Rotation-free computed tomography with orthogonal ray imaging: First millimetric experimental results

Orthogonal ray imaging is a new technique under investigation by our groups. It shows good potential for (1) dose verification in external beam radiotherapy, and (2) very-low-dose computed tomography (CT-like) imaging, the latter being investigated here. We present simulated (Geant4) and experimental orthogonal ray imaging results of the visualization of a heterogeneous phantom irradiated from both sides with a 6-MV photon beam. The phantom was constructed with azimuthal symmetry in respect to the incoming beam directions. This imaging technique, named orthoCT, does not require rotational irradiation of the target. It is based on the detection of photons emitted at almost right angles in respect to the incoming photon flux, the latter being shot at the target only from two diametrically opposed directions. In addition to showing the visual correlation between the detected photon profile and the phantom density profile, we also show by simulation and experiment that a 2-mm phantom displacement is clearly distinguishable with orthoCT. A new, improved collimator is under construction aiming at achieving sub-millimetric accuracy in phantom positioning.