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Preliminary X-ray CT investigation to link Hounsfield unit measurements with the International System of Units (SI)

Published

Author(s)

Zachary H. Levine, Adele P. Peskin, Andrew Holmgren, Edward J. Garboczi

Abstract

Purpose:This paper lays the groundwork for linking Hounsfield unit measurements to the International System of Units (SI), ultimately enabling traceable measurements across X-ray CT (XCT) machines. We do this by characterizing a material basis that may be used in XCT reconstruction giving linear combinations of concentrations of chemical elements (in the SI units of mol/m$^3$) which may be observed at each voxel. By implication, linear combinations not in the set are not observable. Methods and Materials: We formulated a model for our material basis with a set of measurements of elemental powders at four tube voltages, 80~kV, 100~kV, 120~kV, and 140~kV, on a medical XCT. The samples included 30 small plastic bottles of powders containing various compounds spanning the atomic numbers up to 20, and a bottle of water and one of air. Using the chemical formulas and measured masses, we formed a matrix giving the number of Hounsfield units per (mole per cubic meter) at each tube voltage for each of 13 chemical elements. We defined a corresponding matrix in units we call molar Hounsfield unit (HU) potency, the difference in HU values that an added mole per cubic meter in a given voxel would add to the measured HU value. We built a matrix of molar potencies for each chemical element and tube voltage and performed a singular value decomposition (SVD) on these to formulate our material basis. We determined that the dimension of this basis is two. We then compared measurements in this material space with theoretical measurements, combining XCOM cross section data with the tungsten anode spectral model using interpolating cubic splines (TASMICS), a one-parameter filter, and a simple detector model, creating a matrix similar to our experimental matrix for the first 20 chemical elements. Finally, we compared the model predictions to Hounsfield unit measurements on three XCT calibration phantoms taken from the literature. Results: We predict the experimental HU potency valuWe
Citation
PLoS One
Issue
12

Keywords

Medical CT , computed tomography , XCOM , intrinsic dimensionality , beam hardening
Created December 20, 2018, Updated January 27, 2020