82 SUBMILLIMETER NUCLEAR MEDICAL IMAGING WITH A COMPTON CAMERA USING TRIPLE COINCIDENCES OF COLLINEAR BETA+− ANNIHILATION PHOTONS AND GAMMA RAYS

Abstract

PET cameras had their breakthrough as an imaging instrument for clinical application studies due to the large variety of radioactively labeled tracer molecules [1]. These tracers carry an e emitting isotope to e.g. a tumor. Annihilation of the positron into two back-to-back 511 keV photons allows to restrict the source origin in 2 dimensions onto a line of response (LOR). Superimposing the LORs of different decay events locates the source distribution of the emitter in 3D. Modern PET systems reach a spatial resolution of 3-10 mm. A disadvantage of this technique is the diffusion of the positron before its decay with a typical range of ca. 1 mm (depending on its energy). This motion and Compton scattering of the 511 keV photons within the patient limit the performance of PET. We present a nuclear medical imaging technique, able to reach submillimeter spatial resolution in 3 dimensions with a reduced activity application compared to conventional PET. This ’gamma-PET’ technique draws on specific e sources simultaneously emitting an additional photon with the β decay. Exploiting the triple coincidence between the positron annihilation and the third photon, it is possible to separate the reconstructed ’true’ events from background [2,3]. Therefore the spatial uncertainty introduced by the motion of the e or by Compton scattering within the patient can be strongly reduced in the direction normal to the annihilation. Especially Sc is of interest, which β decays into Ca, emitting an 1157 keV photon. With its halflife of 3.9 h, Sc has to be produced from a Ti generator (t1/2 = 60.4 a) [4]. Presently this cannot be performed in clinically relevant quantities, however, this may change with the soon expected availability of highly brilliant gamma beams [2]. Sc has already been applied in patient studies [5]. We combined the Compton camera technique, i.e. the measurement of photon energies and positions of Compton scattering interactions, with a PET camera defining the LOR. Due to the Compton kinematics and subsequent photon absorption, a Compton camera can reconstruct the origin of a primary photon on the surface of a ’Compton cone’ [6]. Superimposing such cones from different events reduces