# In computed tomography (CT), why is 'Inverse Problem of Radon transform' studied?

As I know, the Radon transform is a very important tool in CT by Beer's law. Thus, finding $$f(x)$$ of Radon transform $$Rf(L):=\int_{L} f(x)dl(x)$$ is helpful in CT. Nowaday, the Filtered back-projection formula (FBP) for all dimensions $$n$$ (which can find the $$f(x)$$) is obtained and is used already.

My question is here. Why is people studying 'Inverse Problem' in Radon transform although FBP is obtained?