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Session: CT Dosimetry [Return to Session]

Determining Water Equivalent Diameter and Size Specific Dose Estimates From CT Localizers

C Burton*, St Jude Children's Research Hospital, Memphis, TN


TU-D1000-IePD-F8-2 (Tuesday, 7/12/2022) 10:00 AM - 10:30 AM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 8

Purpose: Water equivalent diameter (WED) is the preferred patient size surrogate needed to calculate the normalize dose calculation (NDC) to determine the size-specific dose estimates (SSDE). The WED can be calculated from computed tomography (CT) axial scans providing the gold standard. A calibration method has been developed that relates the CT localizer pixel values (LPV) to the water-equivalent area calculated from the CT localizer. Utilizing CT localizers will provide an estimate of SSDE prior to the scan that may be used in DRR workflows and will reduce data overhead. We investigate the calibrate method on patient data on two scanners with different vendors with different techniques.

Methods: For calibration, CT localizer and CT axial images of the computed tomography dose index (CTDI) and ACR phantoms were acquired using routine techniques on both scanners: GE Revolution (120 kV and lateral localizers) and SIEMENS Intevo (80 kV and lateral localizers). The water-equivalent area (WEA) and LPV were related. Then CT axial and CT localizer images of patients were acquired using the same technique and the WED was calculated using the calibration method.

Results: The WED calculation and NDC calculation for GE Revolution (R2=0.93) and SIEMENS Intevo (R2=0.98) show good correlation with most data points falling within the 95% confidence interval including the line of unity and AAPM TG Report 220 results. The maximum, mean and minimum absolute percentage differences of NDC for GE Revolution were 21.2, 3.0, and 0.2% and for SIEMENS Intevo were 4.0, 1.6, and 0.4%, respectively. The 21.2% was due to a single outlier whereas most cases were around the average of 3.0%.

Conclusion: This study validates that the calibration method gives a good estimate of WED from CT localizers. The normalized dose coefficient can be determined within 20% as recommended by the AAPM TG Report 220.


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