In image guided radiotherapy (IGRT) two of the most promising recent developments are four dimensional
cone beam CT (4D CBCT) and dynamic optical metrology of patient surfaces. 4D CBCT is now becoming
commercially available and finds use in treatment planning and verification, and whilst optical monitoring
is a young technology, its ability to measure during treatment delivery without dose consequences has led
to its uptake in many institutes. In this paper, we demonstrate the use of dynamic patient surfaces,
simultaneously captured during CBCT acquisition using an optical sensor, to phase sort projection images
for 4D CBCT volume reconstruction.
The dual modality approach we describe means that in addition to 4D volumetric data, the system provides
correlated wide field measurements of the patient's skin surface with high spatial and temporal resolution.
As well as the value of such complementary data in verification and motion analysis studies, it introduces
flexibility into the acquisition of the signal required for phase sorting. The specific technique used may be
varied according to individual patient circumstances and the imaging target. We give details of three
different methods of obtaining a suitable signal from the optical surfaces: simply following the motion of
triangulation spots used to calibrate the surfaces' absolute height; monitoring the surface height in a single,
arbitrarily selected, camera pixel; and tracking, in three dimensions, the movement of a surface feature. In
addition to describing the system and methodology, we present initial results from a case study oesophageal