How can I extent the fan2para parallel beam data?

I am recontructing parallel beam data from incomplete fan beam data, and it works, but the fan beam is 0-360 degrees while the recovered parallel beam is only 0-180.

Does anyone know of any way to:

1. Ensure that the fan2para returns a complete parallel data set, or
2. A function that extends a parallel beam data set, or
3. Is that something I will have to extend by hand?

I am aware of the symetric properties of parallel beam data over the 180 degree point, but I would prefer to use a built in function to be sure of the accuracy.

Thank you!