Management of respiratory motion during radiation therapy requires treatment planning and simulation using imaging modalities that possess sufficient spatio-temporal accuracy and precision. An investigation into the use of a novel ultrasound (US) imaging system for assessment of respiratory motion is presented, exploiting its good soft tissue contrast and temporal precision. The system dynamically superimposes the appropriate image plane sampled from a reference CT data set with the corresponding US B-mode image. An articulating arm is used for spatial registration. While the focus of the study was to quantify the system's ability to track respiratory motion, certain unique spatial calibration procedures were devised that render the software potentially valuable to the general research community. These include direct access to all transformation matrix elements and image scaling factors, a manual latency correction function, and a three-point spatial registration procedure that allows the system to be used in any room possessing a traditional radiotherapy laser localization system. Counter-intuitively, it was discovered that a manual procedure for calibrating certain transformation matrix elements produced superior accuracy to that of an algorithmic Levenberg-Marquardt optimization method. The absolute spatial accuracy was verified by comparing the physical locations of phantom test objects measured using the spatially registered US system, and using data from a 3DCT scan of the phantom as a reference. The spatial accuracy of the display superposition was also tested in a similar manner. The system's dynamic properties were then assessed using three methods. First, the overall system response time was studied using a programmable motion phantom. This included US video update, articulating arm update, CT data set resampling, and image display. The next investigation verified the system's ability to measure the range of motion of a moving anatomical test phantom that possessed both high and low contrast test objects. Finally, the system's performance was compared to that of a four-dimensional CT (4DCT) data set. The absolute spatial and display superposition accuracy was found to be better than 2 mm and typically 1 mm. Overall dynamic system response was adequate to produce a mean relative positional error of less than 1 mm if an empiric latency correction of 3 video frames was incorporated. The dynamic CT/US display mode was able to assess phantom motion for both high and low contrast test objects to within 1 mm, and compared favorably to the 4DCT data. The 4DCT movie loop accurately assessed the target motion for both of the high and low contrast objects tested, but the minimum intensity and average intensity reconstructions did not. This investigation demonstrated that this US system possesses sufficient spatio-temporal accuracy to properly assess respiratory motion. Future work will seek to demonstrate efficacy in its clinical application to respiratory motion assessment, particularly for sites in the upper abdomen, where low tissue contrast is evident.