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Technical Brief

Methods for Post-Hoc Quantitative CT Bone Density Calibration: Phantom-Only and Regression

[+] Author and Article Information
Jacob Reeves

Department of Mechanical Engineering, Western University Canada, 1151 Richmond Street, London, Ontario, N6A3K7, Canada
jreeves3@uwo.ca

Nikolas Knowles

Department of Biomedical Engineering, Western University Canada, 1151 Richmond Street, London, Ontario, N6A3K7, Canada
nknowle@uwo.ca

George S Athwal

Roth|McFarlane Hand and Upper Limb Centre, 268 Grosvenor Street, London, Ontario, N6A4V2, Canada
gathwal@uwo.ca

James A. Johnson

Department of Mechanical Engineering, Western University Canada, 1151 Richmond Street, London, Ontario, N6A3K7, Canada
jajohnso@uwo.ca

1Corresponding author.

ASME doi:10.1115/1.4040122 History: Received July 20, 2017; Revised April 21, 2018

Abstract

Quantitative computed tomography (qCT) relies on calibrated bone mineral density data. If a calibration phantom is absent from the CT scan, post-hoc calibration becomes necessary. Scanning a calibration phantom after-the-fact and applying that calibration to uncalibrated scans has been used previously. Alternatively, density varies with CT settings; suggesting that it may be possible to predict the calibration terms using CT settings. This study compares a novel CT setting regression method for post-hoc calibration to standard and post-hoc phantom-only calibrations. Five cadaveric upper limbs were scanned at 11 combinations of peak tube voltage and current (80-140kV and 100-300mA) with two calibration phantoms. Density calibrations were performed for the cadaver scans, and scans of the phantoms alone. Stepwise linear regression determined if the calibration equation terms were predictable using peak tube voltage and current. Peak tube voltage, but not current, was significantly correlated with regression calibration terms. Calibration equation slope was significantly related to the type of phantom (p < 0.001), calibration method (p = 0.026), and peak tube voltage (p < 0.001), but not current (p = 1.000). The calibration equation vertical intercept was significantly related to the type of phantom (p < 0.001), and peak tube voltage (p = 0.006), but not calibration method (p = 0.682), or current (p = 0.822). Accordingly, regression can correlate peak tube voltage with density calibration terms. Suggesting that, while standard qCT calibration is preferable, regression calibration may be an acceptable post-hoc method when necessary.

Copyright (c) 2018 by ASME
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