7/24/2014
Emerging Practice of
Medical Physics in CT
Ehsan Samei, PhD, DABR, FAAPM, FSPIE
Department of Radiology
Duke University Medical Center
Disclosures
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Research grant: NIH
Research grant: RSNA – QIBA
Research grant: General Electric
Research grant: Siemens
Research grant: Carestream Health
Royalties: Oxford University Press
Share-holder: Zumatek Inc
Consultant: GLG Council
Credits
Justin Solomon
Rachel Tian
Baiyu Chen
Olav Christianson
Josh Wilson
Paul Segars
James Winslow
Duke CIPG
Clinical Imaging Physics Group
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Medical Physics 1.0
• We have done a GREAT job using
engineering and physics concepts to
– Design systems with superior performance
– Ensure minimum intrinsic performance
– Claim compliance
• But…
Why 1.0 is not enough
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Clinical performance?
Optimization of use?
Consistency of quality?
Changing technology?
Value-based healthcare?
1.0 to 2.0
• Clinical imaging physics extending from
–
–
–
–
–
intrinsic to extrinsic
Specs to performance
compliance to excellence
Quality to consistency
Equipment to operation
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Outline
A. Physics implications of new technologies
B. New metrics and metrology
C. Operationalizing medical physics 2.0
A.
Physics implications of
new technologies
Physics and new technologies
1. Hardware
– New detectors
– Operation is extra low dose
– Photon-counting
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Physics and new technologies
2. Acquisitions
– Innovative helical scans
– Wide-beam acquisitions
– AEC and its variants
Physics and new technologies
3. Image processing
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–
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Iterative reconstructions
Kernels
Quantitative CT
Higher order data analysis
• 3D rendering
• CAD
• Functional analysis (eg, perfusion)
Physics and new technologies
4. New designs and applications
– Dual-energy
– Inverse geometry
– Application specific devices
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Dental
MSK
Breast
RT
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B.
New Metrics and
Metrologies
Metrics and metrology
1. Radiometrics
– From CTDI to SSDE and beyond
2. Qualimetrics
– From CNR to d’ and beyond
– Size, contrast, and texture effects
Radiometrics
Metric
Definition
CTDI
Radiation output of a CT system in a standard sized
phantom
SSDE
Radiation output of a CT system adjusted for the average
patient size (for chest, abdomen/pelvis scans)
Organ dose
Dose to individual organs; estimated by simulation or
experimental measurement
Effective Dose
Weighted sum of organ/tissue equivalent dose for
radiation sensitive organs ignoring patient specific factors
Risk index
Weighted sum of organ/tissue equivalent risk for
radiation sensitive organs, accounting for age, gender,
anatomy
Samei, Ped Rad, in press, 2014
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Patient avg
total burden
Modality
generic
Patients
Gender
Patient age
Patient
anatomy
Patient Size
Measure-able
Metric
Scanner model
and factors
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CTDI
SSDE
Organ dose
Effective Dose
Risk index
Virtual human models for organ dosimetry
Segars el al, Medical Physics, vol. 37 (9), 2010
Population Representation
Building towards 400 patient models
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Imaging Simulation
Computer model of
X-ray CT scanner
Images reconstructed
from projections
Tube Current (mA) Modulation
Actual dose distributions
chest exam
abdomen-pelvis exam
Li, Samei et al., Med Phys, 38(1), 397-407 (2011).
Li, Samei et al., Med Phys, 38(1), 408-419 (2011).
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Typical organ dose
values (chest CT)
Error
in
orga
n
dose
Qualimetrics
1. Contrast
2. Lesion size
3. Lesion shape
4. Edge profile
5. Resolution
6. Viewing distance
7. Display
8. Noise magnitude
9. Noise texture
10. Operator noise
Feature of
interest
Image details
Distractors
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Parameters that are measured
by CNR
1. Contrast
1. Contrast
2. Lesion size
3. Lesion shape
4. Edge profile
5. Resolution
6. Viewing distance
7. Display
8. Noise magnitude
9. Noise texture
10. Operator noise
Feature of
interest
?
Image details
8. Noise magnitude
Distractors
Why CNR is not enough:
Noise texture
FBP
IR
Solomon, AAPM 2012
Resolution and noise, eg 1
Lower noise but
different texture
Comparable
resolution
x 10
1
-FBP
-IRIS
-SAFIRE
-FBP
-IRIS
-SAFIRE
2.5
2
0.6
NPS
MTF
0.8
-6
1.5
0.4
1
0.2
0
0.5
0
0.2
0.4
0.6
Spatial frequency
0.8
0
0
0.2
0.4
0.6
Spatial frequency
0.8
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Resolution and noise, eg 2
Lower noise but
different texture
Higher resolution
x 10-5
1
0.5
0.9
0.45
0.8
0.4
0.7
0.35
0.3
0.5
NPS
MTF
0.6
-MBIR
-ASIR
-FBP
0.4
0.3
0.25
0.2
0.15
0.2
0.1
0.1
0.05
0
-MBIR
-ASIR
-FBP
0
0.2
0.4
0.6
Spatial frequency
0
0.8
0
0.2
0.4
Spatial frequency
0.6
0.8
Detectability index
Resolution and
contrast transfer
×
Attributes of image
feature of interest
Image noise magnitude
and texture
(d )
2
'
NPWE
[ òò MTF (u,v)W
2
=
òò MTF (u,v)W
2
2
Task
2
Task
(u,v)E 2 (u,v)dudv
]
2
2
(u,v)NPS(u,v)E 4 (u,v) + MTF 2 (u,v)WTask
(u,v)N i dudv
Richard, and E. Samei, Quantitative breast tomosynthesis: from detectability to estimability. Med Phys, 37(12), 6157-65 (2010).
