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180404 DXA...
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Dual Energy X-Ray Absorptiometry (DXA) –Science, Technology, and Practice
David M. Gauntt, Ph.D.UAB Medical Center
Disclosures
I am a (minor) GE stockholder
I was involved in the design of two DXA machines (Expert and Achilles) when working for Lunar Corporation (now GE Lunar)
My only clinical experience with DXA has been as a patient (no QA experience!)
I grew up in Bedford MA, 2 miles from Hologic HQ
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DXA Bone Densitometers
Image courtesy GE Healthcare
Image courtesy Hologic Corporation
Effective dose
Exam GE Prodigy Hologic QDR 4500
Lumbar spine 0.7 uSv 1.8 uSv (?)
Proximal femur 0.7 uSv 6 uSv
Whole body 0.6 uSv 4 uSv
Source: CRCPD Bone Densitometry White Paper (2006)
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Uses of DXA machines
Bone densitometry
Diagnosis (accuracy)
Therapy response (precision)
Fracture prediction (FRAX application)
Body composition
Total bone, total lean, total adipose
Vertebral morphology
Dual Energy X-ray Absorptiometry
ScienceTechnologyPractice
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Monoenergetic x-rays
ln
Monoenergetic x-rays
, ,
Log attenuation ln
Mass attenuation coefficient
, ≡
Areal density (g/cm2) ≡
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Dual energies
, ,
, ,
,
,
Dual energies
, , , , ,
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Material Decomposition
0.125
0.25
0.5
1
2
4
8
20 40 60 80 100 120 140
Mu/Rho (cm2/g)
Photon energy (keV)
Aluminum
PMMA
Compton
Photoelectric
Material Decomposition
0.125
0.25
0.5
1
2
4
8
20 40 60 80 100 120 140
Mu/Rho (cm2/g)
Photon energy (keV)
Aluminum PMMA Fi ed Aluminum Fi ed PMMA
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Material Decomposition
0
0.05
0.1
0.15
0.2
0.25
0.3
0.9 0.95 1 1.05 1.1 1.15
Coefficent o
f photoelectric
absorp
on
Coeffic
i
ent of Compton Sca ering
Bone
Aluminum
Lean
PMMA Adipose
Material Decomposition
‐0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
‐0.5 0 0.5 1 1.5
Aluminum
equivalence
(g/cm2)
PMMA Equivalence (g/cm2)
Bone
Lean
Adipose PMMA
Aluminum
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Material Decomposition
‐0.2
0
0.2
0.4
0.6
0.8
1
1.2
‐0.2 0 0.2 0.4 0.6 0.8 1 1.2
Bone equivalence
(g/cm2)
Lean Equivalence (g/cm2)
Bone
Lean
Adipose
PMMA Aluminum
Example
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‐0.8 ‐0.6 ‐0.4 ‐0.2
0 0.2 0.4 0.6 0.8 1
1.2
‐5 0 5 10 15 20 25 30
Bone equivalence
(g/cm2)
Lean Equivalence (g/cm2)
Bone
Lean
Adipose
Total
Example
Example
‐0.8 ‐0.6 ‐0.4 ‐0.2
0 0.2 0.4 0.6 0.8 1
1.2
‐5 0 5 10 15 20 25 30
Bone equivalence
(g/cm2)
Lean Equivalence (g/cm2)
Bone
Lean
Adipose
Total
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Fat content correction
Image courtesy Hologic Corporation
Fat content correction
‐0.8 ‐0.6 ‐0.4 ‐0.2
0 0.2 0.4 0.6 0.8 1
1.2
0 5 10 15 20 25 30
Bone equivalence
(g/cm2)
Lean Equivalence (g/cm2)
Lean
Adipose Total so ssue
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Fat content estimation
‐0.6
‐0.4
‐0.2
0
0.2
0.4
0.6
0.8
1
1.2
‐5 0 5 10 15 20 25 30 Bone equivalence
So ssue (25% fat) Equivalence
Bone
Lean
Adipose
Total
Dual Energy X-ray Absorptiometry
ScienceTechnologyPractice
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Technologies - History
• Single Photon Absorptiometry• 1963 – Cameron and Sorenson
• Dual Photon Absorptiometry• ~1970, Cameron, Mazess, et al
• Dual X-Ray Absorptiometry• ~1980s (?); Stein, Mazess, et al
Source: Thorson and Wahner, JNucMed Vol 14, pp163-171US Patent Office
Single Photon Absorptiometry
LM Thorson, HW Wahner, “Single and Dual Photon Aborptiometry...”, J Med Vol 14, Sept 1986, pp163-171
Courtesy of Society of Nuclear Medicine and Molecular imaging.
