POSITRON EMISSION TOMOGRAPHY WITHIN A MAGNETIC-FIELD USING PHOTOMULTIPLIER TUBES AND LIGHTGUIDES

The spatial resolution of positron emission tomography (PET) improves when positron annihilation takes place in a strong magnetic field. In a magnetic field, the Lorentz force restricts positron range perpendic ular to the field. Since positron annihilation occurs closer to its po int of origin, the positron annihilation point spread function decreas es. This was verified experimentally by measuring the spread function of positron annihilation from a 500 mm Ge-68 bead imbedded in tissue-e quivalent wax. At 5 T the spread function full width at half maximum ( FWHM) and the full width at tenth maximum (FWTM) decrease by a factor of 1.42 and 2.09, respectively. Two Nar(TI) scintillation crystals tha t interface to a pair of photomultiplier tubes (PMTS) through long lig htguides detect positron annihilation at zero field and 5.0 T. Photomu ltiplier tubes, inoperable in strong magnetic fields, are functional i f lightguides bring the photons produced by scintillators within the f ield to a minimal magnetic field. These tests also demonstrate techniq ues necessary for combining magnetic resonance imaging (MRI) and PET i nto one scanner.