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Estimating Patient Water Equivalent Diameter From Localizer Images -- Dependency of Calibration Parameters On Various Imaging Parameters

D Zhang1*, X Liu2 , X Duan3 , J Rong2 , A Bankier1 , M Palmer1 , (1) Harvard, Beth Israel Deaconess Medical Center, Boston, MA, (2) UT MD Anderson Cancer Center, Houston, TX, (3) UT Southwestern Medical Center, Dallas, TX


(Tuesday, 7/16/2019) 4:30 PM - 5:30 PM

Room: Exhibit Hall | Forum 1

Purpose: Water equivalent diameter (Dw) is a sound patient-size descriptor and is used to determine SSDE. Calculating Dw from CT localizer radiographs makes it possible to utilize Dw prior to scan initiation and minimizes truncation errors due to limited FOV. However, localizer-based Dw estimation requires calibration to relate localizer pixel values to attenuation. Here we investigated how this calibration is affected by various imaging conditions.

Methods: Localizer and axial images of ACR- and body CTDI-phantoms were acquired on 16 scanners from GE, Siemens, Philips, and Canon. By associating axial images with the corresponding localizer lines, calibration parameters were estimated using an in-house software. Experiments were conducted under various kVp, mA, angle (LAT vs PA), and imaging kernel of localizers, as well axial kVp.

Results: The calibration parameters (slope and intercept) showed dependency on localizer kVp on all CTs. For example, on a Canon Acquilion-One CT, slope changed from 1.47 to 1.64 for localizers from 80 to 135 kVp. Using the same calibration parameters (120 kVp localizer), we simulated the errors in Dw estimation caused by using wrong calibrations: Dw from localizers at 80-135 kVp deviated from the ground truth by 1-5% for the 32cm CTDI phantom and 1-7% for the ACR phantom. Localizer mA and angle only demonstrated small effects on the calibration parameters and Dw results. For Canon scanners, calibration slope depends on the localizer kernel. For the Acquilion-One, Dw calibration slopes under Sharp- and Standard-kernels are identical (diff. < 0.01%) but differ from the Soft-kernel slope by 55%. Using the Sharp-kernel calibration, Dw from Soft-kernel localizer images deviated from the ground truth by 35% for the 32cm CTDI phantom and 42% for the ACR phantom.

Conclusion: Our study showed that Dw calibration is strongly affected by localizer kVp and image kernels, and slightly influenced by other factors.


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