Room: Karl Dean Ballroom B1
Purpose: Instantaneous, beam's-eye-view, high-resolution proton radiography would enable proton range verification and real-time anatomical registration. The implementation of a full-field magnetic-lens-refocusing system partially cancels chromatic aberrations in the proton distribution, and selectively collimates for contrast enhancement, resulting in image resolution on the order of hundreds of microns.
Methods: An 800-MeV proton beam was prepared with a position-angle correlation where it interacts with an object. After transmission through the object, the proton beam traversed two magnetic quadrupoles, sorting the beam by angle at the Fourier plane, where protons scattered beyond 10 mrad by multiple Coulomb scattering in the object are rejected from the system. The beam then traversed two more magnetic quadrupoles, where it was refocused at an imaging plane 9.4 m downstream of the object. The re-imaged proton distribution was read out by a flat panel imager normally used for X-ray detection.
Results: Proton radiographs were acquired with 150-ns pulses of ~10â?¹ protons. Images of a hand phantom demonstrate a 200-Âµm spatial resolution, and a signal-to-noise ratio (SNR) of 6, with an estimated deposited dose of 1 mGy. In an aluminum step-wedge, measured resolution ranges from 180 Âµm for 3 mm Al, to 400 Âµm for 27 mm Al, due to the chromatic effects at an energy loss of 14 MeV. Geant4 simulations of an equivalent 330-MeV system predict a resolution of 500 Âµm through 5 cm of tissue, and 800 Âµm through 10 cm of tissue.
Conclusion: Magnetically refocused full field proton radiography can aid in orienting patients for proton beam therapy and aid in range verification, at proton energies greater than the typical treatment energies utilizing the Bragg peak, of ~200 MeV. Simulations at 330 MeV demonstrate the feasibility of implementing this technique at existing clinical accelerators.