Purpose: Variation in implementation strategies of tube current modulation (TCM), common even within the same CT scanner manufacturer, may result in wide differences in tube outputâ€”yielding differences in patient dose and image quality for clinical exams. This work explored strategies to achieve consistent tube output curves (i.e., mAs vs patient size) on CT scanners with different implementations of TCM.
Methods: Anthropomorphic water phantoms ranging 15-50cm in lateral size were scanned (Scanner A and Scanner B, SOMATOM Force, Siemens Healthcare) using the clinical, low-dose skeletal survey protocol at our institution. The two scanners differed in software version (VA50 and VB10, respectively), and importantly the implementation of TCM. Scans were repeated for each configuration of the vendor TCM strength setting, ranging from very weak to very strong, as well as with two base protocols with different organ characteristics (Abdomen and Pelvis respectively). The average effective mAs from scans of each phantom size was recorded.
Results: The default protocol on Scanner A demonstrated up to 40% reduction in x-ray output compared to the corresponding, identical protocol on Scanner B, due to differences in TCM implementation between the two software versions. Differences in x-ray output between the two scanners varied significantly across phantom size, base protocol, and TCM strength setting. A tube output curve for scanner A that matched the that from scanner B was found through adjustment of TCM settings and the base protocol. The mean absolute percent difference between the two tube output curves was 7%.
Conclusion: By accurately adjusting the three parameters that affect the tube output profile as a function of patient size on the TCM algorithm, namely QRM, organ reference size, and strength values, we were able to obtain matching tube output on two scanners with significantly different default TCM performance, regardless of patient size.
Not Applicable / None Entered.