Dose, noise and view weights in CT helical scans

The amount of X-ray dose expresses itself as the noise level in image volume after reconstruction in clinical CT scans. It is important to understand the interaction between the dose, noise and reconstruction, which helps to guide the design of CT systems and reconstruction algorithms. Based on the fact that most of practical reconstruction algorithms in clinical CT systems are implemented as filtered back-projection, in this work, a unified analytical framework is proposed to establish the connection between dose, noise and view weighting functions of different reconstruction algorithms in CT helical scans. The proposed framework helps one better understand the relationship between X-ray dose and image noise and is instrumental on how to design the view weighting function in reconstruction without extensive simulations and experiments. Even though certain assumptions were made in order to simplify the analytical model, experimental results using both simulation data and real CT scan data show the proposed model is reasonably accurate even for objects of human body shape. In addition, based on the proposed framework an analytical form of theoretically optimal dose efficiency as a function of helical pitch is also derived, which suggests a somehow unintuitive but interesting conclusion that the theoretically optimal dose efficiency generally varies with helical pitch.