Susanne
Schweizer
*a,
Bodo
Hattendorf
b,
Philipp
Schneider
a,
Beat
Aeschlimann
b,
Ludwig
Gauckler
c,
Ralph
Müller
a and
Detlef
Günther
b
aInstitute for Biomedical Engineering, University and ETH Zürich, Switzerland. E-mail: schweizer@biomed.ee.ethz.ch
bInstitute of Inorganic Chemistry, ETH Zürich, Switzerland
cDepartment of Materials, Nonmetallic Inorganic Materials, ETH Zürich, Switzerland
First published on 7th August 2007
Phantoms for the calibration of local bone mineral densities by micro-computed tomography (μCT), consisting of lithium tetraborate (Li2B4O7) with increasing concentrations of hydroxyapatite [HAp, Ca10(PO4)6(OH)2] have been prepared and characterized for homogeneity. Large-scale homogeneity and concentration of HAp in the phantom materials was determined using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), while homogeneity on the micrometer scale was assessed through μCT. A series of standards was prepared by fusion of pure HAp with Li2B4O7 in a concentration range between 0.12 and 0.74 g cm−3. Furthermore, pressed and sintered pellets of pure HAp were prepared to extend the calibration range towards densities of up to 3.05 g cm−3. A linear calibration curve was constructed using all individual standard materials and the slope of the curve was in good agreement with calculated absorption coefficients at the effective energy of the μCT scanner.
Currently, solid phantom materials are in use that contain the bone mineral hydroxyapatite for performing BMD measurements in quantitative computed tomography (QCT)6,7 on millimeter to centimeter scales. Computed tomography is widely used as a diagnostic tool in many medical disciplines.8–11 Specifically, much research has been aimed at the development of CT-based calibration phantoms to mimic the attenuation profile of various tissue types.12–16 Commercially available bone phantom materials, such as the epoxy resin-based SB3 introduced by White et al.17 containing 67% calcium carbonate, allow the accurate calibration for cortical BMD. Several other phantoms have been designed to quantify bone mineral density from CT images,12,18–24 and effects of temperature,25,26 imaging resolution27 and radiation dosage27 have been studied in detail for such phantoms. However, the concentration range covered and the homogeneity of these phantoms make them suitable to only a limited extent for μCT measurements. Owing to the spatial resolution of μCT, assessment of the local bone mineral content of individual bone struts, so-called trabeculae, is possible. Thus, the mass attenuation coefficients that need to be quantified by μCT in bones are significantly higher than the averaged values obtained by QCT, reaching bone mineral densities of up to 1.6 g cm−3.28 However, higher phantom material densities are desirable to perform μCT calibration, to be able to also measure biomaterials (alone and after implantation in bone). These materials are very often ceramics, HAp or tricalcium phosphate (TCP) based.29
Additionally, a suitable μCT reference material to calibrate the scanner for BMD measurements has to mimic the absorption properties of the underlying bone material.28,30,31 Increasing mass attenuation coefficients corresponding to an exactly determined concentration of bone mineral have to be attained. Also, the reference material has to be homogenous on a micrometer scale such that the standard deviation of the attenuation coefficient in a given area is reduced to a minimum.
So far, two commercially available μCT phantoms from the CIRS (Computerized Imaging Reference Systems, Inc., Norfolk, USA) and Scanco (Scanco Medical, Bassersdorf, Switzerland) companies and fluid phantoms containing H2KPO4 are used for quantitative mineralization analysis of bone specimens. However, they all have limited concentration ranges of up to about 1 g cm−3 HAp equivalents only. Higher values of bone volume densities have to be extrapolated, assuming an accurate correction of effects such as beam hardening.32 In addition to their limited concentration range, H2KPO4 phantoms have been shown to form air bubbles and were therefore subjected to changing attenuation properties over time. Replenishment and proper service at regular intervals is needed for those phantoms.33
In this work we describe the preparation and characterization of new phantom materials for the calibration of a μCT scanner for BMD measurements. The materials cover a HAp density range from 0.12 to 3.05 g cm−3 and were prepared using two different methods. One set of samples contained HAp dissolved in a flux of lithium tetraborate (Li2B4O7), at densities between 0.12 and 0.74 g cm−3. Lithium tetraborate is a traditionally used solid solvent for a wide range of minerals and is commonly used for the production of homogenous glassy samples for mineral bulk characterization in X-ray fluorescence spectroscopy (XRF).34 It has a relatively low X-ray absorption coefficient and low fluorescence yield and is thus an ideal solvent for absorption measurements of a wide range of materials. The mineral concentrations (or densities) can be adjusted very easily and precisely by mixing known weights of the flux and the mineral before the fusion.
A second set of standards was prepared by compaction of HAp directly in a hydraulic press at different pressures. The phantoms were measured in-house and the micro-scale homogeneity was validated on a Scanco μCT scanner (μCT 40, Scanco Medical, Bassersdorf, Switzerland). Large-scale homogeneity and mineral contents were determined using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS).
