CT Protocol Optimisation: Balancing Image Quality and Dose

Comments

Transcription

CT Protocol Optimisation: Balancing Image Quality and Dose
CT Protocol Optimisation:
Balancing Image Quality and Dose
Maria Lewis
Guy’s & St.Thomas’ NHS Foundation Trust
Overview
• 
• 
• 
• 
• 
Introduction
Challenges of CT protocol optimisation
Radiation dose optimisation features
Dose audit
Approaches to balancing image quality
and dose
Trends in CT doses
•  Rapid technical
developments and
expanding list of
applications have led to
a dramatic increase in
the use of CT
Hall & Brenner, BJR 2008
1997/8
80
2008
•  CT contribution to
collective dose
•  1988 ≈ 40%
•  2008 ≈ 70%
% Dose contribution
70
60
50
40
30
20
10
0
Conventional
CT
Angiography
Interventional
UK data, HPA, 2008
Trends in CT doses
•  In a well-optimised practice* doses have
been decreasing
Europe 1999
Europe 2004
UK 2003
*Data from Mayo Clinic Rochester - Routine abdomen exam
Trends in CT doses
From Mahesh, AAPM 2010 - New Technologies for image quality improvement and dose reduction
Trends in CT dose
The equipment…
…and how it is used
Optimising dose and image quality
•  Aim
•  achieve desired image quality
•  lowest radiation dose possible
•  Requirement
•  fully utilise the capabilities of the equipment
Optimising dose and image quality
Imaging challenges in CT
Patient motion
Small
structures
Low contrast
structures
Scan time
Spatial resolution
Image noise
Artefacts
Radiation dose
Protocol design
kV Slice width Rota2on 2me Beam width Pitch Flying focal spot Focal spot Recon. interval FBP/ Itera2ve Acquired width Scan FOV mA Recon. kernel Scan Recon. length FOV Protocol design
•  Spatial detail
•  Contrast resolution
Protocol design: example
•  X-ray beam width selection
4-slice scanner
16 x 1.25 mm detector banks
Protocol for CTPA exam
Protocol dessign example
•  How long does it take to cover 300 mm
length?
300 mm
Rot. time (s)
1
1
1
Pitch
1
1
1
Beam width (mm)
5
10
20
1.25
2.5
5
60
30
15
Z-axis resolution (mm)
Total scan time (s)
Protocol design example
•  Is spatial resolution adequate?
300 mm
Rot. time (s)
1
Pitch
1
Beam width (mm)
20
Z-axis resolution (mm)
5
Total scan time (s)
15
Protocol design example
•  Is spatial resolution adequate?
300 mm
Rot. time (s)
1
0.5
Pitch
1
1
Beam width (mm)
5
5
1.25
1.25
60
30
Z-axis resolution (mm)
Total scan time (s)
Protocol design example
•  What about dose?
Beam width
(mm)
z-axis GE
(%)
Relative
dose
20 (4 x 5)
97
1.00
10 (4 x 2.5)
83
1.17
67
1.45
5 (4 x 1.25)
z-axis
4 x 5 mm
4 x 2.5 mm
4 x 1.25 mm
Protocol design example
•  Helical scanning includes extra rotations at
either end of imaged volume:‘over-ranging’
•  Additional dose more significant for wider
beams particularly for short scan ranges
Tzedakis et al, Med. Phys. (2005) 32 (6)
Protocol design example
•  On modern scanners dose from ‘over-ranging’
is reduced with dynamic collimators
•  Collimator blades open and close
asymetrically at start and end of scan
Scan range
Conventional technology
without Dose Shield
Courtesy Siemens Medical Systems
Scan range
SOMATOM Definition AS+ with
Adaptive Dose Shield
Protocol design example
•  Any other implications?
•  Cone beam artefacts with wider beams?
