Accuracy of bone mineral density quantification using dual-layer spectral detector CT: a phantom study

Abstract

Objectives: To investigate the accuracy of bone mineral density (BMD) quantification using dual-layer spectral detector CT (SDCT) at various scan protocols.

Methods: Two validated anthropomorphic phantoms containing inserts of 50-200 mg/cm³ calcium hydroxyapatite (HA) were scanned using a 64-slice SDCT scanner at various acquisition protocols (120 and 140 kVp, and 50, 100 and 200 mAs). Regions of interest (ROIs) were placed in each insert and mean attenuation profiles at monochromatic energy levels (90-200 keV) were constructed. These profiles were fitted to attenuation profiles of pure HA and water to calculate HA concentrations. For comparison, one phantom was scanned using dual energy X-ray absorptiometry (DXA).

Results: At both 120 and 140 kVp, excellent correlations (R = 0.97, P < 0.001) were found between true and measured HA concentrations. Mean error for all measurements at 120 kVp was -5.6 ± 5.7 mg/cm³ (-3.6 ± 3.2%) and at 140 kVp -2.4 ± 3.7 mg/cm³ (-0.8 ± 2.8%). Mean measurement errors were smaller than 6% for all acquisition protocols. Strong linear correlations (R² ≥ 0.970, P < 0.001) with DXA were found.

Conclusions: SDCT allows for accurate BMD quantification and potentially opens up the possibility for osteoporosis evaluation and opportunistic screening in patients undergoing SDCT for other clinical indications. However, patient studies are needed to extend and translate our findings.

Key points: • Dual-layer spectral detector CT allows for accurate bone mineral density quantification. • BMD measurements on SDCT are strongly linearly correlated to DXA. • SDCT, acquired for several indications, may allow for evaluation of osteoporosis. • This potentially opens up the possibility for opportunistic osteoporosis screening.

Keywords: Bone mineral density; Dual energy X-Ray absorptiometry; Dual-energy CT; Dual-layer spectral detector CT; Material decomposition.

Fig. 1

Phantom setup. An anthropomorphic European spine phantom on top of a bone density calibration phantom

Fig. 2

Image analyses and bone mineral density quantification. (A) Axial image with a ROI drawn in the ESP (blue) and BDC (pink and green). (B) Spectral plot for the corresponding ROIs: mean HU versus monochromatic energy level (keV). (C) Attenuation profiles were constructed between 90 and 200 keV in steps of 10 keV. Using in-house developed software, HA concentrations were calculated by fitting the constructed profiles to known attenuation profiles of pure HA and pure water. For this image this concerns ROI S3, an insert with a phantom design value of 200 mg/cm³ HA concentration. Av average, BDC bone density calibration phantom, ESP European spine phantom, HA calcium hydroxyapatite, HU Hounsfield units, keV kilo electron voltage, ROI region of interest, SD standard deviation

Fig. 3

Accuracy of dual-layer detector spectral CT for bone mineral density quantification. Mean relative errors (%) per protocol per nominal design HA concentration are shown. Symbols indicate mean measurement error (%) and error bars standard deviation. * = Phantom specific concentrations (ESP-143; 51.2, 102.3 and 201.2 mg/cm³ HA and BDC-03-29; 104.4 and 206.2 mg/cm³ HA) were used to determine deviation. CTDIvol volumetric CT dose index, ESP European spine phantom, HA calcium hydroxyapatite, kVp kilovoltage peak, mAs milliampere second, mGy milligray, SD standard deviation

Fig. 4

Comparison of BMD measurements on SDCT and aBMD measurements on DXA. Scatter plots with linear fit show a strong correlation. aBMD areal bone mineral density, BMD bone mineral density, DXA dual energy X-ray absorptiometry, kVp kilovoltage peak, SDCT dual-layer spectral detector computed tomography