Comparison of Norland DXA and Lunar DXA results with the Application of a Cross Calibration Equation

1. Comparison of Norland DXA and Lunar DXA Results with the
Application of a Cross Calibration Equation
B. Pejovska, R. Barnett
Department of Nuclear Medicine, PET & Ultrasound Westmead Hospital
Background: Bone Mineral Density (BMD) plays a primary role in the diagnosis and management of osteoporosis. A comparison of results from different
manufacturers of dual-photon X-Ray absorptiometry (DXA) scanners is challenging, which undermines longitudinal assessment results. It is possible to use t-scores
and z-scores, provided different machines use the same reference population. However, this is often not the case, thereby emphasising the importance of applying
and validating an appropriate cross calibration equation.
Aim: To verify that published cross calibration equations can be used to interpret changes in BMD between a Norland XR-800 DXA scanner and GE-Lunar Encore DXA
scanner.
Method: Calibration measurements were verified using a Lunar DPX Series iDXA QC spine phantom (BMD 1.2521 g/cm2) and a standard wrist QC phantom (BMD
0.905 g/cm2). The spine phantom was scanned 10 times and the wrist phantom 20 times, respectively, on the Norland and Lunar scanners. The data for the duplicate
measurements for the spine and the wrist phantom were analysed using the automatic analysis method with the manufacture specific internal program. For the
spine, we plotted the Norland BMD versus the Lunar BMD alongside two calibration equations (Genant et al. 1994, Hui et al. 1997), allowing for meaningful
comparison between BMD measurements. For the wrist, published cross calibration equations implement different methods of region of interest (ROI) placement,
thus making meaningful comparisons of BMD measurements difficult.
Results: SPINE PHANTOM
NORLAND sBMD = 1.0761 BMD
LUNAR sBMD = 0.9522 BMD
The above equations are the original
International DXA Standardisation Committee
(IDSC) algorithms for cross calibration of BMD
machines by Genant et al. These equations
give us the line of comparison after regression
analysis.
NORLAND sBMD = 0.9743 (BMD - 0.969) + 1.0436
LUNAR sBMD = 0.9683 (BMD - 1.100) + 1.0436
The optimal universal standardised
measurements derived by Hui et al. are given
by the above equations. These equations give
us the line of comparison after regression
analysis.
Calibration Equations Genant et al. 1994
Calibration Equations Hui et al. 1997
Figure 1. A scatter plot of Norland and Lunar BMD measurements. Each measurement of the
spine phantom is marked with a red cross. There are 10 Norland and 10 Lunar measurements
for each vertebrae L2, L3, and L4. There is insufficient data to perform regression analysis of
Norland BMD values (g/cm2) versus Lunar BMD values (g/cm2). A line of comparison proposed
by Hui et al. 1997 is plotted in blue (dashed) and represents the equation BMDL = 1.0062 x
BMDN +0.1250. The original line of comparison by the IDSC Genant et al. 1994 is plotted in
green (dashed) and represents the equation BMDL = 1.1301 x BMDN.
Figure 2. A Bland-Altman plot of
average difference after standardisation
between Norland and Lunar BMD
measurements. The Hui et al. (blue
bars) and Genant et al. (green bars)
algorithms are compared by expressing
the average difference of L2, L3 and L4
in mg/cm2; the percentage difference is
also expressed above each bar. The
error bars represent the limits of
repeatability of the measured values
using standard deviation.
g/cm2 PROXIMAL WRIST
DISTAL WRIST
NORLAND 0.89 +/- 0.01 0.35 +/- 0.01
LUNAR 0.87 +/- 0.01 0.40 +/- 0.01
Results: WRIST PHANTOM
Table 2. A table of the Norland and Lunar BMD
measurements with no calibration applied. The results are
expressed in g/cm2 +/- standard deviation. Published cross
calibration equations apply different ROI placement for
the distal and proximal BMD measurements.
