature is important because a major part of the cost of the current cyclotron-based PET systems is the cost of the staff. When technicians instead of accelerator experts and radiochemists are used to operate the system, a substantial saving in operating costs results. Another feature of the invention contributing to its simplicity is the lack of a beam extraction system. That is, no extraction system is required to extract the .sup.3 He.sup.++ beam from the RFQ accelerator as is required to extract a proton/deuteron beam from a cyclotron. Still another feature of the invention allows the presently available and medically-proven and accepted target systems, including the programmable robotic features thereof, e.g., those used in existing cyclotron-based PET systems, to be used therewith. Significantly, however, due to the neutron-poor nature of the .sup.3 He.sup.++ beam and resulting reactions, no shielding around the accelerator and little shielding around the target chambers is required relative to existing cyclotron-based PET systems.