•  E.g. For head scans it may be better to use
20 mm beam width instead of 40 mm
Protocol review and optimisation
Focal kV mA/ Scan Recon. Recon. spot Beam kV AEC length interval interval width Protocol
review
is a
FBP/ complex
task Focal and Slice FBP/ Slice Acquired Itera2ve Acquired width Itera2ve spot width should
undertaken
width be
width Pitch with caution
and as a
Flying team
Scan Scan Recon. Rota2on Rota2on focal FOV FOV FOV 2me Flying 2me spot Recon. focal FOV spot mA/ Pitch AEC Recon. Recon. kernel kernel Beam Scan width length Optimisation – Working as a team
•  Team members:
•  Medical Physicist – technical
•  CT Radiologist – clinical
•  CT Technologist - implementation
•  Cultivate good inter-profession relationships
CT Protocol management and review
•  How team members may work together and in parallel:
AAPM Guidelines: J Applied Clin Med Phys. 2013;14(5):3-12
Protocol review & optimisation
•  Scanner arrives with default protocols – good
starting point
•  Applications training
•  Protocols adapted to local practice (?) with
input from:
•  Lead CT technologist
•  CT radiologists (from different specialists)
•  Medical physicist
•  Use application training and acceptance to
learn about scanner capabilities
Protocol review & optimisation
•  Once in use, review any obvious weaknesses
in image quality?
•  Too noisy, poor contrast, artefacts, high dose…
•  When making protocol changes MUST
understand your scanner
•  Consult colleagues with same scanner
•  Review literature and web resources
•  www.aapm/pubs/CTprotocols
•  AAPM CT Dose summit meetings
•  www.aapm.org/meetings/2013CTS/presentations.asp
•  Perform your own phantom studies if necessary
Trends in CT dose
The equipment…
CT dose optimisation features
•  Radiation dose control is now a priority for
manufacturers
•  Main dose optimisation features:
• 
• 
• 
• 
• 
Automatic tube current adjustment (CT AEC)
Automatic kV selection
Adaptive collimation
Organ specific dose modulation
Iterative reconstruction
AEC in CT
•  Removes guesswork from manual adjustment
for patient size
Longitudinally
Rotationally
high current
attenuation
Patient size
low current
AEC in CT
•  In practice different levels of modulation
are usually combined
• 
• 
• 
Patient size
Longitudinal
Rotational
Implementation of AEC in CT
Implementations of AEC in CT
•  Reference level of image quality must be set
Manufacturer
Image Quality setting
For reduced dose
Philips
mAs/slice
Decrease mAs/slice
Siemens
Quality reference mAs
Decrease Qual. ref. mAs
GE
Noise index
Increase NI setting
(standard deviation)
Increase S.D. setting
AECS.D.can
increase
Tube current
as well
as
Attenuation
decrease
N.I. = 10 the
dose!
Toshiba
400
350
tube current
300
250
200
150
100
50
N.I. = 15
Automatic kV selection
•  Traditionally tube potential of 120 kV used
•  Decreasing tube potential
•  Increases noise
•  Increases contrast between high & low z materials
120 kV
Automatic kV selection
Automatic kV selection
Dynamic collimation
•  Dose from ‘over-ranging’ reduced
•  Collimator blades open and close
asymetrically at start and end of scan
Scan range
Conventional technology
without Dose Shield
Courtesy Siemens Medical Systems
Scan range
SOMATOM Definition AS+ with
Adaptive Dose Shield
Organ-based tube current modulation
•  Siemens X-CARE: dose reduction to sensitive
anterior organs
X-rays OFF
120°
Lungren,
AJR, 2012
In-plane bismuth shields
•  Or use bismuth shields?
•  Use controversial
Kim et al Pediatr Radiol 2010; 40:1739
In plane bismuth shields
•  AAPM statement on use of in-plane bismuth
shields
AAPM Position Statement on the Use of Bismuth Shielding for the Purpose
of Dose Reduction in CT scanning
Policy Text: Bismuth shields are easy to use and have been shown to reduce
dose to anterior organs in CT scanning. However, there are several
disadvantages associated with the use of bismuth shields, especially when used
with automatic exposure control or tube current modulation. Other techniques
exist that can provide the same level of anterior dose reduction at equivalent or
superior image quality that do not have these disadvantages. The AAPM
recommends that these alternatives to bismuth shielding be carefully considered,
and implemented when possible.
www.aapm.org/publicgeneral/BismuthShielding.pdf
Iterative reconstruction
• Process of repeatedly improving an image
by comparison to the measured data
From: Iterative reconstruction methods in x-ray CT
Beister, Kolditz, Kalender, Physica Medica (2012) 28
Iterative reconstruction
•  Can offer improvements compared to FBP
methods
•  Noise reduction without degraded resolution
•  Artefact reduction
•  Improved spatial resolution
•  What’s the downside?
Measured data
•  Computational cost: clinically practical?
