Since the inception of Computerized Tomography (CT), tube thermal performance has been characterized by tube heat storage and dissipation. As the number of CT detector rows has increased from one to sixteen and higher, these parameters have become less relevant to clinical performance. In addition, peak power is quite dependent on focal spot size, so quoting one parameter without the other gives an incomplete picture. We propose a new set of specifications that more completely characterize a tube's thermal performance: the Scan Performance Index (SPI) and the Focal Spot Loadability (FSL). The Scan Performance Index (SPI) is a measure of clinical tube performance over a range of application parameters. The Focal Spot Loadability (FSL) is a measure of peak power as a function of focal spot size. This paper describes these figures of merit and provides some proposed parameter definitions. Calculations of SPI were made based on the mA vs. time performance curves under different assumptions of the thermal cycle repeat frequency and clinically relevant scan time range. Comparison of the results of SPI calculations with the traditional heat storage and dissipation characteristics vs. actual clinical capability leads to the conclusion that SPI is the better indicator of total throughput, especially as tube power and CT fan beam coverage increases, and total scan time decreases. FSL is shown to elucidate the inherent ability of a CT tube to handle high power loads for a given focal spot size. We conclude that these two new CT tube characteristics should be considered by clinicians in place of the traditional tube characteristics in order to benchmark the thermal performance capability of a given CT tube.