Sample | HAp concentration (wt%) (±1 SD)a | HAp density/g cm−3 (±1 SD)b | Linear attenuation/cm−1 (±1 SD)c |
---|---|---|---|
a Average concentrations and standard deviations for the entire samples according to LA-ICPMS analyses. b Combined uncertainty from concentration, mass and volume determinations.37 c Intra-scan standard deviation. | |||
Blank | <0.02 | <0.0005 | 1.10 ± 0.034 |
L05 | 4.96 ± 0.19 | 0.12 ± 0.057 | 1.46 ± 0.037 |
L10 | 9.99 ± 0.28 | 0.23 ± 0.035 | 1.71 ± 0.048 |
L20 | 19.97 ± 0.31 | 0.48 ± 0.018 | 2.33 ± 0.080 |
L30 | 30.03 ± 0.95 | 0.74 ± 0.032 | 2.85 ± 0.097 |
After preparation the fused beads were characterized for large-scale homogeneity by spatially resolved analysis using an established LA-ICPMS method.38 The samples were analyzed at ten equally spaced positions between the center and the rim. There is a slight depletion of HAp towards the inner parts of the samples, which might have been a result of non-congruent solidification of the melt. Nonetheless, the variation of the HAp concentration across the entire sample was less than 10% in all cases (Fig. 1). Blank values were below the instrumental detection limit (0.02 wt% of HAp). Apart from the major constituents of HAp, several potential contaminants were also analyzed. Most trace elements were present at or below the instrumental detection limit, while magnesium [2 mg (kg HAp)−1], aluminium [0.2 mg (kg HAp)−1], manganese [0.015 mg (kg HAp)−1], nickel [0.012 mg (kg HAp)−1], copper [0.008 mg (kg HAp)−1], zinc [0.015 mg (kg HAp)−1], strontium [0.15 mg (kg HAp)−1] and barium [0.01 mg (kg HAp)−1] were detectable at very low levels.
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Fig. 1 Concentrations of HAp when fused with lithium tetraborate, at different locations across the samples L05–L30. Analyses were carried out by LA-ICPMS. Error bars indicate one standard deviation of the individual results, based on counting statistics of the ICPMS data for calibration and measurements. |
Sample | Press load/tons | HAp density/g cm−3 (±1 SD)a | Mass attenuation/cm−1 (±1 SD)b |
---|---|---|---|
a Uncertainty calculated from readability of the balance and caliper. b Intra-scan standard deviation. c Relative uncertainty of the pycnometer. | |||
Pressed powder samples | |||
P05 | 0.5 | 1.24 ± 0.007 | 3.13 ± 0.16 |
P10 | 1 | 1.41 ± 0.007 | 3.53 ± 0.19 |
P20 | 2 | 1.61 ± 0.008 | 4.02 ± 0.20 |
P30 | 3 | 1.77 ± 0.012 | 4.16 ± 0.21 |
P50 | 5 | 1.95 ± 0.013 | 4.68 ± 0.23 |
Sintered powder sample | |||
S50 | 5 | 3.05 ± 0.01c | 7.19 ± 0.36 |
One sample was additionally sintered in a temperature-controlled electrical furnace (Nabertherm 1750 °C; Eurotherm controller, Nabertherm, Lilienthal, Germany) after pressing. The furnace temperature was ramped from ambient to 800 °C at a rate of 2 °C min−1 and kept constant for 30 min. Subsequently the temperature was increased to 1300 °C at 5 °C min−1 and held constant for another 180 min. After sintering, the sample was allowed to cool to ambient temperature at a rate of 2 °C min−1 again.39 Table 2 lists the conditions and resulting densities obtained for these samples.
The density of the pressed pellets was determined from their geometrical volume (calculated from five individual measurements of diameter and height) and the weight of the samples.
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Fig. 2 Histogram of the mass attenuation data for the HAp pellet P50. The solid line represents a fit to a Gaussian distribution. |
The distribution of the attenuation coefficients represents the combined uncertainties arising from the homogeneity of the material and the measurement uncertainty during signal acquisition. It thus constitutes an upper limit for the estimate of uncertainty of the homogeneity of the material. Averaging a larger number of frames may reduce the overall uncertainty but will also increase measurement times dramatically. A compromise has to be made with respect to uncertainty and analysis time. By visually inspecting sample homogeneity on a micrometer scale, most phantoms also proved to be homogeneous at a resolution of 10 μm. In Fig. 3, representative samples from the lithium tetraborate pills and the pressed pills are shown. Only sample P05 shows variation of the attenuation coefficient at the resolution of the scanner. This indicates that the pressure applied is insufficient to compact the starting material to below a range of 10 μm. At higher pressures and in the fused material, however, the density of the material shows an even distribution at the spatial resolution of the scanner.
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Fig. 3 Representative areas (1 × 1 mm) for samples L10, L20, P05, and P20, scaled to the maximum intensity of the system. The pixel resolution is 10 × 10 μm. |
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Fig. 4 Calibration graphs for mass attenuation versus HAp density in the fused pills (left) and pressed pellets (right). Error bars correspond to one standard deviation of the linear attenuation and HAp densities respectively. |
The mass attenuation of the pressed and sintered pellets of HAp also shows a linear dependence on the HAp density (Fig. 4, right). Nonetheless, the slope of this fit yields a smaller slope, corresponding to a higher effective beam energy (27–28 keV).
Fig. 5 shows a combined calibration graph for all samples prepared in this study. Despite the different matrix compositions, the correlation of the measured mass attenuation with the HAp density is very high.
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Fig. 5 Combined calibration curves for fused and pressed HAp samples. Also given are the corresponding prediction intervals (p = 95%). |
Selected regression data are listed in Table 3. The correlation for the combined calibration curve is reasonably good, while the slopes show variations of 12% RSD. According to the regression statistics, the uncertainty of the results obtained against this calibration are in the range of 10% RSD, as indicated by the prediction interval in Fig. 5.
The standards allow the calibration of μCT scanners for the density of HAp with an accuracy of better than 10% relative, which enables a spatially resolved quantitative characterization of the bone mineral content.
Furthermore, the newly prepared phantoms are mechanically stable, allowing them to be handled without special care. The phantom weight did not significantly change over a two-year period (differences below 1%). They can be cut and polished when necessary to adjust them to individual sample holders of μCT scanners.
The research was funded by the Swiss National Science Foundation (SNF) through the SNF Professorship in Bioengineering (FP 620-58097.99).
This journal is © The Royal Society of Chemistry 2007 |