Conclusion: The application of the proposed cross calibration equations allow
for meaningful clinical comparisons of lumbar spine values between studies
acquired on different devices. Future studies investigating new methods of ROI
placement for the wrist will lead to improved calibration factors, thus allowing for
better correlation between different manufactures.
B. Pejovska, R. Barnett
Department of Nuclear Medicine, PET & Ultrasound Westmead Hospital
Discussion: Baseline and follow up examinations must be acquired
on the same make and model of densitometer, however
monitoring the same patient on a different machine comes up in a
variety of clinical situations including device upgrades and when
patients change a primary caregiver. To allow for meaningful
patient follow up and monitoring there needs to be certainty that
measurements from different densitometers are comparable.The
algorithm proposed by Hui et al. was reported as being an
improvement to the algorithm by Genant et al, however, the
average difference in our Bland-Altman comparison (figure 2)
shows that our phantom study did not reveal significant reductions
in error using the improved method. Although the average
differences in our phantom study are comparable to magnitudes
of error reported by Hui et al, a patient study is needed to
determine which algorithm is optimal and the contribution of
error arising from differences in processing techniques and
anatomical variation.
Unfortunately, calibration of BMD measurements for the wrist are
more challenging as manufactures do not reveal their specific
algorithms for methods of bone segmentation, thus making the
quantification of ROIs difficult. Proposed calibration algorithms
for ultradistal, middistal and proximal wrist regions by Shepherd
et al. based on the algorithms of Hui et al. are available, but
several shortcomings exist in the study one of which is the lack of
standardised ROIs. Prevrhal et al. proposes a common ROI before
standardisation of wrist BMD. To be clinically useful
manufacturers would need to provide this standardised,
automatically placed region as part of their processing software.
As per figure 4, the regions from the Norland BMD are processed
differently to the Lunar BMD, hence calibration in the study was
not possible. Comparability of forearm densitometry would
require standardisation of the the ROIs used. Standardised ROIs
would eliminate biological variations between BMD measurements
on different devices, increase confidence and reduce the standard
error estimate in cross calibrating devices.
Figure 3. Lunar DPX Series iDXA
QC spine phantom (BMD 1.2521
g/cm2) scanned on a) Norland XR-
800 DXA and b) GE-Lunar Encore
DXA.
Figure 4. Standard
wrist QC phantom
(BMD 0.905 g/cm2)
scanned on a)
Norland XR-800 DXA
and b) GE-Lunar
Encore DXA.
4a) 4b)
mg/cm2 L2 L3 L4
NORLAND 998 +/- 0.02 1146 +/- 0.02 1284 +/- 0.01
LUNAR 1015 +/- 0.01 1175 +/- 0.01 1336 +/- 0.01
Table 1. A table of the Norland and Lunar BMD
measurements after calibration. sBMD results are
expressed in mg/cm2 +/- standard deviation.
Figure 1.
Figure 2.
Table 1.
Table 2.
3a) 3b)
References:
1. Genant HK, Grampp S, Gluer CC, Faulkner KG, Jergas M,Engelke K, Hagiwara S, Van Kuijk C 1994 Universal standardisation for dual X-ray
absorptiometry: Patient and phantom cross-calibration results. J Bone Miner Res 9:1503–1514.
2. Hui SL, Gao S, Zhou XH, Johnston CC Jr, Lu Y, Gluer CC, Grampp S, Genant H 1997 Universal standardisation of bone density
measurements: A method with optimal properties for calibration among several instruments. J Bone Miner Res12:1463–1470.
3. Shepherd JA, Cheng XG, Lu Y, Njeh C, Toschke J, Engelke K, Grigorian M, Genant HK 2002 Universal standardisation of forearm bone
densitometry. J Bone Miner Re 17:734-745
4. Prevrhal S, Lu Y, Genant HK, Toschke JO, Shepherd JA 2005 Towards standardisation of dual X-ray absorptiometry (DXA) at the
forearm. A common region of interest (ROI) improves the comparability among DXA devices. Calcif Tissue Int 76:348-354
L2
L3
L4
2%
3% 3%
2%
4%
2%