A new hybrid medical imaging technology incorporates a positron emission tomograph (PET) within the uniform magnetic field of a 3-Tesla magnetic resonance imaging machine (MR). When a positron emitting radioactive source is suspended in such a PET/MR, the Lorentz force of the magnetic field causes the emitted positrons to form a well-defined beam across the PET's field of view. This novel separation of the positron production from the positron annihilation has significant advantages for PET source construction. The annihilation of these positron beams in air can form easily imaged 'phantom' objects that are free from the attenuation and scattering effects of conventional sources. Thin targets intersecting these beams can produce intense annihilation sources having the thickness of a sheet of paper, which are otherwise difficult to achieve. In this paper we discuss the physical characteristics of such positron beams in air and present examples of their applications.

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