Method and apparatus for proton computerised tomography

A method of reconstructing a 3-dimensional computerised tomography image, the method comprising:

• a. creating an image reconstruction matrix by:
• from a plurality of scan angles surrounding an object to be imaged:
• i. directing a plurality of particles to traverse the object;
• and
• ii. measuring a trajectory and energy of each particle before and after each particle has traversed the object;
• iii. for each particle, calculating an equivalent path length (EPL) within the object;
• and
• iv. calculating an entry position at which each particle entered the object and an exit position at which each particle exited the object;
• and
• b. adding the EPL to the entry and exit positions of each particle of the image reconstruction matrix or a subset of elements thereof;
• c. applying a spatially varying 2-dimensional filter function to the image reconstruction matrix to generate a filtered reconstruction matrix;
• d. applying a correction factor to the filtered reconstruction matrix to at least partially correct for a finite extent of the filtered reconstruction matrix, and
• e. reconstructing the 3-dimensional computerised tomography image from the filtered reconstruction matrix.