Room: Exhibit Hall | Forum 9
Purpose: To investigate the impact of kV settings on Size Specified Dose Estimates (SSDE), evaluate the necessary corrections for low kV and small patient scans, to improve dose estimates and noise consistency.
Methods: PMMA CTDI â€˜likeâ€™ phantoms with varying size (10cm, 16cm, 24cm, 32cm, 40cm) were used to measure dose for comparison with calculated SSDE based on CTDI body (32cm) according to AAPM TG 202. The phantoms were scanned with varying kV (70kV/100kV/120kV/140kV) with constant mAs, and same kV/mA settings with or without additional bowtie for small patient (object) dose optimization (e.g. for Pediatric).Meanwhile water phantoms with varying size (10cm/15cm/20cm/30cm) are used to evaluate the dose and noise consistency under same scan conditions.
Results: From the results, it could be clearly seen that for low kV scans (e.g. 70kV), the measured CTDI is obvious higher than calculated SSDE for small objects (as PMMA cylinder with 10cm/16cm diameter). The effects or impacts will be smaller for either higher kV or larger objects. In case additional bowtie filter used, such difference in case of low kV + small object could be even bigger, which shows the necessity of corrections based on current SSDE for better size specified dose estimating. The direct measured dose at small CTDI phantom as corrected SSDE could obtain also better dose/noise consistency than non-kV dependent SSDE, and both of them are better than situation which same CTDI applied for different size of object (patient).
Conclusion: Current Dw dependent conversion factor can provide much better estimates with the size specified dose comparing with traditional CTDI, however the deviations between calculated SSDE and measured absorbed dose for low kV and smaller patient size is still relatively large, which shows for pediatric scan measurement based correction will be necessary for more accurate patient specific dose and obtain better nose consistency.
Funding Support, Disclosures, and Conflict of Interest: Authors are employees of Siemens Shanghai Medical Equipment Ltd.(Siemens Heathineers)