•  Change in image texture
Compare
Update
Image 2
n
3
Iterative Reconstruction Techniques, UKRC 2012
Adapted from: Keat, Iterative Reconstruction Techniques, UKRC 2012
Iterative reconstruction
•  Change in image texture can have
significant effect on visualisation of
structures (low contrast detectability)
•  Clinical acceptability of images must be
considered
S Singh, AAPM 3rd CT Dose Summit, 2013. Iterative Reconstruction: Dose it work to reduce noise...
Iterative Reconstruction
•  Vendor specific implementations
Vendor
System
Approach
GE
ASIR – Adaptive Statistical
Image Reconstruction
Statistical method
Veo (MBIR)
Statistical + Geometric
iDose4
Statistical
IMR – Iterative Model
Reconstruction (WIP)
Statistical + Geometric
IRIS – Iterative
Reconstruction in Image
Space
Image based
Philips
Siemens
SAFIRE – Sinogram AFfirmed Statistical
Iterative Reconstruction
Toshiba
AIDR – Adaptive Iterative
Dose Reduction
Statistical
Dose audit
The equipment…
…and how it’s used
Dose audit
•  Collect, for common scans
•  patient weight, sex
•  CTDIvol & DLP for each series
•  Select standard patients
•  60 - 80 kg adults
•  3.5, 9, 19, 32, 35 ± 15% for children
•  Compare to
•  National DRLs
•  Local DRLs
•  Scientific literature
CT TAP
Hospital A
NDRL
CTDIvol
(mGy)
DLP
(mGy)
12
840
12, 14
940
Courtesy Elly Castellano, RMH
Dose audit example
•  District General Hospital with 2 CT scanners
•  GE LightSpeed 32
•  Siemens Somatom Sensation 64
Background
•  Initial review of doses by radiologist
•  Small numbers (8 patients per scanner)
•  No info on patient size
The use of this scanner must be suspended Scan type
Routine Abdo-Pelvis
DLP (mGy.cm)
GE
Siemens
1707
753
National DRL
(mGy.cm)
560
Method
•  Protocols
•  Routine abdo-pelvis: helical, contrast-enhanced
Scan
type
AEC
Beam
width
(mm)
Recon
slice
(mm)
Pitch
kV
Rotation
time
(s)
Noise index/
Qual. ref. mAs
Max/min
mA
Recon
filter
GE
Helical
Smart mA
32 x 1.25
(40 mm)
5
0.969
120
0.8
24.6
750/100
Standar
d
Siemens
Helical
CAREDose 4D
24 x 1.2
(28.8 mm)
5
1.4
120
0.5
200
-
B31f
Dose review methodology
•  From RIS system ~50 consecutive
patients selected from each scanner
•  Patient images reviewed on PACS
•  Measurements of patient size
•  Noise values in ROIs
•  CTDIvol and DLP from dose report
Dose review methodology
•  Patient dimensions – effective diameter
•  Geometric mean (GM) of AP and lat dimensions
•  GM = √36.2*28.2 = 32 cm
36.2 cm
28.2 cm
Dose review methodology
•  Measurement of noise: ROI Level 1
•  Liver and aorta
Dose review results
Scanner
Patient
dimension
(cm)
CT no:
Liver ROI
Noise:
Liver ROI
CTDIvol
(mGy)
DLP
(mGy.cm)
GE
28.9 ± 3.7
97± 20
13.8 ± 2.9
14.4 ± 11.0
689 ± 554
Siemens
28.1 ± 3.0
91 ± 15
12.3 ± 2.3
11.5 ± 2.7
552 ± 141
Mean values ± S.D
•  National DRL = 560 mGy.cm
•  Mean GE doses ~ 25% higher than Siemens
doses
for Siemens routine abdo-pelvis protocol
Results:Doses
CTDI
vol versus patient size
y = 0.7487x - 9.2314
R2 = 0.7606
Siemens
70.0
CTDIvol (mGy)
60.0
50.0
40.0
30.0
20.0
10.0
0.0
20.0
25.0
30.0
35.0
40.0
Doses for GE routine abdo-pelvis protocol
Mean water-equivalent patient diameter (cm)
70.0
CTDIvol (mGy)
60.0
y = 0.2468e0.1354x
2
R = 0.8419
GE
50.0
40.0
30.0
20.0
10.0
0.0
20.0
25.0
30.0
35.0
Mean water-equivalent patient diameter (cm)
40.0
Results
•  Dose variation with patient size sub-group
DLP (mGy.cm)
GE
3000
2500
2000
1500
1000
500
0
Siemens
DRL
20.1 - 25.0
25.1 - 30.0
30.1 - 35.0
Mean patient dimension (cm)
35.1 - 40.0
Tube current modulation approach
1.6
1.0
0.6
se
a
e
cr 0.4
e
d
ag
str e d
on
e
g d cre
ec as
re e
as
e
av
er
im
Sl
k
e
1.2
se
crea
n
i
se
ng
stro ge increa
avera ak increase
we
0.2
0.0
0.5
1.0
Reference
attenuation
1.5
2.0
2.5
3.0
Relative attenuation
Image Quality
reference tube current
1.4
0.8
a
we
co Tub
ns e
tan cu
t im rren
a g t fo
en r
ois
e
1.8
es
Ob
Relative tube current
2.0
Dose audit example: Outcome
•  We suggested setting higher Noise Index
values for large patients (‘Large’ protocol)
•  problems if patient size not assessed
correctly
•  GE recommend controlling dose to large
patients by reducing ‘Max’ mA setting
•  non-uniform IQ along patient
•  Hospital decided that all large patients
scanned on Siemens scanner
Balancing image quality and dose
•  CT dose – how low can we go?
