Development of a head scanner for proton CT

Abstract

We describe a new head scanner developed for Proton Computed Tomography (pCT) in support of proton therapy treatment planning, aiming at reconstructing an accurate map of the stopping power (S.P.) in a phantom and, in the future, in patients. The system consists of two silicon telescopes which track the proton before and after the phantom/patient, and an energy detector which measures the residual energy or range of the proton to reconstruct the Water Equivalent Path Length (WEPL) in the phantom. Based on the experience of the existing prototype and extensive Geant4 simulations and CT reconstructions, the new pCT scanner will support clinically useful proton fluxes.

Reconstructing highly accurate relative stopping powers in proton computed tomography

“…The method allows direct reconstruction of relative stopping power of patient tissues in a 3D-fashion. The pCT collaboration has built first experimental prototypes of pCT scanning systems [1] and has developed approaches to reconstruct proton CT images based on registering the coordinates and water equivalent path length (WEPL) of individual protons traversing the scanned volume. From these data one reconstructs the object boundary (hull) and initial image based on filtered back projection (FBP), calculates a most likely path (MLP) for each proton, and improves the initial image iteratively by solving a large linear system of equations of the form Ax = b using an iterative projection algorithm [2].…”

Advanced proton imaging in computed tomography

“…Nearly all proposed systems use silicon strip detectors (SSDs) derived from highenergy physics experiments as tracking planes. SSDs contain an array of parallel long strip-shaped detecting diodes and return the position in one dimension of a traversing particle with around hundred micron precision and high speed (up to many megahertz, depending on the strip length) (5) . To retrieve the x and y position of the particles, it is necessary to use two orthogonally placed SSDs for the tracking planes for both the entrance and exit sides of the scanner.…”

“…100 kHz for a 1 MHz refresh rate. This means that the best present set-ups, with frame rates of the order of 10 MHz, would take about 10-16 min to record the 10 9 tracks (5) necessary to reconstruct a complete image.…”

“…Among the many solutions proposed to overcome this limitation, the most promising one uses protons also for the 3D imaging of the target, actually realising a proton computed tomography, pCT (2,5) . By tracking protons of high energy (at least 200 MeV, so they do not stop inside the body), it is possible to generate a 3D view of the target, exactly as in an X-ray CT, by moving the proton source around the target (or by rotating the target itself ) and recording at each step the paths and energy loss of the particle.…”

“…Besides the higher precision for hadron therapy guidance, this approach has the advantage of exploiting the same accelerating machine and beam-line used for the treatment, as well as the same gantry to hold the patient. In addition, mathematical considerations and first prototype testing indicate that about 10 9 proton tracks in total are necessary for a precise 3D reconstruction of the target, equivalent to a dose of 1.5 mGy (5) . This compares very favourably to the dose necessary for a usual X-ray CT, which varies in the 15-100 mGy range depending on the target (7) .…”