Comparison of the image quality of various fixed and dose modulated protocols for soft tissue neck CT on a GE Lightspeed scanner

Purpose: Up-to-date CT scanners provide high quality soft tissue imaging of the neck, but scanning protocols often are not optimized regarding radiation dose. Thus, we tried to find a dose-optimized protocol for soft tissue imaging of the neck. Material and methods: 70 patients were scanned with a 16-row CT-scanner (Lightspeed, GE) with seven different protocols. We used four fixed tube current settings (225, 200, 175 and 150 mA; corresponding CTDIvol = 10.6, 9.5, 8.3 and 7.1 mGy) and three z-axis dose modulations with a relatively high, moderate and low dose (calculated CTDIvol = 10.5, 9.1, 7.7 mGy). Representative slices of seven anatomical regions (from the nasopharynx to the aortic arch) were subjectively judged by two radiologists with respect to image quality (five-point rating scale for noise and sharpness). For each protocol and for each judged anatomical region we determined and compared mean values regarding image quality and local tube current. For each protocol, mean values regarding the volume CT dose index (CTDIvol) and the dose-length product (DLP) were statistically compared. Moreover, using the software CT-Expo the respective effective doses and the cumulative organ doses of the thyroid gland were compared. Results: For a fixed tube current of at least 200 mA (CTDIvol = 9.5 mGy) and for dose modulations with a moderate or high dose adjustment (calculated CTDIvol = 9.1 and 10.5 mGy) the image quality was sufficient to excellent. As compared to a fixed tube current of 200 mA, dose modulation with a moderate dose adjustment improved the image quality in regions more vulnerable to noise-related artifacts such as at the level of the shoulder, without a noteworthy difference regarding the DLP. However, the cumulative organ dose of the thyroid gland was 17% lower using dose modulation with a moderate dose adjustment as compared to the fixed tube current of 200 mA. Thus, for a comparison with other scanners, we recommend dose modulation and an averaged CTDIvol < 9 mGy (or a DLP < 250 mGy cm). Conclusion: A combination of dose modulation and an averaged CTDIvol < 9 mGy or a DLP < 250 mGy cm yields sufficient image quality for soft tissue CT-imaging of the neck.