•  What is diagnostic threshold?
•  Dependent on diagnostic task
Karmazyn B et al, AJR Jan 2009
Balancing image quality and dose
Various approaches to determining
diagnostic threshold
•  Gold standard: Blinded human observer
studies at different dose levels
•  Progressive reduction of mAs in small
increments
•  Useful tool: simulation of reduced dose
scans by addition of noise
Balancing image quality and dose
Aquilion 16
120 kV, 200 mAs, 5 mm
Scanned dose: 1
Noise = 7.6 HU
0.8
0.3
0.9
0.7
0.4
0.15
0.1
Simulated dose: 0.2
0.5
0.6
Noise = 24 HU
Images courtesy Y. Muramatsu, NCC Tokyo
Conclusions
•  Modern scanners are complex
•  Every scanner is different – essential to
understand operation of your scanner
•  A team approach is essential in CT
protocol review
•  Make use of information resources
•  Consult: user manuals, manufacturers,
websites, scientific literature and colleagues
•  Dose audit is important and is a good first
step in highlighting optimisation issues
Thank you for listening
[email protected] Developments in technology
CT scanner ~1971 …
• 
• 
• 
• 
4 min rotation time
Recon overnight
2 slices/rot (8 mm)
80 x 80 matrix
…40 years later
• 
• 
• 
• 
< 0.3 s rotation
Real time recon
up to 320 slices (0.5 mm)
512 x 512 matrix
Implementation of AEC in CT
•  Implementation manufacturer specific
Longitudinal
Patient size
GE
Philips
Siemens
Toshiba
Auto mA
DoseRight ACS
DoseRight
Z-DOM
Rotational
Smart mA
DoseRight
D-DOM
CARE Dose 4D
SUREExposure
3D
Iterative loops
•  Steps
1. 
2. 
3. 
4. 
5. 
Acquire raw data-> sinogram
Generate initial image (FBP)
Forward project
Compare
FBP for correction image
(to aid convergence)
6.  Apply image regularization
7.  Update image
8.  Repeat 3-7 as necessary
5
6
4
1
7
2
•  Iterate until suitable convergence reached
Iterative Reconstruction Techniques, UKRC 2012
N Keat, Iterative Reconstruction Techniques, UKRC 2012
3
Highlighting radiation risk
•  CT – An increasing source of radiation
exposure. Brenner & Hall, NEJM, Nov 2007
28th November 2007
Overuse of diagnostic CT scans
may cause as many as 3 million excess
cancers in the USA over the next two
decades, doctors report today...
“normal” dose → hair loss, skin burns, cataractogenesis?
