A controller for use in controlling operation of a spectral computed tomography X-ray imaging apparatus, wherein the imaging apparatus is a kVp-switching apparatus comprising an X-ray generator having at least one X-ray tube, and the generator controlled during operation in kVp switching mode to switch cyclically between a lower tube voltage and an upper tube voltage for acquiring spectral CT scan data, and the apparatus having a set of adjustable first operating parameters including at least the upper tube voltage, the lower tube voltage and a duty cycle; wherein said apparatus is selectively operable in a secondary acquisition mode in which the controller is adapted to: receive a user input indicative of user-defined settings for a set of second operating parameters, wherein the second operating parameters are parameters associated with non-spectral CT imaging, comprising use of a radiation source operated with a fixed x-ray tube voltage, the second operating parameters including at least a value of the fixed tube voltage, kVp, and a tube current, /, apply a conversion procedure for converting the user-defined settings for the second operating parameters into a set of values for the first operating parameters which are determined to result in delivery of a same X-ray radiation dose over a single kVp switching cycle period as would be delivered by a fixed-voltage radiation source, operated with the user-defined second operating parameters, over the same time period; and generate control instructions for causing the X-ray imaging apparatus to perform a CT scanning procedure in accordance with the determined settings for the first operating parameters, and thereby obtain a set of spectral CT scan data. wherein the imaging apparatus is a kVp-switching apparatus comprising an X-ray generator having at least one X-ray tube, and the generator controlled during operation in kVp switching mode to switch cyclically between a lower tube voltage and an upper tube voltage for acquiring spectral CT scan data, and the apparatus having a set of adjustable first operating parameters including at least the upper tube voltage, the lower tube voltage and a duty cycle; wherein said apparatus is selectively operable in a secondary acquisition mode in which the controller is adapted to: receive a user input indicative of user-defined settings for a set of second operating parameters, wherein the second operating parameters are parameters associated with non-spectral CT imaging, comprising use of a radiation source operated with a fixed x-ray tube voltage, the second operating parameters including at least a value of the fixed tube voltage, kVp, and a tube current, /, apply a conversion procedure for converting the user-defined settings for the second operating parameters into a set of values for the first operating parameters which are determined to result in delivery of a same X-ray radiation dose over a single kVp switching cycle period as would be delivered by a fixed-voltage radiation source, operated with the user-defined second operating parameters, over the same time period; and generate control instructions for causing the X-ray imaging apparatus to perform a CT scanning procedure in accordance with the determined settings for the first operating parameters, and thereby obtain a set of spectral CT scan data. receive a user input indicative of user-defined settings for a set of second operating parameters, wherein the second operating parameters are parameters associated with non-spectral CT imaging, comprising use of a radiation source operated with a fixed x-ray tube voltage, the second operating parameters including at least a value of the fixed tube voltage, kVp, and a tube current, /, apply a conversion procedure for converting the user-defined settings for the second operating parameters into a set of values for the first operating parameters which are determined to result in delivery of a same X-ray radiation dose over a single kVp switching cycle period as would be delivered by a fixed-voltage radiation source, operated with the user-defined second operating parameters, over the same time period; and generate control instructions for causing the X-ray imaging apparatus to perform a CT scanning procedure in accordance with the determined settings for the first operating parameters, and thereby obtain a set of spectral CT scan data.