Yi-Shi Hwua 1 & Sheng-Pin Changlai 2 1 Department of Radiological Technology, Central Taiwan...

Yi-Shi Hwua1 & Sheng-Pin Changlai2

1 Department of Radiological Technology, Central Taiwan University of Science and Technology, Taiwan ROC

2 Department of Medical Imaging Diagnostic, Chang Bing Show Chwan Memorial Hospital, Taiwan ROC

E-mail address : [email protected]

The 6th Japan-Korea-ROC Joint Conference for Radiological Technologists and the 23rd Academic Meeting of JART.

The Precision Assessment of The Precision Assessment of Dual-Energy X-ray AbsorptioDual-Energy X-ray Absorptio

metry (DXA)metry (DXA)


Introduction

Bone mineral density (BMD) results are technologist dependent.

Short-term in-vivo precision study measures technologist’s ability to reproduce technical factors from one scan to next on same patient.

Precision refers the ability to reproduce the same numerical result in the setting of no real biologic change when the test is repeatedly performed in an identical fashion.


The precision error (PE) determines how much change in BMD must be seen between measurements before it can be considered a true biological change.

Interpreting physician will be use precision error when comparing baseline and follow-up scans.

This means that the precision must be quantified by performing a precision study.

Introduction


This study was performed to determine the precision of lumbar spine bone mineral density measurements by a new

dual-energy X-ray absorptiometry.

The Purpose


A total of 15 healthy Taiwan women (mean aged 28.4 years of age) with normal anatomy participated in the present study.

Before participation in precision assessment, subjects were informed of the benefits and risks and written informed consent was obtained from each participant.

Each subject also completed a standardized questionnaire designed to document putative risk factors of osteoporosis.

General exclusion criteria were diseases, drugs, fractures and other major determinants known to affect bone metabolism.

Subjects


Bone mineral density was determined by a Lunar Prodigy Vision DXA system (GE Lunar Prodigy Advance, Madison, WI).

A quality assurance (QA) test was performed at the beginning of each scan day to calibrate and verify the correct operation of the densitometer.

All BMD measurements were carried out by one experienced technician.

The DXA scans were obtained by standard procedures supplied by the manufacturer for scanning and analysis.

BMD Measurement


Fig. 1 Positioning for Scout of Lumbar Spine.

Participants had DXA scans of the lumbar spine performed three times each on the same day with repositioning (getting off the table) after each scan.

BMD Measurement


Mean bone density values were calculated as the mean of each subject’s mean value from thee scans.

Precision of the scan techniques is expressed as SD based on root mean square averages as described by Glüer (1995).

The PE is also called the Root Mean Square Standard Deviation (RMS-SD).

For PE, ISCD recommends using g/cm2 (absolute amount rather than %).

Calculation of Precision Error


The LSC is the minimum amount of change needed to be 95% confident that the change in BMD is real, biologically, and not due to random measurement error.

According to the elementary properties of the normal distribution, 95% of the time two successive measurements on the same individual will be within 2.77σof each other (2.77 = 1.96√2), where σ is the SD of measurement error.

In this study, the precision error (RMS SD) and LSC in BMD for lumbar spine were calculated by the “Precision Calculating Tool” which was developed by Dr. Nelson Watts, MD, CCD for ISCD.

Calculation of Least Significant Change


The percent coefficient of variation (%CV) of daily quality assurance was 0.08%.

Their mean height was 158.6 ± 4.9 cm, mean weight was 57.2 ± 8.3 kg and mean body mass index (BMI) was 22.7 ± 2.9.

Precision error (RMS SD) in BMD for L1-L4 was 0.005g/cm2 and L2-L4 was 0.007g/cm2, respectively.

Calculate LSC for the group at the 95% confidence interval were L1-L4 (0.014g/cm2) and L2-L4 (0.018g/cm2), respectively.

Results

Patient’s CharactersP’t no. Age (yr) Weight (kg) Height (m) BMI (kg/m2)

1 32.1 60.6 1.570 24.6

2 33.9 53.5 1.551 22.2

3 35.0 61.3 1.570 24.9

4 32.9 52.0 1.531 22.2

5 23.9 59.4 1.691 20.8

6 23.3 73.0 1.594 28.7

7 28.9 69.2 1.627 26.1

8 37.3 55.7 1.570 22.6

9 22.8 52.3 1.563 21.4

10 22.6 49.8 1.523 21.5

11 21.3 53.7 1.614 20.6

12 24.8 50.6 1.520 21.9

13 26.4 70.8 1.645 26.2

14 32.9 45.3 1.583 18.1

15 28.1 51.5 1.640 19.1

Mean 28.4 57.2 1.586 22.7

SD 5.3 8.3 0.049 2.9



LSC = PE 2.77 = 0.005 2.77 = 0.014

Results (L1-L4)


LSC = PE 2.77 = 0.007 2.77 = 0.018

Results (L2-L4)


Precision overall is excellent and these results were consistent with International Society for Clinical Densitometry (ISCD) recommendations (the RMS SD less than 0.02g/cm2 for spine).

The 2005 ISCD guidelines for precision assessment also recommended that each DXA center perform its own precision study and calculate the LSC for every relevant skeletal site.

In the case of multiple technologists performing densitometry studies, the values for the precision studies from individual technologist be averaged to determine the precision for the facility.

Discussions


How much of a difference in BMD is real?

Calculate your LSC in BMD (g/cm2).

Subtract current BMD from the BMD of the previous scan.

Does that value equal or exceed the LSC?

If yes, the BMD change is significant.

If no, the change is not significant.


Baseline spine BMD 0.783 g/cm2

Repeat spine BMD 0.750 g/cm2

Difference 0.033 g/cm2

Difference exceeds LSC ? Yes

Change is significant.

Serial BMD: Example

LSC 0.022 g/cm2


The variability of precision assessment is due to multiple causes, such as:

1. Technologist related factors (patient positioning and scan analysis).

2. Scanner related factors (accuracy, precision and machine calibration).

3. Patient related factors (artifact, anatomy deformities, patients’ movements, and patient cooperation).

4. Variation due to other unpredictable sources.

Conclusions


The authors deeply appreciate every volunteer was involved in this study.

We also would like to deeply thank Dr. P. U. Chieng and Dr. C. H. Wu for their comments on this manuscript, and all colleagues of the department of Medical Imaging Diagnostic, Chang Bing Show Chwan Memorial Hospital.

This work were supported by Chang Bing Show Chwan Memorial Hospital and in part by a grant from Central Taiwan University of Science and Technology.

Acknowledgements


Thank you for your attentionThank you for your attention

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