•
CT
over-exposure
incidents
Mad River Incident: repeated scans in
single location on baby
2008
Mad
River
incident
••  Class
action
lawsuits
against
multiple
hospitals and vendors
•  repeated head scans on child
• Media attention & requests for CT
•  Skin dose > 7 Gy
experts/ opinions
•  2009 – 2010 brain perfusion
overdose incidents
Cagnon, CT Protocols, AAPM 2012
•  multiple hospitals and vendors
•  ~ x 8 expected dose (3 – 4 Gy)
•  Lack of standardisation and/or
poor understanding of protocol
and equipment capabilities
8
Scan & reconstruction parameters
Focal kV mA/ Scan Recon. Recon. spot Beam kV AEC length interval interval width FBP/ Slice FBP/ Focal Slice Acquired Itera2ve Acquired width spot width width Itera2ve width Pitch Flying Scan Scan Recon. Rota2on Rota2on focal FOV FOV FOV 2me Flying 2me spot Recon. focal FOV spot mA/ Pitch AEC Recon. Recon. kernel kernel Beam Scan width length Defining imaging task
•  Spatial detail
•  Contrast resolution
Defining imaging task
•  Standard-sized patients
•  Non-standard patients
Case study 2: CT screening clinics
•  Dose audit at CT screening provider
•  13 clinics
•  4 scanner models: GE, Siemens, Toshiba
•  Screening examinations
• 
• 
• 
• 
• 
abdo/pelvis
calcium scoring
lung
virtual colonoscopy
+ combinations of the above
Case study 2: CT screening clinics
•  Protocols
•  Weight-based mAs tables used on GE &
Toshiba scanners
•  CARE Dose 4D used on Siemens scanners
Case study 2: CT screening clinics
•  Dose audit
•  Only total DLP values for whole exam
available
•  e.g. calcium sore + virtual colonoscopy
•  Patient weight also documented
Case study 2: CT screening clinics
•  DLPs for virtual colonoscopy + calcium score
Mean DLPALL = 689
Mean DLPSTD = 523
Mean DLPALL = 513
Mean DLPSTD = 502
Mean DLPALL = 624
Mean DLPSTD = 593
Mean DLPALL = 668
Mean DLPSTD = 558
Case study 2: CT screening clinics
•  All sites should adopt automatic exposure
control (AEC).
•  Sites should document the CTDIvol value
as well as the DLP.
•  Sites should document CTDIvol and DLP
for each exam series.
•  CT technologists should receive further
training on CT dose issues and in
particular on operation of AEC systems.
Implementations of AEC in CT
•  Using AEC ≠ Dose reduction
•  dose can increase or decrease depending
on patient size
•  dose can increase or decrease depending
on reference ‘image quality setting’
AEC in CT
•  Tips
•  Centre patient in FOV
•  Consider order of SPRs
- final one usually used
in AEC
Elliptical phantom 16 x 30 cm
SPR
AEC
mode
CTDIvol
(mGy)
Lat then AP
Auto mA
5.7
AP then lat
Auto mA
10.1
Courtesy Elly Castellano, RMH
Courtesy Siemens
AEC in CT
•  Removes guesswork from manual
adjustment for patient size
Automatic kV selection
Nelson, Optimal kV selection. AAPM 3rd CT Dose Summit 2013
Organ-based tube current modulation
•  May not always be effective
•  Another approach: in-plane bismuth shields
Iterative reconstruction
•  Clinicians initially reported
‘waxy’nature to images
•  Change in noise structure
From: Iterative reconstruction methods in x-ray CT
Beister, Kolditz, Kalender, Physica Medica (2012) 28, 94-108
Iterative Reconstruction Techniques, UKRC 2012
used
tential
straint
straint
–5!d",
suggest that the CNR be no less than and the noise be no
higher than those obtained at the reference tube potential.
With this very tight constraint on image noise, the RDFs at
80 kV were 0.780, 1.005, 1.230, 1.897, and 2.905 for XS, S,
M, L, and XL phantoms, respectively. For the requirement of
equal noise, there is a dose reduction at 80 kV for only the
XS phantom size and there is a big dose increase at 80 kV
mA at each kV adjusted to give same CNR
for L and XL phantom sizes. When ! = 1.25, the desired imagefor
quality
at other
tube potentials images
satisfies twoat
conditions:
some
applications
low kV too
Automatic kV selection
•  Relative dose factor (RDF)
all
• 
• 
noisy
RDF with no noise constraint (for iodine contrast)
2.5
ge
2
RDF
Phantom
dimensions (mm):
XS = 150 x 150
S = 300 x 200
M = 350 x 250
L = 400 x 300
XL = 480 x 380
Extra Small
Small
Medium
Large
Extra Large
1.5
1
0.5
140
0
80
Yu et al. Med Phys. 2010;37(1)
100
120
kV
140
Protocol design example
•  Choice of settings is scanner dependent
•  On a 16 - slice scanner can achieve 1.25 mm
slices with 20 mm beam width
z-axis
16 x 1.25 mm