DESIGN CONSIDERATIONS AND INITIAL PERFORMANCE OF A 1.2cm/sup 2/ BETA IMAGING INTRA-OPERATIVE PROBE - Nuclear Science Symposium and Medical Imaging Conference Record, 1995., 1995 IEEE

A novel small area beta (p') detector is under development for nuclear emission imaging of surgically exposed radiolabeled tumor beds. The imaging device consists of an 0.5 mm thick x 1.25 cm diameter CaF2(Eu) scintillator disk coupled to a rigid bundle of 19, 2 mm diameter x 5 cm long double clad optical fibers through a 1.7 mm polystyrene light diffuser. The detector size (1.2 cm2) was determined by the requirement to introduce the probe into small cavities, e.g. during neurosurgical lesion resection, but large enough to produce images of clinical significance. Double clad optical fibers were utilized for both the frontand back-end components, and -75 photoelectrons were obtained through a 1.9 m long flexible optical fiber bundle with CaF2(Eu), indicating that sufficient numbers of photoelectrons were detected at the PMT for positioning and energy information. The long flexible fibers guide the scintillation light to a Philips XP1700 series fiber optic faceplate, multi-channel PMT. The parallel MC-PMT outputs are fed into a variable gain, charge divider network and an i-V pre-amplifiedline driver network, whose resulting four outputs are digitized and histogrammed with standard Anger positioning logic. The various components in the imaging chain were optimized by both simulations and measurements. Line spread functions measured in the 10.8 mm FOV were 0.50 mm f 0.038 mm and 0.55 mm f 0.065 mm FWHM, in X and Y, respectively. For a 20% variation in pulse height, no variation in spatial resolution was observed. The differential uniformity was measured to be f15.6% with -150 ctdpixel. I. I[NTRODUCTION A novel small area (1.2 cm2) beta (p') detector, which overcomes many of the problems associated with gamma-ray detection and imaging [1,2], is under development (Fig. 1) for nuclear emission imaging of intra-operatively exposed, radiolabeled tumor beds [3-71. The beta imaging probe is intended to delineate the boundaries of radiolabeled tumors, after the bulk of the lesion has been removed. Direct intraoperative detection of short ranged p' particles is not a new technique [ 8,9]. However, intra-operative imaging greatly facilitates the mapping of radioactive distributions on tissue surfaces and would aid a surgeon in residual lesion identification for removal, while sparing unaffected tissues. The technique exploits: (1) the inherent short range of p' particles in tissue; (2) direct source contact obviating a collimator; and (3) detector to source proximity, which has the dual advantage of increased sensitivity 121 and improved resolution [ 10,111 in nuclear emission imaging. The success of the beta imaging device depends on detecting a sufficient number of scintillation photons through -2 m of flexible optical fibers at a remotely located photodetector. The optical fibers facilitate miniaturization of the imaging system, and remove all voltage and current sources from the surgical field. 11. DETECTOR COMPONENTS A. Light Sharing Scintillation Detector A continuous disk of CaF2(Eu) scintillator was optimized for <1 MeV /3' particles [6], since use of the device is anticipated with 18F (p' Ep = 635 keV) labeled pharmaceuticals. (A) FRONT-END BACK-END ' C@&Eu) Diffuser Optical Fibers Flexible 0 tical Fibers Philips XP1700 FO 0.5" 0-3" 19 x 2mm @ x 5cm " 57 x 0.929" @ x 2m Multi-Channel PMT '