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Dual Photon Absorptiometry
0
20
40
60
80
100
20 40 60 80 100 120 140
Yield
(%)
Photon energy (keV)
Gd153 spectrum
Detector
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Dual Photon Absorptiometry
0.125
0.25
0.5
1
2
4
8
20 40 60 80 100 120 140
Mu/Rho (cm2/g)
Photon energy (keV)
Bone Lean Adipose
Pulse height spectrum
0
2000
4000
6000
8000
10000
12000
Pulse count
PMT Pulse Height
Raw counts
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Pulse height spectrum
0
50
100
150
200
250 Pulse count
PMT Pulse Height
With 20cm water
Raster scan
Image courtesy GE Healthcare
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DXA Technologies
X-ray source• K-edge filter (GE)• kV switching (Hologic)
Beam geometry• Pencil beam (1st
generation)• Partial fan beam• Fan beam• Cone Beam
Photon counting Detectors (GE)• Na-I / PMT• CdTe (direct detection)• LYSO / SSPM
Energy integrating detectors• Scintillator/photodiode
array (Hologic)• Sandwich detector
Beam geometries
Pencil beam Wide fan beamNarrow fan beam
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Beam Geometries
Image courtesy GE Healthcare
X-ray source technologies
Characteristic GE/Lunar Hologic
Tube voltage 76 kV60kV, 120kV or100kV, 140kV
Filtration K-edgeSpinning filter
wheel*
* Six sector wheel with the following filter combinations• Air• Bone equivalent• Soft tissue equivalent• Copper• Copper + bone• Soft tissue + bone
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K-edge filtration
kV switching
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kV switching
Detector technologies
Photon counting (GE)• NaI with PMT• CdTe with direct conversion• LYSO with solid state PMT
Energy integrating• Ceramic detectors with photodiode
array (Hologic)• Sandwich detector (obsolete)
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K-edge filtration
0 20 40 60 80 100
Rela
ve photon flux
Photon energy (keV)
20 cm Water 1.0 mm Lanthanum
Sandwich detectors
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Sandwich detectors
Dual Energy X-ray Absorptiometry
ScienceTechnologyPractice
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Clinical report
Image courtesy GE Healthcare
is the average for young healthy adults
is the average for adults of the same age and sex
Diagnosis
T-Score WHO Category
-1 or higher Normal
-2.5 to -1.5 Osteopenia
-2.5 or lower Osteoporosis
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Lumbar spine BMD
Lumbar Spine and Proximal Femur Bone Mineral Density, Bone Mineral Content, and Bone Area: United States, 2005-2008
https://www.cdc.gov/nchs/data/series/sr_11/sr11_251.pdf
Femoral neck BMD
Lumbar Spine and Proximal Femur Bone Mineral Density, Bone Mineral Content, and Bone Area: United States, 2005-2008
https://www.cdc.gov/nchs/data/series/sr_11/sr11_251.pdf
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Accuracy
Parameter Value
Absoluteaccuracy
+/- 10%
Inter-modelvariation
+/- 15%
Inter-unit variation +/- 2%
Source: International Society for Clinical Densitometry (ICSD)white paper “Precision Assessment and Radiation Safety for Dual-Energy X-ray Absorptiomentry
Therapy response
Precision• Scanner stability• Technologist skill
• Positioning• Tissue segmentation
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Scanner QA
Shewhart Chart
Avg + 2 Std. Dev.
Avg - 2 Std. Dev.
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Shewhart Rules
Machine failure if• Single BMD outside average ± 3 standard deviation• Two consecutive BMD on same side of average outside avg ± 2 S.D.• Four consecutive BMD on same side of average outside avg ± 1 S.D.• Ten consecutive BMD on same side of average• Two consecutive BMD differing by more than 4 S.D.
Sources
Sydney Lou Bonnick, Bone Densitometry in Clinical Practice: Application and Interpretation, Springer, p.115 (2009)
D.Pearson, S.A.Cawte, “Long-term quality control of DXA: A comparison of shewhart rules and cusum charts”, Osteoporois International, Vol 7 (4), pp 338-343 (1997)
Technologist Precision
In-vivo Precision Measurements• Measure BMD in 30 patients 2 times, or• Measure BMD in 15 patients 3 times
Least Significant Change (LSC)95% confidence interval
Written consent recommendedIRB approval not needed
≡ 2.77
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BMD Precision
Site Coefficient of variation
Leastsignificant
change
Lumbar spine 1.0-1.2% ~3%
Total hip 0.8-1.7% ~3.5%
Femoral neck 1.1-2.2% ~4.5%
Trochanter 1.2-1.5% ~3.8%
Therapy response
Parameter Lumbar spine Femoral neck
Baseline 0.850 g/cm2 0.865 g/cm2
2 years later 0.865 g/cm2 0.858 g/cm2
Difference +0.015 g/cm2 -0.007 g/cm2
LSC for this technologist
0.027 g/cm2 0.030 g/cm2
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Accreditation - ISCD
International Society of Clinical DosimetryAccreditation Program
• Follow manufacturer’s QA recommendations• Periodic (at least once/week) phantom scans• In-vivo precision test for every technologist
• Upon completion of training• When a new DXA system is installed• When the technologist’s skill level has changed.
Accreditation - CBMD
Canadian Bone Mineral Density Facility Accreditation Program(Ontario Association of Radiologists)
• “Approved” medical physicist• Weekend long workshop on DXA QA• At initial visit
• Establish “Shewhart” baselines• Determine LSC for facility/technologist• Measure entrance skin exposure• Measure scatter
• Annual review by physicist• Establish LSC for new staff
Source: Jeff Frimeth, MSc, MCCPM, CIIP
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Accreditation - ANZBMS
Australian and New Zealand Bone and Mineral Society
Periodic tests• Daily phantom accuracy tests• Long term stability testsAcceptance tests and after major repair• Laser light positioning• Scan line and step spacing• Scan time indication• Entrance skin exposureOther• Staff scatter exposure measurements
Acknowledgements
Jeff Frimeth
Hologic Inc. and affiliates
GE Healthcare
Society of Nuclear Medicine and Molecular Imaging
UAB Osteoporosis Clinic