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

Abstract
Philips CT Clinical Science Philips Healthcare • USA

Robbert W. van Hamersvelt, MD

Department of Radiology & Nuclear Medicine

University Medical Center

Heidelberglaan 100

3584 CX Utrecht

The Netherlands

r.w.vanhamersvelt-3@umcutrecht.nl

 

Objectives

Tissues that cannot be distinguished based on attenuation number can be distinguished by using material decomposition algorithms based on spectral CT. Material decomposition imaging (MDI) using spectral CT can be used to improve accuracy of quantitative analysis of bone mineral density (BMD). Therefore the objective of the current study was to investigate the accuracy of BMD quantification using dual-layer spectral detector CT (SDCT) at various scan protocols.

 

Methods

Two validated anthropomorphic phantoms (European spine phantom on top of a bone density calibration phantom) containing inserts of 50, 100 and 200 mg/cm3 calcium hydroxyapatite (HA) were scanned using a 4 cm coverage detector 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 subdivided by HA concentrations (50, 100 and 200 mg/cm3) were -0.9 ± 2.1 (-1.8 ± 4.0%), -3.0 ± 2.7 (-2.9 ± 2.6%) and -10.6 ± 5.3 mg/cm3 (-5.3 ± 2.7%) at 120 kVp and 1.6 ± 1.1 (3.1 ± 2.1%), -0.7 ± 1.8 (-0.7 ± 1.7%) and -6.0 ± 2.7 mg/cm3 (-2.9 ± 1.3%) at 140 kVp. Mean measurement errors were smaller than 6% for all acquisition protocols. Highest mean relative measurement errors were obtained for the protocol with the lowest dose (120 kVp 50 mAs), and were significantly higher than at 140 kVp 200 mAs (P=0.013), 140 kVp 100 mAs (P=0.016) and 140 kVp 50 mAs (P=0.025). No significant differences were found between the other protocols. At both 120 and 140 kVp, 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.

 

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Image analyses and bone mineral density quantification

  1. Axial image with a ROI drawn in the ESP (blue) and BDC (pink and green).
  2. Spectral plot for the corresponding ROIs: mean HU versus monochromatic energy level (keV).
  3. Attenuation profiles were constructed between 90 to 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.

 

* Results from case studies are not predictive of results in other cases; results in other cases may vary.



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Abstract
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