Purpose: The purpose of this work was to develop a technique to measure bone densities using a cadmium zinc telluride (CZT) photon counting detector (PCD).
Methods: Seven bone phantoms were prepared by mixing a suitable amount of calcium perchlorate with water to achieve densities ranging between 1-1.45 g/cm³, which were pipetted into 600 μL microcentrifuge tubes. A 10 cm diameter cylindrical polyethylene phantom with 18 cavities was used as a soft tissue equivalent material. Each bone tube was inserted in the same cavity and the phantom was imaged with a 120 kVp x-ray beam and CZT PCD while the remaining 17 cavities were filled with water. Energy thresholds of 38, 42, 68, 72, 98 and 102 keV were chosen to create 5 energy bins. Planar x-ray images were acquired at a tube current of 1 mA and the acquisition time was 25 ms.
Results: A linear calibration curve was produced by plotting the ratio of the total number of counts in the bone phantom region of interest between two different energy bins as a function of bone density. Between all energy bins, the ratio of bins that resulted in the highest sensitivity was between bins 1 (38-42 keV) and 5 (98-102 keV), which resulted in a slope of 0.17 and an adjusted R² value of 0.976. Five additional bone phantoms with unknown densities were imaged and their densities were estimated from the calibration curve where the mean absolute error was 3.8% and 6.6% for the 40/70 and 40/100 keV ratios, respectively.
Conclusion: It was demonstrated that measuring bone densities was possible by using a single image acquired with a PCD, a new technique that does not rely on dual-exposure imaging or a dual-layer detector. With further optimizations, future clinical applications to measure bone densities in vivo could be achieved.
Funding Support, Disclosures, and Conflict of Interest: This work was funded by Redlen Technologies, the NSERC Alliance Grant and Canada Research Chair program.
Photon Detectors, Image Processing, BMD