Chen et al., Relevance of MTF and NPS in quantitative CT: towards developing a predictable model of quantitative... SPIE2012
d’ vs observer performance
Christianson et al, Radiology, in print 2014
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Predicted
precision matches
empirical
precision!
PRC j  1.96 2Wj2 /Vtrue
 2.77ˆWj /Vtrue
_
 2.77
 ni  Kk
1
(V ijk V ij )2
1
n (K  1)
/Vtrue
Task-based assessment metrology
Mercury Phantom 3.0
• Diameters matching population cohorts
• Depths consistent with cone angles
• Straight-tapered design enabling evaluation
of AEC response to discrete and continuous
size transitions
Wilson et al, Med Phys 2013
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Design: Resolution, HU, noise
• Representation of abnormality-relevant HUs
• Iso-radius resolution properties
• Matching uniform section for noise assessment
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imQuest: image quality evaluation software
HU, Contrast, Noise, CNR, MTF, NPS, and d’ per patient size, mA modulation profile
Wilson et al, Med Phys 2013
Lung texture representation
30 mm
165 mm
Solomon et al, Med Phys, accepted 2014
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FBP
Noise in the uniform phantom
0
10
20
30
20
30
IR
STD (HU)
0
10
STD (HU)
0
STD (HU)
30
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FBP
Noise in the lung phantom
0
10
20
30
20
30
IR
STD (HU)
0
10
STD (HU)
0
STD (HU)
30
C.
Operationalizing
Medical Physics 2.0
Operational
medical physics 2.0
1.
2.
3.
4.
Quality by prescription
Quality by outcome
Training and communication
Pragmatism QC
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Components of quality assurance
System performance assessment
Quality by inference
Prospective protocol definition
Quality by prescription
Retrospective performance assessment
Quality by outcome
kV IR optimization
ACR phantom
Task
Optimal technique
% dose reduction
(wrt 120 kVp FBP)
No Iodine
80/100 kVp with IRIS
36%
With Iodine
80 kVp with IRIS
40%
Feature with iodine
Feature with no iodine
1
0.9
0.9
0.8
0.8
AZ
AZ
1
0.7
0.7
0.6
0.5
0.6
0
2
4
6
8
10
Eff dose (mSv)
0.5
0
2
4
6
8
10
Eff dose (mSv)
Detectability trends
with dose/size
Samei, Richard, Med Phys, in press, 2014
Smitherman, AAPM, 2014
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Protocol optimization
• Setting dose to achieve a targeted task
performance for a given size patient
Smitherman, AAPM, 2014
Protocol optimization
• Setting dose to achieve a targeted task
performance for a given size patient
Smitherman, AAPM, 2014
Quality-dose dependency
Quantitative volumetry via CT
PRC: Relative difference between any two repeated
quantifications of a nodule with 95% confidence
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Noise texture vs kernel
Siemens
GE
Solomon, Samei, Med Phys, 2012
Texture similarity
Sharpness
Solomon, Samei, Med Phys, 2012
Proper dose tracking – with size
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9
12
8
7
10
EDadj (mSv)
ED (mSv)
6
8
6
5
4
3
4
2
2
1
0
0
0
50
100
150
Patient Number
200
250
25
30
35
Effective Diamerter (cm)
40
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Size (cm)
25
30
35
40
Upper
10
17
31
55
Lower
3
6
10
18
55
31
17
18
10
10
6
3
PE Chest Protocol
60
50
CTDIvol (mGy)
40
30
VCT - Old
SOMATOM Definition
20
10
0
20
25
30
35
Effective Diameter (mGy)
40
45
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PE Chest Protocol
60
50
CTDIvol (mGy)
40
VCT - Old
30
SOMATOM Definition
VCT - New
20
10
0
20
25
30
35
Effective Diameter (mGy)
40
45
Noise per slice
FBP
ASiR
Trends in dose and noise
Abdomen Pelvis Exams (n=2358)
50
Scanner 1
45
Scanner 2
Scanner 1
45
Scanner 2
40
Scanner 3
35
35
30
30
GNL (HU)
SSDE (mGy)
40
50
25
20
25
20
15
15
10
10
5
Scanner 3
5
0
0
20
30
40
Effective Diameter (cm)
20
30
40
Effective Diameter (cm)
Christianson, AAPM, 2014
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Trends in dose and noise
Abdomen Pelvis Exams (n=2358)
50
Scanner 1
45
Scanner 2
Scanner 1
45
Scanner 2
40
Scanner 3
35
35
30
30
GNL (HU)
SSDE (mGy)
40
50
25
20
25
20
15
15
10
10
5
5
0
Scanner 3
0
20
30
40
Effective Diameter (cm)
20
30
40
Effective Diameter (cm)
Christianson, AAPM, 2014
Communication
• Insular days of medical physics are over
• We are as good as we can communicate
Pragmatic medical physics
• We need to be smarter with our 1.0
activities to clear space for 2.0 stuff
• Action for the sake of action is not
value-based
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Conclusions: Clinical imaging
physics at the cross-road
• New technologies necessitates an upgrade to
physics metrology
• Clinical needs requires to become more
operationally minded
• New healthcare realities provides us an
opportunity to become more value-conscious
Thank you!
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