Medical Image Reconstruction: A Conceptual Tutorial - Gengsheng Zeng
I chanced upon this book yesterday while searching for “parallel beam to fan beam conversion” and have become a fan of this book already.
http://www.amazon.com/Medical-Image-Reconstruction-Conceptual-Tutorial/dp/364205367X
The concepts are introduced very simply with easy-to-understand diagrams and explanation - the mathematical background is introduced later on. This way the reader can quickly appreciate the major concepts of the algorithms, before diving into the mathematics.