Technological innovations to reduce radiation dose for multidetector CT examinations include improved detector efficiency, pre-patient beam collimation, pre-patient bow tie filters, projection-adaptive reconstruction filters, image post processing filters, and automatic exposure control (3
). These techniques either reduce radiation dose while maintaining acceptable image quality, or improve image quality of low radiation CT examinations (3
). Prior clinical studies have also shown that diagnostic information can be obtained from CT examinations performed with a lower radiation dose (8
The automatic exposure control technique without weight adjustment for chest CT examinations has been reported to reduce radiation dose by 18-26% compared to the fixed tube current technique (7
). Application of the weight-adjusted AEC technique at 140 kVp to abdominal CT examination was also associated with substantial radiation dose reduction (5
). However, to the best of our knowledge, there has been no published study on use of weight-based AEC for chest CT scanning.
We found that the weight-based AEC technique at 120 kVp reduces the radiation exposure associated with chest CT while maintaining the structural and diagnostic integrity of the images. The visualization of all the anatomical structures was also intact with the weight-based AEC technique at 120 kVp. Overall, the weight-based AEC technique at the lower kVp showed a 17% to 43% reduction in the CTDIvol compared with the non-weight-adjusted AEC technique at 140 kVp. A substantially stronger positive correlation was found between radiation dose and patient weight, in addition to transverse diameters with weight-adjusted AEC technique at 120 kVp compared to the non-weight-adjusted AEC technique protocol. This suggests that the weight adaptation provided by the AEC technique is more precise in modulating dose according to patient size.
Interestingly, with the weight-based AEC technique at 120 kVp, radiation dose reduction was the least in patients weighing more than 90 kg. One reason for this finding is that the regional size of patients (transverse diameter) in the weight-adjusted AEC group was significantly greater than in the non-weight-adjusted AEC group (p = 0.02). Also, there was a much greater increase (mean difference = 132 mA) in maximum mA with use of the weight-adjusted AEC technique for patients weighing more than 90 kg over non-weight-adjusted AEC technique at higher kVp. A similar argument, however, cannot be made to explain why the dose reduction with the weight-adjusted AEC technique at 120 kVp was greater for the 61-90 kg category compared to patients ≤ 60 kg. It is conceivable that this anecdotal finding may have been due to disproportionately fewer number of patients in the ≤ 60 kg weight category (n = 17) compared to those in the 61-90 kg category (n = 52). It is possible, though not proven, that with the greater number of patients in the lighter weight category in our study, a different trend in dose reduction was observed. The image noise in the weight-based AEC technique was not statistically different from the non-weight-adjusted AEC technique.
Some limitations exist for the current study. Firstly, it is a retrospective review of the CT examinations performed with weight-adjusted AEC and non-weight-adjusted AEC, in a different set of patients. However, patients in these two groups were similar in terms of gender and weight distribution. Another limitation is that we did not evaluate subjective image quality in the CT examinations performed with the non-weight-adjusted AEC technique at 140 kVp, as these examinations were performed at a higher radiation dose compared to those performed with the weight-adjusted AEC technique. Also, prior studies have already reported acceptable image qualities with the non-weight-adjusted AEC technique for chest CT technique employed in our study (7
). Also, no formal evaluation of the effects of lower radiation dose CT examinations using the weight-adjusted AEC technique on lesion conspicuity or detection was performed in our study. However, the two radiologists did grade diagnostic acceptability of images based on their ability or confidence in identifying any abnormalities or the absence of abnormalities, based on their perceived likelihood in identifying potential abnormalities. We did not separately assess radiation dose reduction with the weight-adjusted AEC technique. Dose reduction reported in our study was a composite effect of lower kVp and the weight-adjusted AEC technique. Another limitation of our study is that the results of our study pertain to a combined modulation technique of one vendor and may not be applicable or relevant on similar techniques for other vendors. This was not feasible as we did not have access to all the different vendors' CT systems.
The implications of the current study proves that 120 kVp is sufficient for chest CT scanning, regardless of patient weight or transverse diameter. Although conventionally, adjustment to the AEC technique is recommended to modify radiation dose and image quality for different clinical indications. We found that the automatic exposure control technique needs to be further adapted to patient weight in order to obtain additional radiation dose reduction compared to the non-weight-adjusted AEC. Lighter patients should be scanned with lower noise index compared to the heavier patients. Use of the non-weight-adjusted AEC technique is not adequate for obtaining the full benefit of the technique. Also, further adjustment to the automatic exposure control technique is necessary to adjust for prospective acquisition of thinner sections, as was performed in our study. Although scanners automatically increase the noise index when section thickness is decreased, the radiologists, physicists and technologists using this instrument need to ensure that such a change in noise index maintains a constant radiation dose regardless of section thickness and that the noise index is not over-written or rejected unless there are over-riding concerns for obtaining higher image quality.
In conclusion, chest CT scanning using a weight-based automatic exposure control technique at 120 kVp helps in substantial radiation dose reduction compared to the non-weight-adjusted AEC technique and 140 kVp. Furthermore, the use of 120 kVp and the weight-adjusted AEC does not compromise image quality, diagnostic acceptability, and visually sharp reproduction of the thoracic anatomic structures.