In interventional procedures, computed tomography (CT) has demonstrated its effectiveness in guiding operators through otherwise difficult tasks. By providing near real time feedback, CT fluoroscopy (CTF) enables the operator to dynamically adjust the location and orientation of the biopsy instrument. The most important performance parameter of CTF is the system temporal response. In this paper, we present a mathematical framework that accurately characterizes the system response of a needle type instrument. Based on the model, a modified halfscan reconstruction algorithm is presented. The algorithm makes use of the redundant nature of CTF data processing and enables efficient generation of CT images. The effectiveness of the algorithm is demonstrated by theoretical analysis and phantom experiments.